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Evolution of the
Earth
Seventh Edition
Prothero • Dott
Chapter 7
Mountain Building and Drifting Continents
Fig. 7.1
Ocean Drilling
Vessel JOIDES
Resolution.
This scientific
drilling ship is
equippped to drill 5
miles below the
ocean surface. To
date, it has drilled
over 500 wells
worldwide. Together
with its earlier
sistership, the
Glomar Challenger,
they have drilled
over 1500 wells.
Mountain Building and Drifting
Continents: Major Concepts
By end of 1857 (“Heroic Age”) models of Earth all
assumed that continents were fixed in place.
Attempts were made to explain mountains by crustal
cooling and contraction of Earth.
However, even then evidence existed that the
continents had moved. By 1950, submarine data had
beg;un to show astonishing sea floor features. This
eventually lead to “sea floor spreading” and “plate
tectonics”, the crowning jewels of geologic thought.
Fig. 7.2
Early (1840) depiction of a cross-section across the Applachian
Mountains. Section is across Pennsylvania from northwest (top) to
southeast (bottom)
Fig. 7.3
Early theories of mountain
building based largely on
geologic observations in the
U.S. Appalachian Mountains.
A. Hall(1857) postulated that
sedimentation depressed the
crust at continental margins.
B. Dana (1873) postulated
that the crust underwent
bending as the interior cooled
and shrank.
Fig. 7.4
Two different interpretations of the structure of the Jura Mountains
(Switzerland).
Top - upper rocks slip along “basement” along a flat shear surface.
(“Thin-skinned” tectonics.)
Bottom - Basement rocks are involved in folding and faulting along
steep (thrust) faults. (“Thick-skinned” tectonics)
Fig. 7.5
Early conception of continental drift (Arthur Holmes, 1910) showing
formation of Cenozoic mountains.
Fig. 7.6
Early attempt to
fit continents
together by
“cutting”
present-day
globe and
rotating and
squeezing
islands and
peninsulas.
(Baker, 1912,
Michigan
Academy of
Sciences)
Fig. 7.7
Alfred Wegener’s famous
reconstruction showing
three stages of continental
drift. (Wegener, 1929)
Fig. 7.8
Wegener’s reconstruction of Permian continents and paleoclimate
zones based on rock assemblages (glacial striations) and
paleoclimate data (salt deposits, fossil ferns, etc.).
Fig. 7.9
Holmes convection current mechanism for continental drift. (A.
Holmes, 1928, Physical Geology)
Fig. 7.10
A. Example of
remnant magnetism
with respect to
present field for two
rocks of different
ages.
B. Restoration of
Cambrian magnetic
field after correcting
for post-Cambrian
folding.
Figure 4.7
How do we reconstruct paleo continental positions?
• Early pioneers had to use geology (glacial
striations, salt deposits), paleontology
(fossils) and geometry
• But all this changed when paleomagnetism
was discovered and used to recreate
positions of continents in the past.
Fig. 7.11
Schematic representation
of components of fossil
magnetism. The
declination angle
provides information on
paleolongitude. The
inclination angle provides
information on
paleslatitude. With paleolat-long coordinates and
the age of the rock, its
position on the Earth can
be plotted.
Fig. 7.12
Relationship of earth’s magnetic field to remanent magnetism in rocks.
I. Present
II. Present discordant (measured) positions
III. Restored positions (paleogeographic locations)
Figure 2.8
Figure 2.9
Figure 2.10
Fig. 7.13
Different positions of
North America
relative to the equator
from Cambrian time
to present. Note
progressive
counterclockwise
rotation and
northward drift
throughout time.
Fig. 7.14
Restoration of
continental positions
of longitude in the
past using
paleomagnetic data.
The declination
angle of a sample
points toward the
paleomagnetic pole.
Continental
positions must be
adjusted until
ancient pole
positions for two
continents coincide.
Having outline for
original continental
margins helps.
Fig. 7.18
The East African rift
system showing the Afar
Triangle as a triplejunction at the intersection
of the Red Sea, Aden and
East African rifts. Possibly
the expression of a mantle
plume. Diverging rifts
starts a new round of
continental drifting and
ultimately “creates” new
ocean floor. Dots indicate
young volcanoes.
Nature of Sea Floor
Prior to the 1960’s most geologists considered the ocean floors to be generally
featureless plains, the oceanic crust to be very old and topographically featureless. It
was also assumed to be fixed in place. By 1970, all this had changed.
Fig. 7.19
Model for sea-floor spreading showing expansion of ocean ridges (divergent) and
arc-trench (convergent) systems. Three lithospheric plates are shown moving
over the weak low-velocity zone of the upper mantle. Magmas are produced in
arcs by heating along the subduction zone. Deep earthquakes are concentrated in
the relatively cool, brittle downgoing slab. Shallower earthquakes occur under
the spreading ridges. The 1000 C contour illustrates the contrast between hot
upper mantle beneath ridges and cooler region beneath the arcs.
Fig. 7.20
Gemini spacecraft photo of Gulf of Aden and southern Red Sea
Fig. 7.21
Comparison of motion
on transform and
transcurrent (strikeslip) faults. Red lines
are spreading ridge
axes. A and B show
two different stages for
each case.
Upper: Plates are spreading away from ridge axis and the transform fault connects
two offset segments of that axis. Segments of adjacent moving oceanic crust slide
past one another along the transform while spreading occurs.
Lower: Sea-floor spreading ceased before A and then a transcurrent fault cut the
ridge. Between times A and B the dead ridge was offset in the direction shown by
the arrows in the opposite sense of displacement from that of the transform fault.
Fig. 7.22
Geometry of spreading ridge axis, transform faults and subduction zones
Lithospheric plates (A, B,
C) move (rotate) around an
imaginary pole.
Transform faults are
perpendicular to the
spreading axis (parallel to
imaginary lines if latitude
around the rotation pole.
Rates of spreading are
indicated by lenghs of
arrows and increase from
rotation pole to (rotation)
equator.
Fig. 7.23
Fig. 7.24
Fig. 7.25
Fig. 7.26
Paleomagnetism helps date age of oceanic crust.
Fig. 7.27
Earthquake epicenters 1961-1967
Note how epicenters outline plate boundaries. Arrows indicate
direction of horizontal motion during earthquakes.
Fig. 7.28
Major Lithospheric Plates
Plates as defined by seismicity (previous slide). Arrows show direction of plate
motion and confirm hypothesis of sea-floor spreading by showing divergence
(extension) away from ocean ridges and convergence (compression) toward
volcanic arc-trench (subduction) zones.
Fig. 7.30
Possible Driving Mechanisms for Plate Tectonics
1. Ocean ridge push
2. Gravity sliding (down
slope of an ocean ridge)
3. Gravitational pull on a
cold plate (down a
subduction zone)
4. Carried on convection
cell.
Fig. 7.31
Types of Plate Interactions
Fig. 7.32
The Six Major Types of Sedimentary Basins
Indonesia
Offshore Calif.
Nevada
E. Africa
E. Coast NA
Michigan Basin
The six major types of sedimentary basins are shown in their platetectonic settings. The major physical cause or causes of subsidence
for each case are shown below the diagram. Some examples are
indicated in top.
Fig. 7.33
Stages in the Development of a Passive Margin
How did we get from B to C?
Fig. 7.34
Detailed Cross-section of a Passive Margin
Atlantic Margin
What is the relative
age of the basalt?
Jurassic salt
Cretaceous &
Cenozoic sediments
Triassic rift valley sediments
Fig. 7.35
Regional Crustal Subsidence due to local sediment loading
Example: Gulf of Mexico
and Mississippi River
Sediments delivered by
major river systems
eventually deposit a nonnegligible load on the crust,
resulting in slight
deformation (subsidence) and
opens accomodation space
for further sediment loading.
(positive feedback).
Fig. 7.36
Formation of an intercratonic basin and a foreland basin
Formation of an intercratonic basin and a foreland basin and an
intervening arch by “thrust loading” e.g thrust faulting a package of
rock onto a continental margin.
Fig. 7.37
Conversion of a passive margin to a convergent margin
A classic passive
margin (A) can be
converted into an
active convergent
margin by collision
with an arc (B).
Thrust loading and
erosion of mountains
produce a foreland
basin (FB).
Cessation of tectonic
activity and deep
erosion can produce a
new passive margin
(C).
GO TO
“SEAFLOOR SPREADING”
SLIDES