Abstract

Conspicuous structural features of the northern Gulf Coast are regional systems of listric normal faults formed contemporaneously with deposition. Periods of main fault movement range from Eocene to Miocene. Initial development of these contemporaneous fault systems is usually attributed to differential compaction and/or gravity sliding within the sedimentary section during clastic sedimentation on a stable shelf. Evidence of recent reactivation of some fault systems indicates that sediments in the region are currently in a tensional state of stress. Infrequent, low- to moderate-intensity earthquakes also are associated with the faulted region. Lithospheric flexure due to sediment loading has been suggested as a possible cause of this extensional regime. For most passive continental margins, sedimentation alone is not sufficient to cause flexurally induced faulting and earthquakes because the sediment load accumulates slowly enough for the stresses to relax. However, sedimentation rates in the northern Gulf Coast have been extremely high since the Pleistocene (12–18 cm/100 yr). A simple flexural calculation yields horizontal extension (normal faulting) in the lower Gulf Coast caused by the weight of Pleistocene sediments. Although the relationship between basement upwarp and surface uplift may be complicated by subsidence due to compaction of sediment, the location of the flexural bulge due to loading by Pleistocene and younger sediments is generally consistent with areas of recent uplift determined from releveling data. Bending stresses caused by the total weight of Mesozoic and Cenozoic sediments predict horizontal compression for this region, indicating that most of these stresses must have relaxed. Relaxation of stresses over time may also explain the absence of large earthquakes.

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