Graduate Thesis Or Dissertation
 

P and S velocity structure beneath the Gulf of Maine

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  • 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.
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