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Distributed fault slip in the Eastern California shear zone: adding a piece to the puzzle

Elizabeth K. Haddon, David M. Miller, Victoria E. Langenheim, Tanzhuo Liu, Elmira Wan, & Laura C. Walkup

Published August 15, 2018, SCEC Contribution #8542, 2018 SCEC Annual Meeting Poster #237

The Lockhart (LF) and Mt. General (MGF) faults are two primary structures accommodating dextral shear across the northern portion of the Eastern California shear zone. Early mapping infers simple fault traces branching NW of the Lenwood fault and anticline, which underlies the city of Barstow. We leverage Structure-from-Motion topography (18–23 cm grid), gravity data, field observations, and geochronology to map geometrically complex fault traces along the margins of southern Hinkley Valley and reconstruct late Pleistocene and Holocene fault slip. Three distinct traces of the MGF deform the fan piedmont on the linear SW flank of Mt. General. Scarps on Holocene alluvial fans are SW-facing and up to ~1.5 m in height, with larger magnitude, down-on-the-SW displacement of Pleistocene surfaces and Tertiary bedrock. Apparent dextral offsets of bars, channels, and levees in Holocene deposits average ~4 m with corresponding vertical offset of ~0.6 m. Regional dating indicates deposition of Holocene alluvium at ~8–13 ka and suggests a Holocene slip rate on the MGF of ~0.3–0.4 mm/yr. Summing cumulative dextral offsets for subparallel fault traces cutting bedrock ridges and late Pleistocene surfaces (~160–170 ka) implies at least ~21–44 m of net dextral slip. Our longer-term dextral slip rate estimate on the MGF is ~0.1–0.3 mm/yr, which lags relative to the Holocene rate. Anastomosing traces of the ~1–2 km-wide LF zone deform a relict braid plain of middle Pleistocene sand and gravel deposited by the ancestral Mojave River between ~200–550 ka, based on tephra identified as 631 ka Lava Creek B ash and varnish microstratigraphy for overlying rockfall. Apparent offsets of inverted bar and channel deposits with subtle ridge-and-swale topography show distributed right-lateral and right-reverse oblique slip with an overall down-on-the-NE component. Cumulatively, fault strands yield ~110–180 m of dextral offset and suggest a long-term dextral slip rate of ~0.2–0.9 mm/yr. Dextral offset between ~8 and 10 m of a Holocene fluvial terrace riser and levee, related to the modern incised Mojave River and likely abandoned by ~8–13 ka, yields a similarly fast Holocene slip rate at ~0.6–1.3 mm/yr. Our preferred late Pleistocene dextral slip budget for the combined MGF and LF (~0.8 mm/yr) compares well with reported rates for the Lenwood fault (~0.8–1 mm/yr) and favors structural connectivity among spatially distributed faults of the MGF and Lenwood–Lockhart fault systems.

Key Words
ECSZ, fault slip rates, distributed shear, Mojave desert

Haddon, E. K., Miller, D. M., Langenheim, V. E., Liu, T., Wan, E., & Walkup, L. C. (2018, 08). Distributed fault slip in the Eastern California shear zone: adding a piece to the puzzle. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology