DEFORMATION HISTORY AT THE EASTERN END OF THE GARLOCK FAULT, CA FROM AVAWATZ MOUNTAINS LOW-TEMPERATURE THERMOCHRONOLOGY (Invited Presentation)

The sinistral Garlock Fault Zone (GFZ) stretches east-west over 250 km before terminating within the Avawatz Mountains at its eastern end. A solid body of geological information exists along the central fault segment, but to the east, the timing of fault onset and its relationship to the intersecting Southern Death Valley Fault Zone (SDVFZ) and Avawatz topography require further investigation. We assess deformation at the eastern tip of the GFZ using a suite of apatite (U-Th)/He and fission tracks samples. Age results span 2.8 to 72.2 Ma with a north-south spatial pattern. In the northern Avawatz, near to the dextral SDVFZ, we find similarly young (late Miocene to Pliocene) ages on both sides of the Mule Springs Fault (MSF), a previously mapped structure thought to build the range. Here the MSF does not demonstrate significant vertical exhumation and is likely a predominantly strike-slip feature. We consider the MSF a continuation of the left-lateral Garlock Fault Zone and suggest that mountain building may instead relate to previously proposed blind thrusting at the edge of the bajada as well as to a system of mapped folds and thrusts along the mountain base. We attribute this complex deformation field to transpression along an east-bending SDVFZ starting ~4 Ma. In contrast, the southern MSF wraps around to an almost N-S strike, eventually forming a steep-relief range-front. Samples collected across the fault and into the high peaks of the southern Avawatz Mountains show younger ages (<16 Ma) on the west side of the fault as compared to older ages (43 to 72 Ma) on the east side. This pattern of ages and geomorphology suggest thrust-sense motion along a west dipping Mule-Springs Fault. Here we attribute Avawatz Mountains topography to this thrust motion, which ultimately accommodates the termination of the Garlock Fault. An age-elevation plot of cooling ages suggests rapid exhumation started between 15 to 7 Ma. We relate this to the onset of mountain building and by proxy, the initiation of Garlock Fault motion. Our results agree generally with a 12 – 10 Ma onset of faulting proposed along the western and central fault segments and suggest an approximately synchronous initiation of the Garlock Fault along its length.