Use of Synthetic and Antithetic Shear to Model the Development of Gulf Coast Listric Normal Faults : ABSTRACT

Abstract

ABSTRACT Geometric models can be used to construct balanced, restorable cross sections of Gulf Coast listric faults. Previous geometric models fail to account for important attributes of listric faults, such as synthetic faults, normal fault drag, upfolding, fault smear and displacement gradients which can range from zero at the top layer to a maximum at depth. The new geometric model consists of distributed simple shear in two orientations, which are synthetic and antithetic to the main fault. As the hanging wall moves through a bend, it is sheared according to previous models of inclined, antithetic shear which causes either and/or reverse drag depending on whether the bend is convex or concave upward. The other active deformation zone is synthetic to the main fault, has a steeper dip than the main fault, and is modeled as a zone of distributed synthetic and antithetic shear. When the dip on this zone of synthetic and antithetic simple shear approaches the dip on a fault segment (e.g. at a convex upward bend), the synthetic shear simulates fault smear. It is well documented that growth sediments above an active listric fault result in a fault displacement which increases with depth. However, synthetic shear in the hanging wall also causes a displacement gradient which also increases with depth. Synthetic and antithetic faults can be modeled by the introduction of narrow zones of intense shear or as discrete faults. With increasing displacement, rocks, which have undergone synthetic and/or antithetic shear, pass into the zone of antithetic shear and are deformed again. Original synthetic faults and fractures are sheared into a lower dip during this second deformation, and layering, which formerly dipped away from the fault, is rotated to dip into the fault. New antithetic faults and fractures could also be formed during this deformation. This new method can thus be used to predict the orientation and possibly the relative intensity of both faults and fractures in regions which have been both singly and multiply deformed.