PLIOCENE CHANGES IN KINEMATICS AND LONG-TERM GEOLOGIC SLIP RATES ALONG THE FURNACE CREEK – FISH LAKE VALLEY FAULT ZONE, CALIFORNIA AND NEVADA

The Furnace Creek-Fish Lake Valley fault zone (FC-FLVFZ) is the longest, active structure in the Eastern California Shear Zone. Cumulative right-lateral offset along the FC-FLVFZ is well established at 45-50 km by an offset Jurassic pluton found in northern Death Valley and southern Fish Lake Valley. Previous studies of long- and short-term slip rates suggest spatial and temporal variability along the FC-FLVFZ, in part due to uncertainties in the initiation of fault movement and in the reliability of offset markers. Using detailed geologic mapping and ⁴⁰Ar/³⁹Ar geochronology of Neogene volcanic rocks in and around a prominent restraining bend uplift along the FC-FLVFZ, we document that (a) the inception of faulting along the northern FC-FLVFZ occurred since 11.5 Ma; (b) that 22 km of right-lateral displacement accumulated in a system of distributed strands of the FC-FLVFZ prior to 4 Ma; (c) that a change in regional kinematics at 4 Ma marked the onset of contemporary transtensional deformation along the FC-FLVFZ; and (d) that 23-28 km of right-lateral displacement along the FC-FLVFZ has occurred since 4 Ma. From these observations we conclude that before the kinematic reorganization, the average long-term geologic slip rate from 11.5 Ma to 4 Ma was 2.9 mm/yr, and that from 4 Ma to the present the average long-term geologic slip rate markedly increased to 5.75 – 7mm/yr, which is in excellent agreement with the late Pleistocene slip-rate of 6.1 +1.3/-1.0 mm yr measured in the same location. These results indicate that the late Quaternary slip rate measured along the northern FC-FLVFZ remained constant since the kinematic reorganization in the mid-Pliocene, and prior to this, the long-term average geologic slip rate was less than half of the contemporary rate.