Microstructural Character and Strain Localization at Initiation of a Low-Angle Normal Fault in Crystalline Basement (Chemehuevi Mountains, SE California)

Abstract:

Exposures of the Miocene Chemehuevi detachment (CDF) system provide a natural laboratory to study the initiation of low-angle normal faults (LANF) 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 low-angle normal faults; the CDF preferentially localized ≥ 18 km of NE directed slip rendering the deepest fault, the Mohave Wash Fault (MWF), inactive after 1-2 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- and microstructural studies of the MWF, sampled over 15 km down dip, provide insight into strain localization at initiation. At outcrop scale, the MWF is defined by a damage zone 10s of meters thick of fractured host rock cut by anastomosing principal slip zones of cohesive cataclasite (≤2 m thick), locally hosting chlorite, epidote and quartz. At structurally shallow levels (T 200-250° C; 6-8 km paleodepth at initiation), the MWF cuts isotropic granitic rocks, and exhibits dominantly cataclastic deformation overprinting localized crystal plasticity. Five km down dip (T 300-350° C), cataclasis remains the primary deformation mechanism; syntectonic dikes show plastic deformation with no brittle overprint. Rare pseudotachylite is present within meters of the principle slip zone. At the structurally deepest exposures of the MWF (T ≥ 400°C; 12-15 km paleodepth), gneissic basement cut by syntectonic dikes host a well-developed mylonitic lineation parallel to the extension direction, both reworked by cataclasis. Oxygen isotope data collected from fault rocks hosting quartz-epidote pairs indicate early infiltration of surface-derived fluids. Calculated oxygen isotope temperatures from the fault rocks and footwall are consistently 50-200° C higher than the ambient footwall temperature at MWF initiation. The elevated temperatures are interpreted to reflect infiltration of fluids through the fractured hanging wall followed by heating, and subsequent migration up the LANF due to enhanced permeability and porosity in the damage zone, likely driven by syntectonic dike emplacement.