Structural, Geochemical, and Thermal Evolution of the Southen San Andreas and Parallel Subsidiary Faults in the Mecca Hills, Southern California

Abstract:

The Mecca Hills, Southern California, is a 30 km-long, 8 km-wide north-plunging anticlinorium related to transpression and dextral/dextral normal faults along the southern San Andreas Fault (SAF). Although an iconic area for studying transpressional deformation and the Late Cenozoic sedimentary record, the long-term history of faulting, significance and kinematics of the subsidiary faults, and relationship between these faults and the main trace of the SAF remain unclear. We examine the petrologic, kinematic, and timing relationships between 4 subsidiary faults and related damage zones that parallel the SAF to evaluate relationships with the SAF and the Eastern California Shear Zone.

At least 6 major faults cut the Mesozoic to Late Tertiary crystalline and sedimentary rocks in the Mecca Hills, including the SAF. Hematite- and clay-coated fracture and slip surfaces are common in damage zones of the subsidiary faults. Slip surface orientation data of hematite-coated surfaces in the Painted Canyon Fault damage zone cluster at 110°, 65° SW and at 196°, 90° W. Similar surfaces in the Platform Fault damage zone cluster at 049°, 69 SE° and 003°, 83° E. Clay-coated slip surfaces in the Hidden Springs Fault damage zone cluster at 195°, 53° W and 196°, 11° W. Multiple slip vector orientations are observed on a single fault surface, consistent with oblique and dip-slip motion on faults in the Mecca Hills. Iridescent hematite and smooth clay surfaces suggest frictional heating on these surfaces, possibly from seismic slip. Preliminary scanning electron microscopy data reveal thin (10s-100s of μm), brecciated hematite slip surfaces. The specular hematite appears originally syn-tectonic and subsequently reworked with host rock and comminuted in multiple slip events. We apply hematite and apatite (U-Th)/He dating from the fault surface and host rock, respectively, to constrain fault thermal evolution and evaluate hematite (U-Th)/He dates as recording hematite formation, fault slip, or erosional cooling in the Mecca Hills. Collectively, these data will provide insight into the geochemical, structural, and temporal evolution of these structures.