AN ABSTRACT OF THE THESIS OF
Daniel Sattel for the degree of Master. of Science in Geophysics presented
on October 4g 1990
Title: P and S Velocity
Abstract approved:
the Gulf of Maine
Redacted for Privacy
Seismic refraction data collected in 1985 by the USGS were used in
this study to derive the P and S velocity structure of the crust beneath the
Gulf of Maine. The data quality differs among instruments and is affected
by surficial lateral heterogeneities, a ringy source signature and
reverberations. Velocity models of the crust were computed by onedimensional raytracing and by wavefield continuation. Pg arrivals were
modeled using both techniques to derive the P velocity of the upper 5-15
km of the crust and give very similar results. Strong Sg arrivals were also
observed, and computed S amplitudes generated from P-S conversion for
different scenarios show that the observed S wave is generated at the
basement top. Two small sediment basins are indicated in the Central
Plutonic Zone and two faults are suggested in the Fault Zone and the
Central Plutonic Zone, respectively. Beneath the sediments the layering is
uniform with dips of less than 2° and a fairly laterally homogeneous
velocity structure, in spite of lateral variations in reflectivity. P and S
velocities increase from 5.3 and
2.8
km/s, respectively, at the basement to
and 3.7 km/s at 10 km depth. A laterally discontinuous low velocity
zone is indicated at 6-10 km depth which might be caused by laccolithic
6.4
granitic intrusions. However, magnetic and gravity data do not show
indications for felsic intrusions where the low velocity zones are observed.
Velocity differences among some instruments suggest anisotropy in the
upper 6 km of the crust, as observed in onshore Maine. These instruments
indicate velocities parallel to the structural grain of the Appalachians of
6.1-6.4 km/s and velocities transverse to the grain of 5.8-6.1 km/s in the
depth range
2-6 km.
Cashes Ledge granite, a site of an intense magnetic
high, has a reduced velocity compared to surrounding rocks and might
extend to at least 10 km depth. Poisson's ratio for the upper crust ranges
from
0.23-0.26.
To derive the velocity structure of the middle and lower crust, wideangle reflections interpreted to be PmP and SmS were modeled by onedimensional raytracing. In addition synthetic seismograms were computed
using the WKBJ method to constrain possible middle and lower crust
velocity models by their PmP and SmS amplitudes. Recorded PmP and
SmS wide-angle reflections have quite different amplitudes and travel-times
among instruments suggesting a heterogeneous lower crust. The crust
below 10 km depth has an average P velocity of 6.5-6.8 km/s and an
average S velocity of 3.7-3.9 km/s. Most instruments indicate a Poisson's
ratio of around 0.25 between 10 km depth and Moho and one instrument
suggests a Poisson's ratio
of 0.28.
Hence, the middle and/or lower crust
under the Gulf of Maine is heterogeneous and represents average crust
modified by mafic intrusions, probably during Mesozoic extension. Moho
depth is indicated between 30 and 37 km depth. Wide-angle reflections
coming from 28 km depth as indicated by two instruments are interpreted
to come from the top of a lower crustal intrusion. This interpretation is
supported by an observed mismatch between the models giving a thickness
of 28 km and the reflection data.
Although it represents a different geological terrane, the velocity and
thickness of the crust beneath the central Gulf of Maine is very similar that
onshore Maine.
P and S Velocity Structure Beneath the Gulf of Maine
by
Daniel Sattel
A THESIS
submitted to
Oregon State University
in partial fufflulment of
the requirements for the
degree of
Master of Science
Completed October 4, 1990
Commencement June 1991
APPROVED:
Redacted for Privacy
Maj QProssor: Associate Professof of Geophysics
Redacted for Privacy
Dean oCollee of
Redacted for Privacy
Dean of Gra(date School
Date thesis is presented October 4. 1990
Typed by the author Daniel Sattel
TABLE OF CONTENTS
INTRODUCTION
.1
TECTONICSETTING .................................................................... 3
PREVIOUSRESULTS .................................................................... 9
DATAACQUISITION .................................................................. 16
DATA PROCESSING AND ANALYSIS METHODS ........................ 17
Wave Field Continuation......................................................... 27
P-S CONVERSION ....................................................................... 31
RESULTS .....................................................................................
UpperCrust ...........................................................................
LowerCrust........................................................................... 58
Results of Wavefield Continuation and Comparison to
Raytracing Results ............................................................ 72
DISCUSSION ................................................................................ 84
UpperCrust........................................................................... 84
LowerCrust ........................................................................... 89
CONCLUSIONS ............................................................................ 95
BIBLIOGRAPHY .......................................................................... 97
APPENDIX ................................................................................ 106
LIST OF FIGURES
Figiire
1.
Page
Locations of USGS seismic refraction lines collected
during cruise GYRE-85-11 .................................................... 4
2.
Tectonic map of the Gulf of Maine and surrounding region....... 5
3.
Simplified magnetic map of the Gulf of Maine ........................ 10
4.
Bedrock lithology of the Gulf of Maine, derived from
gravity and aeromagnetic data ............................................... 11
5.
Summary of velocity models around the Gulf of Maine........... 14
6.
Frequency spectrum of line 7 OBS A2 trace 7121 vertical
component, prior and after resampling at 16 ms ..................... 18
7.
An illustration of fk-filtering ................................................ 21
8.
Raytracing of P velocity model L7A2 shown in figure 16 ........ 22
9a.
Determination of acceptable velocity models of OBS L7A2 ...... 23
9b. Acceptable and unacceptable fit of first arrival at OBS L7A2...24
10.
Comparison of first onset at two distances (OBS L7C3 HYD)...26
11.
P-S conversion ................................................................... 32
12.
Tectonic setting and bathymetry of line 7 ............................... 34
13. OBS L7C3, seismic section and determined P velocity ............. 35
14. OBS L7C3, seismic section and determined S velocity with
corresponding Poisson's ratio ............................................... 36
15.
OBS L7A2, seismic section and determined P velocity ............ 37
Page
Figure
16. OBS L7A2, seismic section and determined S velocity with
corresponding Poisson's ratio ............................................... 38
17.
P and S velocity of the upper crust at line 7 ............................ 42
18.
Tectonic setting and bathymetry of line 8 ............................... 44
19. OBS L8C3, seismic section and determined P velocity ............ 45
20. OBS L8C3, seismic section and determined S velocity with
corresponding Poisson's ratio ............................................... 46
21. OBS L8A2, seismic section and determined P velocity ............47
22. OBS L8A2, seismic section and determined S velocity with
corresponding Poisson's ratio ............................................... 48
23. P and S velocity of the upper crust at line 8 ............................ 50
24. Tectonic setting and bathymetry of line 3 ............................... 51
25. OBS L3C6, seismic section and determined P velocity ............ 52
26. OBS L3C6, seismic section and determined S velocity with
corresponding Poisson's ratio ............................................... 53
27. P and S velocity of the upper crust at line 3 ............................ 55
28. P and S velocity of the upper crust at line 5 & 6 ..................... 56
29. Illustration of Pg, PmP and Pn arrivals .................................. 59
30.
L7C3, P velocity of middle and lower crust determined from
PmParrival ........................................................................ 61
31.
L7C3, synthetic seismograms corresponding to the P velocity
modelsin figure 30 .............................................................. 62
Page
Figure
32.
L7C3, S
velocity of middle and lower crust determined from
SmSarrival ......................................................................... 63
33.
L7A2, P
velocity of middle and lower crust determined from
PmParrival ........................................................................ 64
34.
L7A2, synthetic seismograms corresponding to the P velocity
models in figure
35.
L7A2, synthetic seismograms corresponding to the P velocity
models in figure
36.
33 .............................................................. 65
L7A2, S
33,
using a "ringy" source wavelet ................ 66
velocity of middle and lower crust determined from
SmSarrival ......................................................................... 68
37.
L7A2, synthetic seismograms corresponding to the S velocity
models in figure
38.
36 .............................................................. 69
Pre-processed and muted data sections of L7C3 and L7A2 used
for wavefield continuation .................................................... 73
39.
Pre-processed and muted data sections of L8C3 and L3C6 used
for wavefield continuation .................................................... 74
40.
Tau - velocity wavefield of L7C3 and L7A2 ...........................75
41. Tau - velocity wavefield of L8C3 and L3C6 ...........................76
42.
Velocity - depth wavefield of L7C3 and L7A2 ........................ 77
43.
Velocity
44.
Data recorded at L7C3 and L7A2 overlain by travel-time
depth wavefield of L8C3 and L3C6 ........................ 78
curves obtained by wavefield continuation.............................. 79
Page
Figure
Data recorded at L8C3 and L3C6 overlain by travel-time
curves obtained by wavefield continuation .............................. 80
Comparison of raytracing and wavefield continuation velocity
models.for.L7C3 and L7A2 .................................................. 81
47. Comparison of raytracing and wavefield continuation
velocity models of L8C3 and L3C6 ....................................... 82
Comparison of upper crustal P velocities determined
at line 3, 5, 7 and 8 .............................................................. 86
Upper crustal P velocity in Coastal Maine and in the Gulf of
Maine ................................................................................. 87
1ij
Moho depth and average middle and lower crust
P velocity determined from PmP wide-angle reflections .......... 90
51.
Moho depth and average middle and lower crust
S velocity determined from SmS wide-angle reflections ........... 91
52.
Average Poisson's ratio of middle and lower crust
determined from PmP and SmS wide-angle reflections ............ 92
53.
Determined velocity models superposed on interpreted
line drawing of the migrated reflection data of USGS
line1A ............................................................................... 93
54.
OBS L7A8, seismic section and determined P velocity ........... 106
55.
OBS L7A8, seismic section and determined S velocity
with corresponding Poisson's ratio ...................................... 107
56
OBS L7C4, seismic section and determined P velocity .......... 108
Page
Figure
57.
OBS L7C4, seismic section and determined S velocity
with corresponding Poisson's ratio ......................................
109
58.
OBS L8A8, seismic section showing Pg arrival ..................... 110
59.
OBS L8A8, seismic section showing Sg arrival ..................... 111
60.
OBS L3C4, seismic section and detennined P velocity .......... 112
61.
OBS L3C4, seismic section and determined S velocity
with corresponding Poisson's ratio ...................................... 113
62.
OBS L3A2, seismic section and determined P velocity .......... 114
63.
OBS L3A2, seismic section and determined S velocity
with corresponding Poisson's ratio ......................................
115
64.
Tectonic setting and bathymetry of line
65.
OBS L6A2, seismic section and determined P velocity .......... 117
66.
OBS L6A2, seismic section and determined S velocity
5 & 6 ....................... 116
with corresponding Poisson's ratio ...................................... 118
67.
OBS L6A8, seismic section and determined P velocity .......... 119
68.
OBS L6A8, seismic section and determined S velocity
with corresponding Poisson's ratio ......................................
120
69.
OBS L5A8, seismic section and determined P velocity .......... 121
70.
OBS L5A8, seismic section and determined S velocity
with corresponding Poisson's ratio ......................................
71.
L7C4, P
122
velocity of middle and lower crust determined
fromPmP arrival .............................................................. 123
Page
Figure
72.
L7C4, S
velocity of middle and lower crust determined
fromSmS arrival ............................................................... 124
73.
L8C3, P
velocity of middle and lower crust determined
fromPmP arrival .............................................................. 125
74. OBS L8C3, synthetic seismograms corresponding
to the P velocity models in figure
73 .................................... 126
75. OBS L8C3, S velocity of middle and lower crust determined
fromSmS arrival ............................................................... 127
76.
L8A8, seismic section showing the PmP arrival .................... 128
77.
L8A8, seismic section showing the SmS arrival ..................... 129
78.
L3C6, P
velocity of middle and lower crust determined
fromPmP arrival .............................................................. 130
79.
L3C6,
synthetic seismograms corresponding
to the P velocity models in figure
80.
L3C6, S
78 .................................... 131
velocity of middle and lower crust determined
fromSmS arrival ............................................................... 132
synthetic seismograms corresponding
81.
L3C6,
82.
to the S velocity models in figure 80 .................................... 133
L3A2, P velocity of middle and lower crust determined
fromPmP arrival .............................................................. 134
83.
L3A2, S
velocity of middle and lower crust determined
fromSmS arrival ............................................................... 135
P AND S VELOCITY STRUCTURE BENEATH THE GULF
OF MAINE
INTRODUCTION
During cruise GYRE-85-1 1 performed by the U.S. Geological Survey
in September-October, 1985, seismic reflection and refraction data were
collected in the Gulf of Maine. Located east of New England and south of
the Canadian Maritime Provinces of New Brunswick and Nova Scotia, the
Gulf of Maine is part of the Appalachian Orogen. Most of its area is
associated with Avalonian rocks.
In onshore Maine, seismic data indicate local low velocity zones
(LVZs) and anisotropy in the upper crust (Kiemperer and Luetgert, 1987;
Luetgert, 1985a, 1985b; Stewart et al., 1985). Average velocities below
10 km depth are 6.6-6.8 km/s and Moho depth is 31-34 km (Luetgert,
1985b; Luetgert et al., 1986; Stewart et al., 1985; Unger et al., 1987;
Luetgert et al., unpublished). Reflection data collected in the Gulf of
Maine show a change in the seismic signature from northwest to southeast,
a reflective lower crust and a Moho depth between 10 and 11 s TWTT (two
way travel time) (Hutchinson et al., 1987, 1988).
2
The seismic refraction data investigated in this paper were acquired by
deploying ocean-bottom seismometers and using two 2000 cubic inch
airguns as the sources. Despite the reverberatory nature of the signal due
to an untuned source and a hard seafloor, a good signal/noise ratio was
recorded for P wave arrivals on most instruments. However, the ringy
source signature made it difficult to use Pg amplitude information to detect
triplications. Clear S wave arrivals are also observed on most records.
Because the source, an airgun in water, only generates P waves, the depth
where P to S conversion occurs was determined.
After reviewing the tectonic setting and previous work about the study
area, the seismic refraction data are used to derive crustal P and S wave
velocity models in the central region of the Gulf of Maine especially to
look for the presence of low velocity zones, anisotropy and to determine
the lower crustal velocity. First arrivals constrain only the upper crustal
structure. PmP and SmS wide angle reflections are used to determine the
Moho depth. Constraints on the upper and lower crustal composition are
derived by computing Poisson's ratio. Finally, our results are combined
and compared with the results of previous studies of the Gulf of Maine and
adjacent areas in order to lead to a better understanding of the history and
structure of the Appalachian Orogen.
3
TECTONIC SETTING
Figures 1 and 2 show the location of the seismic refraction profiles
and the tectonic structure of the Gulf of Maine. The refraction profiles
cover mostly the Gulf of Maine Fault Zone and adjacent Central Plutonic
Zone, which are separated by the Fundy Fault. The profiles also extend
into Nashoba and Coastal Maine terrane.
Today the Gulf of Maine has a mean water depth of 250 m (Ballard
and Uchupi, 1975). A thin unconsolidated sediment layer (max. 200 m),
consisting mainly of gravel and sand close to the shore, probably glacial
debris (Hathaway et al., 1965), and silt and clay in deeper parts of the
basins (Uchupi, 1966) overlies the basement. Paleozoic and Precambrian
igneous and metamorphic rocks are exposed along the coast (Ballard and
Uchupi, 1975).
Bedrock samples taken from the central Gulf are
predominantly granitic and have ages between 231 my and 407 my (Ballard
and Uchupi, 1975).
The Gulf of Maine is the eastern part of the Appalachian orogen and is
bounded to the southeast by the Atlantic passive continental margin. The
Appalachian orogen was the result of a sequence of converging and
diverging tectonics and neighbours the Grenville province, a craton
71°W
70°W
44°l
68°W
69°W
'i2
.LINE6
1
sA2
/NE3
A2
A8
C4
.
C6
LINE 5
LINE 7
A8
C4
43,1
A8
LINE 4
A2
43°N
0A2
,
A2/
V
C4
I'
'S
'S
42°N
'
C3
71°W
70°W
69°W
I
A8
c\LINE
42°N
2
68°W
Figure 1. Locations of USGS seismic refraction lines collected during
cruise GYRE-85- 11. The profile midpoints are indicated and give an
idea of the locations of wide-angle reflection points.
740
460
/
",
440
'/
!DV
EXPLANATION
,-.-'- IMAI.
FAULT MESOZOC
BASAU
FAULT ON TERRANE BOUGOART
- DEEP DEIC REFLECTION PROFLE
000 NIRISCAN FRONT
r---
.**
A
BASEMENT HPDE ZONE
ECMA
DEOROCK SAMPLE
RIFT BASIN, EXPOSED
*******0t**
TROUGH
MESOZOIC
-FAULT ZONE{AIAI.DN)
4000
SP
-.
?- -:-
RIFT BASIN. INFERR
FROM SEIOMUC
AUSI MAGNETIC DATA
.010*
OCEANIC
AWN, MAPPED ON
CfIUS
RIFED
2OMA
CENTRAl.
PLUTONC ZONE
RIFTED
SOUTHERN
a- OLOCK
PLUTONIC ZONE
(AVALON? hEGUMA?)
(AVALON)
2000
3000
PLUTO
(MEGUMA)
000
-
---:--------- -'
UPPER MANTLE
KM
Figure 2. Tectonic map of the Gulf of Maine and surrounding region.
The generalized terranes of the Appalachian orogen, the configuration of
the continental margin, and location of the Quebec-Maine-Gulf of MaineGeorges Bank seismic transect (Hutchinson et al., 1988) are indicated.
Abbreviations are: NFZ, Norumbega fault zone; TH, Turtle Head fault
zone; BL, Belle Isle fault; VF Variscan front; FF, Fundy fault; CLMA,
Cashes Ledge magnetic anomaly; NA, Nauset anomaly; ECMA, East
Coast magnetic anomaly; PZ, Plutonic zone. Below, the line drawing of
USGS line 1A is shown (Hutchinson et al., 1988).
1.1 b.y. old, to the west. Descriptions of its development are given by
Taylor and Toksöz (1982), Zen (1983) and Williams and Hatcher (1983),
from which the following summary of the history of the northern
Appalachian orogen is derived:
Tectonic activities can be traced back as early as 820 m.y. ago when
the fonnation of the Iapetus Ocean (proto-Atlantic) started (Rankin, 1976).
Early or Middle Ordovician to Permian time, the proto-Atlantic was
closing and hence the ancient North American east coast represented a
converging margin during that episode. The major tectonic activities
during the closing occurred in three orogenies, namely the Taconian,
Acadian and Alleghanian movement. The Taconian event (about 480 to
430 m.y. ago) was probably an arc-continent collision. The second and
primary event, the Acadian movement (about 360 to 400 m.y. ago) was
most probably a continent-continent collision of the North American and
Avalonian block and was a period of high metamorphism and plutonism.
The Alleghanian event (about 250 to 300 m.y. ago) was probably the
second stage of the continent-continent collision. In addition to
compression, there is evidence for strike-slip motions during the late
Paleozoic (Arthand and Matte, 1977; Mosher, 1983). Continental
divergence started in late Triassic and produced the present North Atlantic
Ocean south of the Azores (Pitman and Taiwani, 1972). Since the Early
Jurassic when the continents were separated and ocean floor started to fill
the gap left behind, tectonic activity decreased significantly (Ballard and
Uchupi, 1975).
7
Basement rock of the Avalon Zone is well exposed onshore, e.g. in
southeastern Massachusetts (Fairbaim et al., 1967). The Avalon Composite
Terrane was a relatively stable shelf or platform during Paleozoic time.
Paleozoic orogenies affected only locally shallow water sediments, which
are interbedded with volcanic rocks and were intruded by granites dated to
be 560-650 my old (Rast et aL, 1976; McCartney et aL, 1966; Fairbairn et
al., 1967). Metamorphic rocks usually belong to the greenschist facies
(Poole, 1967). During the Carboniferous to Permian a basin evolved
which stretches from Boston 250 km northeast into the Gulf and holds
intrusive and sedimentary rocks (Ballard and Uchupi, 1975).
The Appalachian orogen can be divided into numerous terranes with
different developments. Terranes in our study area are, from west to east,
the Dunnage terrane in coastal Maine and the Nashoba, Avalon and
Meguma terranes in the Gulf of Maine. The Dunnage terrane represents
the North American craton and miogeocline and contains remnants of
oceanic crust, whereas the Nashoba, Avalon and Meguma terranes are
composed of continental island arc rocks and continental rocks of the
Avalon block of pre-Africa (Robinson and Hall, 1980; Williams and
Hatcher, 1983; Zen, 1983; Rast and Skehan, 1983). Because the Meguma
terrane was little effected by the Taconian movement, Avalon rocks and
Meguma rocks carry different magnetic and tectonic signatures (Poole,
1967; Kane, 1972). Whereas the boundary between Avalon and Meguma
terrane is clearly indicated in Nova Scotia by large magnetic anomalies
(Keppie, 1985), it is difficult to define this contact in the Gulf of Maine.
Hutchinson et al. (1988) further divide the Gulf of Maine into four
blocks of differing reflection and magnetic character. These are from the
coast seaward; the Gulf of Maine Fault Zone, the Central Plutonic Zone,
the Southern Plutonic Zone and the Rifled Block. The Gulf of Maine Fault
Zone, the Central Plutonic Zone and probably the Southern Plutonic Zone
are part of the Avalon terrane, whereas the block south of the Southern
Plutonic Zone, characterized by rift basin rocks, is correlated with
Meguma rocks of Nova Scotia (Hutchinson et al, 1988; Schenk, 1981).
The Gulf of Maine's Moho may have been produced by Mesozoic
crustal extension (Hutchinson et. al., 1988). Three rift systems from the
Triassic period are identified in the Gulf of Maine by Ballard and Uchupi,
1975: the Fundy Fault system, the Wilkinson Basin system and the Georges
Basin/Bank system. The Triassic to Early Jurassic rift basins found in the
Gulf of Maine contain igneous and metamorphic rocks (Uchupi, 1966;
Ballard and Uchupi, 1975; Klitgord et al., 1982). The central part of the
Gulf probably represents a "structural horst of uplifted basement rock" and
major faults within the Gulf are thought to have formed at the beginning of
the Late Triassic tectonic activity (Ballard and Uchupi, 1975).
Since Tertiary time the history of the Gulf of Maine includes Pliocene
uplift, fluvial erosion, Pleistocene glaciation, glacial erosion (Shepard et
al., 1934, Murray, 1947), ice retreat at least 11,000 years ago (Emery et
al., 1965; Zeigler et al., 1965), sea level rise and sedimentation (Uchupi,
1966; Ballard and Uchupi, 1975)
PREVIOUS RESULTS
Due to its shallowness, thin sediment cover and its interesting location
between coast and continental margin, the Gulf of Maine has experienced
numerous geophysical projects. The refraction profiles of this study are
part of the U.S. Geological Survey Deep Crustal Studies Program Maine
transect, which extends from Quebec to the oceanic crust east of Georges
Bank. The results of gravity, magnetics and seismic investigations are
summarized in the following paragraphs.
An interpretation of gravity and magnetic data collected in the Gulf of
Maine was done by Kane et al. (1972). The high average Bouger gravity
of the Gulf of Maine relative to the surrounding coast is explained by the
relative uplift of older basement (Paleozoic or Precambrian) and deeper
denser crust, or it might imply that the Gulf has a different composition or
a higher proportion of mafic rocks. Maine and the Gulf of Maine have a
slightly positive free-air gravity field, indicating almost isostatic
equilibrium for the area. Both the gravity and magnetic field show a
northeast trend that is parallel to the Appalachians. The Gulf of Maine has
the highest average magnetic field and the most intense anomalies recorded
along the Atlantic coast (see figure 3). Gravity highs are associated with
mafic igneous lithic units, whereas gravity lows are correlated with felsic
10
/
A
o
cL/I
.,
2
ECI*
Figure 3. Simplified magnetic map of the Gulf of Maine (Zietz et al.,
1980; Hutchinson et al., 1988). Deep seismic reflection profiles and the
investigated refraction lines are indicated. Abbreviations are as on figure
2.
11
GEOPHYSICAL FIELD INVESTIGATIONS
E X P LANA TI ON
i.
.. -
Area of pronounced gyarity high ,ndicating
prisoner of mafic pluto,ic rock. or possibly
In
IOsii
places.
ramafsc rocl
Area of moderate gravity lo
rndicatuig
presence ,f this bodies of felsac compo.i.
lieu, or of moderate thickness., of strati-
fied rocks
hicb are at most partly
metamorphosed
'
Ar.a of pronounced areally large, .eronsag-
notic high indicating presence of m.fic
pluton.c rock
Aros of moderate gravity high indicating
presence of mafic rock, probably of volcanic
one.,,
Principal magnetic none boimday
incur none boundary
Area of pronounced gravity lo
probably
caused by felsic plulonic rock. Broad eros
In central porte of zones B and C may be
underlain is part by stratified rocks
X
X
X
Prono,nced linear aeromagnetic high, proDably caused by oltramaf,c rock
Figure 4. Bedrock lithology of the Gulf of Maine, derived from gravity
and aeromagnetic data (Kane et al., 1972). Location of investigated
refraction lines are also shown.
12
igneous rocks or porous sedimentary rocks. Magnetic highs are caused by
igneous rocks, intense anomalies are associated with mafic and ultramafic
rocks. In the central Gulf, mafic and felsic plutonic rocks are abundant
and some of these lithologies occupy broad areas. A summary of the
results is shown in figure 4. Cashes ledge is the location of the gravity and
magnetic peak of the Gulf. A granite sample recovered from there
indicates therefore a felsic rock unit overlying a mafic unit.
Tn 1984, as part of the U.S. Geological Survey Deep Crustal Studies
Program Maine transect, seismic reflection data were collected in the Gulf
of Maine. The location of the recorded profile and results are shown in
figure 2 (Hutchinson et al. 1987, 1988). The data indicate a varying
reflection character across the Gulf of Maine and an essentially flat Moho
between 10-11 s TTWT or 33-36 km depth. Bands of moderately dipping
reflections dominate the northern Gulf. The central section has a fairly
transparent upper crust, an indication for granitic basement rock and shows
discontinuous subhorizontal lower crust reflections. The southern end of
the profile shows dipping and subhorizontal reflections throughout the
crust. The reflection character in the central and southern plutonic zones,
showing a band of subhorizontal lower crust reflections, is typical for
extensional regimes.
In the 1984 Maine-Quebec experiment conducted by the USGS in
cooperation with the Canadian Earth Physics Branch, reflection and
refraction profiles were shot at different sites in Maine. The refraction
13
profiles and determined velocity models are shown in figure 5.(Luetgert et
al., unpublished). The upper crust has a velocity of 5.3 - 6.5 km/s and low
velocity zones are indicated by two profiles (figure 5, profiles 2+4), one
from 7 to 14 km depth and the other from 15 to 21 km depth. Seismic
reflection data across the coastal volcanic belt show the contrast between
reflection-free plutons and reflection-rich metavolcanic rocks (Stewart et
al., 1986).
Refraction data collected in coastal volcanic terrane of
southeastern Maine (figure 5, profile 4) indicate anisotropy in the upper 6
km (Kiemperer and Luetgért, 1987; Kafka and Reiter; 1987). Velocities
are 10% higher parallel than transverse to the structural grain of the
Appalachians (see figure 5 & 49). The surface rocks in the data acquisition
area are metasedimentary schists, and the velocities obtained are
comparable to laboratory velocity results on metamorphic rocks. In the
Northern Appalachians, the crust thickens from 30-35 km in coastal Maine
towards the axis of the Appalachians to 38-40 km (Cipar et al., 1986;
Luetgert, 1985b; Luetgert et al., 1986; Luetgert, 1990; Luetgert et al.,
unpublished; Stewart et al., 1985; Taylor and Toksöz, 1982; Unger et al.,
1987). The crustal thinning is explained by Mesozoic stretching (tJnger et
al., 1987). The average velocity between 10 km depth and Moho is 6.6-6.7
km/s (Luetgert et al., unpublished). According to Luetgert et al. (1987)
and Luetgert (1990) the mean Poisson's ratio for the crust in Maine is 0.25.
However, the middle and lower crust of Maine seems to consist of quartz
rich rocks as indicated by a reduced Poisson's ratio (Luetgert et al., 1988).
The absence of reflections coming from the mantle suggests a gradational
Figure 5. Summary of velocity models around the Gulf of Maine. Tic marcs are 10 km. References:
1-5 Luetgert et al. (unpublished); 6, Taylor et al. (1980); 7, Chiburis and Ahner (1979) as cited in
Taylor and Toksoz (1982); 8, Dainty et al. (1966).
15
crust-upper mantle transition (Luetgert, 1985a,
Luetgert et al., 1987,
Stewartetal., 1985).
Figure 5 also shows velocity models of refraction data in southern
New England (Chiburis and Abner, 1979) and along Nova Scotia (Dainty et
al., 1966). Results from Taylor et al. (1980), who analyzed earthquake
records are also shown.
To summarize previous studies, seismic data indicate a crustal
thickness of 30 - 35 km in coastal Maine, an upper crustal velocity of 5.7-
6.3 km/s, localized low velocity zones and anisotropy in the upper crust.
The average velocity below 10 km depth is 6.6 - 6.7 km/s (Luetgert et al.,
unpublished).
16
DATA ACQUISITION
The data acquisition for this experiment has been described by Tréhu
(1987), and is summarized here.
Data collection of each profile started with the deployment of oceanbottom seismometers (OBS) from the USGS, which were equipped with
one to four components (a hydrophone and three orthogonal geophones).
Next the ship fired two 2000 cubic inch airguns simultaneously at 2000 psi
at a fixed time interval along the profile. For lines 3 and 8, a shot was
fired every 2 minutes, and for lines 4, 5, 6, and 7 every minute,
corresponding to shotspacings of approximately 125 m and 250 m,
respectively. The interval between the clock pulse and the firing of the
port gun was held between 22 and 26 ms. The delay between the two guns
was held
within 5
ms.
A cartridge recorder recorded the events on a pre-programmed
schedule. For each sample, sixteen bits were recorded to achieve a nominal
dynamic range of 132 db. 32 kilobytes storage were provided for each
event. The total recording capacity was 16 megabytes, which translates
into approximately 500 32-s-long 4-component events or 2000 onecomponent events at 8 ms sample rate.
17
DATA PROCESSING AND ANALYSIS METHODS
Figure 6 shows the spectrum of one trace of instrument A2 at line 7.
This trace and other investigated traces show that hardly any energy was
recorded above 25 Hz. Therefore the data were resampled at 16 ms prior
to any filtering to save processing time.
The second step in working on any section was to check the location of
the instrument. Because instruments and shots are located at different
times by smoothed Loran C readings, uncertainties occur due to the
influence of atmospherical conditions. A visual check was used to
determine if the OBS location and ranges calculated from the navigation
were compatible with the first arrivals. If not, the OBS was relocated
using the observed direct wave arrival time from several shots at known
locations to determine the best fit instrument location. Corrected OBS
locations are accurate
within
0.05 km (based on traveltime-, depth- and
water velocity uncertainties and divergence of relocation algorithm) and
were not more than 0.2 km away from their initially determined location.
The seismic data are bandpass filtered from around 5-18 Hz with a 48
dB/octave cutoff minimum-phase filter. Different cutoff frequencies were
Frequency Spectrum
18888
p
(R)
188
ie
31.21
Freq (Hz)
Figure 6. Frequency spectrum of line 7 OBS A2 trace 7121 vertical
component, prior and after resampling at 16 ms.
used for different profiles to eliminate background noise below 3-6 Hz and
noise above 15-20 Hz, mainly generated by whales.
Finally a topographic correction was applied on the data. For detailed
discussion of the method applied see Purdy (1982). The apparent velocity,
which is used to calculate the angle by which shots are projected onto the
sea floor, was set to 6.0 km/s for P-wave sections and 3.5 km/s for S-wave
sections; the assumed water velocity was 1.47 km/s. Unfortunately the
topographic correction does not remove all shallow travel time
perturbations because the basement does not follow the topography, as
assumed by the correction algorithm. Additionally the Gulf of Maine is
reported to have an irregular seafloor (Ballard and Uchupi, 1972), which
makes the raypath more complicated than assumed.
Record sections were then plotted with reduction velocities
appropriate for emphasizing P and S wave arrivals, i.e. 6.8 km/s and 3.9
km/s, respectively. Amplitudes in a given section have been scaled by a
factor of (range)c where 0<a<0.3, depending on the S/N ratio.
To enhance S wave arrivals, especially at close ranges, a velocity filter
was applied on some sections. The passing apparent velocity window went
from 0.8 to 4.0 km/s, the cutoff velocities were set to 0.5 and 5.0 km/s to
taper the transition from pass to stopband. Before being fk-filtered,
sections were bandpass filtered from 5-10 Hz to avoid spatial aliasing. An
example is shown in figure 7. The results show that in some cases the 1kfilter is a powerful tool to enhance S wave arrivals.
An attempt was made to apply an NMO correction to sections showing
PmP and SmS arrivals from the Moho in order to improve the visibility of
these reflections, but without success.
Predictive deconvolution was also attempted, but was not successful
due to the extremely "ringy" nature 'of the signal caused by a hard and
shallow seafloor combined with a very "ringy" source signature.
Velocity modelling started with one-dimensional raytracing (see figure
8) that was used to match the digitized first arrival. The resulting model
was used to generate a traveltime curve that was overlain on the actual
seismic section. If the fit was not satisfactory, the model was changed until
the resulting traveltime curve matched the recorded first arrival.
Discrepancies between model travel times and recorded travel times are
generally less than 0.1 s and greater deviations are most probably local
surface effects. To obtain the whole range of possible and reasonable
velocities the best fitting traveltime curve was used as a reference and
acceptable models had to generate very similar travel time curves (see
figures 9a & b). The allowable departure is variable with offset and ranges
from below 0.1 s at shots where the first arrival is clear to below 0.3 s
where the first arrival is ambiguous or invisible. Hence, an optical check
was preferred over a computational one. Initial S velocity models were
(s)6/x-II
(s)/x-yflj
E
C,)
0
C)
0
41)
rd
I-
41)
21
II
11
11
22
RAYTRACING
1
1
DISTANCE (kin)
eq
'5
10
I5
DISTANCE (Ian)
Figure 8. Raytracing of P velocity model L7A2 shown in figure 16 (bold
line). The upper section shows the close range affects of a soft sediment
layer overlying basement. Refractions observed at a distance of 5 km
penetrate the upper 500 m of the crust. The lower section shows a
triplication between 68 and 98 km distance caused by a layer of increased
gradient between 8.5-11 km depth.
23
p-VELOCiTY (km/s)
50
60
DISTANCE (km)
Model A: Perfect fit
Figure 9a. Determination of acceptable velocity models of OBS L7A2.
The crosses represent fixed data points fitted by the shown models.
Travel-time curves of model B and C are shown on the following page.
24
3
00
k
10
20
30
40
50
60
70
80
90
100
110
70
80
90
100
110
DISTANCE (kin)
Model B: Acceptable fit
3
00
10
20
30
40
50
60
DISTANCE (1cm)
Model C: Unacceptable fit
Figure 9b. Acceptable and unacceptable fit of first arrival at OBS L7A2.
25
calculated from the P velocity models by assuming a Poisson's ratio of
0.25. Depth of layer boundaries for the S velocity model correspond to
depth of boundaries in the P velocity model.
Because of the attenuation of the seismic signal it can be hard to
identify the onset of the first arrival beyond certain distances. Therefore
later arriving, stronger phases of the first arrival can be used as a
reference to pick the invisible first onset. Certainly this is only valid if the
time difference between first onset of arrival and stronger later arriving
phases stays constant over distance. Comparison of phases and amplitudes
of traces at different distances show that indeed a later arriving stronger
phase can be delayed by a constant offset. An example is shown in figure
10.
If the amplitude decay of the first arrival is abrupt rather than
gradual, the presence of a low-velocity zone is assumed.
Amplitude modelling was tried for all ranges, but because of smallscale upper crustal heterogeneity did not provide satisfactory results for the
first arrivaL However, computed amplitudes of PmP and SmS wide-angle
reflections using the WKBJ method (Chapman, 1978) provide some
information about the lower crust, but are limited due to the short offset of
most sections. Upper mantle velocities of 8.0 km/s (Steinhart et al.,1962)
and 4.62 km/s were reasonably assumed to model these Moho reflections.
26
22.2
B
23.2
0
1
2
3
4
3.4
4.4
- X / 6.8 (s)
552
562
0.4
1.4
2.4
TIME - X /6.8 (s)
Figure 10. Comparison of first onset at two distances (OBS L7C3 HYD).
The very first onset dies out, while later arriving stronger phases can still
be correlated. The arrow indicates the observed onset in the upper figure
and the assumed onset in the lower figure.
27
Wave Field Continuation
In addition to raytracing, the data were analysed by transforming the
wavefield into the tau-p domain followed by downward continuation to
obtain the z-p wavefield, where tau is the intercept time and p is the ray
parameter or horizontal slowness, which is the inverse of phase or apparent
velocity v. The first process, the tau-p inversion or slant stack transforms
a seismogram linearly into a tau p wavefield (Chapman, 1978; Phinney et
al., 1980):
S(tau,p)=JU(x, tau+px)dx
Here S(tau, p) is the slant stacked wavefield, U(x, t) is the recorded
seismogram at range x, p is the ray parameter, and tau is the intercept
time. For fixed tau values, amplitudes are summed in the t - x domain
along the slope p. In order to achieve finer sampling at low p's (high
velocities), we preferred to compute the tau p wavefield for constant
velocity increments rather than p increments. We used the slant stack
algorithm described by Henry et al. (1980), which is a 3 dimensional
transformation (by assuming a point-source geometry) and computes the
tau-p (or tau-v) wavefield in the frequency domain.
Artifacts of slant stacking are generally introduced by the finite
aperture of the data set, incoherence of the source signature, spatial
aliasing, lateral heterogeneity and noise (Clayton and McMechan, 1981;
McMechan and Ottolini, 1980). The long offset of our data and a coherent
source signature should reduce these artifacts. However, the lateral
heterogeneity indicated by the seismic sections and spatial aliasing were
expected to produce artifacts. The trace spacing of our data is 125 and 250
m, hence the nyquist frequency is around 20 and 10 Hz, respectively.
Therefore, due the frequency content of our data shown in figure 6 spatial
aliasing is expected. Lateral heterogeneity not only makes an interpretation
p wavefield questionable due to the change of p along the
raypath, but also introduces destructive interference of the first arrival
of a tau
during slant stacking.
The data preparation prior to slant stacking included bandpass filtering
to reduce spatial aliasing, range scaling to compensate for attenuation and
geometrical spreading, and muting to eliminate unwanted energy.
Although the slant stack method is robust for some spatial aliasing
(McMechan et al., 1982), high frequencies were tapered prior to slant
stacking. In addition, trace interpolation doubling the range sampling was
tried on one profile. However, the resulting resolution of the tau
v
wavefield and derived velocity function did not improve significantly. To
apply a range scaling, the data were scaled by a factor of (range)°.3. Next,
the data around the refraction were muted to eliminate noise, multiples and
slower arrivals (see figures 38-39). Unfortunately this muting eliminated
post-critical reflections as well, but the advantage of muting unwanted
energy exceeded the utility of scarce upper crustal reflections. For OBS
L7C3 it was additionally necessary to mute traces contaminated by 20 Hz
29
noise (generated by whales). After slant stacking low frequencies were
filtered out to compensate for the time stretching induced during slant
stacking (McMechan and Ottolini, 1980).
The second step of the analysis is a downward continuation of the tau -
velocity wavefield into the depth - velocity wavefield, imaging the onedimensional velocity function. The equation for downward continuation is
(Clayton and McMechan, 1981; Lyslo, 1988):
s(p, z) = S(tau = iy(p, z), p)
where
iv(p,
z) = 2
p21112
dz
Here s(p, z) is the ray parameter - depth wavefield, S(tau, p) is the tau p
wavefield and v(z) is the user-provided velocity function. We continued to
use constant velocity increments rather than p increments for the
downward continuation to obtain the velocity - depth wavefield s(v, z).
The user-provided velocity function v(z) maps each v (or p) trace from the
tau domain into the depth domain. For a detailed discussion of downward
continuation see Clayton and McMechan (1981). Downward continuation
is an iterative process, repeated until the input velocity function converges
with the locus of energy of the resulting velocity - depth wavefield. For a
causal data set, the velocity function is imaged in the depth domain as the
locus of the onset of the wavelet containing the maximum energy (Lyslo,
1988). Picking the first arrival can be difficult especially in zones of small
velocity gradients, because the tau-p inversion spreads energy towards
lower velocity and higher tau values (Lyslo, 1988; McMechan and
Ottolini, 1980). Because the velocity function is limited to one dimension,
lateral heterogeneity can cause the image to be broken and multivalued
(McMechan et al., 1982). Convergence is independent of the initial
velocity model because each iteration uses the original tau - velocity
wavefield. The resolution of the fmal imaged velocity is defined by the
width of the fitted wavelets at convergence and the coherence of the image
(McMechan et al., 1982).
31
P-S CONVERSION
Clear S waves are observed on all stations. Since the source, an airgun
in water, generates only P waves, it is necessary to determine where the
conversion occurs in order to obtain reliable Poisson's ratio estimates. To
do this, we calculated expected amplitudes of arrivals for several possible
scenarios. The two most likely, i.e. conversion at sediment-basement
interface versus conversion at water-sediment boundary (seafloor) are
shown in figure 11. The parameters of the layers were defined as the
following:
water: Pw=l° g/cm3; vp=l.47 km/s; vs=O.Ol km/s;
sediment: ps=1.8 g/cm3; Vp2.8O km/s; vs=1.05 km/s;
basement: pb=2.4 g/cm3; vp=S.25 km/s; vs=3.0 km/s.
The results indicate that conversion occurred at the sediment-basement
interface. Hence, the S velocity of the sediment layer has still to be
calculated. The average velocity of the sediment layer is then 1.05 km/s,
and knowing the P velocity (2.80 km/s) the S velocity is determined as
v5O.65 km/s.
P-S Conversion at:
Seafloor
Basement Top
0.5I
0.4-
0.4-
:1
:
DISTANCE (km)
I
DISTANCE (km)
Figure 11. P-s conversion. Clear S waves are observed on all stations. Since the source, an airgun in
water, generates only P waves, it is necessary to determine where the conversion occurs to obtain reliable
Poisson's ratio estimates. To do this amplitudes of possible scenarios were calculated. The above figure
indicate that conversion occurred at the sediment-basement interface.
33
RESULTS
Upper Crust
At offsets greater than 2 km, P refracted in the basement is the first
arrival. Because of the rapid increase in velocity at shallow depth, the first
arrivals measure only the velocity of the upper 3-15 km of the crust,
depending on the maximum offset of each station. A detailed line-by-line
discussion follows.
Line 7
Line 7 starts in the Nashoba terrane, crosses the Gulf of Maine Fault
Zone and extends into the Gulf of Maine Central Plutonic Zone. The
bathymetry, tectonic setting and the location of the OBS are shown in
figure 12. Instruments with the best data quality and longest offset are
OBS C3 and A2. Figures 13 to 16 show seismic sections and the derived
velocity models for C3 and A2. The predicted arrival is shown as a bold
line superposed on the seismic section. Dashed lines show the uncertainty
range of the upper crust velocity model, obtained as explained in the
previous chapter. Sections showing S wave arrivals show the S velocity
model
and
corresponding
Poisson's
ratio
model.
NW
BATHYMETRY LINE 7
0
Nashoba>< ------------------------------------------------------
Terrane
fl
1
Avalon Terrane --------------------------------
< --------- Fault Zone ------------------------------------------- Central Plutonic Zone ----------
C4
E
SE
'-1 Fault?
<--->
Abrupt travel-time
decrease to the NW
A8
A2
C3
Basin indicated by refraction data
<--->
Basin indicated by reflection data
1
1
DISTANCE (1cm)
Figure 12. Tectonic setting and bathymetry of line 7. The locations of the OBS are indicated by vertical
lines. Two consistent unmodeled refraction anomalies are indicated. One suggests a NW-dipping fault
close to C4, another suggests a small basin. The exact location and extension of the basin vary among
OBS, hence the entire possible range is indicated. A basin indicated by unpublished reflection data is
indicated for comparison.
L)
-
NW
LINE 7 OBS C3 HYD
SE
3
00
'0
DISTANCE (kin)
P-VELOCITY MODEL
fl
10
4
5
6
7
VELOCITY (km/s)
Figure 13. OBS L7C3, seismic section and determined P velocity. The superposed travel-time curve
corresponds to the bold velocity model.
1
LINE7 OBSC3H1
NW
,-' 3
__
..
-
en
<
SE
2
.. .-.
;
..
1
.' -.. I
-
-.
.t.I
.
.. II'-
0100
96
.
'Y'' '
.
.
'.u.
II...
76
80
66
I
..
56
. i
40
3
DISTANCE (1cm)
POISSON'S RATIO
S-VELOCITY MODEL
0
1
0-
-
NwV
8
SE
10
2
I
\,
I
3
VELOCITY (km/s)
I
4
0I
PO1SSONS RATIO
Figure 14. OBS L7C3, seismic section and determined S velocity with corresponding Poisson's ratio.
LINE7OBSA2V
NW
SE
3
00
P-VELOCITY MODEL
DISTANCE (km)
0
4
5
6
7
VELOCITY (km/s)
Figure 15. OBS L7A2, seismic section and determined P velocity.
(J)
ratio. Poisson's corresponding with velocity S determined and section seismic L7A2, OBS 16. Figure
RATIO POISSON'S
(km/s)
4
VELOCITY
3
2
0
0
12
12
15
15
RATIO POISSON'S
MODEL S-VELOCITY
DISTANCE(km)
1
01
4,
o
4
NW
OBSA2V LINE7
39
The travel time departure recorded at OBS C3 (see figures 13 & 14)
between 17 and 38 km distance shows up on all instruments at that
approximate geographic location and is probably due to a small basin. An
examination of unpublished coincident seismic reflection data (Hutchinson,
personal communication), did indicate the presence of a sedimentary basin
in that area (see figure 12). Other possible small basins are observed, but
do not have a signature in the refraction data. However, the abrupt travel
time decrease in the distance range 62-65 km shows up on all instruments
and is confined to a location 4 km northwest of OBS L7C4. Hence, it
probably represents a fault, that juxtaposes either upward dipping layers
vs. horizontal layers or less sediments vs. more sediments. The depicted
reverse fault is arbitrary and might as well be a normal fault as long as the
relative motion of the two juxtaposed blocks is preserved. The basement
top has P and S velocities of around 5.2 and 3.1 km/s, respectively. The
velocities increase with an increased gradient between 4 and 6 km depth to
6.2 and 3.6
km/s
at 7.5 km depth. A low-velocity zone is indicated at a
depth of 7.5-9.7 km with a velocity decrease by 0.3 to 0.6 km/s. An LVZ
was assumed at that depth range and not shallower, because first arrivals
can be traced as far as 65 km and then end abruptly. A closeup of the first
arrival at a distance of 55 km is shown in figure 10. At 10 km depth the
velocities are 6.3 and 3.65 km/s for P and S waves, respectively. Poisson's
ratio is 0.22±0.04 at the basement top and increases to 0.245±0.01 at 6 km
depth. In the low velocity zone Poisson's ratio estimates range from 0.19
to 0.33 and is constrained to 0.245±0.005 beneath it. The southeastern part
of the section, shows a higher apparent velocity and therefore suggests
either a small dip (1-2°) of the uppermost layers to the northwest or a
velocity increase to the southeast.
Due to the absence of close shots at OBS A2 (see figures 15 & 16), a
location check of instrument A2 is not possible and there is no detailed
information about the upper 0.5 km. A small basin is indicated at a
distance of 45-55 km and the abrupt travel-time decrease at 92 km suggests
a fault, both indicated by C3 and shown in figure 10. The basement P and
S velocities at 0.5 km depth are 5.85 and 3.35 km/s with Poisson's ratio
around 0.25; P and S velocities increase gradually to 6.05 and 3.65 km/s at
8.5 km depth, while Poisson's ratio decreases to 0.214±0.03. Between 8.5
km and 11 km depth an increased P velocity gradient centers Poisson's
ratio back to 0.25±0.015 and causes a travel-time triplication from 70-110
km. However, the amplitude of this triplication is drowned by the ringy
source enhanced by reverberations in the source and receiver area. At 15
km depth P and S velocities are 6.6 and 3.8
km/s,
with Poisson's ratio at
0.25±0.02. An LVZ was not implemented, because of the continuous first
arrival, visible as far as 75 km.
Seismic sections of instruments C4 and A8 are shown in figures 54-57.
Unfortunately data recorded at instrument A8 are very noisy and did not
provide any additional information. Data from OBS C4 are also noisy, but
suggest a P velocity model for the upper 2 km that is similar to the one
derived from C3. Hence, the Fault Zone and the Central Plutonic Zone
have a similar upper crustal velocity structure. A8 and C4 also indicate a
41
small basin and fault at the locations shown in figure 12. A signature of
the Fundy Fault has not been observed by any instrument. All velocity
models determined at line 7 are summarized in figure 17.
Local departures of the data from the model are frequent, due to
surficial heterogeneities, and probably contribute to noise. Both C3 and
A2 show an increased velocity gradient sandwiched between two layers of
lower gradient. The absence of a low-velocity zone at A2 in spite of the
small distance to C3 (28 kin) indicates that this LVZ must be laterally
variable. An explanation for the derived reduced Poisson's ratio at A2
between 6 and 11 km depth may be offered by the Cashes Ledge Granite
which is located between C3 and A2. In this case, the data suggest that the
vertical extension of the granite is limited to the upper 8-11 km. A mafic
or ultramafic material directly underlying the granite, as suggested by
gravity and magnetic data, is not confirmed by the calculated Poisson's
ratio but the offset might be too short to show an increase of Poisson's
ratio caused by such material.
VELOCITY MODELS OF LINE 7
0
0
3
3
16
12
12
15
15
P-VELOCITY (km/s)
Figure 17. P and S velocity of the upper crust at line 7.
S-VELOCrFY (krn/s)
L7A2
L7C3SE
- - - L7C3NW
- - L7C4
Line 8
Line 8 is perpendicular to line 7 and is entirely through the Central
Plutonic Zone in a southwest-northeast fashion. The bathymetry of line 8
and the location of the instruments is shown in figure 18.
OBS C3 has a relatively good S/N ratio and results are shown in
figures 19 and 20. The basement P and S velocities start with 5.95 and 3.4
km/s and increase gradually to 6.0 and 3.5 km/s at 2 km depth. An
increased gradient raises the P velocity from 6.05
km/s
at 3 km depth to
6.45 km/s at 9 km depth. At 12 km depth the P velocity reaches 6.5 km/s.
S velocity information is limited to the upper 3 km and the northeastern
part of the section suggests an S velocity up to 0.2 km/s higher than that
determined from the southwestern branch. Because the P velocity shows
only a small velocity increase on the northeastern branch and the S wave
section has a lower S/N ratio, this discrepancy is questionable. Poisson's
ratio of the upper 3 km is estimated at 0.225±0.025.
OBS A2 still has an acceptable S/N ratio that allows us to determine
the velocity structure of the upper 5 km (see figures 21 & 22). The
southwestern branch clearly indicates shallow anomalies from the close
range out to 25 km distance suggesting a basin, that is indicated in figure
18. However, the anomalies' northeastern extension is hard to define and
must be between 0 and 10 km to the SW of A2. The NE branch shows a
faint first arrival and strong reverberations. Both branches start with P
and S velocities of 5.3 and 2.2 km/s at the basement top and increase to 5.9
SW
BATHYMETRY LINE 8
Avalon Terrane, Central Plutonic Zone
A2
C3
A8
>
Basin indicated by
refraction data
1
DISTANCE (kin)
Figure 18. Tectonic setting and bathymetry of line 8. Consistent travel-time anomalies suggest a fault and
a basin at the indicated locations.
SW
LINE 8 OBS C3 HYD
NE
3
L
80
70
60
40
50
30
20
DISTANCE (kin)
P-VELOCITY MODEL
0
2
4
5
6
7
VELOCITY (km/s)
Figure 19. OBS L8C3, seismic section and determined P velocity.
10
0
10
20
30
rir
C
z
0
o
I
I-
0
E-
-L'
I©z
0
I
CPj
0
C
0
'1
CPj
C,,
0
0
1
.
SI
0
C
0
(vc) aici
0
C')
C
:
0
C
-
tn
0
SW
LINE 8 OBS A2 V
3
¼,
00
>
n
DISTANCE (kin)
P-VELOCITY MODEL
2
10
4
I
VELOCITY (km/s)
Figure 21. OBS L8A2, seismic section and determined P velocity.
NE
SW
NE
LINE 8 OBSA2V
4
DISTANCE (km)
POISSON'S RATIO
S-VELOCITY MODEL
0
0
10
10
2
3
VELOCITY (km/s)
4
POISSON'S RATIO
Figure 22. OBS L8A2, seismic section and determined S velocity with corresponding Poisson's ratio.
km/s and 3.6 km/s at 2 km depth. At 4 km depth the P velocity reaches 6.2
km/s. Poisson's ratio is 0.23±0.02 for the upper 4 km.
OBS A8 is shown in figures 58-59 but has a low S/N ratio and does
not allow conclusive velocity modelling. However, an abrupt travel-time
increase to the NE suggests a basin flanked to the SW by a fault at the same
location as suggested by A2. The reverse fault shown in figure 18 could as
well be a normal fault, as long as the relative motion of the block
containing the basin is downwards. A summary of all line 8 velocity
models is shown in figure 23.
Line 3
Line 3 runs along the Gulf of Maine Fault Zone. Its bathymetry and
the location of deployed OBS are shown in figure 24. Surficial anomalies
are hardly observed at line 3. Seismic data recorded at OBS C6 show a
good S/N ratio (see figure 25 & 26) and suggest a low velocity zone
between 6 and 10 km depth. The velocity model shown corresponds to the
northeastern branch, but was also used to overlay a travel time curve on
the southwestern branch. Although the data from the SW branch show
some deviations from that model the fit indicates a uniform layering
without dip or lateral velocity variation. Basement P and S velocities start
with 5.2 and 2.75
km/s
and increase to 6.34 and 3.69
km/s
at 6 km depth.
The low velocity zone has P and S velocities of 5.85±0.25 and 3.4±0.1
km/s
and is underlain by material with velocities of 6.36 and 3.71 km/s.
Poisson's ratio is 0.24±0.01 in the upper 6 km and below the LVZ. Within
VELOCITY MODELS OF LINE 8
0
0
3
3
16
12
12
/
15
15
S-VELOCITY (km/s)
P-VELOCITY (km/s)
Figure 23. P and S velocity of the upper crust at line 8.
LSC3SW
- L8A2NE
L8C3NE
- - - L8A2SW
U'
0
SW
BATHYMETRY LINE 3
z
DISTANCE (1cm)
Figure 24. Tectonic setting and bathymetry of line 3.
NE
SW
LINE 3 OBS C6 HYD
3
DISTANCE (km)
P-VELOCITY MODEL
10
4
5
6
7
VELOCITY (km/s)
Figure 25. OBS L3C6, seismic section and detemiined P velocity.
NE
SW
NE
LINE 3 OBS C6 Hi
4
DISTANCE (km)
S-VELOCITY MODEL
POISSON'S RATIO
0
10
10
2
3
VELOCITY (km/s)
4
0.15
025
POISSON'S RATIO
Figure 26. OBS L3C6, seismic section and determined S velocity with corresponding Poisson's ratio.
54
the LVZ estimates of Poisson's ratio range. from 0.17 to 0.30 and hence
carry to much uncertainty to allow any conclusion about its composition.
Seismic sections and velocity models of OBS C4 and A2 are shown in
figures 60-63. Both instruments suggest a slightly lower P velocity than
OBS C6 above 6 km depth, but a similar upper crustal S velocity. Lateral
velocity variations are not indicated by the two branches of OBS C4 and
A2. In spite of the long offset recorded at OBS A2, no indications for a
low velocity zone are observed. This suggests that the one determined
from OBS C6 must be local. Poisson's ratio estimated from these two
instruments is 0.24±0.03 for the upper 10 km of the crust. P and S
velocity models determined at line 3 are summarized in figure 27.
Lines 5 & 6
Lines 5 & 6 are coincident with the deep crustal reflection line studied
by Hutchinson et al. (1987, 1988) and run NNW-SSE. Line 5 starts in
Coastal Maine and runs into the Gulf of Maine Fault Zone. From there
line 6 continues and extends into the Central Plutonic Zone. Lines 5 and 6
do not provide enough offset to record any wide angle reflections.
However they provide velocity information of the upper crust up to 6 km
depth.
The bathymetry and seismic sections of lines 5 and 6 with
corresponding velocity models are shown in figures 64-70. A summary of
the derived velocity models is shown in figure 28. The basement P
velocity starts at 5.2-5.6 km/s and increases to 6.3-6.6 km/s at 5 km depth.
VELOCITY MODELS OF LINE 5 & 6
0
0
3
3
16
12
12
15
15
P-VELOCITY (km/s)
Figure 28. P and S velocity of the upper crust at line 5 & 6.
S-VELOCITY (km/s)
L5A8NW
L5A8SE
- - - - L6A2NW
L6A2SE
- - L6A8NW
- - L6A8SE
57
The S velocity starts at 2.6-3.0 km/s at the basement top and reaches 3.73.8 kin/s at 5 km depth. Poisson's ratio in the upper crust is 0.25 ± 0.04.
The layers are indicated to be fairly horizontal and although slight velocity
differences exist between different sections of line 5 and line 6, a pattern
can not be inferred.
Lower Crust
Figure 29 illustrates raypathes of Pg, PmP and Pn arrivals with their
corresponding travel-times. Below 15 km depth, the crustal structure must
be inferred from travel-times and amplitudes of PmP and SmS reflections
rather than from Pn and Sn refractions, because of the limited source-
receiver offset. The locations of reflection points of the wide-angle
reflections are indicated in figure 1 and fall in the Central Plutonic Zone
(for OBS of line 7 & 8) and in the Fault Zone (for OBS of line 3).
Because we are not able to distinguish middle and lower crust we use the
term 'average lower crust velocity' for the average crustal velocity
between 10 km depth and Moho. The quality of recorded PmP and SmS
reflections varies quite a lot among stations, and due to the ringy source
and reverberations that may either occur in the lower crust or in the
sediment column underlying the instruments, it is difficult to pick the first
onset. Because the refracted Pg arrival show that the first onset carries less
energy than later arriving phases, the very earliest arriving fragments of
PmP or SmS arrival were used to model these reflections. Some energy of
SmS reflections will be converted to P at the basement-sediment interface,
which results in slightly shallower Moho depth estimates, but due to the
thin sediment cover this underestimation should always stay below 1.0 km.
A line-by-line discussion follows.
5
PmP
4
0
0
10
20
30
40
DISTANCE (km)
Figure 29. Illustration of Pg, PmP and Pn arrivals. Pg and Pn are basement and upper mantle
refractions, respectively, and PmP is the Moho reflection. The PmP arrival shown is post-critical. In
addition pre-critical PmP were identified on some instruments.
Line 7
PmP and SmS wide angle reflections are visible on all three
instruments and are strongest on instrument C3. Figure 30 shows the
recorded PmP arrival of OBS C3. Velocity models satisfying the travel
time of the arrival are shown and their critical distances are indicated on
the section. Model #1 has its critical angle at a distance of around 75 km,
which comes close to the observed strong amplitude increase. In addition
the three models are tested by looking at their predicted PmP amplitude
with respect to the refracted Pg arrival. Figure 31 shows synthetic sections
generated by using the corresponding velocity models shown in figure 30.
These synthetic sections suggest velocity model #1, which has an average
lower crust velocity of 6.5 km/s and a Moho at 28 km depth. The SmS
arrival is shown in figure 32 and the travel time indicates a Moho depth of
29-31 km and an average lower crust S velocity of 3.8-3.9 km/s.
In spite of being close to OBS C3, instrument A2 shows a quite
different PmP arrival (see figure 33). The curvature of the travel-times of
the identified PmP arrival suggests an average lower crust velocity of 6.95-
7.15 km/s and a Moho depth of 35-37 km. The two possible velocity
models were used to compute the synthetic sections shown in figure 34 and
go along with the recorded relative amplitudes. Hence, the absence of a
strong PmP or Pn arrival, in spite of the big offset, confirms a higher
lower crust velocity. Due to this significantly higher velocity the Moho is
computed to be 4 km lower than it would be assuming an average lower
S.
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(kin) DISTANCE
(km) DISTANCE
(kin) DISTANCE
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0
00
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Model
8
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Sections Synthetic
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Model 1
8
8
6
6
00
oq
<
0
4
2
2
0
0
DISTANCE (km)
DISTANCE (km)
Figure 34. L7A2, synthetic seismograms corresponding to the P velocity models in figure 33.
Synthetic Sections
Model 2
Model 1
JJH j1
IIff
:
60
70
8
90
DISTANCE (km)
100
1il
110
:
60
ii I
70
80
90
100
110
DISTANCE (kin)
Figure 35. L7A2, synthetic seismograms corresponding to the P velocity models in figure 33, using a
"ringy' source wavelet. The travel-time triplication shown in figure 15 is concealed by the interference of
"ringy" arrivals.
67
crust velocity of 6.6 km/s. Figure 35 shows synthetic sections of the same
models generated with a ringy source wavelet in order to simulate the
observed source signature. It shows a reduced resolution due to destructive
interference. The SmS arrival recorded at OBS A2 is shown in figure 36.
Possible velocity scenarios include models with low and high lower crust
velocities. The computed amplitudes as shown in figure 37 support rather
the lower S velocity models (3.8 km/s) with a Moho depth of 33-35 km
than the higher S velocity model. Combined, the determined lower P and S
velocities imply mafic to ultramafic material in the lower crust.
PmP and SmS arrival can also be identified on OBS C4 (see figures
71-72) and suggest a Moho depth between 27.5 and 30 km and average
lower crust velocities of 6.55 and 3.7 km/s for P and S waves, respectively.
If the lower crust has a lower velocity all along line 7, then the differences
in amplitude observed between A2, C4 and C3 suggest lateral variation in
lower crust properties on a scale of less than 28 km. In order to resolve a
truly 2-dimensional velocity-depth profile more data are needed, and some
of this difference may also be due to differences in shallow structure
beneath both source and receiver.
a.
.
A
%Y
II
-
0
0
0
0
0
1
36. figure in shown models velocity S the to corresponding seismograms synthetic L7A2, 37. Figure
(km) DISTANCE
(km) DISTANCE
10
10
8
8
4
4
f,
Model
(km) DISTANCE
0
0
3
h4
n
Iii
8
10
2 Model
1
Model
Sections Synthetic
70
Line 8
Wide-angle Moho reflections are more difficult to identify in the data
collected along line 8 in spite of large offsets recorded. This is in part due
to higher background noise levels.
The PmP arrival identified on
instrument C3 allows a broad range of average lower crust velocities from
6.6 to 7.2 km/s and implies a Moho between 32 and 40 km depth (see
figure 73). However synthetic results generated by the possible velocity
models show that the recorded PmP amplitudes support a lower crust
velocity of 6.6 km/s and a Moho depth at 32-36 km depth (see figure 74).
Travel time of SmS suggest an average lower crust S velocity of 3.7-3.8
km/s and a Moho depth between 30 and 34 km (see figure 75).
Reliable PmP and SmS identifications are not possible at OBS A8 (see
figures 76-77). Although an SmS arrival is suggested in figure 77, its first
onset is too ambiguous to derive the lower crustal velocity and Moho
depth.
Line 3
A clear PmP arrival is recorded by OBS C6 (see figure 78). The
travel-time slope of the reflection indicates an average lower crust velocity
and a Moho at 35.5 km depth. Only one travel time curve
overlying the data is visible, because both velocity models produce exactly
the same travel time. Relative amplitudes of the two models contradict the
of 6.95
km/s
71
recorded amplitudes, which suggest a slower lower crust P velocity and
imply a shallower Moho (see figure 74). Travel time and amplitude from
the SmS arrival suggest an average lower crust S velocity of 3.8 km/s and a
Moho depth of 30 km (see figures 80-81). The shallower Moho depth
derived from SmS suggests that the PmP amplitude data is indeed reliable
and the P velocity probably lower than indicated by the PmP travel time.
Data recorded at OBS A2 show PmP and SmS arrivals. The PmP
reflection indicates an average lower crust velocity of 6.6-6.75 km/s and a
Moho depth of 35-36 km (see figure 82). The SmS reflection suggests an
average lower crust S velocity of 3.8-3.9 km/s and a Moho at 32-33 km
depth (see figure 83).
72
Results of Wavefield Continuation and Comparison to
Raytracing Results
For the data sets we investigated, wavefield continuation provides
good results. The derived velocity models are compatible with results
from raytracing and the differences in velocity structure among the
instruments L7C3, L7A2, L8C3 and L3C6 derived from raytracing are
confirmed by the results. The pre-processed and muted data sections used
for wavefield continuation are shown in figures 38-39.
The tau - velocity and velocity - depth wavefields of L7C3, L7A2,
L8C3 and L3C6 are shown in figures 40-43. The velocity functions that
mapped the tau - v wavefield best into the depth domain are superposed as
bold lines. No attempt was made to resolve low velocity zones because
arrivals coming from below and above the LVZ interfere in the velocity depth domain. The velocity function and the imaged wavelets in the depth
domain have converged and the frequency content of the wavelets suggests
a depth resolution of ± 1-3 km. Convergence was reached after 10-15
iterations. Energy smeared to high velocities is caused by close range
water bottom reflections, precritical reflections and a ringy source wavelet
combined with spatial aliasing (McMechan et al., 1982; Yilmaz, 1988).
The traveltime curves calculated with the velocity functions shown are
superposed on the seismic section in figures 44-45 and show a good fit.
Only for L7C3, the fit is inferior to the one achieved by raytracing in the
distance range 45-60 km (see figure 13).
Figures 46 & 47 show a
3
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100
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20
30
40
50
60
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80
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80
I
I
70
I
I
60
I
I
50
I
I
40
I
I
30
I
I
20
10
DISTANCE (kin)
Figure 38. Pre-processed and muted data sections of L7C3 (above) and L7A2 (below) used for wavefield
continuation.
0
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0
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0
00
0
0
0
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LINE7 OBSC3
0
3
6
12
15
18
5.0
5.5
6.0
6.5
7.0
VELOCiTY (kmfs)
LINE7 OBSA2
4
8
12
16
20
5.0
5.5
6.0
6.5
7.0
VELOCiTY (km/s)
Figure 42. Velocity - depth wavefields of L7C3 and L7A2. The velocity
functions that were used for the wavefield continuation are superposed as
bold lines.
ii
00
'a
I
DISTANCE (km)
3
00
'0
DISTANCE (km)
Figure 44. Data recorded at L7C3 (upper section) and L7A2 (lower section) overlain by travel-time
curves obtained by wavefield continuation. The corresponding velocity functions are shown in figure 42.
3
oc
\0
0
90
80
70
60
50
40
30
20
10
0
10
20
DISTANCE (kin)
3
00
0
DISTANCE (kin)
Figure 45. Data recorded at L8C3 (upper section) and L3C6 (lower section) overlain by travel-time
curves obtained by wavefield continuation. The corresponding velocity functions are shown in figure 43
30
81
LINE7 OBSC3
0
L
-...---
-
3-
I
I
6
-
\
1215-
I'
'
18-
6.5
6.0
5.5
5.0
'
7.0
VELOCITY (km/s)
LINE7 OBSA2
I
I
4I
\'
8121.L
I
6
VELOCITY (km/s)
Figure 46.
Comparison of raytracing (dashed lines) and wavefield
continuation (solid lines) velocity models of L7C3 and L7A2. The outer
dashed lines show the uncertainty range associated with the raytracing
model.
LINE8 OBSC3
0
3
6
12
15
18
5.0
6.0
5.5
6.5
7.0
6.5
7.0
VELOCiTY Ouiils)
LINE3 OBSC6
0
3
6
12
15
18
5.5
I
I
I
5.0
6.0
VELOCiTY (km/s)
Figure 47. Comparison of raytracing (dashed lines) and wavefield
continuation (solid lines) velocity models of L8C3 and L3C6. The outer
dashed lines show the uncertainty range associated with the raytracing
model.
*3
comparison of the velocity models computed by raytracing and by
wavefield continuation. The dashed lines represent the raytracing model as
explained in the previous chapter, the bold lines represent the wavefield
continuation result. The raytracing models were derived from the first
arrival and the PmP wide-angle reflection. For all instruments raytracing
and wavefield continuation results agree well at least in the upper 8 km.
For L7C3 the slower wavefield continuation model below 8 km depth can
be explained by the absence of the LVZ which taken into account gives
both models a similar average velocity. For L7A2 and L8C3 the wavefield
continuation model is slower below 11 km depth.
This might be a
resolution problem of this method, caused by the low frequency content of
the image or it may represent energy that got smeared to higher tau values.
However, wavefield. continuation results agree to differences in velocity
models among instruments found by raytracing. Therefore, the lower
gradient for L8C3 and L7A2 above 8 km and the lower gradient indicated
for L7C3 above 4 km in contrast to the high shallow velocity of L3C6 are
confirmed by wavefield continuation results.
DISCUSSION
Upper Crust
Beneath the sediments the velocity structure of the Gulf of Maine is
laterally fairly homogeneous and is not clearly correlated with the lateral
block structure of the Gulf as indicated by seismic reflection and potential
field data. Moreover, anisotropy as observed in onshore Maine was
indicated by some instruments. A soft sediment layer is indicated in all
seismic sections, and its thickness varies between 50 and 300 m. The P
velocity of this surface layer is 1.5-2.5 kin/s. The inferred S wave velocity
is 0.6-0.9 km/s and Poisson's ratio is around 0.40. The P velocity of the
basement starts around 5.3 km/s and increases to 6.4 km/s at 10 km depth.
The S velocity of the basement is around 2.8 km/s and increases to 3.75
km/s at 10 km depth. Poisson's ratio is estimated at 0.24±0.03 in the upper
crust.
Two of the instruments, L3C6 and L7C3, indicate the presence of a
low-velocity zone in the upper crust around 6-10 km depth. The other
instruments do not show any such layer, which implies that any upper
crustal low velocity zone beneath the Gulf of Maine must be laterally
variable. Localized low velocity layers have been reported in Maine
(Luetgert, 1985a, 1985b; Kiemperer and Luetgert, 1987; Luetgert et al.
unpublished).
Mueller and Landisman (1966), proposed the general
presence of LVZs in the upper crust, which have been reported globally by
a large number of studies in the depth range of 5 to 15 km (Krishna et al.,
1989). Upper crust LVZs have been explained by zones of laccolithic
granitic intrusions (Mueller, 1970; Landisman et al. , 1971). The water
content and pore pressure in these granites keep the density lower, hence
the velocity lower, than the surrounding basement rocks (Mueller, 1977).
The presence of granitic plutons in the Gulf of Maine (Hermes et aL, 1978;
Hutchinson et aL, 1988) and the discontinuity of LVZs suggest that regions
of low velocity in the Gulf are due to laccolithic granitic intrusions.
However, the magnetic or gravity data from the Gulf of Maine do not
indicate felsic material at these LVZs. The determined Poisson's ratio in
the upper crust, i. e. the upper 10 km, is 0.24±0.05, mostly ±0.02 and
indicates a slightly quartz enriched upper crust, as expected from the
presence of granitic plutons.
The orientation of the profiles made this survey suitable to investigate
the refraction data for anisotropy in the Gulf of Maine. Figures 48 and 49
show velocity models of good data determined from all profiles and results
from onshore Maine (Kiemperer and Luetgert, 1987). Although line 7 and
8 are perpendicular they do not indicate anisotropy in the upper crust.
However, comparing line 3 and line 7 suggests anisotropy in the upper 6
km of the crust as observed in onshore Maine. Anisotropy in Maine is
related to the orientation to the strike or structural grain of the northern
LINE 3, 5 & 8
LINE 7 & 8
0
0
3
3
6
6
F
:2
12
15
15
18
18
P-VELOCITY (km/s)
P-VELOCITY (km/s)
Figure 48. Comparison of upper crustal P velocities determined at line 3, 5, 7 and 8.
COASTAL MAINE
GULF OF MAINE LINE 3 & 7
0
0
3
3
6
6
12
12
15
15
18
18
P-VELOCiTY (km/s)
P-VELOCITY (km/s)
Figure 49. Upper crustal P velocity in Coastal Maine (Kiemperer and Luetgert, 1987) and in the Gulf of
Maine. Locations of profile 1 & 2 are shown in figure 5.
Appalachians with the fast velocity along the strike. It might be caused by
the orientation of microcracks and/or mineral alignment (Kafka and Reiter,
1987; Park and Simmons, 1982). The presence of metamorphosed rocks
in Maine confirms this assumption and the observed anisotropy of 10 %
agrees well with velocity determinations of layered crystalline rocks
(Kiemperer and Luetgert, 1987; Birch, 1960; Christensen, 1982). The
low velocities determined at L7A2 and L8C3 above 9 and 6 km,
respectively, can be explained by the Cashes Ledge Granite.
Lower Crust
Figures 50 and 51 show the average crustal velocity between 10 km
depth and Moho determined from different instruments. OBS L7A2
suggests a high average P velocity of 6.95-7.15 km/s. The other
instruments suggest lower P velocities (v<6.8 km/s) and all instruments
indicate low S velocities (v<3.95 km/s) in the lower crust. The
corresponding Poisson's ratios are shown in figure 52. Three instruments
(L3A2, L7C4 & L7C3) determine Poisson's ratio of the lower crust to fall
between 0.24 and 0.27 whereas OBS L8C3 suggests 0.225±0.015 and OBS
L7A2 suggests 0.29±0.01.
Phanerozoic extensional terranes are reported to have a transparent
upper crust and a reflective lower crust (Meissner, 1984; Kiemperer, 1987;
Cheadle et. al, 1987). As figure 53 shows, a highly reflective lower crust
has been reported beneath the Gulf of Maine confirming this trend
(Hutchinson et al., 1988). Figure 53 also shows velocity - two way travel
time (TWTT) models calculated from the determined velocity depth
models projected onto the line drawing. The obtained Moho depths agree
very well for all instruments except L7C3. The Moho depth determined
for L7C3 is 28 km and too shallow when compared with the reflection
data. Determined Moho depths in coastal Maine were 30-35 km and
combined with the reflection results suggest that for L7C3 the reflection
identified as PmP might actually come from the lower crust. This suggests
that for L7C4 as well the wide-angle reflection has been misidentified as
STRUCTURE OF THE LOWER CRUST
25
-
L3A2
- - L7C4
L7A2
L7C3
35
L8C3
40
ru
P-VELOCITY (km/s)
Figure 50. Moho depth and average middle and lower crust P velocity determined from PmP wide-angle
reflections.
STRUCTURE OF THE LOWER CRUST
25
- -------------- L3A2
L3C6
- - L7C4
30
L7A2
L7C3
L8C3
40
S-VELOCITY (km/s)
Figure 51. Moho depth and average middle and lower crust S velocity determined from SmS wide-angle
reflections.
POISSON'S RATIO OF THE LOWER CRUST
POISSON'S RATIO
Figure 52. Average Poisson's ratio of middle and lower crust determined from PmP and SmS wide-angle
reflections.
GULF OF MAINE
USGS LINE IA
>
LU
0
SP
CENTRAL
FAULT ZONE
PLUTONIC ZONE
(AVALON)
(AVALON)
LINE 3 4000
LINE 7 C3 LINE 7 A2
3000 LINE 8 C3
I
'
I
l
I
Pluton
FAU
I -Jw
OLLJCj
H
.F
U
I
0
VELOCITY(kmfs)
.1
V
I
Cd
0
vELoerrY (Iun/s)
I
0
J
Ii0 I_I
1
UPPER MANTLE
0
VEL0CrrY(km/s)
0
10KM
Figure 53. Determined velocity models superposed on interpreted line drawing of the migrated reflection
data of USGS line 1A (Hutchinson et al. (1988).
(J)
PmP. For L7C3 and L7C4, the identified wide-angle reflection might
come from the top of a lower crustal intrusion, which was penetrated by
PmP recorded at L7A2. The increased Poisson's ratio at L7A2 (see figure
52) suggests this intrusion to be mafic.
A mafic to ultramafic, high-velocity lower crust is suggested by the
abundance of plutonic rocks sampled from the central Gulf, which indicates
extensive melting during the Paleozoic which might have lead to
differentiation of the crust (Hutchinson et al. (1987). Poisson's ratio
determined in this study (figure 52) however, suggests an average crustal
composition with one mafic intrusion indicated by L7A2 that probably
occurred during Mesozoic extension. Therefore, for the Gulf of Maine a
heterogeneous lower crust is suggested that represents an average lower
crust that is underplated by mafic to ultramafic material. Due to the scarce
Poisson's ratio estimates in the lower crust it can not be determined if these
intrusions are only locally or occupy most of the lower crust. Data from
Maine suggest an average (Taylor and Toksöz, 1982) to felsic (Luetgert et
al., 1988) middle and lower crust, indicating a similar composition of at
least the middle crust.
95
CONCLUSIONS
No correlation between the velocity structure and the lateral block
structure of the Gulf of Maine determined from seismic reflection data was
observed. The lateral velocity structure is uniform, layers are horizontal
or have only small dips (<2°). P and S-wave velocity models indicate the
presence of a soft sediment layer at the top of the Gulf of Maine having a
thickness up to 300 m and velocities of 1.5 to 2.5 km/s and 0.7 to 0.9 km/s
for P and S-waves, respectively.
The S wave is generated at the
sediment/basement interface by P-S conversion. The velocities for P and
S-waves at the basement top are around 5.3 km/s and 2.8 km/s and increase
to 6.3 km/s and 3.6 km/s
within
the upper 5 km. Between 5 and 10 km the
velocity is around 6.4 km/s and 3.7 km/s for P and S waves, respectively.
Poisson's ratio starts with 0.4 in the soft sediment layer and ranges from
0.23 - 0.26 in the upper crust, which indicates average crustal composition,
slightly quartz enriched by the presence of granitic plutons. Localized low
velocity zones are suggested by two instruments and best explained by
laccolithic granitic intrusions. Anisotropy is suggested by our data, as
observed in Maine with the faster velocity along the strike.
Travel time and amplitude information yield that the Moho is found at
a depth of 28 to 37 km with average P and S velocities of 6.5-6.8
km/s
and
3.7-3.9 km/s in the middle and lower crust, which implies a Poisson's ratio
of around 0.25. One OBS indicates a higher P velocity of 6.95-7.15 km/s
and its increased Poisson's ratio suggests a mafic intrusion. The obtained
Moho depth of 30-37 km is in agreement with the Moho depth inferred in
the Gulf of Maine from reflection data (Hutchinson et al.,1987) and fits
well in the transition between coastal Maine (Unger et al.,1987; Luetgert,
1985; Stewart et al., 1985; Luetgert et al., 1986; Luetgert et al., 1987;
Mann and Luetgert, 1985; Luetgert et al., unpublished) and Western
Georges Bank (Grow et al, 1979; Swift et al., 1987). The differing P and
S velocities and Poisson's ratio estimates from the lower crust suggest
average lower crust formed by Mesozoic extension modified locally by
mafic or ultramafic intrusions.
97
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APPENDIX
NW
LINE7OBSA8V
3
00
1
0
DISTANCE (km)
P-VELOCITY MODEL
10
4
6
5
VELOCITY
7
/s)
Figure 54. OBS L7A8, seismic section and determined P velocity.
SE
50
40
1
-
.,
ratio. Poisson's corresponding with velocity S determined and section seismic L7A8, OBS 55. Figure
RATIO POISSON'S
(km/s)
VELOCiTY
4
2
10
1-_-4.........
30
i
._
-
-
-
.
1
-r.4..;
20
i
:
.
..4
1
10
i
0
......
0
DISTANCE
10
.
'dr...!lIZ"
z
-
0
MODEL S-VELOCITY
RATIO POISSON'S
(kin)
i
-..
;
..
- -
20
-
b..-'-- -
.e....
-
,
30
40
60
*
-
.
-
70
0-
St
4.....LL
-
1
-".
1
2
.
en
.i.L.
NW
OBSA8V LINE7
SE
DEPTH (KM)
0
00
TIME - X / 6.8 (s)
O\
t')
111111 liii
9'
C
CD
C
(-J
CD
Cl)
C
z
tTl
I-
C
Cri
109
C
0
0
o
ci
(vc) iu.ia
C)
C
N
z
C
00
-
LINE 8 OBSA8V
3
oc
10
0
10
20
30
40
50
DISTANCE (1cm)
Figure 58. OBS L8A8, seismic section showing the Pg arrival.
60
70
80
90
00
C
00
z
cn
e1
(s)6/x-a
C
C'
C
00
C
C
_
Cc)
I-
.
C,)
111
LINE3OBSC4V
SW
-
DISTANCE (km)
P-VELOCITY MODEL
VELOCITY (km/s)
Figure 60. OBS L3C4, seismic section and determined P velocity.
NE
SW
NE
L1NE3OBSC4H1
4
0'
1
DISTANCE (km)
POISSON'S RATIO
S-VELOCITY MODEL
0
0
10
2
3
VELOCITY (km/s)
4
5
POISSON'S RATIO
Figure 61. OBS L3C4, seismic section and determined S velocity with corresponding Poisson's ratio.
SW
LINE3 OBSA2V
3
Co
-
0
1
0
DISTANCE (1cm)
P-VELOCITY MODEL
0
10
4
5
6
7
VELOCITY (km/s)
Figure 62. OBS L3A2, seismic section and determined P velocity.
NE
SW
4
3
O\
LINE3 OBSA2H1
-i
';.'
-
?
a-I-
a' I ,_,.
1
:",
-
.
.
;.
,
:
..
:..zr
-
A
V
. __
I
I
ê
.,-..
a. .
.. '
a
tIi
4.._#_'._.
a
I
DISTANCE (kin)
S-VELOCITY MODEL
POISSON'S RATIO
0
0
2
8
10
2
3
VELOCiTY (km/s)
S1
.p_-.';
4
POISSON'S RATIO
Figure 63. OBS L3A2, seismic section and determined S velocity with corresponding Poisson's ratio.
NW
BATHYMETRY LINE 5 & 6
SE
------------------------------------------ Avalon Terrane -----------------------------------------------------zCoastal Maine>< --------------------------- Fault Zone ----------------------------- ><Central Plutonic Zone>
3000
10
20
i
30
i
i
40
i
i
50
do
i
70
i
i
80
i
90
I
1D
DISTANCE (km)
Figure 64. Tectonic setting and bathymetry of line 5 & 6.
0\
LINE6 OBSA2V
NW
SE
ii
00
1
0
DISTANCE (kin)
P-VELOCITY MODEL
0
4
5
6
VELOCITY (km/s)
Figure 65. OBS L6A2, seismic section and determined P velocity.
NW
LINE 6 OBS A2 V
SE
DISTANCE (km)
POISSON'S RATIO
S-VELOCITY MODEL
4
VELOCITY (km/s)
;.i5
025
POISSON'S RATIO
Figure 66. OBS L6A2, seismic section and determined S velocity with corresponding Poisson's ratio.
0.35
NW
LINE6OBSA8V
SE
3
00
0
DISTANCE (kin)
P-VELOCITY MODEL
0
10
4
5
O
VELOCITY (km/s)
Figure 67. OBS L6A8, seismic section and determined P velocity.
NW
LINE 6 OBS A8 V
SE
1
DISTANCE (km)
POISSON'S RATIO
S-VELOCITY MODEL
0
I
I
10
10
2
3
VELOCiTY (km/s)
4
POISSON'S RATIO
Figure 68. OBS L6A8, seismic section and determined S velocity with corresponding Poisson's ratio.
NW LINE5 OBSA8V SE
3
00
I
[C
DISTANCE (kin)
P-VELOCITY MODEL
[C
I
10
4
5
6
/
VELOCITY (km/s)
Figure 69. OBS L5A8, seismic section and determined P velocity.
NW LINE 5 OBS A8 V SE
4
[1
DISTANCE (km)
POISSON'S RATIO
S-VELOCITY MODEL
0
0
10
10
2
3
VELOCITY (km/s)
4
POISSON'S RATIO
Figure 70. OBS L5A8, seismic section and determined S velocity with corresponding Poisson's ratio.
t\)
,
IIs
S
S.
I..S
.
S
S .SS
IS
I.
4.
;
S
II
11
IS
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I,
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V
U
&
C
C
a.
a
V
'V
a
a
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I
4
I
.
.
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I.
I
S .SII
III
I.
4
dl-
I
I
14
I'
I,
I
I,
tz
A
I'
C
a.
I
r
a
I
1
a.
a
.
.1
.
e
0
0
0
-I
Model 3
Model 2
Model 1
8
8
8
6
2
2
0
0
DISTANCE (kin)
Model 4
0
DISTANCE
0cm)
Model 5
DISTANCE 0cm)
Model 6
[
DISTANCE (kin)
DISTANCE (kin)
DISTANCE (km)
Figure 74. L8C3, synthetic seismograms corresponding to the P velocity models in figure 73.
0\
II
S
s.
-
i'
e
is
is 115.
N
*
I
14
I,
-
N U
I
a
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'
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-
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-
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.
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.
.5,
S
::
-
I
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0
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e
55
'
I'
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II
.
1
S.
S.
S
S
I
S
'.11
.
-
.55
-
;.
-
I
ts
I,
u
V
I,
I
S
0
.
I.
II.
,
S .SS
I
II
I:
I'.
.1
S.
S
S
s
i
- I,
I,
I,'
I
I
'I
III
a
I
I
Synthetic Sections
Model 2
Model 1
8
8
6
6
oq
00
2
0
0
DISTANCE (km)
DISTANCE (kin)
Figure 79. L3C6, synthetic seismograms corresponding to the P velocity models in figure 78.
1
S
P.
I
.
-
Si .ii
S
S.
I
I
I,
ii
I:
is
P
''..PTa ''4)I.t
'I
I
5U .1I
I
Ii,
Ii,
I!
lid
I
:
'I
I
I
S
I
Synthetic Sections
Model 2
Model 1
10
10
-6
oq
%0
oq
'0
-.6
DISTANCE (kin)
DISTANCE (kin)
Figure 81. L3C6, synthetic seismograms corresponding to the S velocity models in figure 80.
.
'I
(
,_
S
SI'
511
5
S
-
I
S
51
I.
I,
I'
I.
O
I'
a
aI
ii
aI
iv
I
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