Inherited Structural Fabrics and the Impact on Oblique Slip Faults in the South Texas Laramide Foreland

Abstract

The extent of a compressional orogenic event is often determined by the last folds or faults toward the hinterland. However, the stress conditions which allow for compressional deformation are gradational into the “undeformed” stable craton resulting in a zone of high horizontal stress beyond the mapped front. This zone is of tremendous importance in petroleum systems as this can become the unconventional reservoir “sweet spot” in the basin. The orientation of faults should be predictable given knowledge of the shortening direction. However, inherited pre-existing fabric can influence the orientation and intensity of the younger deformation. South Texas has experienced many of the major tectonic events that define the southwestern US including PC-Cambrian rifting, orogenic uplift in the Penn and E. Permian associated with the Marathon-Ouachita Orogeny, Triassic-Jurassic rifting, compressional folding in the Tertiary Laramide Orogeny, and extensional rift fault development in the Neogene during Basin and Range rifting. Boundaries that define the deformation associated with these events overlap and can be difficult to discern. A well exposed road cut north of Sanderson, Texas contains significant oblique extensional faults (rake 5-25 deg) with NW (315) and NE- (020) oriented faults. However, satellite imagery over the exposure reveals that only a few of the faults are visible linear elements in satellite images. Scaling back from the outcrop, more than 30,000 mappable fractures/faults define a 60-km wide zone between the last Laramide fold and the stable craton. When exposed, the faults within this zone have limited offset (most are less than 3 m, subseismic resolution) because of their oblique offset. Fault and fracture orientations vary along this zone due to the fabric of older, Paleozoic subsurface structures. Observation of slickensides, massive sparry calcite development and fault damage zones suggest that the NW faults are shearing along distributed faults which allows for significant extensional opening of the NE fault sets. No element in outcrop honors “ideal” orientations, likely a result of the reactivation of inherited Paleozoic structural fabric. Much work remains to fully characterize the zone of faulting ahead of the so called “Laramide Front” but outcrops provide valuable insight into the hierarchy of fracture development expected in similar age rocks along orogenic fronts in both surface and subsurface systems.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016