BROADBAND SEISMIC STUDY OF THE TEXAS CONTINENT-OCEAN BOUNDARY: A PILOT PROJECT

Seismic investigations indicate that the upper crust or basement beneath the Gulf Coast Plain (GCP), a Jurassic rifted margin, is very thin (10 km) and this structure has generally been interpreted to be attenuated continental crust caused by rifting in either the early Mesozoic or the early Paleozoic, although some models interpret this feature as an underplated double thickness of oceanic crust that formed during subduction related to the earlier formation of Pangea (Ouachita Orogeny). However, seismic work at the Texas-Louisiana border (west of the Mississippi embayment) indicates an upper mantle low velocity anomaly that is consistent with a depleted mantle or a rift pillow. This observation supports the interpretation of this region as severely attenuated continental crust overlain by 10 to 20 km of Gulf Coast sediments. Another possibility is that the transitional lithosphere is a volcanic rifted margin, analogous to the Greenland-Norway margin or E. Coast of the US.

Over the northern Gulf of Mexico ocean-to-continent transition (OCT) a broad and thick sedimentary succession has built upward and seaward where, because of the great depths of these sediments, little is known about the transition from craton to oceanic crust. In an effort to provide additional constraints on the lithospheric structure across the OCT, five broadband seismographs were deployed for a period of six months (February and September 2008) from Junction, TX to San Antonio, TX. This is the narrowest portion of the OCT, and is an are for which there exist active-source reflection data, vintage refraction data, and a proposed geophysical model. This deployment allows us to (1) apply, validate, and fine-tune receiver function techniques for imaging the crust and upper mantle beneath deep sediments, (2) test a geophysical model proposed for this region, and (3) collect additional constraints that will aid in the design of an effective and cost-efficient proposal to learn how continental and oceanic lithosphere merge and, hopefully, to better reconstruct the extensive tectonic history of this passive margin.

Receiver functions and shear-wave splitting measurements from the five stations will be presented, along with interpretation in the context of the geophysical model and regional shear-wave splitting measurements presented previously.