DEVELOPING ALLUVIAL FAN CHRONOLOGIES TO DETERMINE RATES AND PATTERNS OF DEFORMATION IN THE EASTERN CALIFORNIA SHEAR ZONE-WALKER LANE

Accurate and precise alluvial fan chronologies are critical to assessing rates and patterns of deformation along plate boundary fault systems. Fan deposits are commonly used to determine fault slip rates due to their pervasiveness in the landscape, ability to preserve deformation, and because they often contain dateable material. Recently, the transtensional eastern California shear zone (ECSZ) has received considerable attention with respect to the role it plays in accommodating strain along the Pacific-North America (PAC-NAM) plate boundary. The best-studied structure within the ECSZ is the NNW-SSE striking ~300-km-long Death Valley-Fish Lake Valley (DVFLV) fault system, which now has numerous sites with late Pleistocene to mid-Holocene geochronologically-determined slip rates. Key to this are the many alluvial fans that have been dated and placed into a consistent late Quaternary stratigraphic framework to determine the temporal and spatial distribution of strain along this part of the PAC-NAM plate boundary. In some cases, the extensive dating of fans in this region highlights problems associated with cosmogenic nuclide (TCN) techniques and emphasizes the need for multiple, different methods to accurately determine landform ages. Combining TCN and luminescence (OSL) dates from offset alluvial fans with fault displacement measurements reveals that the right-lateral slip rate along the DVFLV fault is fastest (6 mm/yr) along the central section and decreases to 2 - 2.5 mm/yr at the southern and northern ends of the structure. Summation of the rates of all major faults in the ECSZ at the latitude of northern Death Valley shows that the geologic rate is indistinguishable from the geodetic rate. To the north and south slip is likely transferred onto other structures (e.g., the down-to-the-NW Lone Mountain fault where rates of deformation from TCN-dated fans appear to be increasing over late Pleistocene to recent timescales), reflecting distributed deformation rather than transient strain accumulation in these regions. This complex pattern of strain accommodation may result from the evolution of this part of the PAC-NAM plate boundary towards a structurally simpler zone of dextral shear that utilizes well-established right-lateral faults, which are linked by distributed zones of extension.