CONSTITUTIVE BEHAVIOR OF GRANITIC ROCK WITHIN A CONTRACTIONAL FAULT STEP AT THE BRITTLE-DUCTILE TRANSITION

This project takes a comprehensive field, laboratory, and numerical approach to investigate the constitutive behavior of granitic rock at the brittle-ductile transition (BDT). The BDT likely plays a critical role in earthquake nucleation and rupture mechanics, yet the constitutive behavior of crustal rock within this lithospheric level remains poorly understood. The Bear Creek field area (central Sierra Nevada, CA) provides an excellent opportunity to study exhumed strike-slip faults and shear zones that developed at the BDT. We focus on Outcrop SG10, which features a 10cm thick aplite dike that is offset 0.45m through a contractional step between two sub-parallel left-lateral faults. Within the step, the aplite undergoes extensive thinning (stretch ~1/10) and the granodiorite displays a well-developed foliation. Thin-section analysis indicates that deformation within this shear zone is characterized by an S-C mylonitic fabric, in which quartz and biotite plastically flow around larger grains of feldspars, hornblende and opaque minerals. Electron backscatter diffraction (EBSD) analysis gives a more quantitative representation of the micromechanisms active throughout the step and provides constraints for the mechanical modeling. We use Abaqus, a commercial finite element software, to test several constitutive laws that may account for the deformation observed both macro- and microscopically throughout the step. The initial geometry of the model is based on a kinematic restoration of the outcrop, and material properties for the aplite and granodiorite are taken from the rock mechanics literature. Boundary conditions are employed in two steps: (1) isotropic pressure of 100 MPa to simulate the lithostatic load at 4 km depth; and (2) displacement oblique to the faults to induce left-lateral slip. Using the same starting configuration and boundary conditions, we experiment with several inelastic constitutive laws (e.g. Mohr-Coulomb, Drucker-Prager, cap plasticity, viscoplasticity, creep flow laws) in order to eliminate those that fail to capture the observed deformation. Through this elimination process, we obtain a new understanding of the constitutive properties that likely govern the localization of deformation in the BDT.