WHERE ARE THE HIGH SLIP-RATE FAULTS OF THE EASTERN CALIFORNIA SHEAR ZONE THROUGH THE MOJAVE DESERT?

The Eastern California Shear Zone (ECSZ) presents an intriguing example of a plate boundary fault system where known geologic fault-slip rates do not appear to add up to the geodetic rate of strain accumulation. Geodetic observations support that 10 to 14 mm/yr, or upwards of 20% of the total Pacific-North America plate motion passes into the Mojave Desert and drives the ECSZ. Geologic reconstructions of the Mojave Desert and southeastern California require 40 to 125 km of dextral shear, or about 10 to 20% of total Pacific-North America plate motion, since 10 Ma. Although the geodesy and geology yield similar strain rates, critical information to link these time scales is often missing: Quaternary slip rates on active faults. Paleoseismic investigations of the southern Mojave Desert suggest long earthquake recurrence intervals and low (sub mm/yr) fault slip rates. Sparse slip rate measurements from offset syntectonic markers also fail to yield fault-slip rates in the Mojave Desert that exceed 1 mm/yr. In order to reconcile the high rate of geodetic strain accumulation with geologic data requires either a single, previously unrecognized or under-appreciated fault to accommodate 5 to 7 mm/yr of slip, or that half of the strain in the Mojave Desert occurs through continuum shear by slip on numerous small faults or by block rotation. In the southern Mojave Desert, the Calico fault is the most likely high slip-rate candidate, whereas evidence for continuum deformation is insufficient to explain a high strain rate over time. From new detailed and reconnaissance mapping of the Harper Lake and Blackwater faults, the northwest Mojave desert probably does not accommodate more than 20% of the total geodetic shear strain rate. Rather, most of the active strain accumulation in the northern Mojave Desert must occur as clockwise block rotation of the Fort Irwin region. A zone of crustal shortening expressed from west of Barstow to the Calico Mountains transfers right-lateral faulting in the southern Mojave Desert northeastward into Fort Irwin. These preliminary results support that the pattern of westward migration of fault activity in the southern Mojave Desert does not appear in the northern Mojave Desert and is probably not linked to westward migration of fault activity in the Death Valley, Panamint, and Owens Valley fault systems.