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 one-dimensional
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
6.4 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
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, wide-angle
reflections interpreted to be PmP and SmS were modeled by one-dimensional
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. / Graduation date: 1991
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29140 |
| Date | 04 October 1990 |
| Creators | Sattel, Daniel |
| Contributors | Trehu, Anne M. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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