LATE HOLOCENE MULTI-FAULT RUPTURES ALONG THE PINTO MOUNTAIN AND MESQUITE LAKE FAULTS, EASTERN CALIFORNIA SHEAR ZONE, SOUTHERN MOJAVE DESERT, CALIFORNIA

Geomorphic and geologic mapping, in combination with geochronologic investigations, reveal evidence for late Holocene multi-fault surface ruptures on two intersecting faults, the Pinto Mountain (PMF) and Mesquite Lake (MLF) Faults within the Eastern California Shear Zone (ECSZ). Our data suggest the surface ruptures were nearly concurrent and provide kinematic and temporal displacement histories. Our mapping for this study is focused along the west-central PMF where it transects the Sunfair and Twentynine Palms 7.5’ quadrangles. The PMF is a sinistral east-trending fault that forms a domain boundary between east-striking sinistral faults to the south and primarily northwest-striking dextral faults to the north. From west to east, the major northwest-striking faults that intersect the PMF within the map area are the Copper Mountain Fault, the Calico-Hidalgo Fault Zone, and Mesquite Lake Fault. We used our detailed mapping in conjunction with remote sensing interpretation to develop a chronostratigraphic framework that we applied to evaluate the area’s deformation history. We identified multiple surface ruptures in early Holocene and Late Pleistocene deposits and to constrain the timing of deformation we employed luminescence age dating. Within the Sunfair quadrangle, deformed alluvial fans along the PMF yielded ages of 1,400 – 1,750 years, whereas an adjacent undeformed fan yielded an age of 910 years. To the east within the Twentynine Palms quadrangle, deformed deposits along the PMF yielded ages of 5,140 to 1,410 years with an overlying unfaulted deposit with an age of 1,270 years. Even farther east along the PMF near its intersection with the MLF, three late Holocene alluvial fan deposits between 1,400 and 3,130 years are sinistrally offset by 3-4 m along the PMF. Along the MLF, a sheared eolian dune deposit yielded an age of 1,270 years, coincidentally the same as the age of the unfaulted deposit along the PMF. To the SE along the MLF, several alluvial fans with age ranges of ~1, 970 years and ~2,280 years are right laterally offset by 3-4 m and are adjacent to an unfaulted fan with a 900-year-old age. These data bracket the most recent multi-fault rupture along the PMF and MLF to ~1,000 to 1,500 years and are consistent with known multi-fault ruptures elsewhere in the southern ECSZ from previous investigations.