COMPLEX QUATERNARY DEFORMATION AMONG INTERSECTING SETS OF STRIKE-SLIP FAULTS NEAR TWENTYNINE PALMS, SOUTHERN CALIFORNIA

We present geomorphic, structural, stratigraphic, and geophysical data indicating complex time-variant strain developed in a region of intersecting right- and left-lateral faults in the Eastern California Shear Zone (ECSZ) near Twentynine Palms, CA. Here two NW-trending, right-lateral faults of the ECSZ in the central Mojave Desert, i.e., the Mesquite Lake (MLF) fault on the east and the “airstrip” fault directly to the W, intersect the EW-trending sinistral Pinto Mountain fault (PMF) of the Eastern Transverse Range (ETR) fault domain. Major strain patterns include: (a) mismatches between fault traces and gravity-defined subsurface basins that suggest changes in fault organization and deformational style (from transtension to transpression); (b) transpressive deformation (linear ridges, pop-ups, and tilted or folded sediments) along the eastern PMF and SE part of the MLF; (c) contractional deformation and uplift localized at intersections of the dextral faults with the PMF; (d) significant off-fault deformation (asymmetric uplift and tilting from the SW to NE block margins, widespread fault-related folding) of the intervening blocks between the dextral faults north of the PMF; and (e) large arch-like regional uplift (>600 m) rising southward from the Mojave lowland across the PMF and the entire northern range front of the ETR. New stratigraphic, structural, geomorphic and geochronologic data suggest that much of this deformation developed after mid-Quaternary time. We propose the following geologic history: (1) deposition of fine-grained muddy and sandy sediment (containing a .layer of Bishop Ash (~760 ka age), identified by paleomagnetism and tephrochronology) within a regional depositional system as yet not understood in terms of ECSZ history, but discordant with younger mid-late Quaternary deposits; (2) deposition of gravelly and sandy sediment that spread eastward from the rising San Bernardino Mountains (SBM), probably post 760 ka; (3) disruption of the SBM depositional system by surface deformation and uplift associated with the ECSZ and its intersection with the sinistral faults of the ETR. This latter event clearly is post 760 ka, and probably younger than 300-200 ka. Deformation has spanned late Pleistocene to Holocene time, as indicated by deformed deposits dated using OSL (80 ka to 1 ka).