A.F. Christensen, 1983. "An Example of a Major Syndepositional Listric Fault", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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A major Miocene growth fault system (Figure 1) trends subparallel to the Texas shoreline 30 to 70 km (20 to 42 mi) offshore of Matagorda Island in the Gulf of Mexico. It has been called, among many names, the Brazos Ridge Fault System, the Post-Discorbis (Miocene) Flexure, the Big Hum Trend (Oxidental and Mesa), and the Corsair Fault System (Shell). The system is characterized by tremendous thickening of the Upper Miocene section across the fault(s).
The system is seen on Shell's high resolution migrated depth seismic sections (Lines A and B) as a normal listric growth fault with fault plane angles varying from 170 to nearly horizontal. Parallel and near-parallel synthetic and antithetic faults are numerous in the shallower measures and decrease in number with depth.
Movement along the fault system measures more than 5,200 m (17,000 ft) of vertical displacement and greater than 16.5 km (10 mi) of horizontal displacement. An excellent fault plane reflection (acoustic interface) is seen because of significant differences in rock velocities on opposite sides of the fault. Moderately pressured (0.65 FPG) sandstones, siltstones, and shales in the downthrown fault block are displaced against hard-pressured shales (0.8 FPG) upthrown.
Hydrocarbons are found primarily in two types of traps: shallow gas accumulations occur in sandstone reservoirs within the keystone faulted zone and often on the upthrown sides of major antithetic faults; while deeper gas zones (see schematic map) occur within regressive wedges of interbedded sandstone and shale that have anticlinal closure associated with rotation into the fault plane.
Downdip of this growth fault system is another interesting trend composed of chevron faulting (see Line A). The opposing fault directions resulted from an early salt roll with westerly dipping faults on its flank. Then, with withdrawal of salt and collapse of the younger measures, basinwarddipping faults developed in the upper interval. This withdrawal of salt also provided the space filled by the downthrown sediments of the Brazos Ridge Fault and the resulting huge heave.