Fault-related deformation within the brittle-ductile transition [electronic resource] : field observations, microstructural analysis, and mechanical modeling
Keywords: Geological and Environmental Sciences.

The "brittle-ductile transition" is an interval of Earth's crust where the primary mechanism of rock deformation gradually changes from brittle fracturing to viscous flow with increasing depth. Fault-related deformation within this transitional zone significantly influences earthquake rupture nucleation and propagation, yet remains poorly understood due to uncertainty in the constitutive equations that govern deformation by simultaneous brittle and viscous mechanisms. This dissertation provides a new perspective on the topic by integrating detailed field observations, microstructural analysis, and mechanics-based numerical modeling of geologic structures. The Bear Creek field area (central Sierra Nevada, CA) contains abundant left-lateral strike-slip faults in glacially polished outcrops of Lake Edison granodiorite (88±1 Ma) that were active under brittle-ductile conditions. Secondary structures near fault tips include splay fractures in extensional regions and an S-C mylonitic foliation in contractional regions. Microstructural observations (including electron backscatter diffraction analysis), titanium-in-quartz analysis, and thermal modeling of pluton intrusion and cooling indicate that the temperature during early faulting and mylonitization was 400-500°C. The faults remained active until 79 Ma as the pluton cooled to 250-300°C, as evidenced by cataclastic overprinting of the mylonitic foliation and the presence of lower greenschist minerals within faults. Kinematic and mechanical models of the Seven Gables outcrop provide insight into the constitutive behavior of fault-related deformation under brittle-ductile conditions. The outcrop contains a 4 cm-thick leucocratic dike that is offset 42 cm across a 10 cm wide contractional step between two left-lateral strike-slip faults measuring 1.1 m and 2.2 m in length. Within the step, the dike is stretched and rotated about a non-vertical axis, and a mylonitic foliation develops in the dike and surrounding granodiorite. The geometry and kinematic model for this outcrop serve as a basis for a 2D mechanics-based finite element model (FEM). The FEM tests five potential constitutive equations for brittle-ductile deformation: Von Mises elastoplasticity, Drucker-Prager elastoplasticity, power law creep, two-layer elastoviscoplasticity, and coupled elastoviscoplasticity. Models with plastic yield criteria based on the Mises equivalent stress are most successful in reproducing the outcrop deformation. Frictional plastic yield criteria (i.e., Drucker-Prager) and power-law creep are incapable of reproducing the outcrop deformation and can be excluded from further consideration. In addition, the effect of distributed inelastic deformation on fault slip and slip transfer through fault steps is investigated using field observations, microstructural analysis, and FEM results. Distributed plastic shear strain (i.e., mylonitization) near fault tips effectively lengthens faults, allowing for greater maximum slip and greater slip gradients near fault tips. Furthermore, distributed plastic shear strain facilitates slip transfer between echelon fault segments, particularly across contractional steps where plastic shear strain is greatest. However, fault segments separated by contractional steps also have significantly reduced slip in the step-bounding portions of the faults, because shear offset is accommodated by distributed shearing within the step. Thus, off-fault distributed inelastic deformation significantly impacts fault behavior within the brittle-ductile transition.