Late Pleistocene Slip Rate along the Panamint Valley Fault System, Eastern California Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/sb397b54m

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  • The Eastern California Shear Zone (ECSZ) is a broad zone of dextral shear inboard of the North American - Pacific plate boundary. Despite decades of study, the significance of a mismatch between geodetic velocities and geologic fault slip rates across the ECSZ remains incompletely understood. Geodetically determined interseismic strain across the ECSZ has been determined to be 9-11 mm/yr, but across much of the ECSZ, the geologic rates sum to only about 50% that rate. At the latitude of 36°-37°N, geologic slip rates have been constrained along the length of Owens Valley and Death Valley fault systems; however, the neighboring Panamint Valley fault system (PVFS) has limited geologic slip data. Existing estimates of Late Pleistocene - Holocene slip rates along the PVFS are relatively slow (~2-3 mm/yr) and come from a single site along the southernmost fault segment. In contrast, geodetic measurements through GPS and InSAR suggest that the northernmost segment of the PVFS, the Hunter Mountain fault, may experience slip rates as high as 6 mm/yr. Offset Holocene geomorphic markers along the Hunter Mountain fault have been proposed to record 3-4 mm/yr of slip, albeit with a qualitative age constraint; however, slip rates along the central PVFS are completely lacking. In this thesis, I present results from an investigation of displaced alluvial fan surfaces along the central PVFS, at the mouth of Jail Canyon. This site is characterized by a relict channel system associated with a large alluvial fan complex: The channel is displaced dextrally across a ~500 m wide zone of distributed deformation. Restoration of the channel with the aid of LiDAR digital topography suggests 118 m of net slip, oriented 303°, across the PVFS. The degree of soil development and the degraded morphology of channel walls place bounds on the abandonment ages of displaced surfaces. Soil ages are based on a set of soils in twelve alluvial deposits of known age in Panamint Valley and the Mojave Desert (Hoffman, 2009). Profile development indices (PDI) of these soils suggest a strong correlation of soil development with age; interpolation of this empirical relationship to soils at Jail Canyon suggests that soils in the displaced surfaces are likely 11-22 ka (and up to 29 ka within 95% confidence). To model the age of the offset channel walls at Jail Canyon, a range of diffusivity constants, which controls the rate of change in elevation, was locally calibrated for Panamint Valley. Calibration with fault scarps and wave-cut scarps of known age suggests a best-estimate diffusivity range of 1.2 - 2.5 m²/ka. Applying this range to the degradation of channel risers at Jail Canyon results in a best-estimate age of 9 - 19 ka, consistent with soil ages. Using an age range of 20-30 ka, bounded by the conservative end of age estimate from each method, the minimum slip rate along this segment of the PVFS is 4-6 mm/yr. This rate is faster than any existing geologic slip rate along the PVFS and confirms previous suggestions that (1) the PVFS carries a large fraction of dextral shear in the ECSZ north of the Garlock Fault, (2) slip along the PVFS has been accelerating in the past 3-4 Ma, and (3) all three major fault systems north of the Garlock decrease in slip southward despite the steady geodetic velocities.
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