(U-Th)/He thermochronometry reveals Pleistocene punctuated deformation and synkinematic hematite mineralization in the Mecca Hills, southernmost San Andreas Fault zone

Amy C. Moser, James P. Evans, Alexis K. Ault, Susanne U. Janecke, Kelly K. Bradbury, & Peter Reiners

Published September 22, 2017, SCEC Contribution #7223

Phases of past, punctuated deformation and synkinematic mineralization in strike-slip fault systems are challenging to resolve in the rock record. The Mecca Hills lie adjacent to the southernmost San Andreas Fault where faults cut Plio-Pleistocene sedimentary rocks and underlying crystalline basement, producing active deformation and exhumation. We document the spatiotemporal patterns of San Andreas Fault-related deformation as recorded in crystalline basement rocks of the Mecca Hills using integrated fault microstructural data, geochemical observations, and hematite (n = 24) and apatite (n = 44) (U-Th)/He (hematite He and apatite He, respectively) thermochronometry data. Reproducible mean hematite He dates from minor hematite-coated fault surfaces in the Painted Canyon Fault damage zone range from ~0.7-0.4 Ma and are younger than ~1.2 Ma apatite He dates from adjacent comparatively unaltered crystalline basement host rock. These data reveal concomitant Pleistocene pulses of fault slip, fluid flow, and syn-kinematic hematite mineralization. Hematite textures, crystal morphology, and hematite He data patterns imply deformation on these fault surfaces was intermittently accommodated by cyclic crack-seal and creep. Apatite He data from crystalline basement rocks define distinct date-eU patterns and suggest differential cooling and exhumation across discrete fault blocks in the Mecca Hills. Uniform, ~1.2 Ma apatite He dates regardless of eU values are exclusively located between the Painted Canyon and Platform faults. Outside of this fault block, samples yield individual apatite He dates from ~30-1 Ma that define a positive apatite He date-eU correlation. These patterns document spatially and temporally localized fault block exhumation, including focused exhumation away from the main trace of the San Andreas fault ~ 1.2 Ma. Low-temperature thermochronometry of fault-related rocks thus provides an unprecedented window into distinct phases of punctuated deformation, deformation processes in hematite-coated faults, and the million-year-scale record of San Andreas Fault-related deformation in the Mecca Hills.

Key Words
Fault geochemistry, textures, thermal history

Citation
Moser, A. C., Evans, J. P., Ault, A. K., Janecke, S. U., Bradbury, K. K., & Reiners, P. (2017). (U-Th)/He thermochronometry reveals Pleistocene punctuated deformation and synkinematic hematite mineralization in the Mecca Hills, southernmost San Andreas Fault zone. Earth and Planetary Science Letters, 476, 87-99. https://doi.org/10.1016/j.epsl.2017.07.039

Related Projects & Working Groups
Fault and Rupture Mechanics