SPATIOTEMPORAL EVOLUTION OF SAN ANDREAS FAULT-RELATED DEFORMATION IN THE MECCA HILLS, SOUTHERN CALIFORNIA, FROM INTEGRATED FAULT ZONE CHARACTERIZATION AND LOW-TEMPERATURE THERMOCHRONOLOGY

Quaternary deformation and exhumation in the Mecca Hills, southern California, results from transpression on the adjacent San Andreas Fault and faulting on a suite of sub-parallel subsidiary faults. The sedimentary record of the Mecca Hills fault zone development spans ~3-4 Ma, but the spatiotemporal evolution of crystalline basement-hosted fault zones is unclear. We integrate field, microstructural, and geochemical studies with low-temperature hematite (HeHe), apatite (AHe), and zircon (ZHe) (U-Th)/He thermochronology to characterize the pattern of deformation on these structures.

Major dextral faults of the Mecca Hills fault zone cut crystalline and sedimentary bedrock on multiple strands with well-developed clay gouge zones. Faults exhibit a bimodal distribution of strike-slip and dip-slip slip vectors. NE-SW striking strike-slip faults are at high angles to the main traces of the major dextral faults, whereas dip-slip faults strike E-W. These data imply separate structures accommodate strike-slip and dip-slip motion and transfer slip. Results from X-ray diffraction and X-ray fluorescence of fault damage zones are consistent with hydrothermal and epithermal mineralization associated with fault slip. Mean HeHe dates from hematite-coated slip surfaces in major dextral crystalline basement fault damage zones are younger than crystalline host rock mean AHe dates. Painted Canyon Fault (PCF) samples yield mean HeHe and AHe dates of 0.4 ± 0.1 Ma and 1.2 ± 0.3 Ma, respectively, and a similar data pattern exists at the Platform Fault, east of the PCF. AHe data imply basement rocks cooled within ~1 km of the modern surface by ~1 Ma. Scaly hematite morphology, hematite-calcite crack-seal textures, absence of independent textural evidence of elevated fault surface temperatures, and HeHe-AHe date patterns collectively suggest that HeHe dates may reflect syn-kinematic hematite mineralization along subsidiary faults ~0.4 Ma. HeHe dating of fault zone material thus suggests a late Pleistocene phase of concomitant fluid flow, mineralization, and brittle faulting in the Mecca Hills. Additional AHe, ZHe, and HeHe data will illuminate the spatiotemporal pattern of cooling, unroofing, and deformation across and along strike of the major dextral structures as the San Andreas Fault zone in the Mecca Hills evolved.