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The Gulf of Mexico (GoM) and Bay of Bengal (BoB) passive margins are the sites of our planet’s largest accumulations of sediments. These sediments form deep blankets that, among other things, harbor vast reserves of hydrocarbons, create fertile soils, and produce flat terrains that millions of people call home. They also obscure the structure and composition of the crust and mantle beneath the sediments, so that, even now, their geological histories, compositions, and tectonic processes are poorly understood.
There are several reasons to study lithospheric structure and composition in efforts to infer the tectonic processes that created passive margins. First, they are the locales of continent-ocean transitions, so our understanding of plate tectonics requires that we understand the nature of and controls on this transition. Second, understanding details of the margin’s structure is critical to accurate reconstructions, which is a major pursuit in both industry and academia. Third, “Wilson cycles” (the opening and closing of ocean basins and formation of supercontinents) require that continents rupture repeatedly and these ruptures (rifts) tend to occur on pre-existing zones of weakness. Understanding previous rifted margins throughout the cycle is crucial to understanding the entire cycle. Lastly, passive margins are likely to be where future subduction is initiated.
Two-dimensional seismic tomography across the “dry land” portion of the Texas-Gulf of Mexico margin using data from several sources reveals the crustal thickness and the structure of the Moho across the transect and reveals several unexpected features. For example, we used data from teleseismic and local earthquakes, postcritical SsPmp arrivals, and direct P wave energy identified with seismic interferometry that were recorded by a broadband, three-component array and partially-overlapping short-period, vertical-component array. The postcritical SsPmp phase, which is sensitive to the structure of the Moho, helps constrain the discontinuity, which a previous receiver function study suggested was absent beneath the seaward portion of this transect. A high velocity body is observed in the crust below the Luling uplift, indicating possible magmatism during rifting. The crust thins from NW to SE, indicating that extension occurred mostly to the south of the Ouachita orogeny. The non-uniform extended crust suggests that the rift moved southward with time. Our model suggests a new role for the enigmatic Balcones Fault Zone.
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