--> Abstract: Numerical Analyses Of Trishear Fault-Propagation Foldsimplications For Geometry, Growth Strata, Strain Evolution, And Fracturing, by R. W. Allmendinger; #90928 (1999).

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ALLMENDINGER, RICHARD W.
Dept. of Geological Sciences, Snee Hall, Cornell University, Ithaca, NY

Abstract: Numerical Analyses of Trishear Fault-propagation foldsImplications for Geometry, Growth Strata, Strain Evolution, and Fracturing

Fault-propagation folds can act as important hydrocarbon traps in both contractional and extensional tectonic settings. During the last fifteen years, models of fault-propagation folds have been dominated by the kink fold geometry. But this geometry does not simply explain some commonly observed geometries (footwall synclines, forelimb thickness changes) and provides no direct way to predict fractures associated with such structures. Although receiving almost no attention, the trishear model first proposed by Eric Erslev for basement structures addresses these shortcomings.

Unlike the graphical kink fold method, trishear geometries must be calculated numerically. We have developed an extremely flexible computer program to produce forward and inverse numerical models of extensional (i.e., "extensional forced folds") and contractional trishear structures. The inverse method uses a sixparameter grid search to extract the best fit trishear model for a structure of partially known geometry. Then, forward modeling using the same parameters can fill in the geometric details in areas of poor resolution. During forward modeling we track, the evolution of strain throughout the structure and can model growth strata as well. Highly deformed zones in competent rock units should correspond to zones of intense fracturing. Potential fracture orientation can be inferred from the lines of no finite elongation. In general, zones of greatest deformation occur at the tip lines of the faults, in the footwall beneath thrust faults, and in the hanging wall above normal faults. Preliminary tests of the models have been made using analog models, outcrop-scale structures, and cross sections based on seismic and well data.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas