APPLICATION OF A CALIBRATED SOIL CHRONOSEQUENCE TO THE STUDY OF ACTIVE FAULTS: SLIP RATES ALONG THE PANAMINT VALLEY FAULT SYSTEM, CALIFORNIA

Application of terrestrial cosmogenic nuclide (TCN) dating of alluvial fan surfaces and deposits reveals intriguing variations in both the spatial and temporal patterns of fault slip over timescales ranging from 10⁴ – 10⁵ yr. However, epistemic uncertainties associated with variable TCN inheritance and post-depositional modification of alluvial surfaces can severely limit the utility of TCN exposure ages to address these problems. Moreover, the expense of obtaining ages often limits their application to a single site along a given structure. Soil profile development indices (PDI) can be readily acquired, and calibration of soil development in select sites can provide a reliable chronologic framework for alluvial deposits across a region of similar climate despite variations in catchment lithology. In eastern California, we present a soil chronosequence that utilizes well-preserved and dated surfaces from Panamint Valley and the western Mojave Desert ranging in age from ~10³ - 10⁵ yr. These surfaces have been dated using a combination of ¹⁴C (wood and tufa), optically stimulated luminescence (OSL), and ¹⁰Be depth profiles. Morphologic descriptions of soil profiles at each site were combined using a PDI approach. These observations establish local stratigraphic relationships and allow evaluation of post-depositional modification of surfaces (erosion or deposition). In southern Panamint Valley, displacement of a debris-flow levee provides a precise estimate of dextral-oblique fault slip along the Panamint Valley fault zone. Exposure ages of boulders on the levees range from 20 – 40 ka. The degree of soil development and local cross-cutting relationships with dated Late Pleistocene lacustrine shorelines indicate that this debris flow is only ~9 – 12 ka. Slip rates along the southern segment of the fault system are thus constrained to be 2.4 ± 0.7 m/ka. The cosmogenic nuclide concentrations in these boulders are interpreted to reflect significant variability in inheritance accumulated in the high-relief but arid catchment. Application of the soil chronosequence to younger alluvial deposits provides constraints on the number and timing of Late Holocene surface ruptures along this fault, demonstrating the utility of establishing calibrated soil stratigraphic frameworks in the study of active fault systems.