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


Kinematic models can be used to construct balanced, restorable cross sections of Gulf Coast listric normal faults. Previous kinematic models fail to account for important attributes of listric normal faults, such as synthetic faults, "normal" fault drag, fault smear and displacement gradients which can range from zero at the top layer to a maximum at depth. It is well documented that growth sediments above an active listric normal 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. The new kinematic models consist of two active deformation zones or zones of distributed simple shear, which are fixed in space in the hanging wall at fault bends. As the hanging wall moves through a bend, it is sheared according to previous models of inclined, antithetic shear which causes either normal and/or reverse drag. The other active deformation zone is synthetic to the main fault, is also fixed in space at fault bends and has a steeper dip than the main fault. When the dip on this zone of synthetic simple shear approaches the dip on a fault segment (e.g. at a convex upward bend), the synthetic shear simulates fault smear. Synthetic faults can be modeled by the introduction of narrow zones of intense synthetic shear. With increasing displacement, rocks, which have undergone synthetic shear, pass into the zone of antithetic simple shear and are deformed a second time. Original synthetic faults and fractures are sheared into a lower dip during this second deformation, and new antithetic faults and fractures are formed. 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.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana