The Port Isabel fold belt lies in the southern and central Port Isabel protraction area in the westernmost deepwater Gulf of Mexico, offshore Texas. The northeast termination of the trend into the northeast corner of the Port Isabel and Southwest corner of the East Breaks protraction areas is a unique structural trend with a distinctive structural style. This northern extension of the Port Isabel fold belt is a structurally complex, linked fault system that has been significantly affected by regional salt tectonism and shallow shale diapirism. Large-scale capture of Miocene deposition updip of an Oligocene-age extensional zone, concurrent with evacuation of salt and ductile shale, has resulted in structural inversion and overprinting phases of deformation. The structural style of this zone is characterized by an updip trend of deep Miocene basins flanked by downdip large-scale rollover anticlines. Frio sediment-cored rollover anticlines are fringed on the downdip edges by thrusts, shale diapirs, or detachment folds. Inversion within this zone is expressed by faults with senseof- motion reversal, rollover anticlines with basinward vergent thrusts, pop-up structures, and shearing of large portions of the section.

Ductile shale deformation has increasing importance along trend from southwest to northeast. The Anahuac shale is an important detachment zone within the trend and it is diapiric over much of the northern Port Isabel fold belt trend. The unit is very well imaged because of its shallow position in the section and the high quality seismic data available over the area, and it thereby provides a rare opportunity to view the internal deformation of a ductile, diapiric shale. The shale displays many characteristics similar to the deformation style of salt, including mini-basin formation during early deposition, reactive diapirism of the shale layer triggered by regional

Figure 1. Line A-A' is a regional dip-oriented section across the northern Port Isabel fold belt trend. The shaded unit is the Anahuac Shale. Frio-age sediments lie below the Anahuac Shale and Miocene-age sediments lie above it. The line illustrates the regional structural elements in the trend. A large Miocene basin that is in contact with the regional detachment (the Port Isabel Decollement) is located on the northwest side of the line and a rollover-anticline of Frio age sediments is located on the southeast portion of the line. The Anahuac Shale above the large rollover-anticline is diapiric, forming several large shale diapirs.

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extension, shale-cored detachment fold formation, and contractional diapirism, as well as more unique characteristics such as close juxtaposition of brittle and ductile behavior. The ductility of the Anahuac shale at shallow depth is unique in that it is not caused by overpressure, as is assumed of most diapiric shales.

The trend has economic hydrocarbon accumulations in traps created by the ductile Oligocene Anahuac shale. Future remaining exploratory plays will be to evaluate early Miocene turbidites in ponded basins immediately above the Anahuac, continued amplitude tests of the middle Miocene, and sub-Anahuac Frio structures. The Anahuac shale also has application as a uniquely shallow, and well imaged, structural analog for exploration targets in shale tectonic settings such as offshore Nigeria, Brunei, and the southern Caspian region.

Figure 2. The Anahuac Shale is ductile and diapiric within the trend. Unique structural features associated with salt tectonics are developed in the shale such as mini-basins and early contractional folding of the mini-basin rims (Line B-B').

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