HOLOCENE FAULT SLIP RATES INDICATE LOCALIZATION OF EASTERN CALIFORNIA SHEAR ZONE DEFORMATION BETWEEN RIDGECREST AND BARSTOW

Surface ruptures related to the July 2019 Ridgecrest earthquake sequence occurred on mostly unrecognized faults in Indian Wells Valley and re-emphasize the importance of detailed Quaternary fault mapping in eastern California. The dextral Lenwood–Lockhart fault (LLF) and Mt. General fault (MGF) are among five primary structures in the eastern California shear zone, including the Helendale, Harper Lake, and Blackwater faults, that extend south of Indian Wells Valley and the Garlock fault. The LLF and MGF underly the cities of Lenwood and Barstow and pose a hazard to urban populations and critical lifelines to southern California, including the natural gas compressor station at the locus of groundwater contamination in Hinkley Valley. Earlier mapping depicts single traces of the LLF and MGF concealed by Holocene terraces and deposits of the Mojave River. We use high-resolution Structure-from-Motion topography and gravity data in combination with field observations to identify multiple surface traces and buried faults that locally crosscut Holocene landforms and increase mapped fault zone widths by up to 2 km. Geomorphic offsets along a Holocene-active trace of the 142-km-long LLF indicate dextral displacement of 4–6 m. A feldspar infrared stimulated luminescence (IRSL) age from a faulted fluvial terrace and levee implies at least one earthquake after 3540 ± 880 ka and a Holocene slip rate for the LLF of ~1–2 mm/yr. Holocene-active traces of the 19-km-long MGF are marked by southwest-facing scarps and dextral offsets of 4–5 m on alluvial fans, with down-on-the-southwest vertical offset of ~0.3 m. Summing dextral displacements across subparallel fault strands yields a maximum of 7–8 m. A feldspar IRSL age on fan alluvium that is faulted against younger inset alluvial fans indicates at least one earthquake after 11,380 ± 1700 ka. These data yield a Holocene slip rate for the MGF at 0.3–0.6 mm/yr, possibly ranging up to 1 mm/yr. Paleoseismic trenches at its southern terminus, proximal to the LLF and Lenwood anticline, exposed faults that are buried by early Holocene alluvial fan deposits. These observations imply a net Holocene dextral slip rate for faults in Hinkley Valley of ~1–3 mm/yr that is higher than published slip rates along-strike to the south (0.5–1 mm/yr) possibly due to slip transfer from faults to the east.