STRAIN ACCOMMODATION ABOUT STRIKE-SLIP-FAULT DISCONTINUITIES IN GRANITIC ROCK UNDER BRITTLE-TO-DUCTILE CONDITIONS

Brittle and ductile structures adjacent to fault terminations and echelon fault steps of left-lateral strike-slip faults in the Lake Edison Granodiorite of the central Sierra Nevada, California are related to stress perturbations caused by fault slip. The structures associated with fault terminations change from dilatant splay fractures only, to splay fractures and ductile fabrics on opposing sides of the fault, to ductile fabrics ahead of the fault with decreasing distance from a younger neighboring pluton and, presumably, with increased temperatures. The distributions of deformation mechanisms correlate with the inferred local magnitudes and spatial distributions of the maximum tensile stress, the mean stress, and the maximum shear stress. Displacements along the faults are transferred across echelon stepovers by mineralized dilatant fractures in extensional steps. To accommodate slip transfer across contractional steps, rock apparently was squeezed vertically out of the step by ductile flow; concentrations of mobile and immobile elements indicate that diffusive mass transfer was not significant. Increased mean stress in contractional steps apparently enhanced crystal-plastic how of quartz. A how law that includes a pressure effect through correlation with pressure-dependent solidus temperatures is more successful than other experimentally derived flow laws in predicting the observed distribution of ductile fabrics.