AN ABSTRACT OF THE THESIS OF
Ellen T. Drake for the degree of Doctor of Philosophy
in oceanography presented on 4th March 1981
Title:
ROBERT HOOKE AND THE FOUNDATION OF GEOLOGY: A
COMPARISON OF STENO AND HOOKE AND THE HOOKE IMPRINT
ON THE HUTTONIAN THEORY. THE TECTONIC EVOLUTION OF
THE OREGON CONTINENTAL MARGIN: ROTATION OF SEGMENT
BOUNDARIES AND POSSIBLE SPACE-TIME RELATIONSHIPS
- /-JZG
IN THE CENTRAL HIGH CASCADES
Abstract Approved:
G-
Paul D. Komar
Based on the principle that the history of a discipline
is important to the discipline itself, this thesis devotes
two chapters to ROBERT HOOKE AND THE FOUNDATION OF GEOLOGY
and two chapters to modern geology, viz. THE TECTONIC EVOLUTION OF THE OREGON CONTINENTAL MARGIN. The first part
of this abstract covers the historical section of the thesis
and the second part the scientific section.
Robert Hooke was much more than the originator of Hooke's
Law or an inventor who invented or perfected meteorological
instruments and who pioneered equipment design for sounding
the depths of the ocean and collecting ocean water samples
at various depths. He supplied Isaac Newton with the concept
of centripetal force which allowed Newton to formulate his
Laws of Gravitation; he was the first to demonstrate the
pressure and volume relationships of gases which were called
Boyle's Law.
His geological contributions had a profound
influence on the development of geology, but they have been
largely ignored by modern historians and geologists. The
17th-century Dane Nicolaus Steno has been honored by
geologists as a founder of geology and the 18th-century
Scotsman James Hutton is widely recognized as the father
of modern geology.
In a comparison of Steno's geological
contributions with those of Hooke, the latter emerges as
having made a more extensive and profound contribution.
Furthermore, Hooke's system of the earth as presented in
his Cutlerian Lectures, published posthumously as Discourse
of Earthquakes in 1705, is almost identical to the theory
James Hutton announced to the Royal Society of Edinburgh
in 1785.
This similarity is not a coincidence.
That
Hutton was thoroughly aware of Hooke's writings is shown
not only by the extent to which the intelligentsia of the
18th century cited, quoted and adopted Hooke's ideas, but
also by Hutton's own text in both his Abstract of 1785 and
his Theory of the Earth in 1788.
In the few places where
Hutton disagreed with Hooke, Hutton's style became polemical.
He seemed to argue against specific points originated by
Hooke, which then act as a Hookian signature on the Huttonian
Theory.
Hooke's influence in the development of geological
thought and especially on the foundation of the pre-continental-drift paradigm was a significant one.
Robert Hooke,
therefore, like Steno, deserves recognition by geologists
as a founder of their science.
*
*
*
*
The tectonic evolution of the Oregon continental margin
centers around the process of subduction of the Juan de Fuca
plate. Models have been advanced to explain the complex
tectonic history of this area. The imbricate thrust model
seems to test well at the Oregon margin. This model, however,
is complicated by the additional processes of segmentation
and rotation. This synthesized scenario of the Cenozoic
tectonic evolution of the Oregon margin considers the subduction process, including volcanism in the Cascades, as a
near-extinction system.
This area presently could be
experiencing a period of transition from compressional
underthrusting to strike-slip motions resulting from extensional forces as the Juan de Fuca plate is coupled with
the North America plate. A model involving the rotation
of segment boundaries is proposed which may shed some
light on time-space and petrologic relationships in
The positions of these segment boundaries
are supported by bathymetric data on the continental
the Cascades.
shelf and by the deformation of marine terraces along the
Oregon and Washington coasts. Plane vector triangle
solutions calculated by Riddihough (1977) for point interactions between the Juan de Fuca plate and the America
plate show that the direction of convergence has not been
constant over the last 8.5 m.y. The data suggest that
the subducted slab had rotated at least 9° in a clockwise
direction during the last 1.5 m.y.
The model suggests
arrival of a segment boundary in coincidence with
conditions favorable for eruptions may have built the high
Central Cascade peaks during the Pleistocene.
that the
ROBERT HOOKE AND THE FOUNDATION OF GEOLOGY: A COMPARISON
OF STENO AND HOOKE AND THE HOOKE IMPRINT ON THE HUTTONIAN
THEORY. THE TECTONIC EVOLUTION OF THE OREGON CONTINENTAL
MARGIN: ROTATION OF SEGMENT BOUNDARIES AND POSSIBLE SPACE-
TIME RELATIONSHIPS IN THE CENTRAL HIGH CASCADES
by
Ellen T. Drake
A THESIS
submitted to
Oregon State University
Corvallis, Oregon
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Completed March 1981
Commencement June 1981
APPROVED:
Professor of Oceanography in Charge of Major
Dean of the School of Oceanography
Thesis presented on 4th March 1981
Typed by Ellen T. Drake for Ellen T. Drake
ACKNOWLEDGMENT
I am grateful to Professor Paul D. Komar who accepted
me as a student in spite of my maverick interests in the
history of science. Paul guided me through the tortuous,
uncharted, black waters of the Styx in the Underworld of
sleepless Gollums and Zombies known as Graduate Students
and the trauma of realization that I was one of them.
His
consistently high standards and his unerring sense of what
is science and what is history kept me from faltering onto
the primrose path of muddy thoughts enjoyed by many who
call themselves humanists.
Special thanks are due to all the members of my Committee
--besides Paul, Dean George H. Keller, Professor Cyrus W.
Field, Professor Harold E. Enlows, and Professor Frederick J.
Smith--who have been without exception, helpful, patient
and kind.
I would like to express my appreciation of Professors
Richard W. Couch, Edward M. Taylor, Robert A. Duncan, and
La Verne D. Kuim who critically read parts of this thesis
and took time to discuss points of interest with me.
Many others need to be acknowledged for various kindnesses they have shown me that gave me strength throughout
my graduate career:
John, long-time friend and counselor.
Ross, for his belief that the history of a discipline
is important to the discipline itself.
Miriam Ludwig, for her help in finding sources and her
constant moral support.
Gail Davis, for her helpful attention to students.
All my friends in the School of Oceanography (exempli
2ratia, Gordo), the Departments of English and Physics,
and the Computer Center who rooted for me.
My parents for being near.
Judy, Bob, and Linda, for being my friends all their lives.
And Charlie,...
.
Like other members of the human race, scientists
are capable of prejudice. They have occasionally
persecuted other scientists, and they have not
always been able to see that an old theory, given
a hairsbreadth twist, might open an entirely new
vista to the human reason.
I say this not to defame the profession of learning
but to urge the extension of education in scientific
history. The study leads both to a better understanding of the process of discovery and to that kind
of humbling and contrite wisdom-which comes from a
long knowledge of human folly in a field supposedly
devoid of it. The man who learns how difficult it
is to step outside the intellectual climate of his
or any age has taken the first step on the road to
emancipation, to world citizenship of a high order.
He has learned something of the forces which play
upon the supposedly dispassionate mind of the
scientist; he has learned how difficult it is to
see differently from other men, even when that
difference may be incalculably important. It is
a study which should bring into the laboratory and
the classroom not only greater tolerance for the
ideas of others but a clearer realization that
even the scientific atmosphere evolves and changes
with the society of which it is a part. When the
student has become consciously aware of this, he
is in a better position to see farther and more
dispassionately in the guidance of his own research.
A not unimportant by-product of such an awareness
may be an extension of his own horizon as a
human being.
--Loren Eiseley, The Firmament of Time
TABLE OF CONTENTS
Page
Part I: A COMPARISON OF THE GEOLOGICAL CONTRI-
BUTIONS OF STENO AND HOOKE
1
ABSTRACT
2
Introduction
The lives and scholarship of Steno and
3
Hooke
3
Geological contributions of Steno and
Hooke compared
On the organic origin of fossils
On strata
On the origin of mountains
On crystallography
On Scriptural chronology and the
Deluge
8
8
1.3
16
18
21.
Discussion and Conclusion
24
References
27
Part II: THE HOOKE IMPRINT ON THE HUTTONIAN THEORY 30
ABSTRACT
31
Introduction
Hooke's system of the earth
32
Hutton's awareness of Hooke's system
Hutton's knowledge of Hooke's ideas:
36
evidence in his own text
Part
32
42
Discussion and Conclusion
50
References
53
III: TECTONIC EVOLUTION OF THE OREGON CONTI-
NENTAL MARGIN
56
ABSTRACT
57
Introduction
General discription of the Oregon Margin
58
61
Page
Present view of the tectonic setting
68
Evidence of undert_hrusting and conti-
nental accretion
76
Fossil and sediment evidence
76
Structural evidence
78
Magnetic anomalies
Seismic refraction profiles and
82
gravity data
84
Earthquakes
92
Heat flow data
94
Volcanism
Migration of the Oregon continental
96
98
margin
Westward migration
The Cascadan Orogeny
Northward migration
A model of the trench inner slope: the
imbricate thrust model
98
102
105
107
Testing the model
Tectonic history of the Oregon
113
margin in light of the model
115
Segmentation: an additional complication
The segmentation model
120
120
Segmentation across the Oregon
margin
Rotation: a further complication
Conclusion: a synthesized scenario of
the Cenozoic tectonic evolution of the
Oregon margin
Bibliography
121
125
129
134
Part IV: ROTATION OF SEGMENT BOUNDARIES IN OREGON
AND WASHINGTON AND POSSIBLE SPACE-TIME
RELATIONSHIPS IN THE CENTRAL HIGH CASCADES
152
Page
ABSTRACT
153
Introduction
154
Direction of plate convergence
155
Rotation of the subducted crust
157
Segment boundaries indicating possible
age relationships among Central High
Cascades volcanoes
161
166
A corollary
Correlation of marine terraces deformation
with segmentation boundaries
168
Discussion and Conclusion
173
References
175
LIST OF ILLUSTRP.TIONS
Figure
Pa e
Part I
1
Portraits of Nicolaus Steno
2
Hooke's drawings of giant ammonoid fossils 12
3
Steno's diagrams of crystal forms
Hooke's diagrams of crystals
4
5
7
19
20
Steno's diagrams of the six aspects of
Tuscany
23
Caricature of James Hutton
33
Major physiographic provinces of western
Oregon and submarine features offshore
62
Part II
1
Part III
1
2
3
4
Diagrammatic geologic cross-sections from
Willamette Valley to continental shelf
near latitude 440 N
63
Geologic map of Oregon continental shelf
and coastal area
65
Morphology of the Oregon continental
shelf and slope
5
6
7
Major lineaments of the simatic crust
of the Pacific Ocean basin
69
Tectonic elements of the Pacific Ocean
basin
70
Configuration of plate boundaries 53 m.y.
ago and present plate configuration
near Juan de Fuca ridge
8
9
6c
Relative motions of the Pacific, Juan de
Fuca, and North America plates
72
74
Vector diagram for deducing relative
motion between Juan de Fuca and North
America plates
74
Page
Figure
10
Reflection profiles across Washington
continental margin showing deformation
of slope sediments
11
80
Raff and Mason's (1961) summary diagram
of the magnetic anomalies in the
northeast Pacific
12
13
14
Location map for crustal and subcrustal
sections shown in Figs. 13 and 14
83
85
Hypothetic crustal and subcrustal section
87
along line AA' in Fig. 12
Hypothetic crustal and subscrustal section
88
along line BB' in Fig. 12
15
Locations of continental margin during
100
16
the Mesozoic
Discordant tectonic patterns of the
104
17
northwest
Location of the trench inner slope in
relation to other features of the
fore arc
18
Trench margin model showing landwarddipping, concave-upward thrust faults
19
110
Development of fan structure and
slumping during inbricate thrusting
20
108
111
Trench slope model and late Cenozoic
imbricate thrusting and sedimentary
relationships at the Oregon margin
114
21
Alignment of Cascade volcanoes and seg123
22
ment boundaries
Segment boundaries in Oregon and
123
23
Washington
Pacific Northwest in middle Eocene time
showing clockwise rotation of Coast
Range block
126
Page
Figure
24
Diagrams showing clockwise rotation of
California during Nevadan orogeny
25
Schematic diagram showing major crustal
elements in early Eocene
26
128
131
Schematic configuration after collision
of the spreading ridge with the continent
scuth of the Mendocino fracture zone
132
Part IV
1
Convergence directions of the Juan de Fuca
2
plate with respect to the North America
plate over the last 8.5 m.y.
156
Segment boundary wedge positions in the
northwest over the last 8.5 m.y.
160
ROBERT HOOKE AND THE FOUNDATION OF GEOLOGY: A COMPARISON OF STENO AND HOOKE AND THE HOOKE IMPRINT ON THE
HUTTONIAN THEORY. THE TECTONIC EVOLUTION OF THE OREGON
CONTINENTAL MARGIN: ROTATION OF SEGMENT BOUNDARIES AND
POSSIBLE SPACE-TIME RELATIONSHIPS IN THE CENTRAL HIGH
CASCADES
PART I
A COMPARISON OF THE GEOLOGICAL CONTRIBUTIONS OF
STENO AND HOOKE
Presented at the Annual Meetings of the Geological Society
of America, San Diego, California, November 1979.
With Paul D. Komar. Accepted for publication by the Journal
of Geological Education.
2
ABSTRACT:
Nicolaus Steno, the 17th-century Dane, has been honored
by geologists as a founder of geology. In a comparison of
Steno's geological contributions with those of the 17thcentury Englishman Robert Hooke, however, Hooke emerges as
having made a more extensive and profound contribution.
While both stood out from their contemporaries in expressing
an organic origin for fossils, Hooke went further by dis-
cussing the extinction and transmutation of species and that
fossils might provide a key to the chronology of natural
events.
Hooke went beyond Steno's concept of strata in
launching a clear concept of the cyclic nature of the
dynamic processes that alter the surface features of the
earth. On the origin of mountains, instead of limiting
himself to the various ways the original horizontal strata
could be tilted or faulted, as Steno had, Hooke placed the
forces occurring in the earth in a universal context calling
forth such concepts as polar wandering, shifts in earth's
center of gravity and magnetism, isostatic compensation-subjects much discussed in the 20th century. On the subject
of crystallography Steno was content to express the surficial
relationships of crystals while Hooke believed that the external form of crystals was an expression of the internal
arrangement of particles. Finally, while Hooke could not
conceive of the modern vast geological time-scale, he was
not, as Steno was, bounded by Scriptural limitations.
Robert Hooke, like Steno, therefore, deserves recognition
by geologists as a founder of their science.
3
INTRODUCTION
This article is concerned with the very foundation of
modern geology.
My purpose is to show that while Niels
Stensen, better known as Nicolaus Steno, is recognized by
most geologists as a founder of geology, the work of
Robert Hooke, more extensive and more profound than Steno's,
has been generally ignored by both historians and geologists
despite a very likely and enormous influence his writings
had on the development of geology.
THE LIVES AND SCHOLARSHIP OF STENO AND HOOKE
The differences between Steno and Hooke lie as much in
the way others perceive their lives and their works as in
the different personalities possessed by the two men.
Steno
can best be characterized as a scientist-humanitarian.
was known as an anatomist, having
He
written 24 treatises on
anatomy; his reputation in geology is based largely on only
one substantial contribution, his De Solido intra Solidum
Naturaliter Contento Dissertationis Prodromus, referred to
as the Prodromus, published in 1669.
services were sought after by
Steno's scholarly
some of the most powerful
courts of Europe, especially by the Grand Dukes of Tuscany.
After his conversion to Catholicism, however, Steno gave
up his scientific pursuits and led a life of self-denial;
he gave all his personal wealth to the poor and suffering
from fasting and poverty he died in 1686 at age 48.
4
Cosimo III had Steno's body interred in the famous
San Lorenzo, Chapel of the Medicis.
Today, on the
cloister wall of San Lorenzo there is a medallion portrait
of Steno, surrounded by a marble wreath, with a Latin inscription under it.
This carving was provided by geologists
who convened in Bologna at the International Geological
Congress in 1881.
Apparently more than a thousand scien-
tists from many parts of the world contributed to it and
then made a pilgrimage to his tomb from Bologna to Florence
to pay homage to this man "of surpassing distinction among
Geologists and Anatomists."
Robert Hooke, a contemporary of Steno, was quite a
different sort of man.
as a scientist-humanist.
He could perhaps be characterized
Although he is known to most
scientists via Hooke's Law, geologists generally are not
aware of his contributions in geology. Hooke's scientific
contributions are too numerous to summarize here.
He was
one of the world's great inventors, having invented or
perfected, just to name a few things, the spring-controlled
balance wheel for watches, the universal joint, the clockdriven telescope, the air pump, practically all the meteorological instruments like the thermometer, the wheel barometer,
the wind gauge, and many others.
In his Micrographia (1665) he described accurate observations and presented perceptive discussions on interference
colors, such as occurring in flakes of mica and air films
5
between glasses, which ironically were then named "Newton's
Rings" and were the cause of a dispute with Newton.
Apparently without the ability or inclination to derive
rigorously the mathematical relationships, but with great
leaps of imagination, Hooke anticipated Newton in the con-
cepts behind the Universal Law of Gravitation, even as to
the inverse-square relationship, which became another
point of contention.
His
fertile and imaginative mind
allowed him to conceive the centripetal force as an opposite
and equal force to the familiar centrifugal effect.
This
fundamental concept communicated to Newton by Hooke allowed
Newton then to proceed to complete his Universal Law.
R. S.
Westfall (1967) wrote,
The major step forward in the treatment of curvilinear motion from Huygens' Horologium to Newton's
Principia was embodied in the substitution of the
word centripetal for the word centrifugal.
I do
not know of any document in which Newton employed
either the word or the concept before Hooke instructed him to do so.
Without the concept of
centripetal force, the theory of universal gravitation was inconceivable, and Hooke's contribution
to gravitation was not then insignificant.
I. B. Cohen (1964) gives convincing proof that "Boyle's
Law" was first demonstrated with accurate experimentation
by Hooke.
He also made important experimentation on com-
bustion and respiration, skillfully demonstrating his ideas
with dissection and vivisection, self-sacrificially experi
6
menting with himself in a partially evacuated vessel.
Hooke's geological writings were published posthumously
in 1705 as Discourse of Earthquakes, but they had been
read years earlier to the Royal Society of London in a
series of lectures.
In addition to all these scientific
and technological contributions, Hooke was also an architect
of note, having worked side by side with Christopher Wren
in designing and re-building London after the Great Fire
in 1666.
To discuss his architectural achievements, however,
would take another article of some length ('Espinasse, 1956).
In his personal life, Hooke, unlike Steno, enjoyed
himself, regularly frequented taverns and pubs, had affairs
with his maid servants and a young niece, and died in 1703
at age 68.
He left a huge chest full of money, some of which
he had intended to bequeath to the Royal Society, but, like
many of his intentions, never got around to doing it.
He
was buried in St. Helen's church in London, but unlike
Steno's case, there is no stone to mark the site of his
grave, and there is no record of even where the site might be.
Steno's likeness, besides having been carved on the walls
of the San Lorenzo cloister, was painted at least twice, an
earlier one now hanging in the Pitti Palace and a later one
painted while he was Vicar of Schwerin (Fig. 1).
No
portrait of Hooke is known to exist, but his physical appearance was described by Richard Waller (1705) as "crooked,"
"very pale and lean," "his Eyes grey and full, with a sharp
Figure 1.
Nicolaus Steno before and after he gave
up science to devote full time to religion (from White, 1968).
8
ingenious look," and "he went stooping and very fast having
but a light Body to carry, and a great deal of Spirits and
Activity, ...
."
He needed to be fast to accomplish all that he did.
Even so, critics complain that he started many things but
finished few.
But because Hooke's interests were so broad
and he distinguished himself in so many areas, it is quite
understandable that he left many other areas unfinished.
An individual specialist, only concerned with his own specialty, might indeed find Hooke's achievements lacking in that
particular field.
Reading Hooke's descriptions of specific
experiments he performed, one is often frustrated by his
sudden abandonment of a project because of illness or pressures
of other work.
The totality of his achievements, however,
is almost astronomical relative to the scale of human capabilities.
His work gave impetus to the development of so
many fields that to do him justice risks the label of
Hooke-glorification.
GEOLOGICAL CONTRIBUTIONS OF STENO AND HOOKE COMPARED
On the Organic Origin of Fossils
Typical of the prevailing notions on fossils in the
17th century were those of Martin Lister (1671), that fossils
possessed a "plastick vertue" inherent in the environment
in which they were made; the fact that they resembled real
organisms was simply a sport of nature.
Although some earlier
9
men, e.g. Leonardo da Vinci, recognized the organic origin
of fossils, Steno and Hooke stood out from their contemporaries in expressing this belief; Hooke pre-dated Steno by
a few years.
Steno stated in his Prodromus,
those bodies which are dug from the earth and which
are in every way like the parts of plants and
animals, were produced in precisely the same manner
and place as the parts of the plants and animals
were themselves produced.
In his understated and modern style and with a generous
and open-minded attitude towards others who might disagree
with him, Steno had written in 1667 (p. 43):
While I show my opinion to be probable, I do not
accuse the supporters of the opposite view of falsehood.
The same phenomenon may be interpreted in
various ways; indeed, Nature in her processes
gains the same end by various means. Hence it would
be unwise to regard just one method of them all
as true, and condemn all others as false.
Hooke, perhaps a more colorful character than Steno,
wrote in a more polemical and literary style, not sharing
Steno's tolerance, but demonstrating no less a modern
attitude.
He complained that some people who had tried to
explain the origin of fossils had "so far rambled from the
true and genuine Cause of them, that they have left the Matter
much more difficult than they found it."
He dismissed all
the "astrological and magical" fancies as "fantastical and
groundless," and although some other hypotheses and conjectures
to
were "somewhat more tolerable" than these,
yet most of them have recourse to some vegetative
or plastic.' Vertue inherent in the Parts of the
Earth w.-iere they were made, or in the very parcels
of which they consist., which, to me, seems not at
all consonant to the other workings of Nature;
for those more curiously carved and beautiful
Forms are usually bestow'd on some vegetable or
animal Body.
(Earthquakes, p. 288.)
Dom Remacle Rome (1956) in writing about the correspondence
between Steno and the Royal Society of London, summarized
Hooke's ideas on fossils, "Pour lui les fossiles e'taient
d'origine animale... (For him the fossils were of animal
origin...)" then added, "Hooke tenait bon, malgre le peu
de succes de ses explications aupres de ses collegues; it
ne pouvait manquer de voir en Stenon un puissant allie.
(Hooke held fast in spite of the little success his ex-
planations had in the opinion of his colleagues; he could
not miss seeing in Steno a powerful ally.)"
There is no
evidence, however, that Hooke felt particularly supported
by Steno's assertions.
His own convictions based on his
own observations were sufficient for him.
Hooke, in fact,
had accused Henry Oldenburg, secretary of the Royal Society,
of sending his ideas in Earthquakes to Steno allowing the
latter then to plagiarize him (Hooke, 1677).
Both Steno and Hooke, then, agreed that fossils have
an organic origin.
his thinking
Hooke's imagination, however, carried
much farther.
Fossils, he maintained, could
11
act as records of the past; just as old coins could
"inform an Antiquary that there has been such or such a
place subject to such a Prince," so fossils could
certify a Natural Antiquary, that such and such
places have been under the Water, that there have
been such kind of Animals, that there have been
such and such preceding Alterations and Changes
of the superficial Parts of the Earth; And
methinks Providence does seem to have design'd
these permanent shapes, as Monuments and Records
to instruct succeeding Ages of what past in preceding. And these written in a more legible
Character than the Hieroglyphicks of the ancient
Egyptians, and on more lasting Monuments than
those of their vast Pyramids and Obelisks. (Earthquakes., p. 321.)
In a later passage (p. 411) he wrote, with almost clairvoyance,
it must be granted, that it is very
difficult to read them, and to raise a Chronology
(Hooke's italics) out of them, and to state the
And though
intervalls of the Times wherein such, or such
Catastrophies and Mutations have happened; yet
'tis not impossible, but that, by the help of those
joined to other means and assistances of Information,
much may be done even in that part of Information
also.
This passage shows that although he was not a stratigrapher,
Hooke had an instinctive feeling for the usefulness of
fossils in determining a sequence of events. His drawings
of some giant ammonoid fossils found in England have not been
surpassed by later paleontologists (see Fig. 2).
12
z 63 -
a.
/
S9.
.
L
Figure 2.
f-A
Ilooke's drawings of giant armmnncoid
(from Discourse of ,arthquakes).
fossils
1=J
Beyond the organic origin of fossils and that these
fossils might be used to "raise a chronology," Hooke also
speculated on the evolution of species and generation
of varieties. He recognized that the giant ammonites
found in Portland quarry stone represent extinct species
and there were "Shells of divers Fishes Petrify'd in Stone,
of which we have now none of the same kind," because of
the destruction of their environment. By the same token
"divers new varieties" of the same species could be generated by a change in the environment. (Earthquakes, p. 327.)
His "eleventhly" proposition (p. 291) stated succinctly,
that there have been many other Species of Creature
in former Ages of which we can find none at present;
and that 'tis not unlikely also but there may be
divers new kinds now, which have not been from the
beginning.
On Strata
Steno's ideas on sedimentary strata are clearly,
concisely and methodically stated. He defined his terms
and expressed general truths in a modern attitude. He
wrote (Prodromus, p. 30),
At the time when any given stratum was being formed,
all the matter resting upon it was fluid, and,
therefore, at the time when the lowest stratum
was being formed, none of the upper strata existed.
All strata, then, except the lowest, were bounded by two
planes parallel to the horizon, and it follows that
14
strata either perpendicular to the horizon
or inclined toward it, were at one time parallel
to the horizon.
The Prodromus is largely a statement of such seemingly
self-evident truths, self-evident to us because we are so
familiar with these ideas.
But such statements as the law
of superposition made geology a science.
Hooke's ideas on strata were implicit rather than
explicit; for example, he spoke of sediments as layers,
e.g., the "Layers of the several Earths, Sands, Clays,
Stones, Minerals, etc. that are met with in digging
Mines and wells." (Earthquakes, p. 325.)
He assumed
superposition and original horizontality of sediments;
thus, "many parts are cover'd and rais'd by Mud and Sand
that lye almost level with the Water ..." (Earthquakes,
p. 313.)
He realized, however, that water was not the
only agent of deposition; he recognized burial by volcanic
ash and that motion of the air could transport dust from
place to place.
Possibly Hooke's quick mind was a step ahead of himself so that he had assumed the truths expressed by Steno,
as we assume them today.
Instead of setting his assumptions
down as Steno had done so systematically, his mind raced
ahead and he discussed at length not only the depositional
features but conversely the erosional features.
It is typical
of Hookes' mental modus operandi to penetrate quickly beyond
what was apparent to him to a more profound understanding.
15
So he proceeded from the formation of layers of sediments
to all agents that cause alterations to the surface of the
earth.
wind, rain, snow, tides, currents all contribute
to changes in the earth's surface.
As floods from rivers
carried away all things that stood in their way and covered
the land with mud, "levelling Ridges and filling Ditches,"
so tides and currents in the sea washed away cliffs and
wasted the shores.
As the Nile delta was enlarged by
sediments, the Gulf of Venice "choak'd with the Sand of the
Po," and the mouth of the Thames grown shallow from sand
brought down with the stream, so "Cliffs that Wall this
Island do Yearly founder and tumble into the Sea."
As
many parts are covered by sediments, "others are discover'd
and laid open that for many Ages have been hid."
Hooke
was keenly aware that these processes had been going on
"as long standing as the World."
In describing the cyclic rhythm of nature, saying
that "all things almost circulate and have their Vicissitudes,"
Hooke, who wrote with literary grace, waxed poetic:
Generation creates and Death destroys;
Winter reduces what Summer produces:
The Night refreshes what the Day has scorcht,
and the Day Cherishes what the Night benumb'd.
(Earthquakes, p. 313.)
While Hooke was possibly the first actualist (rather
than a strict uniformitarian) since classical times and
described the "almost" steady state of deposition and erosion,
16
subsidence and uplift, or the cyclic nature of rain, he
sensed in the universe a perpetual change that is progressive
and, in a way, teleological.
Not knowing the Second Law
of Thermodynamics or of radioactivity, in speaking of the
fuel that caused "the Subterraneous Flame or Fire, or
Expansion"--he did not know the exact nature of subterranean heat--he said that this fuel gets spent, that is,
"converted to another Substance, not fit to produce any
more the same Effect."
In these mega-thoughts, the Dane was silent.
On the Origin of Mountains
To Steno the major cause of the formation of mountains
was attributable to the various effects of changing the
original horizontality of strata.
He clearly understood
the concept of faulting, as strata could change their
position by
spontaneous slipping or downfall of the upper strata
after they have begun to form cracks, in consequence
of the withdrawal of the underlying substance, or
or foundation.
(Prodromus, p. 31-32.)
Steno recognized that mountains could also be formed "by
eruption of fires which belch forth ashes and stones
together with sulphur and bitumen."
He thought that moun-
tains could also be formd by "the violence of rains and
torrents."
His explanation for this case is unclear but
seems to mean the piling up of debris washed down by water
17
from higher places.
By these processes the uneven surface
of the earth was produced "while in the bowels of the
earth subterranean passage and caverns
are produced."
(Prodromus, p. 31-33.)
Hooke's ideas did not differ fundamentally from
Steno's as to the origin of mountains, but again his fertile mind raced ahead.
Uplift and subsidence are essential
characteristics of his concept of tectonics.
He
attri-
buted mountain-building to earthquakes by which term he
meant any kind of earth movement
violent at times or at
others progressing slowly, in his words, "by degrees."
As Edwards (1936) and Carozzi (1970) noted, Hooke's notion
corresponds to the modern sense of diastrophic movements,
not particularly to the restricted sense of seismic disturbances.
Clearly, Hooke has been misinterpreted by some
historians in his use of the word earthquakes, as he was
called "the first catastrophist" by the historian of science,
Richard Westfall (1972).
In his earlier discourses (1668) Hooke's purpose was
to account for the presence of marine fossils in high and
dry places; later he wove his thoughts into a theory of
global dynamics.
Not stopping with setting down the laws
of sedimentation or tilting of the strata as Steno did, he
placed the dynamic processes of the earth in an universal
context.
He
took into account the motion of the earth
around the sun in the plane of the ecliptic and of the tilt
18
of the axis of diurnal rotation.
He described polar
wandering and possible shifts in the earth's center of
gravity.
Such shifts could cause not only "sliding, sub-
siding, sinking and changing of the internal parts of the
earth as well as external" and thereby causing earthquakes
and changes in sea level, altering the positions of sea
and dry land, but could also cause changes in the "magnetical
Power and Vertue of the Body of the Earth."
Judging from
the type of fossils, he suggested England must have been
under water and in a torrid zone at some time in its past.
Although the term isostatic compensation was unknown to
Hooke, he had an idea of its action.
He wrote,
'tis very probable that whensoever an Earthquake
raises up a great part of the Earth in one place,
it suffers another to sink in another place.
(Earthquakes, p. 320.)
On Crystallography
Steno is credited with the fundamental crystallographic
concept, the constancy of interfacial angles.
To Steno a
crystal grows by accretion, i.e., "new crystalline matter
is being added to the external planes of the crystal already
formed."
(Prodromus, p. 39.)
He showed by diagrams (Fig. 3)
that the length and number of the sides may vary but the
angles between faces remain unchanged.
Hooke, on the other hand, assumed "corpuscularity" of
the internal structure of solids.
His diagrams (Fig. 4) show
19
0
2
1
0
5
8
4
3
I
6
9
10
13
Figure 3.
Steno's diagrams showing the various forms
of crystals, figures 1-13 in his Prodromus. Steno's figures
7 and 13 particularly show that he believed that crystals
grow by accretion.
I,
Figure 4.
Hooke's diagrams of crystals.
graphia, Schema VII.)
(From Micro-
Hooke believed the external form of
crystals to be an expression of the internal arrangement of
particles.
21
clearly that he believed the external form of crystals to
be an expression of the internal arrangement of particles
which he referred to as "bullets" or "globules."
Hooke's
diagrams not only express Steno's constancy of interfacial
angles but also Haty's law of rational axial intercepts
(Schneer, 1960).
Hooke published his diagrams in 1665
in his famous book Micrographia, four years before the
Prodromus in which Steno published his diagrams.
Steno
had rejected the idea that the shape of the whole crystal
should resemble the shape of the arrangements of particles.
He might well have contemplated over Hooke's diagrams and,
as Schneer suggested, redrew them omitting the corpuscles.
On Scriptural Chronology and the Deluge
The most striking difference between Steno and Hooke,
however, is their different attitudes towards Scriptural
chronology and the Deluge.
Steno, a deeply religious man,
even before his conversion to Catholicism, could not have
denied the Scripture.
He therefore had to contort geologi-
cal events into the six thousand years or so time limit.
He divided the origin of the earth into six "aspects"
corresponding to the six days of Creation, the fourth aspect
being the Universal Deluge.
He considered the remains of
Tertiary extinct animals, including elephants, found in the
Arno Valley, as the pack animals Hannibal's army brought on
its way to Rome that died after being mired in the mud (Prodromus, p. 64-65).
He freely alluded to Genesis for
22
confirmation of his arguments.
For example, he wrote, "in
regard to the first aspect of the earth, Scripture and Nature
agree in this, that all things were covered with water."
(Prodromus, p. 69.)
The second and third aspects of his
history were a dry plane and a surface of great relief,
respectively.
With regard to the time of the Flood, the
fourth aspect, Steno wrote,
secular history is not at variance with sacred history
which relates all things in detail.
The ancient
cities of Tuscany, of which some were built on
hills formed by the sea, put back their birthdays
beyond three thousand years; in Lydia, moreover,
we come nearer to four thousand years so that it
is possible thence to infer that the time at which
the earth was left by the sea agrees with the time
of which Scripture speaks.
(Prodromus, p. 72.)
Huge plains again appeared after the flood for the fifth
aspect, and "Nature proves that those plains existed, and
Scripture does not gainsay it."
(Prodromus, p. 74.)
The
sixth aspect of the earth, then, is "evident to the senses,"
as it represents the present configuration of the surface
of the earth.
Steno's diagrams showing the six aspects of
Tuscany are depicted in Figure 5.
Hooke's attitude toward Scriptural chronology and the
Deluge was, by contrast, a very practical one.
He certainly
would have denied a literal interpretation of the Bible, as
he mistrusted the account of anything before the invention
of writing.
He wrote,
23
F
20
B
D
C
A
I2
C
21
C
F
23
F
24
F
25
Figure 5. Steno's diagrams showing the six aspects
of Tuscany, Figures 20-25 in his Prodromus. Note that
although Steno labeled the diagrams in numerical order from
the top down, like a true geologist, he depicted the events
chronologically from the bottom up.
24
The great transactions of the Alterations, Formations, or Dispositions of the Superficial Parts
of the Earth into that Constitution and Shape which
we now find them to have, preceded the invention of
Writing, and what was preserved till the times of
that Invention were more dark and confused, that
they seem to be altogether Romantick, Fabulous,
and Fictious, and cannot be much relied on or heeded,
and at best will only afford us occasions of
Conjecture.
(Earthquakes, p. 334.)
He did not deny the Creation of the world, but he referred
to this act of God as "'that extraordinary Earthquake."
He accepted that Noah's flood was possible, but he said
"several other Floods we find recorded in Heathen writers"
could also make alterations on terrestrial surface features.
Certainly, he insisted, that the fossils in rocks could
not have been placed there either by "Men's Hands" or "the
General Deluge which lasted but a little while."
Although
Hooke's conception of time did not extend to the hundreds
and thousands of millions of years of the geological time
scale, he was at least ahead of his own time, as well as
ahead of some subsequent times, in refusing to be bounded
by the Scriptural chronology, as Steno was.
DISCUSSION AND CONCLUSION
In spite of Steno's relatively short scientific career
and the paucity of his geological writings, scientists and
historians generally agree that his geological contribution
25
has survived to modern times.
The consensus among writers
who refer to Hooke's geological work (e.g. Lyell, 1830;
Carozzi, 1970; Turner, 1974), on the other hand, seems
to be that his geological writings were forgotten or
neglected by later scientists.
Why Hooke's geological writings had no influence for
the first fifty years of their existence is not the concern
of the present paper.
Possibly the greater reputation of
Newton and the known controversy between Hooke and Newton
made Hooke's writings unpopular during the first half of
the 18th century.
But like the seemingly sudden appearance
of a variety of life-forms at the base of the Cambrian,
by the middle of the 18th century, it is clear that Hooke's
Discourse of Earthquakes was a well known piece of literature.
Hooke's reputation as a scientist by then was also widely
respected.
Not only was the discourse not forgotten,
therefore, but it was cited, quoted, and in some cases
adopted by geologists in the 18th century (History and Philosophy of Earthquakes, 1757; Raspe, 1763; Whitehurst, 1786).
In particular, James Hutton, Considered the "Father of
Modern Geology," was apparently keenly aware of Hooke's legacy
to him, although he never once extended credit to Hooke or
even mentioned his name.
The Huttonian Theory, as announced
to the Royal Society of Edinburgh in 1785, was almost identical to Hooke's system of the earth (Davies, 1964).
similarity was not a coincidence.
This
26
Evidence of Hooke's impact on the Huttonian Theory is
the subject of another article (Part II of this thesis and
Drake, in press).
My conclusion in that article is that
was it almost impossible for Hutton not to have
been aware of Hooke's writings, but Hutton's own text in his
Abstract of 1785 and Theory of the Earth of 1788 indicates
that he must have studied Hooke's ideas very carefully; he
referred to singularly Hookian concepts and in the few
places he disagreed with Hooke, Hutton's style was polemical.
Robert Hooke is important in the history of geology
because his writings reveal the extent of pre-Hutton and
not only
pre-Werner geological
those of
knowledge, and because they, like
were transmitted by later geologists.
The continuity of the development of geological thought
is intact and James Hutton and his 18th century colleagues
were the essential links. The notion that there were the
vague beginnings and then there was Hutton is simply not
the
Steno,
case.
If my interpretation is
correct, then Robert Hooke
had an enormous influence on the development of geology and,
with
Steno,
science.
deserves to be recognized as a founder of this
27
REFERENCES
Carozzi, A. V., 1970, Rcbert Hooke, Rudolf Erich Raspe, and
the concept of "earthquakes": Isis, v. 61, p. 85-91.
Cohen, I. B., 1964, Newton, Hooke, and 'Boyle's Law': Nature,
v. 204, p. 618-621.
Davies, G. L., 1964, Robert Hooke and his conception of
earth-history: Proceedings of the Geologists' Associa-
tion, London, v. lxxv, p. 493-498.
Drake, E. T., in press, The Hooke imprint on the Huttonian
Theory: American Journal of Science.
Edwards, W. N., 1963, Robert Hooke as geologist
lutionist: Nature, v. 137, p. 96-97.
and evo-
'Espinasse, M., 1956, Robert HHooke: London, William Heinemann, 192 p., Chapter 5, p. 83-105.
History and Philosophy of earthquakes, from the remotest
to the present times: collected from the best writers
on the subject, 1757: London, J. Nourse, 351 p.
Hooke, R., 1665, Micrographia: or some physiological descriptions of minute bodies made by magnifying glasses,
with observations and inquiries thereupon: London,
Jo. Martyn and Ja. Allestry, printers to the Royal
Society. Reprinted 1966, Bruxelles, Culture et Civi-
lisation, 246 p.
, 1668-ca. 1699, Lectures and discourses of earthquakes, and subterraneous eruptions, explicating the
causes of the rugged and uneven face of the earth;
and what reasons may be given for the frequent finding
of shells and other sea and land petrified substances,
scattered over the whole terrestrial supervicies: in,
Wailer, R., 1705, The posthumous works of Robert Hooke,
M.D. S.R.S. Geom. Prof. Gresh &c. containing his Cutlerian
lectures, and other discourses, read at the meetings
of the illustrious Royal Society: London, Smith &
Walford, printers to the Roy. Soc., p. 279-450.
1677, Lampas, or descriptions of some mechanical
improvements of lamps and waterpoises, together with
some other physical and mechanical discoveries: London,
John Martyn, printer to the Royal Society, 54 p. In,
Gunther, R. T., 1931, Early science in Oxford, v. VIII,
The Cutler Lectures of Robert Hooke: Oxford, p. 154-208,
postscript.
28
1678, Lectures, De potentia restitutiva or
Of Spring, explaining the power of springing bodies:
London, John Martyn, printer to the Royal Society,
25 p.
In, Gunther, R. T., 1931, Early science in
Oxford, v. VIII, The Cutler Lectures of Robert Hooke:
Oxford, p. 331-356.
Hutton, J.,1785, Abstract of a dissertation read in the
Royal Society of Edinburgh, upon the seventh of March,
and fourth of April, 1785, concerning the system of
the earth, its duration, and stability: Reproduced
in facsimile in, White, G. W., ed. Contributions to
the history of geology, v. 5: Darien, Conn., Hafner,
1970, 30 p.
1788, Theory of the earth, or an investigation
of the laws observed in the composition, dissolution,
and restoration of land upon the globe: Transactions
of the Royal Society of Edinburgh, v. I (Part II),
p. 209-304.
,
Lister, M., 1671, A letter of Mr. Martin Lister: Philosophical Transactions, v. VI, p. 2281-2285.
Lyell, C., 1830, Principles of geology, being an attempt
to explain the former changes of the earth's surface,
by reference to causes now in operation: London, John
Murray, p. 31-35.
Raspe, R.E., 1763, Specimen historiae naturalis globi terraquei, praecipue de novis e maris natis insulis, et
ex his exactius descriptis et observatis, ulterius
confirmanda, Hookiana telluris hypothesi, de origine
montium et corporum petrefactorum: Amsterdam and Leipzig,
J. Schreuder and P. Mortier, 191 p. Translated and
edited by Iversen, A. N., and Carozzi, A. V., 1970:
New York, Hafner.
Rome, D. R., 1956, Nicolas Ste'non et la "Royal Society of
London": Osiris, v. 12, p. 244-268.
Schneer, C., 1960, Kepler's new year's gift of a snowflake:
Isis, v. 51, p. 531-545.
Steno, N., 1667, Canis Carcharia dissectum caput, translated
as The earliest geological treatise by Garboe, A., 1958:
London, Macmillan, 51 p.
1669, De solido intra solidum naturaliter contento
dissertationis prodromus: Florentiae, 76 p. English
,
29
version with notes by Winter, J. G., 1.916: New York,
Macmillan, p. 169-283. Reproduced in facsimile in,
White, G. W., 1968 (see G. W. White reference below).
Turner, A. J., 1974, Hooke's theory of the earth's axial
displacement: some contemporary opinion: The British
Journal for the History of Science, v. 7 (no. 26),
p. 166-170.
Walter, R., 1705, The life of Dr. Robert Hooke, in The
Posthumous Works of Robert Hooke: London, Sam. Smith
& Benj. Walford, printers to the Royal Society,
p. i-xxviii.
Westfall, R. S., 1967, Hooke and the law of universal
gravitation--a reappraisal of a reappraisal: The British
journal for the History of Science, v. III, pt. 3
p. 245-261.
(no. ii)
,
1972, Robert Hooke: Dictionary of scientific
biography, v. VI, p. 481-488.
White, G. W., ed., 1968, Contributions to the history of
geology, v. 4, Nicolaus Steno: Prodromus of a dissertation concerning a solid body: New York, Hafner, p. vii,
169-283.
Whitehurst,
J.,
1786,
An inquiry into the original state
and formation of the earth, deduced from facts and the
laws of nature, 2nd edition: London,
W. Bent, 283 p.
Reprinted
1978,
New York, Arno Press.
30
PART II
THE HOOKE IMPRINT ON THE HUTTONIAN THEORY
Presented at the Annual Meetings of the Geological Society
of America, Atlanta, Georgia, November, 1980.
Accepted for publication by the American Journal of Science.
31
ABSTRACT:
Robert Hooke's system of the earth as presented in his
Cutlerian lectures, published posthumously as Discourse
of Earthquakes in 1705, is almost identical to the theory
James Hutton announced to the Royal Society of Edinburgh
in 1785.
This similarity had been noted by Gordon L.
Davies (1964).
The similarity, however, is not a coincidence.
That Hutton was thoroughly aware of Hooke's writ-
ings is shown not only by the extent to which the intelli-
gentsia of the 18th century cited, quoted and adopted
Hooke's ideas, but also by Hutton's own text in both his
Abstract of 1785 and his Theory of the Earth of 1788.
In the few places where Hutton disagreed with Hooke,
Hutton's style became polemical.
He seemed to argue
against specific points originated by Hooke, which then
act as a Hookian signature on the Huttonian Theory. Hooke's
influence in the development of geological thought and
especially on the foundation of the pre-continental-drift
paradigm was a significant one.
32
INTRODUCTION
Most geologists and historians of science credit
James Hutton (1726-1797)
(Fig. 1) with the initial estab-
lishment of a working modern hypothesis in geology in which
all geologists ascribed until the advent of sea-floor
spreading and plate tectonics.
But the progress of scien-
tific development is rarely due to the inspiration of one
great personality.
Actually the foundation of modern geology
was firmly laid in the 17th century.
While the Dane Nicolaus
Stensen (1638-1686), better known as Steno, is generally
recognized as a founder of geology, the geological work of
Robert Hooke (1635-1703), more extensive and more profound
than Steno's, has not gained the recognition it deserves.1
My purpose is to show that Hooke's geological writings were
not forgotten as some historians claim (Lyell, 1830; Carozzi,
1970; Turner, 1974), but were read, quoted, cited, and adopted
by later writers.
on Hutton.
In particular, they had a direct influence
The similarity between Hooke's system of the
earth and the Huttonian Theory, as recognized by Davies (1964),
is not a coincidence.
HOOKE'S SYSTEM OF THE EARTH
Hooke's ideas on geology were mostly published posthu-
mously as Discourse of Earthquakes (1705) composed largely
1The comparison of the geological contributions of Steno
and Hooke is the subject of another article (Part I of this
thesis and Drake and Komar, in ms.).
33
I.
Figure 1.
-4-
A caricature made by John Kay in 1787 showing
James Hutton "rather astonished at the shapes his favourite
rocks have suddenly taken."
(From Wright, 1865.)
34
of the Cutlerian lectures he read to the Royal Society
of London over a period of more than 30 years.
Hooke's
original emphasis in his writings was on the organic
origin of fossils.
References to this subject appear as
early as 1664 and most notably in his famous book
Micrographia (1665).
While the idea that fossils have an
organic origin was not original with him (earlier scholars,
e.g. Leonardo da Vinci, had similar thoughts on the subject),
Hooke and Steno stood out from their contemporaries in
this issue.
The prevailing belief was that fossils some-
how materialized in situ through some "plastick vertue."
Much ahead of his time, in discussing the various types
of fossils, Hooke also speculated on the evolution of
species and their generation and extinction, saying (1705,
p. 291) that "there have been many other Species of Creatures
in former Ages, of which we can find none at present,...and
that there may be divers new kinds now, which have not been
from the beginning."
In presenting the various ways dead organic material
could be laid down in water, buried in sediment,and then
turn into rock as the sediment become consolidated and raised
as dry land, Hooke developed a clear concept of the cyclic
nature of the dynamic processes that alter the surface
features of the earth.
He was possibly the first actualist
(rather than uniformitarian) since Classical times.
He
described cycles of deposition and erosion, subsidence and
35
uplift.
He attributed mountain-building to earthquakes by
which term he meant any kind of earth movement--violent
at times or at others progressing slowly "by degrees."
He wrote (1705, p. 313), "All things almost circulate and
have their vicissitudes," and "Nor are these Changes now
only, but they have in all probability been of as long
standing as the World."2
He then placed these dynamic processes in a universal
context taking into account the motion of the earth
around the sun in the plane of the ecliptic and the tilt
of the axis of diurnal rotation.
He described polar
wandering and a possible shift in the earth's center of
gravity that could cause earth movements and changes in
the "magnetical Power and.Vertue of the Body of the Earth."
The term isostatic compensation was unknown to Hooke,
but he had a clear idea of its action.
Hooke was also in advance of his age in his attitude
towards the age of the earth.
Although his concept of time
did not extend to the millions and billions of years of the
modern geological time-scale, he refused to be bounded by
the Scriptural chronology and claimed a much longer timescale than his contemporaries, including Steno, would allow.
2Clearly, Hooke's concept of the term earthquake has been
misinterpreted by some historians; Richard Westfall (1972),
for example, labeled Hooke "the first catastrophist."
36
In many respects, therefore, Hooke in the 17th century
delineated what came to be the pre-continental-drift
geological paradigm.3
HUTTON'S AWARENESS OF HOOKE'S SYSTEM
Without actually ascribing any influence on Hutton from
Hooke, Gordon L. Davies (1964) stated that "by 1668 Hooke
had formulated the shadowy outline of a theory of the earth
that is almost identical with the theory that James Hutton
presented to the Royal Society of Edinburgh in 1785. Hooke
deserves to be remembered as a precursor of Hutton."
D. R.
Oldroyd (1972) interprets Davies as asserting that "in a
number of instances Hooke should receive the credit for ideas
which are usually believed to have originated in the work
of James Hutton." The suggestion that Hutton was familiar
with Hooke's hypotheses is compelling.
3The term "pre-continental-drift paradigm" as used here
simply refers to the working hypothesis of geologists
before the acceptance of drift theory. It does not imply
that many of the important concepts incorporated in pre-drift
geology were not carried over into the plate tectonics
paradigm. Although the emphasis on vertical displacements
in geology eventually shifted when large-scale horizontal
movements became accepted, such important ideas as uniformity
live on and are as much a part of the new paradigm as of
the old. Kuhnian interpretations, therefore, are not
specifically implied in the use of this term.
37
Hutton made no reference to the writings of other
workers in his 1785 Abstract and only a few in his Theory
of the Earth communicated to the Royal Society of Edinburgh,
in the same year but published in 1788.
excuses
E. B. Bailey (1967)
Hutton by saying, "This might give the impression
that he was ungenerous; but it was probably largely due
to ignorance of what others had written up to date."
It is almost certain, however, that Hutton was aware
of Hooke's writings in geology.
The spread of geological
knowledge in the 18th century was much more extensive
than some historians have suggested (Eyles, 1969; Porter,
1977).
Furthermore, Playfair (1803) tells us that Hutton
had "carefully perused almost every book of travels from
which anything was to be learned concerning the natural
history of the earth."
Nor does the argument that Hooke was a forgotten
author in
Hutton's days seem
valid.
A number of well-known
naturalists wrote about Hooke's ideas in the 18th century,
some in the most matter-of-fact manner, thus giving the
impression that Hooke was well known to naturalists for his
hypotheses and needed no further explication.
John Whitehurst
(1786), for example, famous for his geologic work in Derbyshire, quoted freely from "Dr. Hooke's Post." all through
the first part of his book before he finally identified the
citation on p. 132 as "the posthumous work of the learned
Doctor Robert Hooke."
38
Another contemporary, Rudolf Erich Raspe, was so
convinced of the correctness of Hooke's ideas that he decided to publicize them and add corroborating data and
observations.
In 1763 he published Specimen Historiae
Globi Terraguei,..., displaying the words Hookiana Telluris
Hypothesi prominently in the long title.
On the strength
of the publication of Specimen, Raspe was elected Fellow
of the Royal Society of London in 1769.
This recognition
shows that the Royal Society accepted the publication
as significant and, more important, that Hooke's posthumous works were still alive.
Raspe, a colorful character, famous as the author of
the Travels of Baron M8nchausen, was not unimportant.
He
was the among the first to recognize the significance of
Desmarest's discovery of the volcanic origin of basalt
in the Auvergne published in 1774 in the memoirs of the
Academie des Sciences in Paris, which had been communicated
to the Academie in 1765.
Raspe immediately saw that the
basalts in the vicinity of Kassel were also of volcanic
origin and communicated his idea in a letter to the Royal
Society of London which was read on February 8, 1770.
In
1774 he published his findings and interpreted the Habichtswald as the remains of an ancient volcano.
This work was
hailed by no less a personage than Goethe as epochal because
it introduced the idea of the volcanic origin of basalt in
Germany (Raspe, 1771, 1774; Iversen and Carozzi, 1970).
39
The story of the versatile Raspe's subsequent disgrace,
arrest, and escape from jail, expulsion from the Royal
Society, and ingenious restoration of reputation is
a
fascinating page in the history of science, and the reader
is referred to other accounts (Hallo, 1934; Carswell, 1950;
Iversen and Carozzi, 1970).
The relevant fact for us here
is that during Raspe's social re-instatement, in 1787,
he was introduced to the high society of Edinburgh, which
included James Hutton and his close friend Joseph Black.
Raspe's publications, not to speak of his notoriety, must
have been known to Hutton before his introduction to him
in 1787, a year before the actual publication of his communication to the Royal Society of Edinburgh, The Theory of the
Earth.
Furthermore, while Hutton made scant mention of
other workers in 1785, he did, in the later volumes of
The Theory, cite the names of several writers including
that of Raspe.
Hutton could hardly have missed Raspe's Specimen, even
before 1785, if he was as thorough in his reading as Playfair
claimed he was.
Raspe, who was a first-class linguist,
fluent in Latin, Italian, German, French and English, had
translated the travels and observations of some important
contiental naturalists, and these translations, with an
abundance of notes, introductions and prefaces by the
translator, were published under the aegis of the Royal
Society of London.
That Raspe managed to secure the
40
publication arrangments in spite of his expulsion from
that Society is a testimony to his ingenuity as well as
to the eagerness of the Society for "Englishing" important
foreign publications.
My point is, if we can believe Playfair, and there is
no reason to disbelieve him, that Hutton had indeed "carefully perused almost every book of travels from which
anything was to be learned concerning the natural history
of the earth," he could hardly have missed the very books
of travels translated by Raspe and published by the
prestigious Royal Society.
Two translations were of travels
of J. J. Ferber published in 1776 and travels of I. von Born
published in 1777.
In both books Raspe took the opportunity
to review Hooke's ideas on earthquakes and volcanoes,
albeit sometimes claiming them as his own, but referring to
Hooke by name, nevertheless.
Could a scholar like Hutton
not have consulted the original publication of Hooke to
verify the claims of Raspe?
The publication dates of these
translations were in advance of the 1785 publication date
of Hutton's Abstract and his communication to the Royal
Society of Edinburgh.
At least one other book of importance that could hardly
have escaped Hutton's attention was published in London in
1757 entitled The History and Philosophy of Earthquakes, from
the Remotest to the Present Times: Collected From the best
Writers on the Subject.
The authorship was given as
41
"By a Member of the Royal Academy of Berlin," but the
author was probably John Bevis, the well-known astronomer.
The immediate impetus for the publication of this collection
was the great Lisbon Earthquake of 1755.
Of 334 pages of
text, this book devotes 106 to Hooke's Discourse of Earthquakes.
Also, on the title page were two prominent quota-
tions on the scientific method, one in Latin by Verulam from
the Novum Organum and the other from "Dr. Hooke's Method
of improving Natural Philosophy."
The juxtaposition of
Hooke's words with Francis Bacon's again shows how highly
Hooke was regarded in the 18th century.
Another line of evidence showing that Hutton could
not have avoided knowing Hooke's writings through his
contemporaries is that Hutton was in constant touch with
members of the active Lunar Society of Birmingham which
promoted the importance of knowledge about the earth to
industrial, technological, agricultural and transport
developments.
As a visiting geologist, Hutton was enter-
tained by the Society members among whom were John Whitehurst
and Rudolf Raspe.
Roy Porter (1977) reports that the
membership "kept up extensive correspondence with other
devotees at home and abroad."
Among other visiting geologists
was J. J. Ferber whose very book on travels was translated
by Raspe.
42
HUTTON'S KNOWLEDGE OF HOOKE'S IDEAS: EVIDENCE IN HIS
OWN TEXT
Aside from such bibliographic evidence, Hutton's own
writings show indeed not only that he was aware of Hooke's
writings, but that he must have studied them very carefully.
Hutton's style is a polemical one and his arguments
are directed against some of Hooke's specific lines of
thinking, both in his Abstract of 1785 and his Theory of 1788.
The two theories, Hooke's and Hutton's, are so similar
that it is difficult to find where they differ, but where
they do differ is precisely where Hutton becomes polemical.
It is fundamental to both Hooke's and Hutton's theses
that marine fossils found in high places indicate that the
rocks in which the fossils are imbedded were at one time
loose sediment at the bottom of the sea.
Hooke states
(1705, p. 298),
Many Parts which have been Sea are now Land, and
others that have been Land are now Sea; many of
the Mountains have been Vales, and the Vales
Mountains, &c.
Hutton similarly states (1785, p. 5),
The solid parts of the present land appear, in
general, to have been composed of the productions
of the sea...that, while the present land was
forming at the bottom of the ocean, the former
land maintained plants and animals.
43
Hooke has four species under the category of "raising
of the superficial parts of the Earth above their former
level"; these are:
(1)
the raising of a part of a country
which lay level with the sea,
(2)
the raising of the bottom
of the sea above the surface of the water,
(3)
the raising
of considerable mountains out of a plain and level country,
and (4) the raising of parts of the earth by the "throwing
on" of a great excess of new earth covering the former
surface and adding to the thickness, or height, by many
fathoms.
Conversely, he also cites four ways of lowering
the level of the land.
These are:
of the land to form a lake,
the sea level,
(3)
(2)
(1) the sinking of part
the sinking of land below
the sinking of the sea bottom to form
"vast Vorages and Abysses," and (4)
the uncovering of
land by "throwing away" material as a result of "subterraneous
Motion" or by washing away (1705, p. 298-299).
Hutton seems to go along with Hooke's four species of
raising of the land but has difficulty with the converse
action of sinking.
He seems to be arguing with Hooke when
he writes (1788, p. 265),
the sinking the body of the former land into
the solid globe, so as to swallow up the greater
part of the ocean after it, if not a natural
impossibility, would be at least a superfluous
exertion of the power of nature.
Such an
operation as this would discover as little wisdom
in the end elected, as in the means appropriated
44
to that end; for, if the land be not wasted and
worn away in the natural operations of the globe,
why make such a convulsion in the world in order
to renew the land? If, again, the land naturally
decays, why employ so extraordinary a power, in
order to hide a former continent of land, and
puzzle man?
Hutton, therefore, favors denudation and therefore lowering
of the land by natural processes rather than by sinking
in the different senses that Hooke expresses.
Hutton must
also be aware, however, that elsewhere in his discourses,
Hooke takes pains to express at length the whole concept
of erosion of the land by various agents such as water,
wind, and ice.
To explain how loose sediment turns into rock, Hooke
again has four. causes
(1705, p. 290):
(1) some kind of
"fiery Exhalations arising from subterraneous Eruptions or
Earthquakes"--i.e. heat and fusion;
(2)
"Saline Substance,
whither working by Dissolution and Congelation, or Crystallization, or else by Precipitation and Coagulation"--i.e.
aqueous solution;
(3) some glutinous or bituminous matter,
"which upon growing dry or settling grows hard, and unites
sandy bodies together into hard stone"; and (4)
"a very long
continuation of these Bodies under a great degree of Cold and
Compression."
Hutton generally agrees (1788, p. 223) that,
Besides an operation, by which the earth at the
bottom of the sea should be converted into an
45
elevated land, or placed high above the level of
the ocean, there is required, in the operations
of the globe, a consolidating power, by which the
loose materials that had subsided from water,
should be formed into masses of the most perfect
solidity.
He combines Hooke's second and fourth causes in the
idea of aqueous solution, thus including, in Hutton's
words (1788, p. 225), "congelation from a fluid state
by means of cold."
Instead of the four species of causes
of consolidation that Hooke enumerated, however, Hutton
would admit to but two possibilities, heat and fusion or
aqueous solution, but he attacks the latter idea.
In showing that water could indeed be "petrifying"
Hooke uses the example of the formation of stalactites
and stalagmites in subterraneous caverns of England (1705,
p. 293) :
The water itself does, by degrees, produce several
conical pendulous Bodies of Stone, shap'd and
hanging like Icicles from the Roof of the Vault;
and dropping on the bottom, it raises up also conical
Spires, which, by degrees, endeavour to meet the
former pendulous Stiriae.
Here, Hutton argues against Hooke stating that water could
only consolidate substances that are soluble in water,
and (1785, p. 11) "having found strata consolidated with
every species of substance, it is concluded, that strata
in general have not been consolidated by means of aqueous
solution."
Then in his Theory (p. 229), he states,
46
We have strata consolidated by calcareous spar,
a thing perfectly distinguishable from the
stalactical concretion of calcareous earth, in
consequence of aqueous solution.
He is, therefore, referring specifically to Hooke's example
of stalactite formation.
If Hutton were simply presenting
a new and original theory, he would not be compelled to
argue against straw men of his own making.
He could just
state, "the loose sediment is consolidated by heat and
fusion."
It has been generally supposed that Hutton's
attack against aqueous solution as a process of consolidation
is an attack against neptunism.
But at the time of the
publication of his Abstract in 1785, neptunism versus
plutonism had hardly become an issue.
Werner was only
thirty-five years old although he had been lecturing on
"Geognosy" for a few years.
He had just finished writing,
but not published, his "Kurze Klassifikation."
His paper
on the aqueous origin of basalt was not written until 1788.
R. H. Dott, Jr.
(1969)
reminds us that the issue
I
neptunism vs plutonism predated both Hutton and Werner
and therefore "the neptunism attacked by Hutton himself was
largely pre-Wernerian." It is pre-Wernerian in the sense
that it is directed against Hooke.
Hutton is compacting
Hooke's theory into a Huttonian theory and must throw out
those parts of Hooke which to him are untenable.
Another specific point of contention between the two
which Hutton superfluously brings out is Hooke's idea that
47
there might have been shifts in the earth's axis of rotation.
The question is, and it is asked by Hutton (1788, p. 222):
"How such continents, as we actually have upon the globe,
could be erected above the level of the sea?"
To Hooke,
of course, earthquakes, with his broad definition of the
word, including any movements of the terrestrial crust
whether violently by faulting and volcanic eruptions or
slowly and imperceptibly by degrees, are the general answers.
These could have been caused, he conjectures, by major
changes in the integrity of the earth movement itself in
the planetary system.
It is not impossible, Hooke asserts,
since the position of the North Star is known not to have
been constant through the ages, that there might have been
shifts in the
center of gravity of the earth and the position
of the axis of rotation.
If there is a change in the axis
of rotation there would be a change in the distribution of
land and sea, for there is, as Hooke says (1705, p. 347),
a more than ordinary swelling or raising of the Sea
in those Parts which are near the Aequinoctial, and
a sinking and receeding of the Sea from those which
are near the Poles; so that as any Parts do increase
in their Latitudes, so will the Sea grow shallower,
and as their Latitudes decrease, so must the Sea
swell and grow high; by which means many submarine
Regions must become dry Land, and many other Lands
will be overflown by the Sea,...
Hutton's answer to Hooke across a century is that (1788,
p. 222),
48
no motion of the sea, caused by this earth revolving in the solar system, could bring about
that end; for let us suppose the axis of the
earth to be changed from the present poles, and
placed in the equinoctial line, the consequence of
this might, indeed, be the formation of a conti-
nent of land about each new pole, from whence the
sea would run towards the new equator; but all
the rest of the globe would remain an ocean.
Some new points might be discovered, and others,
which before appeared above the surface of the
sea, would be sunk by the rising of the water; but,
on the whole, land could only be gained substantially
at the poles.
Such a supposition as this, if
applied to the present state of things, would be
destitute of every support, as being incapable
of explaining what appears.
Having demolished this thought with such finality, Hutton
seems at a loss as to what to do with it since he must
realize that no one except Hooke himself would debate this
issue, so he weakly continues (1788, p. 223),
But even allowing that, by the changed axis of the
earth, or any other operation of the globe, as a
planetary body revolving in the solar system,
great continents of land could have been erected
from the place of their formation, the bottom of
the sea, and placed in a higher elevation, compared with the surface of that water, yet such
a continent as this could not have continued
stationary for many thousand years...
also the process of consolidation
of the loose sediment--a point which is not denied by Hooke.
--without, he continues,
49
One wonders, then, why Hutton should bring up the whole
idea of axis-shift which is original with Hooke4 and
which is so superfluous to Hutton's general thesis that
having brought it up, he must retreat from it by sidestepping the issue.
Furthermore, the idea of the earth's
axial displacement was without support from Hooke's contemporaries5 and the idea was not prevalent in the 18th
century.
Why then, should Hutton refer to it, much less
argue against it?
4Thomas Burnet, Hooke's contemporary, famous for his
book Sacred Theory of the Earth (1691) also wrote of the
tilt of the earth's axis, but in an entirely different
context. Burnet's axis-tilt occurred as a result of God's
wrath; in its paradisaical days the earth had no tilt in
its rotational axis and therefore there were no seasons
to complicate the lives of Man.
5According to Turner (1974), Edmond Halley, on February
15, 1687, sent an account of Hooke's theory of a non-spherical
earth and of axial displacement to John Wallis to be communicated to the Oxford Philosophical Society for discussion.
Hooke had delivered his lectures on the subject to the
Royal Society of London shortly before this date. Wallis
wrote two letters to Halley dated the 4th of March and the
26th of April summarizing the reaction of the Oxford Society.
The consensus was, "sure we are, that there is no evidence
in history that ye top of ye Alps was ever sea; Except in
Noah's Floud," and the notion of the earth changing its axis
"seems too extravagant for us to admit." For supportive
evidence Wallis related that a "Dr. of Physick of good credit"
showed the group what seemed the shell of a fish taken by
the good doctor from the kidney of a woman. It is more likely, the Society concluded, that the fish-shell was formed
there in the woman's kidney, "than that this kidney had
once been sea."
50
DISCUSSION AND CONCLUSION
The evidence suggests that Hooke's hypotheses were so
ingrained in Hutton's mind because of his careful perusal
of them, that he almost unconsciously brought up the Hookian
points with which he disagreed---not because these were
necessary to the Huttonian Theory, but because they obviously
were disturbing to him and had to be disposed of.
The parts
of Hooke's system with which he agreed, which include
practically
everything else in his theory, especially
the cyclic nature of sedimentation and denudation and
including such concepts as the unconformity (Davies, 1964)
credited to his originality by some writers (Dott, 1969),
Hutton left intact or expanded with great skill, illustrating them with astute field observations in his later
volumes.
It is a bit of historical irony that Hutton's debate
with Hooke regarding the aqueous solution as one of Hooke's
four processes of consolidation should have been inadvertently and fortuitously assumed by all who followed as an
attack on neptunism.
Hutton might have been more surprised
than anyone that his argument with a man who lived a
century before should have become the point around which
the plutonists rallied their forces.
While Hutton's importance to science is readily admitted
by scientists and historians alike, one wishes, nevertheless,
that he could have given credit where it was due.
Even if it
51
were possible that he missed reading the publications like
those of Raspe and Whitehurst, his contemporaries, in which
Hooke was championed, quoted, or cited, the evidence is
still clear that Hutton had read Hooke directly.
The
notion that axial displacement could be the cause of the
exchange of land and sea areas originated with Hooke and
he apparently had no followers in it of consequence; even
Raspe, Hooke's champion, had, in his Specimen, dismissed
the idea as too philosophical and to be ignored.
The idea
itself, so vehemently denied by Hutton, therefore, acts
like a Hooke signature stamped on the Huttonian Theory.
This article is not intended to be iconoclastic.
Hutton is unquestionably an important figure in the history
of science, even in the light of his contributions vis-a-vis
Hooke's.
Hooke was brilliant, original, and imaginative,
but he threw out thousands of ideas from his fertile mind
(many disarmingly on target in the context of present
knowledge) to challenge other natural philosophers to prove
or disprove.
Hooke served his purpose; Hutton refined
Hooke's hypotheses to an elegant degree.
Playfair and later
Lyell eloquently publicized the theory to the world.
The
pre-continental-drift paradigm in modern geology was certainly established then.
Hooke's contributions to the development of geology as
a science, however, should not be forgotten, not only because
his writings reveal the extent of pre-Hutton and pre-Werner
52
geological knowledge, but also because they were transmitted by later writers and therefore demonstrate the continuity of the development of geological thought.
The notion
that there were the vague beginnings and then there was
Hutton is simply not the case.
53
REFERENCES
Bailey, E. B., 1967, James Hutton--The Founder of Modern
Geology: Amsterdam, Elsevier, 161 p.
von Born, I., 1777, Travels through the Bannat of Temeswar,
Transylvania and Hungary in the Year 1770; described
in a Series of Letters to Professor Ferber, on the
Mines and Mountains of these Different Countries by
Baron Inigo Born, Counsellor of the Royal Mines, in
Bohemia; to which is added John James Ferber'sMineralogical History of Bohemia, translated from
the German with some Explanatory Notes, and a Preface
on the Mechanical Arts, the Art of Mining, and its
present State and future Improvement by R. E. Raspe:
London, G. Kearsley.
Burnet, T., 1691, The Sacred Theory of the Earth: Reprinted
as a Centaur Classic, Carbondale, Southern Illinois
University Press, 1965, 412 p.
Carozzi, A. V., 1970, Robert Hooke, Rudolf Erich Raspe, and
the concept of "earthquakes": Isis, v. 61, p. 85-91.
Carswell, J., 1950, The Prospector, being the life and times
of Rudolf Erich Raspe (1737-1794): London, Cresset,
277 p.
New York edition has title: The Romantic Rogue;
being the singular life and adventures of Rudolf Eric
Raspe, creator of Baron Munchausen: New York, Dutton,
277 p.
Davies, G. L., 1964, Robert Hooke and his conception of
earth-history: Proceedings of the Geologists' Assoc.,
London, v. lxxv, p. 493-498.
Dott, R. H., Jr., 1969, James Hutton and the concept of a
dynamic earth, in Schneer, C. J., ed. Toward A History
of Geology: Cambridge, M.I.T. Press, p. 122-141.
Drake, E. T. and Komar, P. D., in ms., A comparison of the
geological contributions of Steno and Hooke.
Edwards,
W.
N.,
1963,
Robert Hooke as geologist and evolu137, p. 96-97.
tionist: Nature, v.
V. A., 1969, The extent of geological knowledge in
the eighteenth century, and the methods by which it
was diffused, in Schneer, C. J., ed., Toward A History
of Geology: Cambridge, M.I.T. Press, p. 159-183.
Eyles,
54
Hallo, R., 1934, Rudolf Erich Raspe, ein Wegbereiter von
deutscher Art and Kunst: Stuttgart, W. Kohlhammer.
History and Philosophy of Earthquakes, from the remotest
to the present times: collected from the best writers
on the subject, 1757: London, J. Nourse, 351 p.
Hooke, R., 1665, Micrographia: or some physiological
descriptions of minute bodies made by magnifying glasses,
with observations and inquiries thereupon: London,
Jo. Martyn and Ja. Allestry, printers to the Royal
Society.
Reprinted by Culture et Civilisation,
Bruxelles, 1966, 246 p.
1705 (posthumously, 1668-ca. 1699, Lectures and
Discourses of Earthquakes, and Subterraneous eruptions,
explicating the causes of the rugged and uneven face
of the earth; and what reasons may be given for the
frequent finding of shells and other sea and land
petrified substances, scattered over the whole terrestrial supervicies: in, The Posthumous Works of Robert
Hooke, M.D. S.R.S. Geom. Prof. Gresh. &c. containing
his Cutlerian lectures, and other discourses, read
at the meetings of the illustrious Royal Society,
edited by Richard Waller, Roy. Soc. Secretary: London,
Smith & Walford, printers to the Roy. Soc., 1705,
p. 279-450. Reprinted in The Sources of Science,
No. 73, New York & London, Johnson Reprint Corp., 572 p.
,
Hutton, J., 1785, Abstract of a dissertation read in the
Royal Society of Edinburgh, upon the seven of March,
and fourth of April, 1785, concerning the system of the
earth, its duration, and stability: Reprinted in facsimile in, White, G. W., ed., 1970, Contributions to
the History of Geology, v. 5: Darien, Conn., Hafner, 30 p.
1788, Theory of the Earth; or an investigation
of the laws observable in the composition, dissolution,
and restoration of land upon the globe: Trans. Roy. Soc.,
Edinburgh, v. I (Part II), p. 209-304.
,
Iversen, A. N. and Carozzi, A. V., 1970, Editors' Introduction
to Raspe, R. E., An Introduction to the Natural History
of the Terrestrial Sphere, principally concerning new
islands born from the sea and Hooke's Hypothesis of
the earth on the origin of mountains and petrified
bodies to be further established from accurate descriptions and observations: New York, Hafner, cx p.
55
Lyell, C., 1830, Principles of Geology, being an attempt
to explain the former changes of the earth's surface,
by reference to causes now in operation: London,
John Murray, p. 31-35.
Oldroyd, D. R., 1972, Robert Hooke's methodology of science
as exemplified in his 'Discourse of Earthquakes':
Brit. J. Hist. Sci., v. 6 (no. 22), p. 109-130.
Playfair, J., 1803, Biographical account of the late James
Hutton, F.R.S. Edinburgh: Trans. Roy. Soc., Edinburgh,
v. V(Part III), p. 39-99.
Porter, R., 1977, The Making of Geology; Earth Science in
Britain 1660-1815: Cambridge, Cambridge University
Press, 288 p.
Raspe, R. E., 1763, Specimen Historiae Naturalis Globi
Terraquei, Praecipue de novis e maris natis insulis,
et ex his exactius descriptis et observatis, ulterius
confirmanda, Hookiana Telluris Hypothesi, de origine
montium et corporum petrefactorum: Amsterdam &
Leipzig, J. Schreuder & P. Mortier, 191 p. Translated
and edited by Iversen, A. N. and Carozzi, A. V., 1970:
New York, Hafner.
1771, A letter containing a short account of
some basalt hills in Hassia: Phil. Trans. Roy. Soc.,
,
London,
v.
LXI,
p. 580-583.
' 1774, Beytrag zur aller&ltesten and natdrlichen
Historie von Hessen oder Beschreibung des Habichswaldes
and verschiedner andern Niederhessischen alten Vulcane
in der Nachbarschaft von Cassel: Cassel, Johann Jacob
Cramer.
Turner, A. J., 1974, Hooke's theory of the earth's axial
displacement: some contemporary opinion: Brit. J.
Hist. Sci., v. 7 (no. 26), p. 166-170.
Westfall, R. S., 1972, Robert Hooke, in Dictionary of
Scientific Biography, v. VI: New York, Charles Scribner's
Sons, p. 481-488.
Whitehurst, J., 1786, An Inquiry into the Original State and
Formation of the Earth, deduced from facts and the laws
of nature, 2nd ed.: London, W. Bent, 283 p. Reprinted
1978, New York, Arno Press.
Wright, T., 1865, A history of caricature & grotesque in
literature and art: London, Virtue Brothers, p. 462.
56
PART III
TECTONIC EVOLUTION OF THE OREGON CONTINENTAL MARGIN
With Paul D. Komar
57
ABSTRACT
Various
lines of evidence, including fossil and
sediment evidence, structural evidence, magnetic anomalies,
seismic refraction profiles, earthquake data, heat flow
data, and volcanism, seem to support the process of underthrusting and continental accretion at the Oregon continental margin from the early Cenozoic time to very recently.
This process may still be taking place intermittently.
All of Oregon, in fact, seems to have been built by the
general westward migration of the continental margin due
to continental accretion.
Models have been advanced to
explain the complex tectonic history of this area.
The
trench inner slope, or the imbricate thrust model, seems
to test well at the Oregon margin.
This model, however,
is complicated by the additional processes of segmentation
and rotation.
A synthesized scenario of the Cenozoic
tectonic evolution of the Oregon margin is given in the
light of the various processes proposed.
This narrative
considers the entire subduction process with all its
implications and ramifications, including volcanism in
the Cascades, as a near-extinction system, Mt. St. Helens
notwithstanding.
58
I. INTRODUCTION
The northeast Pacific is an unique region in the
general scheme of plate tectonics.
Movements of the Juan
de Fuca plate, a remnant of the ancient Farallon plate,
seem to be the cause of many of the processes that have
taken place at the Oregon continental margin.
The complexity
of the tectonic processes of this region is a challenge
to investigators in unraveling its tectonic evolution.
What has happened to the continental margin of Oregon
over the Cenozoic era, i.e. over the last 65 million years
or so?. Has it always been where it is now or was it slowly
built by continental accretion?
processes that formed the margin?
What were the crustal
Are they continuing today?
How are the Coast and Cascade Ranges related to the crustal
movements of this part of the world?
What are some models
proposed to explain the myriad data and phenomena observed
by investigators?
These are some of the questions that this paper attempts
to answer in a synthesized and integrated view of the tectonic
regime of the margin area off the Oregon coast.
This paper
first gives a brief description of the area and presents the
present tectonic setting.
The various lines of evidence for
underthrusting and continental accretion are then discussed;
they include (1) fossil and sediment evidence;
(2) structural
59
evidence;
(3) magnetic anomalies;
profiles and gravity data;
(4) seismic refraction
(5) earthquakes;
(6) heat flow
data; and (7) volcanism.
Almost all of Oregon seems to owe its existence to
the process of continental accretion, its edge having
migrated progressively westward and northward since the
Triassic.
This migration of the margin, along with the
Cascadan orogeny, is discussed.
The trench inner slope or the imbricate thrust model
is then considered, including an application of the model
to the Oregon continental margin as a test of the model and
an iteration of the tectonic history of the Oregon margin
in the light
of this model.
A complication to the imbricate thrust model, the
segmentation model is then presented, and the Oregon margin
on the basis of this new concept is examined.
An additional complication, that
of rotation, an issue
under much current discussion, is then described.
This paper concludes with a synthesized scenario of
the Cenozoic tectonic history of the Oregon continental margin.
This scenario takes into account most of the observations and
ideas put forth by two decades of investigators.
The author hopes that this synthesis of the literature
on the tectonics of the Oregon continental margin is a
contribution to marine geology in that it presents to students
60
of geology a picture of what perhaps has happened here
tectonically speaking and sets forth the various views of
the problems and the uncertainties that exist. This paper
thus could serve as a starting point for future investigators.
The bibliography also should be a useful, up-to-date
compilation for researchers interested in investigating
the complex but fascinating Oregon continental margin.
61
II.
GENERAL DESCRIPTION OF THE OREGON MARGIN
Several investigators have described the overall
setting of the Oregon continental margin (Byrne, 1962,
1963a,b, & c; Maloney, 1965; Snavely et al., 1968,1977,1980a,b;-
Braislin, 1971; Kulm and Fowler, 1974; Kulm and Scheidegger,
1979; and others).
The major physiographic provinces of
western Oregon, the continental shelf, the continental
slope, and surrounding abyssal features are shown in Figure 1.
The continental shelf off Oregon is narrow (from about
17 km at the southern end to about 56 km at the north,
just south of the Columbia River); it has a slope of from
about 10' to 43' and a depth at the shelf edge of 145-183m.
Coast Range geology continues at least to the continental
slope.
Braislin et al.
(1971) depict sectional offshore
projections of formations in Figure 2.
More than 7 km of
Tertiary sedimentary and volcanic rocks accumulated in an
elongate trough that extended from Vancouver Island to the
Klamath Mountains.
The western boundary of this eugeosyncline
was probably near the base of the present continental slope
and the eastern border is now covered by lava flows of the
Cascade Range.
The geologic history of this trough records
long periods of deposition as well as major periods of
uplift and erosion; the sequence is further complicated by
both submarine and subaerial volcanic eruptions and the
intrusion of extensive gabbroic sills.
Figure 3 is an areal
62
132°
1 3 0'
;26'
12e'
SUBMARINE FEATURES
MOUNTAINS
-
IIIIIIINOMP
HILLY AREAS
DEPRESSIONS > 400 m
LAND
FEATURES
DRAINAGE DIVIDES ....
GORDA
BASIN
CALIFORNIA
FRACTURE ZONE,
J30
126"'
Figure 1. Major physiographic provinces of western Oregon, the
continental shelf, the continental slope, and surrounding abyssal features.
The cross-hashured pattern delineates the continental shelf area off
Oregon. Numbers 174, 175, and 176 refer to Deep Sea Drilling sites.
(From Kuhn and Scheidegger, 1979.)
63
M EOCENE
SEA LEVEL
j
Ip
-5,000
-10,000
-15,000
-20000
-20000
10
10
20
30 MILE S
INDEX MAP
SEA LEVEL
SEA LEVEL
-5,000
-5,000
-10,000
-10,000
-15,000
-15,000
0
10 MILES
INDEX MAP
Figure 2. Top section: diagrammatic cross section from Willamette
Valley to continental shelf off Oregon near latitude 44° N. Bottom
section: cross section from central Coast Range to continental shelf
off Oregon near latitude 44° 45' N. (From Braislin, 1971.)
64
geologic map of the coastal and shelf areas.
Several submarine banks with folded structures jut
out from the seaward edge of the shelf.
These are the
Nehalem Bank just south of the 40° N. latitude, the HecetaStonewall Bank between about 44° 40
Coquille Bank at 43° N latitude.
and 44° N. latitude, and
In the central part of
the shelf, between the shelf edge and the coastline, are
a series of interconnecting synclines filled with Pliocene
and Pleistocene sediments.
Topographic depressions also
occur along the continental slope.
These serve as basins
for sediment accumulation and are considered "accretionary"
basins by Kulm and Scheidegger (1979) after Seely and
Dickinson (1977).
These basins are relatively small
structural features of a few tens of kilometers long and a
few kilometers wide.
Erosionally resistant headlands composed of volcanic
rocks jut out intermittently along the generally straight
coastline.
Plio-Pleistocene marine terraces made of un-
consolidated beach and dune sands line the central and
southern Oregon coast.
Near Cape Blanco, the youngest
terraces are elevated about 60 m above sea level, lowering
both to the north and south of this point.
The continental slope off Oregon has a declivity of
1° 11' to 1° 351
.
It can be divided morphologically into
three regions: northern, central and southern (Figure 4).
65
EOCENE OLIGOCENE
LATE-MIDDLE
FAULT
EOCENE
PRE-LATE
MIOCENE
CROSS SECTIONS
BRAISLIN 91S
1971
Figure 3. Geologic map of the Oregon continental shelf and
to cross secand coastal area. Dashed lines marked A and B refer
(From
Kulm and
2.
shown
in
Figure
tions by Braislin et al. (1971)
Fowler, 1974.)
66
WASHINGTON
pr00
I
A.
10
0
+-=-
20 30
KM
® HILL, BANK
B.
O VALLEY
BENCH
ESCANPMENr
C.
120,100'
14.
Figure 4. Detailed morphology of the Oregon continental shelf
and slope (from Kulm and Scheidegger, 1979, after Kulm and Fowler, 1974).
Dotted lines marked A, B, and C refer to seismic reflection profiles
shown on the right (from Kuim and Scheidegger, 1979).
67
The northern region, from 45° N. to 46° 30' N. latitude,
is characterized by a bench or terrace, the Cascade Bench,
between 366 m to 549 m, and a series of north-northwest
trending ridges with intervening basins which form the
lower slope at greater than 915 m.
Astoria Canyon starting
on the shelf at 110 m cuts the slope to more than 1830 m
and connects with Astoria Fan.
The central region of the slope, from 43° 30' N. to
45
0
N. latitude, is characterized by steep escarpments and
a series of small hills and basins.
South of Heceta Bank
the slope is gradual and there are few hills while north
of Heceta Bank numerous hills and small seamounts are present.
A small seamount near the edge of the shelf opposite Yaquina
Head, according to Byrne (1962), is the only one that extends
into water less than 183 m deep.
The southern region, from 42° N. to 43° 30' N. latitude,
is characterized by submarine valleys offshore from Cape
Blanco to Cape Sebastian and benches or plateaus to the
north and south of the submarine valley area.
The Rogue
Canyon is the largest and most distinct valley; it heads
near the edge of the shelf about 19 km northwest of the mouth
of the Rogue River, cuts across the slope, and merges with
the abyssal plain.
68
III.
PRESENT VIEW OF THE TECTONIC SETTING
The Pacific basin tectonic pattern derived by Rance
(1967), Figure 5, from torsional deformation model experi-
ments is remarkably similar to the actual general pattern
of tectonic elements as plotted by Moody et al.
Figure 6.
(1976),
The crust of the Pacific Ocean basin has been
fragmented into polygonal blocks the boundaries of which
form part of the earth's regmatic shear pattern.
In
addition, the regmatic shear pattern seems to transcend
geologic features resulting from such fundamental differences in the earth's crust as those between continental
and oceanic crust.
And according to Moody et al., the
pattern even apparently transcends the differences between
the western margins of the Pacific and Atlantic basins.
Practically all of Oregon seems to have been created
from the evolution of the tectonic elements that dominate
in the northeast part of the Pacific Ocean.
The East
Pacific Rise, which had migrated and disappeared beneath
the North American plate at the Gulf of California, crops
out north of the Mendocino Fracture as a series of spreading
ridges offset by fracture zones.
To explain the phenomenon
of a spreading ridge itself disappearing under a continent,
presumably consumed by a trench, Atwater (1970) postulates
that the Juan de Fuca plate is a remnant of a once larger
plate, the "Farallon plate," earlier described by McKenzie
and Morgan (1969), Figure 7.
The eastern side of the Farallon
69
Figure 5. Major physiographic lineaments in the exposed portion
of the simatic crust of the Pacific Ocean basin, compared with theoretical failure curves. (From Rance, 1967.)
TECTONIC ELEMENTS OF
THE PACIFIC OCEAN BASIN
-- MAGTUIKS (AULTS. BATNYMCTMC UN[ABS)
-}- U t$ 4L
UPUrTS. MOMS. ACS. Su
in LINEUP& ISLAND OWNS
+-TmouGNS. BASINS
-- 'OLDS
we MM IM IMTNTYCTAK UAr_ M4
Figure 6. Tectonic elements of the Pacific Ocean basin as plotted by
Moody et al. (1976). Compare with theoretical failure curves in Figure 5.
71
plate was consumed by a trench off the coast of North America
at a rate faster than it was being created at the ridge.
In this way, large segments of the ridge itself moved under
the continent.
Figure 8 shows the present kinematics of
the plates off the northwest American coast
of the ridges and fracture
and the names
zones.
Gorda Ridge is bounded by the Mendocino Fracture
Zone on its southern end and the Blanco Fracture Zone on
its northern end; Juan de Fuca Ridge, displaced to the
northwest from the Gorda Ridge by about 350 km, is between
the Blanco Fracture Zone at its southern end and the
Savanco Fracture Zone on its northern end; Explorer Ridge,
with many questions as to the exact location of its spreading
center, is further offset to the northwest starting at the
Savanco Fracture and, undergoing some additional smaller
offsets, terminates at the Queen Charlotte Island Fault.
Wilson (1965a, 1965b) was the first to suggest that the
Queen Charlotte Islands fault is a large right-lateral
transform fault.
Data from the Alaska earthquake of 1964
and data by Tobin and Sykes (1968) support this interpretation.
Both the Savanco and the Blanco Fracture Zones are also
interpreted as dextral transform faults that connect the
two spreading centers, the Juan de Fuca and Gorda Ridges;
i.e. motion along these transform faults, including the
San Andreas in California, are all in a right-lateral
sense--the relative motion between the North America plate
72
Figure 7. Left: Configuration of plate boundaries at the time
of anomaly 21, about 53 m.y. ago. Right: Present plate configuration
and motions near the Juan de Fuca ridge.
(From Atwater, 1970.)
73
and Pacific plate is southeast on the American side
and northwest on the Pacific side.
The Juan de Fuca plate is apparently underthrusting
beneath the North America plate along the base of the
continental slope off Oregon, Washington and southern British
Columbia.
The direction of motion of the Juan de Fuca plate
is toward the continental slope off Oregon but oblique
to the strike of the slope.
Atwater (1970) has estimated
the rate of convergence to be about 2.5 cm/yr.
She arrived
at this figure by calculating a 5.8 cm/yr convergence rate
from magnetic anomalies for the Juan de Fuca plate moving
parallel to the Blanco Fracture Zone (Figure 7);
this motion
added vectorially to the 5.8 cm/yr motion of the North
America plate with respect to the Pacific plate parallel
to the San Andreas fault gives the resultant motion of the
Juan de Fuca plate with respect to the North America plate
as a north-northeastward compression of about 2.5 cm/yr,
Figure 9.
Riddihough (1977) also calculated the convergence
direction of the Juan de Fuca plate to be oblique in a northeasterly direction towards North America, although the degree
of obliqueness, according to Riddihough, has not always
been constant.
The spreading rate also probably has not been constant.
Rea and Scheidegger (1979) present arguments for fluctuations
in the spreading rate over the past 2.4 m.y. with decreasing
rates prior to the Jaramillo to Olduvai time interval
74
NORTH
AMER/CAN
,:.
PLATE
Figure 8. General directions of relative plate motions of
the Pacific, Juan de Fuca, and North America plates (from Couch, 1979.)
Figure 9. Vector diagram for deducing relative motion between the
Juan de Fuca and North America plates. Jo, motion of the Juan de Fuca
with respect to the Pacific plate; Ap, motion of the North America with
respect to the Pacific plate; JA, the resultant motion.- of the Juan de
Fuca with respect to the North America plate (from Atwater, 1970).
75
(0.92-1.73 m.y. ago) and increasing rates since then.
They conclude that during times of more rapid spreading,
increased shear and melting along lithospheric boundaries
may give rise to increased volcanic activity, whereas
during times of slower spreading, volcanic activity may
be less intense.
76
IV.
EVIDENCE OF UNDERTHRUSTING AND CONTINENTAL ACCRETION
Geophysical evidence generally substantiates the
plate tectonics interpretation of the major features in the
The tectonics of the
northeast Pacific outlined above.
Oregon continental margin seem to be almost totally the
result of the motion of the plates described.
Underthrusting
of the continental margin has been the central element in
characterizing the tectonics of this region.
The crucial
problem for many investigators, therefore, has been to produce evidence that oceanic crust has actually underthrust
the continental margin off the northwestern coast.
If
subduction has occurred, how long has this process been
going on, and is it still going on?
Fossil and Sediment Evidence
Byrne et al.
(1966) studied sedimentary rocks containing
fossil benthic foraminifera collected from the continental
shelf and slope off the central coast of Oregon.
These
fossils of Pliocene and Miocene age are indicative of water
depths of 600 to 1000 m.
Yet, the depths from which these
fossils were collected were in every case shallower than
those at which the fauna lived.
This evidence led the au-
thors to conclude that there has been significant uplift of
later Tertiary sediments, as much as 1000 m, along the sea-
77
ward side of the continental margin since their deposition.
An uplift of this magnitude is interpreted by Byrne et al.
(1966) as representing an early stage of continental accreThey further estimated that the maximum horizontal
tion.
accretion, based on the assumption that this distance is
equal to the horizontal distance between the sample location
and the position on the present continental slope equivalent to the samples' paleodepth, was 5 to 50 km
with an
average of 16 km.
In 1971 the deep-sea drilling vessel, the Glomar
Challenger, on its leg 18, drilled in the northeast Pacific
(Kulm et al., 1973).
Site 175, located in a narrow trough
between two lowermost anticlines on the Oregon continental
slope (see Figure 1), yielded additional evidence to
support the ideas of Byrne et al.
(1966).
von Huene and
Kulm (1973) report a significant change in benthic foramiferal assemblages at 80 m downcore indicating a change in
paleodepth at this site.
Above 80 m the indicated paleo-
depth is close to the present water depth of 2000 m, but
below 80 m the assemblages are typical of water depths
greater than 2200 m.
The change occurred 0.3 to 0.4 m.y.
ago based on radiolarian and diatom stratigraphy (Schrader,
1973).
This sequence implies that the sea bottom at this
site was greater than 2200 m deep and was then uplifted,
passing the 2200 m level 0.3 to 0.4 m.y. ago.
Additionally,
78
the lithology is characteristic of the abyssal plain,
suggesting that possibly part of the abyssal plain has been
incorporated onto the slope.
Evidence of uplift also exists at Site 176 located
in 193 m of water on the western flank of Nehalem Bank
on the northern Oregon continental shelf.
Here, von Huene
and Kulm (1973) report the presence of unconformities in
the sediment section ranging in age from late Eocene to
Pleistocene.
These unconformities displaying angular
discordance indicate major periods of uplift and erosion.
A Pleistocene transgressive and regressive sequence of silty
clays and coarse sediments discordantly overlies a Pliocene
fissile shale.
Again, the foraminiferal assemblages
indicate a paleodepth for the shale of at least 500 m
deeper than the present depth at this site.
Structural Evidence
Byrne et al.
(1966) noted that sub-bottom profiles
show the presence of anticlines and synclines near the
outer continental shelf and upper continental slope,
indicating the possibility of forces operating normal to
the continental margin.
Seismic reflection records obtained
during Deep Sea Drilling Project (DSDP) Leg 18 near Site 175 on the Oregon
lower continental slope show some continuous reflectors
that indicate upwarping of Astoria Fan sediments to form
the first fold of the slope.
In studying the structure
79
of the continental margin off northernmost California by
reflection profiling and bottom sampling, Silver (1969)
concludes that all the evidence suggests Cenozoic underthrusting of the margin.
A series of north- to northwest-
trending anticlines crops out on the continental slope and
dams turbidites that form the surface of a marginal plateau.
Silver's reflection profiles (1972) across the Washington
continental margin show deformation of Cascadia basin
strata against the continental slope (Figure 10).
Un-
conformities shown in the profiles suggest that deformation
of the slope edge is episodic rather than continuous.
Silver (1975) believes that there is evidence for both
Quaternary and early Tertiary subduction.
Both landward
and seaward dipping reverse faulting occurred, causing
deformation of the scraped-off sediments.
The landward
dipping faulting has slowly built the deformation slope
to over 2 km relief off northern California, Oregon, and
Vancouver Island, which constitutes a tectonic barrier to
sediment flow.
A large slope basin has resulted.
The sea-
ward dipping thrusts, on the other hand, by "imbricate
stacking of thrust sheets" have rapidly built out the
margin with about a one-km relief, forming a plateau that
is smoothed by sedimentation.
Carson (1977) describes similar
situations off Washington and northern Oregon.
Silver (1975)
also notes rapid sediment dewatering and increasing shear
strength from west to east across the deformation front.
x
,n
Q)
S4
04 M
ON
C) 44
OM
(1 5
rviQ -4 Q
11 w
a)
a) U)
r. 04 Q4 r4 a)
O QJ C) -''a
U +cS 0 m --I
4.) 44 (15 0
x
to
U) -4
Ui
a)
Ul U -1 r-I
U)
a)
1-4
144
11
.- :-4 r-4 w W
ra
(1)
m O Q)
. ,.C
QJ
114
U) 41 r-1
-4
300
0
4 U) 0
8 0)
80
81
While Silver (1975) supports subduction processes as
the dominant influence on continental margin structure north
of the Mendocino fault (40.5° N.), he believes translational processes are the dominant influence in shaping
the margin to the south. Folding of older strata is seen
on the margin south of the Mendocino fault but is buried
by Miocene and younger sediment and
"overprinted"
by basin
transcurrent faulting, and slumping on steep
California borderland basin formations are inter-
formation,
slopes.
preted as corresponding to a change in motion between the
Pacific and American
plates.
The change from N 22° W
before ten million years ago to N 30° W after 10 million
years ago suggests an extensional component movement
across the transcurrent faults that then existed.
The seismically active part of the Mendocino fracture
appears to be the seaward extension of the San Andrews fault,
the transform fault that connects the Gorda Ridge with the
crest of the East Pacific Rise in the Gulf of California
Although a clear Benioff zone of
earthquakes has not been detected, in the opinion of Tobin
and Sykes,these large strike-slip displacements cannot
occur both on the San Andreas fault and on the Mendocino
fracture without some underthrusting of the sea floor
(Tobin and Sykes, 1968).
near the coast of Oregon and
Washington.
LePichon (1968)
compared the trends of the fracture zones at both ends of
the Juan de Fuca ridge with the trends predicted by a
82
center of rotation located at 530 N and 470 V]; a systematic
disagreement of 200 is apparent.
In his opinion this
discrepancy implies compression and deformation of the
continent adjacent to the Juan de Fuca plate.
Magnetic Anomalies Evidence
The classical zebra stripes of magnetic anomalies
(Figure 11) support sea floor spreading and underthrusting
in this area.
Emilia et al.
(1966, 1968) conducted a magne-
tic survey off the Pacific northwest coast.
They note the
Suboceanic
large variation in depths of anomaly sources.
extension of coastal volcanics is indicated by high magnitude positive anomalies off the northern and central Oregon
coast.
Two of the magnetic lineations documented earlier
by Vacquier et al.
(1961) and Mason and Raff (1961) extend
under the continental slope to the shelf break.
These
suggest underthrusting by the oceanic plate.
Silver (1969) mapped magnetic anomalies over the
continental slope off the California-Oregon border.
The
easternmost positive anomaly is correlated with anomaly
number 3 of the Mason-Raff anomaly pattern of northeastern
Pacific with an estimated age of 5 m.y.B.P.
The presence
of this anomaly beneath the continental slope, according
to Silver (1969), is further evidence for underthrusting of
the continental margin during the late Cenozoic.
Silver (1972)
83
135'
130'
125°
Figure 11. Raff and Mason's (1961) summary diagram of the magnetic anomalies in the northeast Pacific. Arrows indicate the axes of the
three short ridge lengths in the area and straight lines indicate faults
offsetting the anomaly pattern (from Vine, 1966).
84
traced magnetic anomalies on the Juan de Fuca plate 40 to
100 km eastward under the slope and he estimates that
approximately 50 km of the outer continental slope may have
been formed by accretion in Pleistocene time.
Seismic Refraction Profiles and Gravity Data
Dehlinger et al.
(1968) constructed hypothetical
crustal and subcrustal sections from the Tufts Abyssal Plain
west of Oregon across the Juan de Fuca ridge, the continental margin, the State of Oregon, to central Idaho
Their seismic refraction results at sea
(Figure 12).
provide data on crustal layering and velocities in the mantle
across the Juan de Fuca Ridge, the Cascadia Basin and the
continental margin. These data show that the mantle beneath the continental margin is shallow (depths of 10 to
11 km) on the oceanic side of the shelf where the water is
22 km deep.
At the outer edge of the shelf where the
average water depth is
00 m, the mantle is at an approxi-
mate depth of 14 km and has a velocity of 8.0 km/sec.
Nearer
the coastline, where the water is 120 m deep, the mantle is
at an average depth of 17, km with a velocity of 8.1 km/sec.
These data generally agree with the seismic refraction studies
of Shor et al.
(1968) off Oregon and northern California.
It
appears that the Mohorovicic discontinuity at the continental
margin dips under the continent.
85
Figure 12. Location map showing regional structures. Crustal and
subcrustal sections (Figures 13 and 14) were constructed along AA' and BB'
(from Dehlinger et al., 1968).
86
Free-air gravity anomaly maps for this area generally
agree with the crustal and subcrustal sections constructed
by Dehlinger et al. (1968) and suggest that the northeast
Pacific region as a whole and its major provinces are
essentially in isostatic equilibrium. Along the coast
small positive anomalies on the continental side and large
negative anomalies on the oceanic side of the continental
slope indicate a thickening of the crust beneath the slope
(Dehlinger et al., 1968). The asymmetry of the anomalies
is due to the dipping mantle beneath the continent. The
observed anomaly varies along the coast suggesting variations in both the steepness of the continental slope and
in the dip of the Moho.
The sections (Figures 13 and 14) show the existence
of a low-density upper-mantle material beneath the ridge
and basin. The gravity data require a thin crust beneath
Oregon and Washington.
The low density with accompanying
low velocity east of the ridges is attributed to high heat
flow as well as variations in chemical composition in the
upper mantle (Dehlinger et al., 1968). The low-density
mantle beneath the ridge and basin is shown as nearly vertical blocks approximately 5 km thick.
The crust thickens eastward and appears to extend
beneath the continental slope and shelf off Washington.
Couch (1972) suggests that the relatively thin crust in
100
JUAN DE FUCA RIDGE TO IDAHO
CRUST AND SUBCRUSTAL CROSS SECTION AX
ALONG GGGO' GOLAN LAn NUOT FAG. 1. .Gar LON.OIAG rO h .afr LTNGl0UON
FREE-AIR AND ROUGUER GRAVITY ANOMALY PROFILE
GGfa.vfG ANwu. mm ANOxur 0 0
anf.K .frAACrNw L.cf aaG+wa anN rG Lnf.f r
.nrtf ae LurAf
arcNL
OREGON STATE UNIVERSITY
wam+nv
Figure 13. Hypothetical crustal and subcrustal section along line AA' in Figure 12.
Numbers in the section are densities of the different petrologic materials. (From Dehlinger
et al., 1968.)
100
50
Q
-100
-150
Q -150
Oc
CD
-200
-200
5
5
SWIE
1.03
103
2.67
-J5
2.74
10
to 3.30
15
15
3.26\.
20
20
3.02
25
25
3,02
30
301JUAN DE FUCA RIDGE TO SOUTHERN WASHINGTON
CRUST AND SUBCRUSTAL CROSS SECTION 8B'
35P
35
AMWONTAL SCALE
0
100
30
ALONG 46.19 NORTH LATITUDE FROM 131 WEST LONGITUDE TO 121' WEST LONGITUDE
FREE-AIR AND BOUGUER GRAVITY ANOMALY PROFILE
OBSERVED ANOYALT -
40
COIMUTED AIOMALY 0 0
3.40
KILOMETERS
40
SEISMIC RETRACTION LINES SHOWING DEPTH TO LAYERS
DENSITIES OF LAYERS IN G/CM'
45
OREGON STATE UNIVERSITY
3.40
45
AUGUST196T
501
50
Hypothetical crustal and subcrustal section along line BB' in Figure 12.
Numbers in the section are densities of the different petrologic materials. (From
Dehlinger et al., 1968.)
1
Figure 14.
89
the region between the continental shelf and the Cascade
Range of Oregon and Washington is characteristic of the
transition from oceanic to continental structure, a result
of underthrusting of the continent by the Pacific lithospheric plate.
Layers of density 2.76 to 2.90 g/cm3 are interpreted
by Couch et al. (1978) as the top basaltic layer of the
oceanic crust that forms the basement of Cascadia Plain
and extends into the continental margin off Washington.
Overlaying the 2.9 g/cm3 layer of the slope and shelf are
layers of 2.7 and 2.6 g/cm3 presumed to be oceanic volcanics
and sediments.
The 2.6 g/cm3 is probably of Eocene age
overlying the 2.7 g/cm3layer which is probably composed
of marine sediments and volcanics of Oligocene and Miocene
age.
Couch et
al.
rocks, therefore,
(1978)
conclude that the older Eocene
must have been emplaced by "imbrication"
during plate subduction.
A totally different interpretation is advanced consistent with Maxwell's (1968) idea suggesting that the lowdensity, hot material intrude as diapirs along zones of
and such intrusions of varying sizes may occur
almost anywhere, along mid-ocean ridges, in the continental
weakness,
margin,
as well as in mountain ranges.
this idea, Dehlinger et al.
(1968
In accordance with
interpret the local
compressional folds in sediments along the shelf and slope
90
of central and southern Oregon as the result of local
diapirism along the edge of the steep slope in that region.
These authors consider the local compression to represent
adjustments to dominantly vertical forces rather than
to regional east-west shortening of the crust. They
believe that fault-plane solutions over the entire northeast Pacific area indicate a relative tension (maximum
principal stress) in an east-west direction, produced by
a westerly migration of blocks of crust and upper-mantle
Differential westerly motion of these blocks
material.
then produces zones of weakness along approximately north-
south lines.
Morgan (1968b)
of blocks near
also proposes such a westerly migration
ridges.
One of these
blocks,
part of the
is triangular, bounded by faults which
strike obliquely to the main trends and offset the magnetic
anomaly pattern (Figure 11), also noted by Raff and Mason
Morgan believes this block is anomalous in that
(1961).
it has moved eastward into North America with the crust
Juan de Fuca
block,
thickening beneath the Coast Range and the Cascades.
But there
may be many small blocks bounded by faults that move
ferentially with each
dif-
in which case it would not be
anomalous for this triangular block to have moved eastward.
The Coast Range, then, in Morgan's opinion, is probably
other,
the fill of an uplifted trench while the Cascades are the
91
volcanic system behind the trench.
The recurrence of
andesitic volcanism in the Cascades suggests the existence
of a downwelling oceanic plate, although Morgan (1968b)
speculates that this is a near-extinct system.
Dehlinger et al.
(1968)
speculate that the cause of
general large-scale westerly migration of blocks might be
related to the concept of polar wandering.
Migration of
the earth's magnetic poles evidenced by paleomagnetic measurements might result from a westward slippage of the outer
earth shell along the upper mantle low-velocity channel.
Such slippage, surmise Dehlinger et al., might be related
to the earth's rotation.
MacFarlane (1975) calculated the thermal regime of a
postulated descending lithospheric slab.
The gravity
anomaly associated with such a structure has a large
amplitude and a long wavelength.
anomalies in the
Observed free-air gravity
Pacific Northwest are not in good agree-
ment with expected values.
MacFarlane's conclusion is that
either such a slab does not exist in the Pacific Northwest
or that its effects are masked by noise and/or compensation.
Although the majority of investigators do interpret
the available data as evidence of subduction at the continental margin of the Pacific Northwest, it appears that
conclusions drawn from crustal profiles and gravity anomalies
regarding the occurrence of underthrusting may be largely
92
speculative.
Such data seem to yield widely different
interpretations from different investigators.
Earthquakes
The plate tectonics model requires a Benioff zone
of deep earthquakes where the crust is being
consumed.
The northeast Pacific is anomalous in that no clear Benioff
zone has been detected.
Chandra (1974) determined focal
mechanism solutions for 24 earthquakes (mb
between January 1963 to July 1973.
5.5) occurring
These solutions generally
confirm the tectonic regime of the northeast Pacific.
The solutions for an earthquake near Vancouver Island and
one near the Puget Sound region in Washington indicate
normal faulting and, in the opinion of Chandra, suggest
the presence of a downgoing slab of lithosphere
region.
in this
He interprets the normal faulting as the result
of tension caused by the bending of the plate as it dips
under the continent and tension within the sinking slab.
Solomon and Butler (1974)
in "prospecting for dead
slabs" postulate that the lithospheric slab at a subduction
zone that has only recently ceased to be active may retain
a seismic velocity greater than that of the surrounding
mantle.
These authors examined the variation with propagation
direction of P-wave travel time residuals from sources at
various distances and azimuths to seismograph stations in
Washington and California.
They conclude that an eastward-
93
dipping band of anomalously early arrivals at several
stations on the western flank of the Sierra Nevada and
California Cascades may imply that a dead slab is present
beneath California.
The authors believe that north of
the Mendocino fracture zone, however, subduction may still
be occurring, but at a slow rate.
Although Couch (1972) maintained that focal mechanisms
of earthquakes which have occurred in the Pacific Northwest
indicate that the principal minimum stress is east-west,
suggesting the region is presently undergoing areal dilation,
Couch et al.
(1978) believe that continuing earthquake
activity, at least around southern Vancouver Island and
Olympic mountains of western Washington, suggests that the
tectonism is due to subduction and associated crustal
compression between interacting plates, i.e. the Juan de
Fuca and North America plates.
Atwater (1970) speculates that since the Juan de Fuca
plate is young and therefore not very thick and cool, it may
reheat quickly, producing few deep earthquakes.
Other
theories to explain the lack of deep earthquakes include
the possibility that because of the high rate of sedimentation at the continental margin in this region, the subducting
block is actually being lubricated by the constant addition
of sediments and therefore underthrusting smoothly and
without great seismic disturbance
(L. Kulm, oral comm., 1979).
94
Based on composite focal mechanisms for three groups
of events in the region of the Puget
Sound,
Crosson (1972)
concludes that crustal consumption may have ceased in this
area.
The north-south compression suggested by the solu-
tions,
in the opinion of
Crosson,
reflect the regional
stress field associated with the movement of the America
plate with respect to the Pacific plate as defined by
slip on the San Andreas and Queen Charlotte Islands fault
systems.
Earthquake information, therefore, seems to be incon-
clusive as to the occurrence of subduction in the northwest
today, although most authors agree that the process has
taken place in the geologically recent past.
Heat Flow
Heat flow data generally support the concept of subduction along the Pacific northwest coast. Blackwell (1973)
shows that the pattern here is similar to other Pacific
subduction zones where heat flow is very high in the Pacific
and low for 100 to
300 km behind the trench area or over the subduction zone.
The Coast Range provinces show low heat flow (less than
1.0,ucal/cm2 sec). High heat flow is again apparent in a
region several km wide behind the band of low heat flow
(but still over the subduction zone).
Ocean (greater than 3.0,ucal/cm2
sec)
95
Interarc volcanism and the caic-alkaline intrusive
activity is typical of the region of high heat flow behind
the outer arcs, and the first volcanic arc usually coincides with the transition of heat flow from high to low.
The Cascade Range, according to Blackwell, represents this
transition.
Keen and Hyndman (1979) state that heat flow across
subduction zones shows a characteristic pattern, consisting
of a band of low heat flow which extends from the trench
to the volcanic arc (about 200 km inland) and a zone of
much higher than normal heat flow further inland.
The low
zone is produced by the cold sinking oceanic lithosphere
while the high heat zone is caused by the upwelling of
magma and convective transport of heat from the sinking
slab which begins to melt at a depth of about 100 km.
In studying the heat-flow pattern for southwestern British
Columbia and western Washington and Oregon, Keen and
Hyndman conclude that the characteristic pattern is exhibited in this area.
The volcanic line, the Cascades,
representing the transitional zone from low to high heat
flow, also corresponds to a transition from high to low
Bouguer gravity anomalies.
Heat flow data, therefore, suggest that the Cenozoic
tectonic history of the Northwest is the result of subduction along the Pacific Coast.
The continent has almost
96
reached the Juan de Fuca ridge, and the high temperature
of the young oceanic plate being overrun could possibly
explain the lack of deep earthquakes along the subduction
zone.
Volcanism
Lipman et al. (1972) and Christiansen and Lipman (1972)
show that at least until the late Oligocene the volcanism
of the entire western United States was predominantly
andesitic and formed two belts of calc-alkalic igneous
rocks that extended parallel to the continental margin.
The wide zone of early and middle Tertiary volcanism is
related to the evolution of a low-dipping imbricate subduction zone under the continental margin from the late
Cretaceous to the late Oligocene.
These authors show that a fundamental change in the
nature of this igneous activity corresponded closely with
the evolution of an extensional tectonic regime. The
development of northwest-trending right-lateral faults
near the continental margin in the late Cenozoic occurred
after the intersection of the East Pacific Rise with the
American trench about 30 m.y. ago.
Normal faults developed
further inland and produced oblique basin and range structures.
As the oblique opening widened, the igneous activity
changed to several types of basaltic volcanism characteristic
of other regions of major extension.
97
By middle Miocene time basin-range structure develop-
ment had advanced northward behind the continental margin
arc and continued through the Miocene and Pliocene, with
most of the younger activity in Oregon.
In the opinion of these authors (Ligman et al., 1972,
and Christiansen and Lipman, 1972), the Cascade-northern-
Sierra calc-alkalic volcanism may have been related to
oblique subduction of the constricted but still active
part of the Juan de Fuca plate between the Mendocino and
Blanco Fracture Zones.
Christiansen and Lipman believe,
as Morgan (1968a) does, that the Cascade Range is near
a state of extinction.
They suggest that the underthrusting
plate at the coasts of northern California, Oregon and
Washington is becoming smaller, increasingly fragmented,
and partially coupled to the America plate.
This reasoning
is consistent with their belief of the apparent waning of
continental-margin volcanism and the progressive growth
of basin-range structures behind the arc.
98
V.
MIGRATION OF THE OREGON CONTINENTAL MARGIN
If subduction and continental accretion are occurring,
or at least have occurred in the recent past, the question
must come to mind how long have these processes been going
on?
How much have we added to our continental margin?
Where was our margin at earlier stages?
Westward Migration
Based on the successively younger belts of ultramafic
rocks westward, Maxwell (1974) concludes that there was
a westward growth of the continental margin of the craton
starting in late Proterozoic time by sedimentation, orogeny,
and continental accretion.
All of Oregon, in fact, seems
to have been created by a series of island arcs and conti-
nental accretion. According to Brooks (1979) the approximate
locations of the continental margin during the Mesozoic has
progressively shifted westward as shown in Figure 15.
The
mountainous region of the U. S. west of the Rocky Mountains,
known as the Cordilleran Orogen, is divided into two belts
which roughly parallel the present continental margin.
The
eastern belt consists mainly of marine sedimentary rocks
deposited on the margin as a result of erosion of continental
rocks.
The western belt is a melange of pieces of oceanic
crust and volcanic arcs with their associated sedimentary
deposits that were accreted to the continent as a result of
99
subduction of the oceanic plate.
According to Brooks
the pre-Cenozoic rocks in
the Blue Mountains of Oregon are among the easternmost
(1979)
exposures of the western belt of accreted terranes.
The
oceanic crust, or ophiolite, and volcanic island arc
terranes of this region were accreted to the continent
during the Late Triassic to Early and Middle Cretaceous
times.
divides the pre-Cretaceous rocks in the
Blue Mountains into four terranes: oceanic crust terrane,
Brooks
(1979)
Wallowa-Seven Devils volcanic arc terrane, Huntington
volcanic arc
terrane,
and forearc basin
terrane.
He con-
cludes that both the volcanic arc terranes are of intraoceanic origin and were accreted to the continent after
the deposition of the youngest associated rocks which are
of Late Triassic for the Huntington arc and Early Jurassic
ages for the Wallowa-Seven Devils
arc.
Mid-Cretaceous
sedimentary rocks in the western part of the province
contain erosional debris, both from the oceanic crust and
forearc basin assemblages as well as from Mesozoic plutons.
Brooks concludes that all the pre-Cenozoic terranes had
been deeply eroded prior to the deposition of the oldest
continental volcanic and sedimentary rocks in early Tertiary
time.
Coleman (1972)
states that Mesozoic interactions between
100
Edge of continent in:
-early Triassic
- early Jurassic
m early Cretaceous
Figure 15. Map showing locations of continental margin at
different times during the Mesozoic (from Brooks, 1979).
101
the Pacific and America plates are recorded in the south-
western Oregon Coast Range as a cyclical series of trench
and/or shelf accumulations which were later metamorphosed
and accreted to the continent.
The oldest rocks, about
150 m.y., are isolated tectonic blocks of high-grade
blueschiSts.,eclogite and amphibolites that may, in Coleman's
opinion, represent fragments of cryptic oceanic crust.
Greenschist metamorphism and folding of the Galice Formation
(Late Triassic) and the associated ophiolites were followed
by intrusion of diorite plutons about 145 m.y. old.
At
the beginning of the Cretaceous, masses of Galice containing
the diorite intrusions probably thrust westward over the
Dothan and Otter Point Formations and partly recrystallized.
Conglomerates and shelf-type sediments were deposited on
allochthonous Galice during the Early Cretaceous in a
southwesterly arc from Myrtle Creek to Port Orford.
Renewed
subduction of the Pacific plate under the America plate
in late Cretaceous produced "further imbrication of distinct
allochthonous masses."
By the beginning of the Tertiary the continental margin,
especially in the southwestern part of Oregon, had migrated
fairly close to the present location.
In writing about
the Eocene stratigraphy of southwest Oregon, Baldwin (1974)
notes that subduction and underthrusting of the oceanic
plate caused asymmetrical folds and reverse faults with
102
relative movement of the overriding continental block
toward the west-northwest. By the end of the early Eocene
northeast-east structural trends formed in the Roseburg
and older formations parallel to the northern edge of
the Klamath Range. Serpentinite and diapiric masses were
squeezed along faults into Roseburg_ strata during this early
Eocene deformation.
Baldwin surmises that mountains must
have been formed but were worn down before the onlapping
basal conglomerate of the Lookingglass Formation was laid
on the older Cenozoic rocks and the Mesozoic rocks of the
Klamath mountains.
The post-early Eocene orogeny was then
the culmination of Klamath Mountain orogenies.
The Cascadan Orogeny
According to Dott (1969), following the late Mesozoic
and early Cenozoic Cordilleran mountain building, there was
a 10 to 15 million-year interval of relative structural
Baldwin (1974) reports deposition of early
Miocene sediments in the South Slough syncline in southwestern Oregon during this period.
In late Cenozoic time, beginning about 20 or 25 million
years ago, a new style of deformation occurred "characterized by wholesale fragmentation" of the crust, acquiescence.
companied by very widespread volcanism, and has continued
to the present (Dott, 1969).
Block faulting in the basin
103
and range region along with subsidence of the Snake River
lava plain began about this time.
The Cascade volcanoes,
which began to form about 10 to 15 million years ago, are
aligned along a north-south fracture zone that is markedly
different from the older structural trends (Figure 16).
Volcanism and intense faulting occurred in western
California in Miocene and Pliocene times.
faulting occurred in the Miocene.
San Andreas
The Gulf of California
began to open up in late Miocene time, about 15 million years
ago, by rifting between Baja California and Mexico as part
of the sea-floor spreading of the East Pacific Rise.
At
the same time the Rocky Mountains and Colorado Plateau
regions underwent igneous activity and upwarping.
Dott (1965)
designates these Late Cenozoic disturbances the "Cascadan
orogeny" as they are so distinct from earlier trends.
Baldwin (1974) notes that the deformation and erosion
near the end of the Miocene in southwestern Oregon was
followed by onlap of the Pliocene Empire Formation which
was also folded along the axis of the South Slough syncline
later in the Pliocene and early Pleistocene.
Baldwin (1966)
postulates that the folding of the South Slough syncline
has continued to the present.
Eustatic movements of sea
level accompanying the glacial stages produced terraces
which also exhibit a gentle dip toward the axis of the
syncline.
Post-Empire faulting has also taken place near
104
CASCADE VOLCANOES
-- PRE-CASCADE TRENDS
MAJOR FAULTS
CASCADE PLUTONS
Q EARTHQUAKE EPICENTERS
`3Ys
!;-
-* o
o
%
i
\
I.!\l 'I
/
,
80p 0
Figure 16.
'(
3rd
m
Discordant tectonic patterns of the region.
The Mesozoic-early Cenozoic arcuate pattern is transected
by late Cenozoic faulting and volcanism.
(From Dott, 1969.)
105
Cape Blanco,
and Baldwin postulates
that
the warping of
the Pleistocene terraces continues today.
Northward Migration
Based on an 8000 km seismic reflection survey and
250 rock samples, Fowler and Kulm (1972) established a
stratigraphic history of the Oregon continental margin from
Miocene to Recent. They observe that the rich foraminiferal faunas in the samples display close affinities to
the late Miocene to Pleistocene biostratigraphy of California. They conclude, therefore, that the provenance
and paleoenvironment of lower slope samples suggest a
north component of motion for the Gorda-Juan de Fuca plate.
They estimate a total range of vertical tectonic movement
since the late Miocene of approximately 1200 m.
In studying the central Oregon continental margin,
Snavely et al. (1980) report the juxtaposition of a lower
Eocene graywacke sequence on the west in the outer continental shelf against lower and middle Eocene volcanics
on the east. The line of contact is a dextral transcurrent
fault, and Snavely et al. estimate a dextral separation
of about 200 km is required. The original site of deposition for the lower Eocene wackes must have been in coastal
northern California or farther south.
Other investigators postulate still more complicated
106
rifting of continental blocks.
In studying the Paleogene
stratigraphy of western Oregon and Washington, McWilliams
(1972) hypothesizes that the area north of Roslyn, Wash-
ington (in the middle of the Cascades), rifted away from
the area south of Port Orford, Oregon, a distance of about
600 km and that the lower to middle Eocene formations,
including the Siletz River volcanics filled the gap.
The
Roslyn and Tyee Formations were deposited over the volcanics.
Later rifting resulted in the formation of Narizian? (upper
to middle Eocene) to Ulatisian (middle to lower Eocene)
Crescent Formation and equivalents in northwestern Oregon
and southwestern Washington.
The data from both the land and the sea, therefore,
suggest not only a eastward motion of underthrusting of
the oceanic plate but a northward motion as well.
The
resultant motion of the Gorda-Juan de Fuca plate then has
been oblique in a northeast direction toward the continent
since the Miocene.
The continental block, on the other
hand, seems to have exhibited not only a westward overriding of the oceanic plate but
at least during the Paleogene.
also a northward motion,
107
VI.
A MODEL OF THE TRENCH INNER SLOPE:
THE IMBRICATE THRUST MODEL
By the early 1970s, some investigators began to
feel the need for a new model for interpreting the barrage
of data concerning the dynamics of plate convergence at
continental margins.
Seely et al.
(1974) constructed a
model of the trench inner slope which they believe is
applicable to many parts of the world.
Figure 17 shows
the location of the trench inner slope in relation to the
other major features of the fore arc.
Seeley et al. maintain that the trench inner slope is
structured by thrust faults and compressional folds, a
conclusion that had been reached by some investigators
cited previously.
Evidence for uplift of sediments in
such locations have also been reported.
The structure is
a result of underthrusting of the oceanic plate.
The outer
highs are the most prominent expression of compressional
deformation caused by the underthrusting.
Seely et al.
(1974)
demonstrate with examples from the mid-America trench that
the thrust faults become progressively older and more
steeply dipping in a landward direction, away from the trench.
The most active faults are at the foot of the trench slope.
The progressively steeper dip of the thrust faults are
attributed to the insertion at the foot of the trench inner
slope of wedge-shaped slices displaying asymmetric folds
108
INNER SLOPE
OUTER HIGH
(TERRACE/
OUTER SLOPE
SHELF-EDGE
TRENCH
HIGH OR
OUTER ARC)
FORE-ARC
BASIN
V.E. 5:1
Figure 17. Location of the trench inner slope in relation
to the other major features of the fore arc (from Seely et al., 1974).
109
between closely spaced,landward-dipping, concave upward
thrust faults (Figure 18).
As the lower slices underthrust
the older wedges they push the latter higher on the inner
slope tilting them so that the older thrusts have a steeper
and steeper landward dip.
In this manner, fan structures
are produced (Figure 19).
Slumping of the lower reaches of trench inner slopes
contributes to the trench turbidite sequence (see Figure 19).
Where slumping does not remove and destroy the tops of folds
and thrusts, perched basins may form upslope from the
prominences.
Sediment accumulation is gnerally more likely
on the upper parts of most trench inner slopes, where thrust
activity has diminished or stopped, than on the lower parts.
The ratio between the rate of trench
sedimentation
and the rate of underthrusting seems to determine the
volume of trench sediments (Dickinson, 1973).
Where this
ratio is high a greater volume of sediment is present than
where this ratio is low.
Where both the sedimentation rate
and the underthrusting rate are high, rapid continental
accretion results.
In cases of rapid continental accretion, seaward-building
of the trench inner slope pushing trench sediments in front
of it is coupled with the outbuilding of shelf and upper
slope sediments on top of the "thrust stack."
If under-
thrusting stops while high depositional rate continues,
110
FACIES 1 TIME TRANSGRESSIVE!
Q SHELF
c
SLOPE
TRENCH
OUTER
HIGH
ABYSSAL PLAIN
Figure 18. Trench margin model showing landward-dipping, concave-upward thrust faults. A: facies pattern; B: time stratigraphy.
(From Seely et al., 1974.)
111
Figure 19. Diagram showing the development of fan structure.
Slumping contributes to the trench turbidite sequence (from Seely
et al., 1974.)
112
section; 3) sediment-filled synclinal basin which was
created by late Pliocene-Pleistocene subsidence of inner
continental shelf; 4) discordant contacts in sedimentary
basins on lower slope indicating post-orogenic sediment
ponding and tectonism contemporaneous with rapid turbidite
deposition; 5) incorporation of abyssal plain and submarine
fan turbidite deposits onto the lower and middle continental
slope, and older slope sediments exposed on the outer
continental shelf; 6) Pleistocene sediments on the lower
slope showing age progression from youngest at the base
to oldest near the middle of the slope; 7) exposure of
older deposits by slumping of younger material on the lower
slope escarpments which were created by rupturing of
uplifted trench sediments as they fold over the incoming
oceanic plate.
Figure 20 depicts the inner trench slope model and the
sequence of events occurring at the Oregon continental
margin showing successive stages of imbricate thrusting.
The diagrams show that over a two-million-year period the
abyssal silt turbidites, ancient fan sand turbidites,
Astoria Fan sand turbidites and even pelagic ooze have been
successively thrust up higher and higher on the slope in
a fanning out process depicted schematically in Figure 19.
Pleistocene sediments on the lower slope thus show an age
progression of from young to old from the base of the slope up.
113
sediments can again accumulate in the trench and on the
trench inner slope; these sediments can then also be
structured with thrusts and folds when underthrusting
resumes again.
To add to the complexity of the situation,
plutonic intrusions may occur locally between the volcanic
arc and trench.
As Seely et al. note, the trench inner slope model,
or the imbricate thrust model, gives insight into the
facies and time-stratigraphy found on many slopes, the
volume of sediments in various trenches, the rate of continental accretion, thrust-fault age relationships, and the
development of fan structure.
After many millions of years of such processes varying
in rates, duration, and intensity, the net result is that
geologists may be faced with unraveling a highly complicated
and convoluted series of events.
Testing The Model
Using all the available data, Kulm and Fowler (1974)
provided an unraveling of the Oregon continental margin in
light of the imbricate thrust model constructed by Seely et
al.
(1974) as a test of the model.
They found the inter-
retations of the geologic data collected from the margin
in close agreement with all the requirements of the model;
1) Late Cenozoic uplift of the entire Oregon continental
margin; 2) major erosional unconformities in the sediment
114
A TRENCH SLOPE MODEL
D
1.2- 0.5 MY.
FACIES (TIME TRANSGRESSIVE)
o SHELF
0 SLOPE
TRENCH
0 ABYSSAL PLAIN
OREGON MARGIN
ASTORIA FAN
SAND TURBIDITES (E,,2)
CONTINENTAL SLOPE
El HEMIPELAGIC MUDS (D)
ABYSSAL SILT
13 TURBIDITES (Ct,s)
U
0.5-0.3 M.Y.
ABYSSAL SILT
TURBIDITES (C,)
ANCIENT FAN
SAND TURBIDITES (B)
PELAGIC OOZE (A)
? 2.0 M.Y.
Figure 20. The trench slope model and schematic diagrams
showing the sequence of late Cenozoic events of imbricate thrusting
and sedimentary relationship at the Oregon margin. (From Kulm and
Fowler, 1974.)
115
Tectonic History of the Oregon Continental Margin In
Light of the
Imbricate Thrust Model
The tectonic history of the Oregon continental margin
as interpreted by Snavely et al. (1968) and Kulm and Fowler
(1974) has the following scenario:
Middle Eocene turbidites of the Tyee Formation were
deposited in a geosyncline at bathyal depths (1500 to >
2000 m) on top of a thick sequence of middle Eocene pillow
basalts and breccia of oceanic tholeiitic basalt compo-
sition.
The area then experienced a major episode of tectonism
which resulted in uplift and erosion during the mid-late
Eocene, a dominant element of which is imbricate thrusting
by the oceanic plate. Volcanism was renewed at this time
within the depositional basin as well as in the ancestral
Cascades to the east where volcanism continued into late
Oligocene time.
The Coast Range was uplifted by the late Oligocene
and thick gabbroic sills were emplaced.
then shifted westward.
Marine deposition
Siltstone and mudstone were de-
posited on the Oregon margin during early Miocene time.
Thick nearshore sediments were also deposited during the
middle and late Miocene.
In mid-late Miocene time, another intense period of
116
thrusting occurred and the marine deposits on the outer
continental margin were uplifted.
Erosion of these de-
posits produced a major angular unconformity which is
dated at about 10 m.y.B.P.
This date coincides with the
worldwide change in plate motion at this time.
The rate
of subduction may also have increased during this time.
Parts of the early Eocene Oceanic crust with overlying
sedimentary deposits were scraped off the descending
oceanic plate and uplifted.
The scraped-off material
may have been uplifted higher at Heceta and Coquille
banks than at other marginal areas.
(Positive free-air
gravity anomalies characterize both of these banks suggesting the presence of a denser mass below.)
Silts and clays were deposited on the upper continental
slope during the Pliocene and were subsequently folded and
uplifted at places to form massive siltstone and claystone
submarine banks at the edge of the continental shelf.
The
folded structures were later covered with Pleistocene
sediments.
At the same time, subsidence of the inner shelf
created a broad shallow syncline which filled with shallowwater Pliocene and Pleistocene sediments.
By repeated thrust faulting of wedge shaped slices
at the base of the continental slope, late Pliocene to
early Pleistocene abyssal fan turbidites were uplifted
between one and two km to a present water depth of about
1100 m on the upper continental slope.
During the imbricate
117
thrusting, basins on the upper continental margin were
created by the uplifted thrust sheets and filled with
fine-grained hemi-pelagic sediments.
As the underthrusting
younger wedges raised the older deposits higher on the
slope, the younger units with their thrust planes dipped
progressively more steeply while the whole stack of thrusts
became more concave upward as they fanned out as if swinging
around a hinge located somewhere below the continental
margin and to the east.
Turbidite deposition occurred in the Cascadia Basin
throughout the Pliocene and Pleistocene so that sediment
access to the basin was always present.
These abyssal-
plain turbidites were downwarped 1.2 to 0.5 m.y. ago,
producing a trench at the base of the continental slope
and thrust beneath it.
Astoria Fan deposition started
immediately and continued to the beginning of the Holocene.
As thrust faults developed on the eastern edge of
the fan, continued underthrusting caused uplifting of
wedge slices producing the ridge-and-trough topography
characteristic of the lower continental slope.
Rapid
continental accretion is postulated for the northern
Oregon slope where the Astoria Fan deposits attain a thickness of greater than one km.
According to Kulm and Fowler (1974) the above chronological sequence of events for the Oregon continental margin
118
demonstrates that this area is characteristic of a fore-arc
structure produced by imbricate thrusting. The Seely et
al. (1974) inner trench slope model, therefore, tests
well at the Oregon margin.
Snavely et al. (1980) provide a revised interpretation
of the Cenozoic geologic history of the central Oregon
continental margin. They agree with Kulm and Fowler (1974)
on the underthrust model for the continental shelf and
slope.
In their opinion the underthrust boundary between
the Pacific and North America plates "jumped" from its
presumed early Eocene position under or east of the Cascade
Range to the present inner continental shelf. The deep
margin basin that formed east of the new underthrust boundary filled with more than 2000 m of middle Eocene turbidite
sandstone and siltstone which were later uplifted into the
present Coast Range. They maintain, however, that the
underthrusting process was interrupted by periods of major
dextral-slip faulting in late middle Eocene and early late
Eocene and by periods of extension during the late Eocene
to middle Miocene and late Miocene to early(?) Pleistocene.
During the first major dextral-slip faulting the lower
Eocene graywacke sequence (disscussed earlier), which must
have originated in coastal northern California or farther
south,
was juxtaposed with lower and middle Eocene volcanics
along a north-south
transcurrent
fault.
Late Eocene to
119
late middle Miocene is inferred by Snavely et al.
(1980)
to have been a period dominated by extension along the
Oregon continental margin accompanied by regional subsidence
and sedimentation in the marginal basin and small outer,
shelf basins.
Extension of tholeiitic basalts occurred
during middle Miocene time.
The Coast Range started to be
uplifted in middle Oligocene time; gabbro sills were intruded
along north-south zones of tensional rifting.
Underthrusting resumed in late Miocene time and middle
Miocene, and older strata were uplifted and folded on the
inner shelf, truncated by erosion, before downwarping and
deposition of upper Miocene and Pliocene marine strata.
Continental accretion and underthrusting continued and
Pleistocene turbidite sands were broadly folded to form
two anticlines separated by a syncline.
Imbricate east-
dipping low-angle thrust faults cut and uplifted Pliocene
and Pleistocene abyssal sediments.
The authors believe,
however, part of the uplift may be due to diapiric action,
an idea expressed earler by Maxwell (1968) and Dehlinger
et al.
(1968).
120
VII.
SEGMENTATION: AN ADDITIONAL COMPLICATION
The Model
Adding to the already complicated scheme of the
imbricate thrust model, Carr et al.
(1974) propose that
studies of the focal area of large, shallow earthquakes that
occur along the upper 20 to 60 km of the inclined seismic
zones demonstrate the existence of transverse structures
across island arcs.
These structures divide island arcs
into segments about 100 to 1000 km long which may act somewhat
independently of each other.
The segments are detected by
changes in strike and offsets of trench axes and other
geologic structures.
Great earthquakes occur when a segment of the overlying
plate rebounds after a period of several decades of being
elastically deformed by an underthrusting oceanic plate.
Depending on the time since the last earthquake, the state
of stress can be different in different blocks.
Deep crustal
faults develop in the overlying plate at segment boundaries.
These faults could extend as far back from the trench as
the volcanic belt.
At this distance the descending litho-
sphere probably cannot affect crustal structures except by
magmatic intrusions along the slab.
A segment of underthrusting lithosphere separated from
neighboring segments by tear faults can descend into the
121
mantle with a different strike and dip.
The deep seismic
zone associated with this segment would also have a different strike or dip from that of adjacent segments.
A line of active volcanoes derived from melting of
the descending slab would have a different alignment if
that part of the slab has a different strike from a neighboring segment.
Or if the dip is different, the melt
zone could occur at a different distance from the trench
and the volcanic line would appear offset.
The locations of active volcanoes seem to be strongly
influenced by the pattern of discontinuities in the deep
seismic zone.
Catastrophic volcanic eruptions of historic
time have frequently occurred at long-dormant volcanoes
on segment boundaries.
For example, the Krakatau Volcano
is located at a prominent transverse structural break in the
Indonesian Arc.
The concentration of volcanoes, therefore,
may represent surface clues to the locations of segment
boundaries.
Segmentation Across The Oregon Continental Margin
Couch (1977) notes that geologic and geophysical data
collected along continental margins of active subduction
yield differences in sedimentary structure along the strike
of the continental shelf.
These differences, according to
Couch, may be due to time-varying differential displacements
122
of segments of the margin during plate subduction.
The
amount and manner of accreticn of oceanic sediments and
crust to the continental margin may differ for different
segments of the margin.
He believes that these differential
displacements may be reflected in the configuration of coastal
land forms of Oregon and Washington.
Hughes et al.
(1980), from petrologic relationships,
eruptive style and alignment of the Cascade volcanoes,
determined the segment boundaries in the Cascade chain.
The six or seven segments proposed by these authors vary
between 110 and 240 km long (Figure 21).
The strike of
the segment boundaries is not well defined by distinct
topographic or structural features, but they are parallel
to the direction of plate convergence, N 50° F.
This
direction is oblique to the continental margin and is consistent with earlier considerations already discussed.
The authors claim that lines drawn through points of change
in strike in the volcanic chain extending seaward parallel
to the direction of plate convergence seem to intersect
at points of change in strike and offsets of the base of
the continental slope (Figure 22).
Hughes et al.
(1980) then correlated the positions of
the volcanoes in relation to the segment boundaries with the
volcano categorization of McBirney (1968).
The "coherent"
large stratovolcanoes that erupt dominantly porphyritic
pyroxene andesites with notable absence of rhyolite are all
N
Figure 21. Alignment of Cascade volcanoes
and segment boundaries. L, Lassen Peak; S, Mt.
Shasta; ML, Medicine Lake; M, Mt. McLoughlin;
C, Crater Lake; N, Newberry Crater; T, Three
Sisters; J, Mt. Jefferson; H, Mt. Hood; SV, Simcoe volcanic field; A, Mt. Adams; SH, Mt. St.
Helens; R, Mt. Rainier; GP, Glacier Peak; B, Mt.
Baker; G, Mt. Garibaldi.
(From Hughes et al.,
after McBirney, 1968.)
Figure 22. Dashed lines represent segment
boundaries which strike about N 550 E. Note lines
intersect at points of change in strike and offsets
of the base of the continental slope (from Hughes
et al., 1980).
N
124
located within the blocks between segment boundaries and
on the volcanic front.
The "divergent" volcanoes that erupt
siliceous andesites and dacites with later basalts and
rhyolites all lie at or near a segment boundary.
These
segment-boundary volcanoes are further grouped into those
along the volcanic front and those behind the volcanic
front to the east.
Other investigators have also found transverse structures across the arc system in the northwest.
Ewing (1980),
for example, describes Eocene transcurrent faulting in
Washington and British Columbia which is superimposed on
a pattern of arc volcanism.
Such superimposition, according
to Ewing, implies large lateral movements in a compressional
subduction regime.
125
VIII.
ROTATION: STILL MORE COMPLICATED
As if segmentation on top of imbrication on top of
subduction is not sufficiently complex, Simpson and Cox
(1977) from paleomagnetic evidence in Eocene rocks conclude
that an Oregon coastal block, at least 225 km in length
extending from just north of the Klamath Mountains to north
of Newport, Oregon, had rotated 500 to 700 east of the
expected Eocene field direction (Figure 23).
Evidence
for rotation is found not only in the lower Eocene Siletz
River Volcanic Series but also in the middle Eocene marine
sediments of the Tyee and Flournoy Formations.
Cox and Magill (1979) claim that, in fact, all of
Oregon west of the Cascades has undergone rotation as a
single coherent block of lithosphere.
They believe that
this entire terrain has rotated clockwise 600 about a
vertical axis since the lower Eocene.
This block constitutes
a single microplate extending from the Klamath Mountains
of southwestern Oregon to southern Washington.
Paleomagnetic
studies by Burr and Beck (1977) of the Gobel volcanics in
southwestern Washington are consistent with this model.
Plumley and Beck (1977) describe
a constant rate of
rotation from 50 to 30 m.y.B.P. in the Oligocene intrusive
rocks in the Coast Range of Oregon.
Beck et al.
(1977)
conclude that the Tertiary microcontinental tectonic history
U.S.
Tyee-
-71 Flournoy
Coast
Block
Eocene
North
K.
Baiholith
Franciscan
Type
Metamorphic
Belts
Left: Pacif is Northwest in middle Eoc ene time, showing Coast Range block as part
Figure 2 3
of small plate related to nor thward migratio n of Kula-Faral Ion-North Ame rica triple j unction.
Right: General i zed geologic a nd tectonic map of northwester n U. S. SRV, Siletz River Volcani c
Series; Y, Yach ats basalt; TF , Tyee and Flou rnoy Format ions ; OP, Olyirpic Peninsula. Fault
dre`i$ .
zones and line ations: V, Vale B, Brothers; ED, Eugene-Deni o; M, McLough lin; SA, San
(From Simpson a nd Cox, 1977.)
.
;
Ml
127
of rotation and northward movement in the western North
Based on paleo-
American Cordillera ceased by the Miocene.
magnetic results from three areas of Eocene volcanic exposure
west of the Cascade Mountains in Washington, Beck et al.
(1979)
suggest accretion and clockwise rotation of as many as three
thickened oceanic crustal fragments in response to oblique
subduction of the Farallon plate during the early and middle
Cenozoic.
The amount of rotation shown in the Eocene basalt
exposures north of the Columbia River, however, is considerably
less than that in the Oregon Coast Range.
Rotation has also been noted elsewhere.
Aune (1980)
describes the clockwise rotation of California during the
Nevadan orogeny as a rotating subduction vector (Figure 24).
Also working in California, Luyendyk et al.
(1979) propose
a model in which many crustal blocks bounded on both sides
by east-west trending sinistral faults have undergone
rotations of 70° to 80 0 within the Pacific-American rightlateral shear couple since the Miocene.
Kamerling and
Luyendyk (1979) claim that paleomagnetism of middle to late
middle Miocene volcanic rocks from regions of the western
Transverse Ranges, California, suggests considerable subsequent clockwise tectonic rotation.
Remanent inclinations
also suggest northward movememt of about 10
Miocene time.
0
since early
128
Rotating North
Arc-trench gap
X-section
170 m. Y.
American Plate
145 M.Y.
85 M.Y.
zoo K_
Figure 24. Diagrams showing clockwise rotation of California
during the Nevadan orogeny as a rotating subduction vector. (From
Aune, 1980.)
129
IX.
CONCLUSION: A SYNTHESIZED SCENARIO OF THE CENOZOIC
TECTONIC HISTORY OF THE OREGON CONTINENTAL MARGIN
From this review of the literature on the tectonics
of the Oregon continental margin, the present writer has
synthesized the following scenario for the Cenozoic:
Near the beginning of the Cenozoic, in the early Eocene,
the Farallon plate was an oceanic lithospheric plate of
respectable size.
The East Pacific Rise, located more
towards the middle of the Pacific, separated the Farallon
plate and the Pacific plate.
A trench was located at the:
convergent margin along western United States and Mexico
between the Farallon plate and the America plate.
The trench
consumed the Farallon plate at a rate faster than the rate
of its creation at the ridge.
Eventually the collision
of the ridge and trench caused the annihilation of parts
of the ridge.
Subduction at the trench was still active
in mid-Tertiary time when the San Andreas fault system
developed around 30 m.y. ago.
After that, subduction south
of the Mendocino fracture zone slowly ground to a halt,
leaving a dead slab under the continental margin.
In Oregon, the underthrusting of the Farallon plate
in a direction parallel to the fracture zones not only caused
imbricate thrusting and uplift but also caused the rotation
of a geosynclinal sedimentation basin on the continental slope
filled with lower Eocene volcanics and middle Eocene sediments
130
of the Tyee and Flournoy Formations.
The pivot point was
at the southern end near the Klamath Mountains which served
as a firm anchor for the pivotal motion, the
of Simpson and Cox (1977)
(Figure 25).
"ball bearing"
At the same time,
the underthrusting oceanic plate caused asymmetrical folds
and reverse faults in southwestern Oregon with overriding
of the continental block toward the west-northwest.
Vol-
canism was widespread during this period of intense tectonic
disturbance.
By the late Oligocene the Coast Range had
rotated almost 60° clockwise and was uplifted; thick gab-
broic sills were emplaced and marine deposition shifted
westward.
After the ridge and trench collided south of the
Mendocino fracture
zone, only a remnant of the Farallon
plate was still present north of the Mendocino.
Spreading
of this much smaller plate, the Juan de Fuca plate, continued off Oregon and Washington but in a different direction
and at a different rate.
The direction of motion became
northeast and oblique to the continent by Miocene time.
Although underthrusting was still powerful enough to uplift
marine sediments on the outer continental margin and cause
continental accretion, the force was no longer great enough
to accomplish rotation, given the new geometry and the uplifted
mass of the Coast Range (Figure 26).
The subducting block, smaller and less active, began to
develop transverse features which caused different segments
131
America
Plate
Plate
Plate
Figure 25. Schematic diagram representing major crustal elements
in the early Eocene as interpreted by E. T. Drake (present paper).
132
Nort., Americo. 1 totQ_
?tote.
V
0
J
Mendocinu
Z. I
Figure 26. Schematic configuration after collision of the
spreading ridge with the continent south of the Mendocino fracture
zone. Rotation of the Coast Range has completed. (Interpretation
by E. T. Drake, present paper.)
133
of the block to act independently of each other.
While
underthrusting is possibly still proceeding very slowly
now, lateral and extensional movements have also taken place
so that the Oregon continental margin could be experiencing
a transition from compressional underthrusting forces to
strike-slip motions as a result of extensional forces and
the coupling of the Juan de Fuca plate with the North
America plate.
The relative motion of the North America
plate, no longer overriding an oceanic plate in a westnorthwesterly direction, changed to a southeasterly one,
corresponding to the motion along the present San Andreas
Fault System.
This scenario has tried to take into account the
observed phenomena reported by various investigators.
It is
consistent with the present lack of a well-defined Benioff
zone, the presence of a downwarped region, a trench, though
masked by sediments, deformation of slope sediments, focal
mechanisms solutions of some earthquakes, heat flow data,
volcanism, gravity and magnetic anomalies, the processes
at one time or another of subduction, obduction,
segmentation, and rotation.
imbrication,
This narrative also considers
the entire subduction process with all its implications and
ramifications, including volcanism in the Cascades, as a
near-extinction system, Mt. St. Helens notwithstanding.
134
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152
PART IV
ROTATION OF SEGMENT BOUNDARIES IN OREGON AND WASHINGTON
AND POSSIBLE SPACE-TIME RELATIONSHIPS IN
THE CENTRAL HIGH CASCADES
With Richard W. Couch
153
ABSTRACT
Segment boundaries
within the subducting slab were
determined to be parallel to the direction
of convergence,
around N 50° E, by Hughes and others (1980), and the boun-
daries intersect the base of the continental slope where
offsets in strike of the slope base occur.
The positions of these segment boundaries are supported
by bathymetric data on the continental shelf where they seem
to be generally aligned with escarpments.
The positions are
further supported by deformation of marine terraces along the
Oregon and Washington coasts.
Plane vector triangle solutions calculated by Riddihough
(1977) for point interactions between the Juan de Fuca plate
and the America plate, however, show. that the direction of
convergence has not been constant over the last 8.5 m.y.
The
data suggest that the subducted slab with its segments had
rotated at least 9 0 in a clockwise direction during the last
1.5 m.y.
The hypothesis is advanced that the rotation of the
segment boundary positions exerted a profound influence on
the loci and eruptive history of the Cascade volcanoes during
the last two million years.
The model suggests that the coin-
cidence of the arrival of a segment boundary with conditions
in the upper mantle favorable for an eruption triggered the
massive eruptions that built the high Central Cascade peaks
during the Pleistocene.
154
INTRODUCTION
The existence of transverse structures across island
arc systems that divide the arcs into segments was described by Carr and others (1974).
These segments may
act somewhat independently of each other causing changes
in strike and offsets of trench axes and possibly changes
in ather geologic structures which then can aid in the
detection of the locations of the segment boundaries.
Deep crustal faults develop in the overlying plate at
segment boundaries.
These faults could extend as far
back from the trench as the volcanic belt.
A line of active volcanoes derived from melting of the
descending slab would have a different alignment if that
part of the slab has a different strike from a neighboring
segment.
Or if the dip is different, the melt zone could
occur at a different distance from the trench and the
volcanic line would appear offset.
The locations of active volcanoes seem to be strongly
influenced by the pattern of discontinuities in the deep
seismic zone.
Catastrophic volcanic eruptions of historic
time have frequently occurred at long-dormant volcanoes on
segment boundaries.
For example, the Krakatau Volcano is
located at a prominent transverse structural break in the
Indonesian Arc.
The concentration of volcanoes, therefore,
may represent surface clues to the locations of segment boundaries.
155
Hughes and others (1980), from petrologic relationships, eruptive style and alignment of the Cascade
volcanoes, determined the segment boundaries in the Cascade chain.
The strike of the segment boundaries is not
well defined by distinct topographic or structural features,
but they are parallel to the direction of plate conver-
gence which they take to be approximately N 50° E.
These
segment boundaries, extended seaward parallel to the
direction of convergence, seem to intersect the base of
the continental slope at places where the base of the slope
changes in strike and is offset.
Vacillation in direction of motion of the Juan de
Fuca plate with respect to the America plate, however, is
implied in the series of vectors drawn by Riddihough (1977)
for the last 8.5 million years (Figure 1).
The situation,
therefore, is much more complex than visualized by Hughes
and others (1980).
The present article correlates the plane
vector triangle solutions of Riddihough for the Juan de
Fuca/America plate interaction with positions of segment
boundaries in Oregon and Washington and attempts to determine whether these segment-boundary positions can provide
some clues to the age and petrologic relationships of the
Cascade volcanoes.
DIRECTION OF PLATE CONVERGENCE
Riddihough
(1977)
calculated plane vector triangle
156
CONVERGENCE DIRECTIONS
10
8
4
1
Juan d e Fuca
Ame rica
10
9
8
6
5
4
m years BP
0
Figure 1.
5 cm per year
Convergence directions of the Juan de Fuca plate
with respect to the North America plate over the last 8.5 million
years (from Riddihough, 1977).
157
solutions for point interactions between the Juan de
Fuca and the America plates, assuming spreading direction
at the ridge is always orthogonal to the ridge.
Atwater
(1970), however, assumed the spreading direction to be
parallel to the Blanco fracture zone which is not orthogonal to the Juan de Fuca ridge.
Atwater's vector dia-
grams show the resultant direction of motion of the Juan
de Fuca plate with respect to the America plate to be
in a north-northeast direction and the compression rate
is 2.5 cm/yr if the motion
of the Juan de Fuca plate
with respect to the Pacific plate and that of the America
plate with respect to the Pacific plate along the San
Andreas are both assumed to be 5.8 cm/yr.
If Oregon and
Washington are moving at a rate less than this, then Atwater's
vector solution shows the compression rate to be faster and
in a more easterly direction.
Variations in any parameter,
therefore, produce variations in convergence direction,
and Riddihough maintains especially the necessity of
establishing more accurate spreading directions.
Having provided this caveat, we now proceed to correlate
the plane vector triangle solutions of Riddihough for the
Juan de Fuca,'America plate interaction in relation to
positions of segment boundaries in Oregon and Washington.
ROTATION OF THE SUBDUCTED CRUST
Riddihough's vectors for the period 8.5 to 6.5 m.y.b.p.
158
show a change in direction from approximately N 390 E
to N 500 E.
Starting about the end of the Miocene or the
beginning of the Pliocene to some time just before the.
Pleistocene, circa 5.5 m.y. to 2.5 m.y. ago, the direction
of convergence was fairly consistent, between about. N 460 E
to N 490 E.
But between 2.5 m.y. and 1.5 m.y. ago the
direction changed 90 to a more northerly direction and between 1.5 m.y. ago and the 0.5 m.y.-present epoch the direction
changed back (i.e. easterly) 9 degrees.
The data suggest first a clockwise rotation of the
subducting block from 8.5 to 6.5 m.y.b.p., then a period
of relatively minor back and forth adjustments from 6.5 to
2.5 m.y.b.p., then a counterclockwise rotation from 2.5
m.y. to 1.5 m.y.b.p.,
followed by a clockwise rotation
The motions seem
from 1.5 m.y. to 0.5 m.y.-present.
to be analogous to those an individual might make in
trying to slide a thin book under
a stack of other books,
pushing a corner in one way then the other corner the
other way, wiggling it back and forth into place to overcome
frictional obstructions; no anthropomorphism in plate motions,
however, is implied in this analogy.
The assumption of Hughes et al.
(1980) that the points
of offsets along the base of the continental slope are
on segment boundaries is probably a valid one.
with
Starting
these points of offsets, we have drawn lines represen-
ting segment boundaries at 1.5 m.y.b.p. when the direction
159
of convergence was about N 390 E and segment boundaries
at 0.5 m.y.-present at about N 480 E (Figure 2).
Sup-
portive evidence for using the slope offsets as starting
points is given by the bathymetry of the Oregon continental terraces (Byrne, 1962, 1963a, 1963b) which reveals
that the segment lines intersect areas of disturbed topography and are aligned with strikes of escarpments.
Radar, Landsat, and Seasat photography, however, is
indeterminate as to segment-boundary positions.
Several
northeast-southwest structural trends, as well as trends
in northwest-southeast, and north-south directions can
be discerned in these photographs,,- but there is no way of
knowing whether the northeast trends represent surface
expressions of segmentation.
The positions of segment boundaries, however, are
supported by deformation of marine terraces on the Oregon
and Washington coasts.
The evidence for such deformation
is discussed in a later section of this article.
Although vector solutions are usually reported in the
literature without indicating limits of error, an error of
±2o is assigned to each of the convergence directions
plotted, based on the uncertainties reported by Minster and Jordan (1978)
for point interactions between the Pacific and America plates.
We have plotted pairs of such wedges representing the positions of segment boundaries for these epochs (Figure 2).
160
Figure 2. Four groups of segment boundary positions in the northwest during the 1.5 m.y.b.p. epoch and the 0.5 m.y.-present epoch shown
as pairs of wedges. Hashured areas represent the 1.5 m.y. positions and
stippled areas represent the recent (0.5 m.y.-present) positions. Dashed
lines represent segment boundary positions at 5.5 and 7.5 m.y.b.p. A
clockwise rotation of the segment boundaries since 1.5 m.y. ago is
implied. Dots represent Cascade volcanoes: B, Mt. Baker; GP, Glacier
Peak; R., Mt. Rainier; SH, Mt. St. Helens; A, Mt. Adams; H, Mt. Hood;
W, Mt. Washington; T, the Three Sisters; N, Newberry Crater; C, Crater Lake.
161
These four groups of segments are referred to as the
"1.5-m.y. wedges" and the "recent wedges" respectively.
The positions at 2.5, 3.5, 4.5, 5.5, 6.5, and 7.5 m.y.b.p.
show that they coincide with the recent wedge (with the 5.5
and 7.5 m.y. lines a little to the north).
As can be seen from Figure 2, if the i.5-m.y. wedges
rotated through an angle of from 9° to 13° to the position
of the recent (0.5 m.y.-present) wedges during the last
1.5 m.y., practically all the Cascade volcanoes would have
been at a segment boundary at some time or another over
the last 1.5 m.y., at least for the area for which we have
drawn segment-boundary wedges.
The relationship between
petrologic types of the volcanoes with segment boundaries
claimed by Hughes et al.
(1980), therefore, would appear
to be unjustified.
We are not prepared to delineate the segment situation
further south, as the relative motions near the southern
end of the Gorda Rise and the triple junction at the Mendocino fracture zone are too complex for us to assume that
the direction of convergence would be parallel with that
further north.
SEGMENT BOUNDARIES INDICATING POSSIBLE AGE RELATIONSHIPS
AMONG CENTRAL HIGH CASCADE VOLCANOES
It is reasonable to suppose that the arrival of a
segment boundary in the subducted crust at a zone of mantle
162
with melted continental and oceanic crust under great
pressure could trigger massive eruptions or extrusions which
built the volcanoes by weakening the upper material and
supplying a path of least resistance.
Dehydration within
the descending slab releasing rising fluids may generate
hydraulic fractures (Fyfe and McBirney, 1975).
It is also
reasonable to assume that weaknesses in the upper crust
left by earlier and older segment-boundary residence or
fissures caused by other means could become mended by lava
flows coming up through them; many of the present Cascade
volcanoes are underlain by older lava flows.
Volcanoes on
the recent wedges, for example, are underlain by the 2.5
to 7.5 m.y. segment boundaries, and old flows of these
ages do occur beneath the present mountains (Fyfe and
McBirney, 1975; Taylor, 1980).
Volcanoes on the 1.5-m.y. segment-boundary wedges, however,
show no other known earlier segment boundary posi-
tions except one at 8.5 m.y. ago (ages older than 8.5 m.y.
are beyond the scope of the present study).
Note that the
large stratovolcanoes, Mt. Baker, Mt. Rainier, Mt. Hood,
North Sister, and the ancestral Middle and South Sisters,
are all situated on or near this segment-boundary wedge of
the respective groups (Figure 2).
According to our hypo-
thesis, these mountains should be about 1.5 m.y. old and
were probably formed more or less synchronously, as the
163
1.5-m.y. segment lines all presumably arrived at their
positions at approximately the same time, given some
adjustments to differential frictional forces.
Eruptions
enhanced by the presence of a segment boundary do not
preclude other eruptions at other fissures and weakened
areas of the crust and at other times.
But we are postu-
lating that the coincidence of the presence of a segment
boundary with conditions favorable for an eruption may have
caused the creation of the High Cascades as distinct peaks
and loci for later activity.
In other words, even though
volcanic activity can occur unpredictably all along the
volcanic front, the massive eruptions may have been caused
by the coincidence of a segment boundary position with
conditions favorable for eruptions.
Mt. Jefferson is between the l.5-m.y. wedge and the
recent wedge of Group 3, and it could have been formed either
while the 2.5-m.y. position rotated northerly to the 1.5-m.y.
position so that its age would be 2 ± 0.25 m.y. old or it
could have been formed more recently during the migration
of the 1.5-m.y. line to the recent position, or about 1 ±
0.25 m.y. old.
Rocks five miles east of Mt. Jefferson dated
by Armstrong et al.
(1975) by K/Ar technique possibly support
the 2-m.y. date of the formation of Mt. Jefferson.
Glacier Peak lies
between the recent wedge of Group 1
and the 1.5-m.y. wedge of Group 2 in Figure 2.
Its first
164
eruptions are undated but its rocks all show normal
magnetic polarity, so it is tentatively placed within the
recent wedge of Group 1.
Mt. St. Helens and Mt. Adams
are on the recent wedge of Group 2, Mt. Washington is on
the recent wedge of Group 3, and Newberry Volcano and
Mt. Mazama (Crater Lake) are on the recent wedge of Group
4.
These mountains, therefore, according to our estima-
tion, should have been formed during the last half million
years.
The present cone of Mt. St. Helens is of very recent
origin, probably within the last millenium, and is the only
currently active volcano in the United States.
Helens
But St.
is situated on an ancestral volcano whose age, if
our hypothesis is considered, should be more or less synchrpnous with the other volcanoes on the recent wedges of
the four groups, i.e. within the last 0.5 m.y.
Being on the recent wedge, of course, does not preclude
older lava flows underneath the present mountains, but if
this model is used, the older flows could show ages of
2.5 to 7.5 m.y.
when segment boundaries of these ages
resided in the vicinity.
Mt. Baker, Mt. Rainier, and Mt.
Hood, on the 1.5 m.y. wedge, on the other hand, could show
ages of 8.5 m.y.
(or older) under them.
A K-Ar age of around
8.2 m.y. assigned by Bikerman (1970) to the Laurel Hill
Pluton on the lower reaches of the southwest part of Mt. Hood
seems to support this interpretation.
The Jefferson-Three
165
Sisters area is complicated by the intersection of the
recent wedge of Group 3 with the 1.5-m.y. wedge of Group
4 so that deciphering the order of events here is difficult,
although this model is not inconsistent with the K/Ar
determinations made by Armstrong et al.
(1975).
The few
published radiometric ages available for the Central High
Cascades (compiled by Laursen and Hammond, 1974, 1978, 1979),
while not definitive in confirming this hypothesis, in
general do not conflict with age-relationships predicted
by this model.
It should be remembered that these age
relationships do not hold for all the areal space covered
by the wedges of segment-boundary positions--only for
the volcanoes on the volcanic front.
This model is also not inconsistent with the suggestion
of Fyfe and McBirney (1975) and Taylor (1980) that a range
of Pliocene composite volcanoes is buried beneath the
Pleistocene High Cascade platform.
This idea is based on
data on the Deschutes and the Outerson Formations much of
which were derived from volcanoes near the present High Cascade
axis whose lavas and ashes flowed eastward and westward over
4.5 m.y. ago.
These proto-Cascades, dubbed by Taylor the
"Plio-Cascades," seem to have been downfaulted as a graben
around 4.5 m.y. ago; fault scarps on both east and west side
of the High Cascades support this interpretation.
The High
Cascade platform, according to Taylor (1980), should be viewed
166
as a "Pleistocene fill within a Pliocene graben."
While
this situation is not necessarily continuous in time or
space, he postulates its applicability from about Mt. Adams
south to Crater Lake.
A Corollary
An additional and intriguing time-space relationship
in the Cascades may be correlative with our hypothesis of
segment-boundary migration.
Much recent activity has
occurred on or near all the Cascade volcanoes regardless
of the original ages of formation of the peaks.
For a
majority of the "dormant" peaks (thus ruling out Mt. St.
Helens) in the section of the Cascade Range under discussion,
from the northern border of Washington to Central Oregon,
the most recent massive activity seems to have been on the
south to east side of the mountain.
Mt. Baker has its more
recent Sherman Crater and Sherman Peak on its southern side;
Glacier Peak's most recent activity occurred on the south
side forming Disappointment Peak; the most recent cone
at the summit of Mt. Rainier is on the east-southeast side
of the mountain; the most recent eruption of lava from Mt.
Adams occurred from the vicinity of South Butte; Mt. Hood
erupted its most recent Crater Rock on the south side; Mt.
Jefferson erupted Forked Butte Cinder Cone and extruded the
Bear Butte lava south of the mountain; the youngest of the
167
Three Sisters, South Sister, exhibits a series of postglacial fissures and vents on its southeast slope.
Al-
though much recent activity has occurred within the New-
berry Caldera and on its outer slopes, the most recent
flow, the obsidian Big Flow, started on its south side.
These observations tantalizingly suggest a space-time
causal relationship.
If the most recent sweep of segment
boundaries is clockwise from the 1.5-m.y. line to the
recent line, i.e. in a southeastward direction, it is possible
that new eruptions would occur more likely at the point of
weakness developed in the crust in the
vicinity of the
volcano than elsewhere around the volcano, barring the
existence of other fissures or weakened areas of the crust
caused by other forces.
have
Earlier lava flow presumably would
mended and reinforced more northerly positions of
weaknesses passed through and left by the segment boundary.
The fact that the area north of North Sister exhibits
a large amount of very recent volcanic activity may at
first seem to contradict such a suggestion.
But as was
earlier stated, the 1.5-m.y. segment wedge of Group 4
intersects the recent segment wedge of Group 3 here so that
very young activity indeed could have taken place in the
area just north of North Sister under the influence of the
recent segment boundary wedge of Group 3.
K-Ar dates of Armstrong et al.
The widely varying
(1975) for this area do not
conflict with this interpretation.
168
While this hypothesis of segment-boundary migration
might provide some tentative clues to possible space-time
relationships among the Central High Cascade peaks,
predictability
speculative.
of any re-newed activity is still highly
In accordance with the present model, however,
such activity would occur more likely within the recent
wedges than elsewhere.
CORRELATION OF MARINE TERRACE DEFORMATION
WITH SEGMENTATION BOUNDARIES
Couch (1977) noted that geologic and geophysical data
collected along continental margins of active subduction
yield differences in sedimentary structure along the
strike of the continental shelf.
These differences,
according to Couch, may be due to time-varying differential
displacements of segments of the margin during plate subduction.
The amount and manner of accretion of oceanic
sediments and crust to the continental margin may differ
for different segments of the margin.
He stated that these
differential displacements may be reflected in the configuration of coastal land forms of Oregon and Washington.
Palmer (1967) constructed a longitudinal marine-terrace
profile off California, Oregon and Washington, showing that
deformation of marine terraces is closely related to the
pattern of past deformation within geological provinces.
169
While relative regional stability of terraces is demonstrated in Washington and Oregon in contrast to areas adjacent to major faults of the San Andreas System, terrace
deformation in Oregon has been noted by a number of investigators (Smith, 1.933; Pardee, 1934; Allen and Baldwin,
1944; Baldwin, 1945; Dicken, 1961; Baldwin, 1976; and others).
Near the point of intersection of the southernmost
segment wedge with the coastline, near the town of Brookings,
the elevation of terraces rises from 15 m to 30 m northward
up to about
100 m at Cape Ferrelo.
There seem to be two
other series of higher terraces at about 122 m and 244 m
which extend without change in elevation from the north to
Cape Ferrelo but they are abruptly absent southward indicating possible faulting.
Warping of the lower level
terrace, however, is strongly suggested by morphology,
according to Palmer (1967).
Points of intersection of the segment boundary wedges
of Group 3 with the Oregon Coast at Cape Blanco bracket the
area of the clearest warping of marine terraces on the
Oregon Coast.
Baldwin (1945) plotted the elevations of
the top of the wave-cut platform and the top of the terrace.
At Cape Blanco the top of the wave-cut platform has apparently
been uplifted at least 62 in.
The level lowers to 31 m at
Cape Arago and then to near or below sea level in a short
distance both north and south of this area.
Palmer's data
170
generally corroborate the presence of terrace warping
suggested by Baldwin.
He traced two terrace levels which
lie persistently at about 70 m and 125 m along most of the
central part of the Oregon Coast but rise to about 100 m
and 155 m respectively through the northern Bandon and
Empire Quadrangles, just north of the Cape Blanco area.
From just north of Cape Arago to Lincoln City is a
stretch of the Oregon Coast of about 200 km that is not
intersected by a segment boundary; interestingly also,
this stretch of the coastline shows no deformation of marine
terraces.
From Lincoln City to about Tillamook Bay the two lines
of segment boundary direction again intersect the coast.
While the evidence for terrace deformation is not as
spectacular here as at Cape Blanco, warping or tilting still
is apparent.
Cape Meares village is built on a terrace
north of Cape Meares.
The surface rises from an elevation
of approximately 8 m at the north end to an elevation of
nearly 12 m at the south (Frye, 1976); Short Beach, presumed
to be a stretch of the same terrace, just south of Cape Meares,
is at an elevation of about 35 meters.
The most prominent
terrace in this area, lying along the east side of Netarts
Bay, has an elevation of 18 to 20 m along the bay shore, but
rises inland to an elevation of 21 to 24 in.
The 124 m terrace
traced by Palmer has as its abrasion platform at 102 m
rising to 118 in in the sea cliffs at the Cape Meares Lighthouae.
171
Additional evidence of warping in this area is exhibited in the upstream elevation increase of river terrace
levels above the present rivers (Frye, 1976).
These river
terraces exhibit a steeper slope (1.4 m/km) than the
average gradient of the modern rivers (0.4 m/km).
While
climatic changes and eustatic changes in sea level can
change the gradient of rivers so that older terraces show
a steeper surface gradient than observed in the modern
rivers, tectonic uplift of the land by warping or tilting
cannot be ruled out.
It is the combination of the gentle
warping of the marine terraces with the apparent tilting
of the river terraces coincident with the intersection of
the segment boundaries through this area that is significant.
In the State of Washington, deformation of Pleistocene
deposits near Port Angeles was noted as early as 1906 by
Arnold and confirmed by Baldwin (1939).
The 1.5-m.y.
segment boundary passes through this area which is on the
northern shore of the Olympic Peninsula on the Juan de Fuca
Strait.
Baldwin (1939) recognized a
lower and an upper
member of the Pleistocene along the west coast of the
Peninsula, and deformation marked the interval between
deposition of the two members.
Warping, in his opinion,
has continued during the deposition of the
and possibly also during post-glacial time.
upper member
He noted that
deformation is differentiated from general emergence of
172
the coast by the variable amount of emergence at different
places.
The area of greatest emergence is around Cape
Elizabeth, about 30 km north of Grays Harbor.
The 1.5-m.y.
segment wedge and the recent segment wedge of Group 1
intersect the coast from about
end of Willapa Bay.
Cape Elizabeth to the north
Cape Elizabeth has emerged at least
72 m while at Kalaloch, about 27 km to the north, the coast
has emerged only 12 m, and to the south of Cape Elizabeth,
near Grays Harbor, the emergence has been very small.
Deformation of coastal terrace deposits, therefore,
generally corroborate the positions of segment boundaries.
The mechanism of such warping of the coast can be visualized
if there has been some displacement of the segment blocks
in relation to each other along the transverse fissures.
In
response to differential frictional obstructions along the
entire length of the subducted block, some segments might
be tilted or "ride higher" than others, so that even though
spreading is at the same rate along the Juan de Fuca ridge,
as the oceanic crust dives below the continent and transverse
structures develop, each segment could behave slightly
independently of its neighbor.
model of Carr et al.
According to the segmentation
(1974), the state of stress can be
different in different blocks depending on the time since
the last earthquake.
Warped coastal terraces, then, may
be the surface expressions of the forcible entry of such a
segmented block into the depths beneath the continent.
173
DISCUSSION AND CONCLUSION
Hughes et al.
(1980) correlated the positions of the
volcanoes in relation to the segment boundaries with the
volcano categorization of McBirney (1968).
According to
them the "coherent" large stratovolcanoes that erupt dominantly porphyritic pyroxene andesites with notable absence
of rhyolite are all located within the blocks between
segment boundaries and on the volcanic front.
The "divergent"
volcanoes that erupt siliceous andesites and dacites and
later basalts and rhyolites all lie at or near a segment
boundary.
These segment-boundary volcanoes are further
grouped into those along the volcanic front and those
behind the volcanic front to the east.
These authors have
treated petrologic relationships as a function of space only.
The present model, suggests that the situation is much
more complex; the dimension of time must also be considered.
The reason the coherent andesitic volcanoes have a different
chemical composition from the divergent volcanoes is not
because they are located in the middle of the segments and on
the segment boundaries respectively, but because the coherent
volcanoes erupted at least a million years earlier than the
divergent ones.
Two and a half million years ago the segment boundary was
in the recent position and eruptions occurred then of andesitic
composition.
Presumably during the eruptive period, pools of
174
the parent magmas formed fairly near the surface and these
would differentiate over time.
The segment boundaries then
migrated to the north so that by 1.5 m.y.b.p. they were in
the 1.5 m.y. positions and eruptions occurred then from
their parent magmas of andesitic composition, which make
up these large stratovolcanoes.
Then the segment boundaries migrated back to the southern
recent positions.
Eruptions occurring then and up to the
present at these positions where the parent magmas had
differentiated would naturally be of a more silicic composition typical of dacites and rhyolites of the divergent
volcanoes.
This model is a refinement of a part of the theory of
plate tectonics, viz. the general process of subduction and
specifically at the continental margin of the U. S. northwest.
The concept of migrating or rotating segment boundaries could
perhaps provide fresh insights into the relationship between
subduction and volcanic chains and space-time/petrologic
relationships.
Predictability of volcanic activity is highly speculative.
But this model suggests that in the Central High Cascades,
at least, such activity, though near-extinction in the geolo-
gical time-frame because of the coupling of the Juan de Fuca
plate with the North America plate, would occur more likely
within the recent wedges and possibly on the south to southeast
sides of the mountains than elsewhere.
175
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Geologic Field Trips in Western Oregon and South-
western Washington: Oregon Department of Geology and
Mineral Industries Bulletin no. 101, 232 p.