APATITE (U-TH)/HE THERMOCHRONOLOGY FROM THE COACHELLA FANGLOMERATE: PRELIMINARY CONSTRAINTS ON MIOCENE BASIN INVERSION AND EXHUMATION ALONG THE SOUTHERN SAN ANDREAS FAULT SYSTEM, CALIFORNIA (USA)

We present new detrital apatite (U-Th)/He thermochronology data from the Miocene (ca. 12–7 Ma) Coachella Fanglomerate near San Gorgonio Pass (California, USA) to constrain the timing of basin reheating, inversion, and exhumation during dextral transpression along the southern San Andreas Fault (SSAF) system. Despite extensive research conducted on the SSAF, a major plate boundary between the Pacific and North American plates, debate persists regarding the slip history and basin paleogeography along the Mission Creek-Coachella Valley (MC-CV) fault strand of the SSAF. The Coachella Fanglomerate is an excellent marker for fault slip reconstructions because cross-fault correlations from prior studies indicate the deposit is offset from its source by ~160–200 km. Dextral faulting along the MC-CV and other structures has resulted in northwestward translation of the Coachella Fanglomerate within the Pacific plate to its present location in the complex transpressive zone in the San Gorgonio Pass.

We collected detrital apatite (U-Th)/He thermochronology data from sandstone and a ~160 Ma quartz monzonite clast contained in the lower member of the Coachella Fanglomerate in Whitewater Preserve. Specific goals are to 1) evaluate post-depositional burial heating, and 2) constrain timing of basin inversion, deformation, and inferred exhumation during dextral displacement along the MC-CV. We explore the hypothesis that Coachella Fanglomerate basin inversion and exhumation took place due to transpressive deformation during Pliocene development of the San Gorgonio Pass structural knot. Preliminary apatite He dates from sandstone 18JGWW01 and clast WWQMC1 range from ~4 to 28 Ma. The clast yields a more restricted range of dates ~18–28 Ma. Observed He date correlations with grain size and effective uranium concentration suggest that these factors are important controls on intrasample dispersion. Some of the detrital apatite He dates are younger than depositional age, pointing to partial He loss during burial reheating. Thermal history modeling of the clast requires low-temperature (~30–50 °C) reheating prior to Pliocene cooling. Further data collection and modeling will resolve the timing and magnitude of basin reheating and cooling as they relate to structural inversion along the SSAF system.