Influence of the state of stress on the brittle-ductile transition in granitic rock; evidence from fault steps in the Sierra Nevada, California

Left-lateral strike-slip faults in the Lake Edison granodiorite (central Sierra Nevada, California) are composed of en echelon segments. Relative displacements across the faults apparently are transferred between segments by ductile shearing at right steps, and by extensional fracturing at left steps. The granodiorite within right steps displays mylonitic foliation, and thin sections show textures in quartz associated with dislocation glide, recovery processes, and dynamic recrystallization, whereas textures in feldspar are related to fracturing. Only centimetres outside the right steps, the rock fabric is approximately isotropic and deformation is accommodated by mineralized opening- mode fractures. The stress field calculated for the right-step geometry, when a boundary element model is used, shows an increase in mean compressive stress of up to 25 MPa within the step relative to that outside. This difference in stress apparently produced the contrasting behaviors of the granitic rock. Experimentally derived power-law flow laws do not predict these behaviors.