Strain partitioning and timing of strike-slip faulting in the central Mojave Desert, CA, indicated by newly dated Pliocene and lower Pleistocene deposits

Abstract:

New insights into ages for sinistral-slip faults and how they operate in simple slip mode even after 60 degrees of vertical axis rotation are revealed by study of coarse gravels in the area within ~40 km of Barstow, CA. Using 31 new U-Pb dates of opaline soil precipitates and crosscutting veins in faults, we demonstrate that many deposits are in the 1-4 Ma age range. Clast provenance and transport direction from sedimentary structures indicate that in the Pliocene a line of uplifts lay from western Fort Irwin to the Daggett Ridge area SE of Barstow, forming an irregular north-trending divide. East of the divide, deposition occurred in broad east-flowing (in modern coordinates) stream valleys coincident with sinistral faults. This relation is best demonstrated from the Manix fault northward into Fort Irwin, and is also suggested for the Cady fault. West of the divide, data are limited but consistent with the interpretation that north-and south-flowing streams met in a central, W- or WSW-flowing system in an axial valley near the modern Mojave River. Folded Pliocene deposits and the paleogeography indicate that strain was partitioned into sinistral fault slip and folds parallel to faults, with synclines forming valleys 7-10 km wide. Folds associated with the dextral faults west of the divide are much broader, with wavelengths of ~50 km and ~westerly trends. Pliocene uplifts that formed the divide have mostly persisted as topographic highs that lie roughly along the boundary between sinistral and dextral domains. The uplifts may reflect block interactions along the boundary including differential vertical-axis rotation of blocks in the sinistral domain. Partitioning of strain into folds and faults helps resolve the conundrum of why sinistral faults that rotated ~60 degrees out of Coulomb failure orientation, and have little resolved shear stress, persist as simple strike-slip faults. It may also explain why the central Mojave Desert is more mountainous than the western Mojave Desert, where there are few sinistral faults. U-Pb ages of fault-related opal demonstrate that strike-slip faulting in both domains began well before 4 Ma, preceding the uplift of the eastern Transverse Ranges.