3-DIMENSIONAL NUMERICAL SIMULATIONS OF CRUSTAL-SCALE WRENCHING USING A NONLINEAR FAILURE CRITERION

We have developed a three-dimensional finite element model to study wrench deformation of the crust regarded as an elasto-plastic material obeying Murrell's extension of Griffith's failure criterion. Numerical experiments using this model predict that the imposed basal wrenching is accommodated by an array of oblique Riedel-like shears and Y-shears (parallel to the direction of wrenching). The partitioning of deformation between the two types of structure depends on the width of the zone of imposed basal wrenching and the existence of a component of deformation in the x-direction (normal to the direction of wrenching). The Riedel shears arc arranged in spiral-like structures that root into the basal wrench zone. In cross-section. the Riedel shears resemble wedge-shaped flower structures similar to those often observed in seismic cross-sections. The 'polarity' of the flower structures is positive (or palm-tree-like) in transpression experiments and negative (or tulip-like) in transtension experiments. The orientation of the Riedel shears throughout the crust obeys Mohr's hypothesis for incipient faulting combined with Murrell's failure criterion. The model also predicts plastic dilatancy inversely proportional to the square root of the confining pressure; this result agrees qualitatively with field observations and the results of sand-box experiments and quantitatively with direct measurement of dilatancy during high-pressure rock-deformation experiments.