BROADBAND SEISMIC STUDIES OF SOUTHERN LAURENTIA: LITHOSPHERIC STRUCTURE FROM THE RIO GRANDE RIFT TO TEXAS’ GULF COAST

Most of Texas and Oklahoma are part of the ~1.4 Ga Laurentia craton that extends from Texas to Canada, which has been reworked by a variety of orogenic and rifting processes. These events formed the main features of the region, including Precambrian exposures of the Llano uplift in central Texas, the Ouachita Deformation Front that extends from southern Texas to the Wichita Mountains of southern Oklahoma, and the Southern Oklahoma Aulacogen. The Texas Gulf Coast now forms the southern edge of the stable North American craton while the much-younger (~35 Ma) Rio Grande Rift marks its southwestern boundary.

Studies of the deep structure of the TX-OK-NM region have been hampered historically by a low density of broadband seismic stations. However, station density was temporarily increased in the past ten years by EarthScope’s Transportable Array and contemporaneous deployments. Data from these stations allow us to image the structure of the lithosphere and asthenosphere, to investigate the relationship between surface manifestations of previous tectonic events and lithospheric structure, and to make inferences about processes that are currently modifying the region.

For example, numerical modeling suggests that an abrupt change in lithospheric thickness, which creates a step change in densities, may produce a gravitational instability that leads to thicker mantle lithosphere dripping off into the lower density asthenosphere. As the mantle deforms it alternately thickens and thins the crust, producing topographic anomalies that correlate with the disappearing mantle lithosphere. The northward time progression of rifting of the Rio Grande Rift allow us to evaluate this mechanism’s role in the evolution of this rift-craton transition.

Tomography and receiver function (RF) imaging reveal an 80-100 km wide “lithosphere-asthenosphere transition zone” the Gulf Coastal Plain, which may result from partial melt and shearing in a softened or underdeveloped lithosphere. Sp and Ps RF images show a Moho beneath the Llano uplift but not beneath the Balcones Fault Zone. RFs also found a low velocity zone at 50-70 km depth near the Gulf of Mexico shoreline, which may be a piece of subducted lower crust emplaced during the Ouachita orogeny.