AmericanChemicalSociety.com

Geodynamic models demonstrate that structural fabrics inherited from Precambrian and Late Paleozoic continental rifting and collision episodes on the U.S. Gulf of Mexico continental margin controlled the development and migration of Mesozoic depocenters during opening of the Gulf of Mexico oceanic basin.  Preexisting heterogeneities in the lithosphere controlled the location, timing, and geometry of the syn-rift Interior Salt Basin and outboard Wiggins Arch south of the Paleozoic Ouachita orogen between the Late Triassic and Late Jurassic Periods.  Dynamic changes in the strength of the lithosphere that occurred during rifting led to later focusing of the depocenter south of the Wiggins Arch beginning in the Late Jurassic Period.  The ensuing evolution of the northern Gulf of Mexico continental margin appears to be unique.  Hyperextended continental crust extends ca. 600 km outboard of the basement hingeline, whereas most continental margins contain 200-300 km of extended crust outboard of the hingeline.  Other continental margins that have regions of hyperextended crust greater than 300 km are typically characterized by a lack of substantial syn-rift magmatism and anomalous seismic velocities in the pre-rift crystalline rocks in the distal parts of the margin.  The Iberian margin is a well studied example.  On the Iberian margin, the anomalous seismic velocities have been associated with subcontinental mantle exhumed by low-angle faulting during the late stages of rifting.   The U.S. Gulf of Mexico margin also lacks substantial syn-rift magmatism and has anomalous seismic velocities beneath the distal syn-rift sedimentary section.  The similarities between the Gulf of Mexico and Iberian margins prompt a hypothesis that the Gulf of Mexico is an amagmatic continental margin, with hyperextended crust created by low-angle detachment faulting.  The comparison further raises the question as to whether the central Gulf of Mexico crystalline crust is oceanic, as previously interpreted, or is instead exhumed subcontinental mantle or highly stretch continental crust. During this reporting period, we have created directionally filtered aeromagnetic maps (reduced to the pole) of the central Gulf as well as corresponding directionally filtered isostatic and bouguer gravity maps. The maps reveal no fabrics indicative of the creation of oceanic crust in the central Gulf. Work currently underway involves 2.5D gravity and magnetic modeling of profiles crossing the Gulf to better understand the nature of the crust.