TRANSPRESSION WITHIN THE EASTERN CALIFORNIA SHEAR ZONE AND LOW-TEMPERATURE THERMOCHRONOMETRY OF THE AVAWATZ MOUNTAINS, SOUTHERN CALIFORNIA, U.S.A

The Avawatz Mountains of Southeastern California, are located south of Death Valley within the Eastern California Shear Zone. Uplift of the Avawatz Mountains results from active transpression at the intersection of the left-lateral Garlock Fault and the right-lateral Southern Death Valley Fault. Quaternary fault scarps that thrust bedrock onto alluvium illustrate recent activity of the range-bounding Mule Spring Fault. Apatite (U-Th)/He ages from the Avawatz Mountains range from 4 Ma at 1010 m elevation to 7.1 Ma at the crest of the range (1850 m). This age-elevation distribution records an exhumation rate of 0.27 mm/yr for the last four million years in the Avawatz Mountains. Background ages of circa 7 Ma occur on peaks throughout the sampling transect as well as at the range front. These background ages probably represent the pre-orogenic age of the range block. Background ages at the range-front are probably due to thermal effects as the hanging wall overrides cooler rocks in the footwall. Southwestward tilting and folding of the Miocene Avawatz formation record shortening within the Avawatz Mountains. The distribution of helium ages across the Avawatz Mountains suggests an anticlinal structure within the deformed hanging-wall. The thermochronologic data in this study suggest that transpressive deformation began no earlier than 7 Ma in the Avawatz Mountains. Slip along the Garlock Fault initiated between 12 Ma and 14 Ma, which suggests that the Avawatz Mountains are not related to the long-term tectonics of the Death Valley region. Initiation of slip within the right-lateral Eastern California Shear Zone (ECSZ) is not well constrained, and ages between 14 Ma and 2 Ma have been cited. In previous models, the Avawatz Mountains represent the leading edge of the Mojave block as it moves eastward between the San Andreas and Garlock faults. We hypothesize that uplift of the Avawatz Mountains is related to initiation and continued slip in the ECSZ, and that the transpressive nature of the Garlock-Death Valley Fault Zone intersection is related to a strike-slip regime younger than the Garlock Fault where the Garlock merely acts as a passive rupture barrier within the more active ECSZ.