Enormous volumes of hydrocarbons are contained in fractured rocks in structurally simple, detached anticlines above basal detachment faults in thrust-fold belts

Broadly interrelated styles of geologic and geophysical anomalies are typically characteristic of petroleum provinces of similar origin. But zones of basal detachment faults are structures resulting in independent styles of deformation in the rocks above and below the fault.

Because the styles of deformation and associated hydrocarbon traps above and below the basal detachment can be interpreted prior to exploration, well understood structural styles can be of great value to exploration teams in new frontier regions.

Building our knowledge of the tectonic styles related to the kinematics of basal detachment, traps and depositional histories

Basal Detachment and Wrench Faulting

Horizontal displacements in transcurrent faults represent one of the fundamental modes of accommodation of deformation in the crust, crosscutting the entire lithosphere.

Studies of wrench faults eroded down to the middle to lower crust show that strain remains localized in transcurrent shear zones, and there is strong evidence that suggests a coherent deformation of the entire lithosphere in major intracontinental wrench faults.

The preservation of wrench fault fabrics within the upper mantle has major effects on the subsequent tectono-thermal behavior of continents, This anisotropy may explain the frequent reactivation, at continent scale, of ancient lithospheric-scale wrench faults and transpressional belts during subsequent tectonic events.

Higher tectono-thermal activity associated with wrench faults is linked to significant hydrocarbon source maturity and traps are associated with en echelon folds associated with such wrench faults.

Wrench faults have both compressional and extensional features and result from the orientations of their boundaries relative to regional plate motion.

Wrench faulting also commonly occurs in active margins or in collisional domains, but it is less clear whether intra-continental strike-slip fault systems generated in active margins or in collisional domains are only crustal structures or are rooted in the upper mantle. The penetration of a “wrench-fault type” tectonic fabric (i.e., a vertical flow plane associated with a horizontal flow direction) deep into the upper mantle may have major geodynamic implications,

A vertical zonation is common at structural levels above a basal detachment. Older basement structures commonly control development of younger structures.

Basal Detachment Structures

Thrust-fold assemblages related to major wrench faults are an essential element of many convergent plate boundaries and occur as wide zones of deformed sedimentary cover above basal detachment faults on the external side of many collision belts. Ductility contrasts are responsible for localization of the fault surfaces so that thrusts are parallel or sub-parallel with bedding in incompetent rocks and oblique to bedding in competent rocks.

Common structures in thrust and fold belts are a low-angle or near horizontal plane of basal detachment or decollement, that act as detachment areas and separate a folded and fractured upper part or cover from a relatively undeformed basement.

Thrust sheets typically consist of repeated slabs. Anticlines with imbricate thrusts and broad folded thrust-fault structures occur across the breadth of a thrust-fold belt.

Divergent Margin Structures

Rift systems are a prolific hydrocarbon setting where oil and gas traps depend on the faults and their associated drape flexures. Such normal faulting dominates divergent margins in early stages of development, and the oceanic crust formed at spreading centers.

The Red Sea typifies the basic style observed at divergent margins and intraplate grabens, where regional faults create intersecting systems with zig-zag edges. Pre-existing zones of crustal weakness often influenced the later position of normal faults. On a regional scale, the normal faults form intra-plate rifts that range in complexity from simple fault troughs to grabens with many junctions. Divergent plate boundaries inherit their outline mainly from these trends of earlier intra-plate rifts.