HR: 1340h
AN: G53A-0877    [Abstracts]
TI: A Rapidly-Slipping Juvenile Strike-Slip Fault in the Eastern California Shear Zone: Lenwood Fault, Mojave Desert, California
AU: * Strane, M D
EM: strane@unc.edu
AF: University of North Carolina, CB#3515, Chapel Hill, NC 27599 United States
AU: Oskin, M
EM: oskin@email.unc.edu
AF: University of North Carolina, CB#3515, Chapel Hill, NC 27599 United States
AU: Perg, L
EM: lperg@umn.edu
AF: University of Minnesota, Pillsbury Hall, Minneapolis, MN 55455 United States
AB: One of the unresolved questions in understanding earthquake hazards is reconciling the strain rate measured by geodetic data with geologic slip rates and paleoseismic histories. The 65 km-long Lenwood fault is potentially high slip-rate member of a system of dextral faults that comprise the Eastern California shear zone (ECSZ) across the central Mojave Desert with potential to resolve discrepant geologic and geodetic strain rates in this region. The fault is host to several measurable offsets through the underlying Miocene section, Quaternary basalts, and Pleistocene age alluvial fan deposits. The most important of the offsets occurs in the Meridian Wash section of the fault, where an alluvial fan surface (Qtk) is offset. Two alternate reconstructions are proposed ranging with a minimum offset of 175m and a maximum offset of 350m. Cosmogenic dating of the alluvial fan has produced a preliminary date of 75 ± 25 ka, suggesting a preliminary minimum slip rate of 2.3 ^+0.8/_-0.5mm/yr and 4.7 ^+2.2/_-1.2 for the maximum slip rate, which represent 20% to 50% of the total strain rate across the ECSZ. Several other offset markers allow a view into the earlier neotectonic history of the Lenwood fault. Total offset is recorded by a thin volcaniclastic ash bed displaced 1.87 ± 0.09 km. The ash bed is found near the base of a sequence of Early Miocene interbedded, monolithologic landslide megabreccias that also match across the fault. An olivine-rich basalt uncomformably overlies the Miocene rocks and is offset up to 1.1 km. Because this offset is greater than half the total observed on the fault in the area, dating this will not only give a long-term slip rate but will also give insight into the inception of activity on the fault. Low total offset, despite high slip rate, and a discontinuous trace suggest that the northern Lenwood fault is a juvenile member of the ECSZ fault array. Active folds, a graben, and a zone of distributed faulting indicate ongoing linkage between distinct sections of the fault.
DE: 1130 Geomorphological geochronology
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 8107 Continental neotectonics (8002)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8175 Tectonics and landscape evolution
SC: Geodesy [G]
MN: Fall Meeting 2005