INITIATION AND STRAIN LOCALIZATION ALONG A LOW-ANGLE NORMAL FAULT IN CRYSTALLINE BASEMENT

Exposures of the Miocene Chemehuevi detachment fault system (SE CA) provide an opportunity to study the initiation and incipient slip on low-angle normal faults (LANFs) active near the base of the seismogenic zone (5-15 km paleodepth). The regional fault system formed at ≤20° dip in heterogeneous gneissic and granitoid rocks, with ambient footwall temperatures from 200 to >400°C. The fault system is characterized by three stacked, anastomosing low-angle normal faults; one of these (the Chemehuevi detachment – CDF) preferentially localized ≥ 18 km of NE directed slip rendering the deepest fault, the Mohave Wash Fault (MWF), inactive after <2 km of slip. At outcrop scale, damage zones to each fault are planar, but at map scale both the MWF and CDF are corrugated parallel to slip.

Detailed macro- to microstructural studies of fault rocks associated with the MWF, sampled over 15 km down dip, provide insight into strain localization at initiation. Throughout its exposure, the MWF is characterized by a principal slip zone of chloritic cataclasite ≤2 m thick, with an asymmetric damage zone tens of meters thick localized in the hanging wall. At structurally shallow levels (T 200-250° C; 6-8 km paleodepth at initiation), the MWF cuts isotropic granitic rocks; cohesive cataclasites and semi-brittle phyllonites show subgrain development in quartz, with fractured plagioclase locally hosting deformation twins and kinking. Five kilometers down dip (T 300-350° C), crystal plastic deformation intensifies but is still overprinted by cataclasis; syntectonic dikes show crystal plastic deformation with minimal brittle overprint. Rare pseudotachylite is present within meters of the principal slip zone. At the structurally deepest exposures of the fault (T ≥ 400°C; 12-15 km paleodepth), the MWF juxtaposes Cretaceous granitoids against gneissic basement hosting dikes with a well-developed mylonitic lineation parallel to the extension direction. Here, mylonitic dikes and shear bands cutting the gneiss display microstructures indicative of subgrain rotation recrystallization, cut locally by zones of fine cataclasite hosting reworked mylonitic clasts.

The distribution of preserved increasingly 'hot' deformation mechanisms down dip (SW to NE) implies slip on the MWF was maintained at a dip of ≤20° throughout the seismogenic zone.