TIMING AND RATES OF HOLOCENE NORMAL FAULTING ALONG THE BLACK MOUNTAINS FAULT ZONE, DEATH VALLEY, USA

Alluvial fans displaced by normal faults of the Black Mountains fault zone (BMFZ) at Badwater and Mormon Point in Death Valley were mapped, surveyed and dated using optically stimulated luminescence (OSL) and 10Be terrestrial cosmogenic nuclide (TCN) methods. Applying TCN methods to Holocene geomorphic surfaces in Death Valley is challenging because sediment flux is slow and complex. However, OSL dating produces consistent surface ages, yielding ages for a regionally recognized surface (Qg3a) of 4.5±1.2 ka at Badwater and 7.0±1.0 ka at Morman Point. An OSL age of 4.1±0.4 ka on fault-scarp colluvium at Mormon Point brackets the last earthquake at 4-7 ka. Comparison to other studies indicates that the BMFZ has not ruptured along its entire length during the most recent events. Holocene faults offsetting Qg3a yield horizontal slip rates directed towards 323^o of 0.8+0.3/-0.2 mm/a and 1.0±0.2 mm/a for Badwater and Mormon Point, respectively. These slip rates are slower than the ~2 mm/yr dextral slip-rate of the southern end of northern Death Valley fault zone (DVFZ) and are half as fast as NNW-oriented horizontal rates documented for Panamint Valley fault zone (PVFZ). This indicates that additional strain is transferred southwestward from DVFZ and BMFZ onto the oblique-normal dextral faults of the PVFZ; consistent with published geodetic modeling showing that current opening rates of central Death Valley along the BMFZ are about three times slower than for Panamint Valley. This suggests that less than half of the geodetically determined ~ 9-12 mm/yr of right-lateral shear across the region at the latitude of central Death Valley is accommodated by slip on well-defined faults and that distributed deformational processes take up the remainder of this slip transferred between the major faults north of the Garlock fault.