EXAMINING A NEW ROCK RECORD OF QUASI-STEADY CREEP IN THE COACHELLA SEGMENT OF THE SAN ANDREAS FAULT

The Coachella segment of the San Andreas fault (SAF) system is a major source of seismic hazard in southern California. Although this segment has produced large earthquakes in the past, geodetic and geomorphological observations indicate that slip has occurred exclusively as quasi-steady creep for at least the last 300 years. These observations are limited to only the most recent interseismic period, however, and the longer-term contribution of shallow creep to slip-rate estimates is unknown. This research is actively exploring the rock record of shallow creep along the main strand of the Coachella segment in the Mecca Hills. In the study area, several pristine gouge zones of the primary SAF are well exposed. These gouge zones are notable for two reasons: 1) they exhibit a decimeter wide scaly-fabric with clay-rich microslip surfaces indicative of distributed shear; and 2) they contain syntectonic gypsum veins with internal fabrics and surface lineations indicative of right-lateral shear. Optical microscopy of the gypsum veins reveals an antitaxial growth history with variable crack-seal and/or Taber-growth microstructures. Thus, vein formation is genetically linked to a series of (sub)micron-scale displacements, and the curvature of the observed crystals likely tracks the overall displacement path / strain dynamics operative during shallow creep along the SAF. We combine these microstructural constraints on strain dynamics from the vein samples with U-Th disequilibrium dating in addition to coefficient of friction experiments on representative samples of surrounding clay gouge. Collectively, these diverse data sets provide new insights into local creep mechanics, and their spatial and temporal scales of relevance in the Coachella segment.