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GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 38 (1988), Pages 207-215

Deep Crustal Structure of the Northwestern Gulf of Mexico

Yosio Nakamura (1,2), Dale S. Sawyer (1), F. Jeanne Shaub, Kevin MacKenzie (1), Jurgen Oberst (1,2)

ABSTRACT

We conducted a large-offset seismic experiment in the northwestern Gulf of Mexico, offshore of Texas, using large-capacity air guns and ocean-bottom seismographs in order to map the salt-disrupted structure of the thickly sedimented shelf and slope. Five seismic lines, each approximately 90 km long with four or more instruments, covered an area which extended from mid-shelf to the continental rise. We analyzed and interpreted the acquired data using two-dimensional ray tracing to match the theoretical and observed arrival times of refracted and reflected waves, refining both shallow structures as well as the depths to the deeper refractors of initial models constructed with conventional methods.

The entire study area is covered with sediments, whose thickness varies from 13 to 15 km along the shelf and slope, thinning to about 11 km on the continental rise. The most prominent reflector in the sedimentary column, interpreted to be the middle Cretaceous unconformity (MCU), lies at a depth of 7 to 11 km. The seismic velocity in the sediment column above the MCU (1.7-1.9 km/s near the sea floor increasing to 3.0-4.0 km/s just above the MCU) is appropriate for clastic sediments. The velocity below the MCU (3.7-4.7 km/s) suggests the presence of carbonate and possibly salt. The basement, or the top of the crust, as inferred from higher velocity refracted arrivals, is found at a depth of 13 to 16 km. The observed basement relief shows a NW-SE trending ridge that may constitute a seaward extension of the San Marcos Arch, and a trough that runs south of the ridge. The systematic increase in basement velocity from 5.0-5.2 km/s in the north to 5.6-5.8 km/s in the south represents a broad transition from continental to oceanic crust. The Moho is observed at a depth of 20 km under the continental rise and deepens towards the slope.


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