A TALE OF TWO SETTINGS: TWO CONTRASTING EPISODES OF SLIP BEHAVIOR OF THE LEFT-LATERAL GARLOCK FAULT FROM A ROCK-CONSTRAINED 4-D EVOLUTION MODEL, EASTERN CALIFORNIA

The Garlock fault is an active left-lateral fault with total net slip of 64 km. Published slip markers are from Paleozoic and Mesozoic features, much older than the Miocene initiation of slip on the Garlock fault making slip history constraints, fault behavior with time, and underlying tectonic forcing uncertain. In addition, the Garlock fault crosses the active Eastern California shear zone (ECSZ) and it bounds the western central Basin and Range (WCBR) from the Mojave Desert (MD). This study identifies numerous Cenozoic age offset markers along the central portion of the Garlock fault (CGF) and adjacent faults in the WCBR and MD. The combination of new ages and kinematic data allows a much more detailed understanding of CGF structure and slip history. The results show that the CGF is a fault zone up to 20 km wide recording two distinct episodes of deformation. Left-lateral slip of 8 km occurred on one CGF fault during slip initiation in the Late Miocene. The early fault did not localize sedimentation patterns, as south-derived arkosic sediment occurs in similar basins with similar paleocurrents to the north and south of the CGF. This slip was contemporaneous with normal faulting in the adjacent WCBR. The younger slip episode began in the Pliocene and is still active. Slip occurs on many structures within the CGF for a total sinistral offset of 56 km. Fault slip on numerous NW-trending dextral and dextral-oblique ECSZ faults in the WCBR and MD is contemporaneous with the CGF. The younger sedimentation system is very different in that local sources are dominant and sediments are deposited in restricted basins along the CGF, some of which have been subsequently uplifted. The CGF zone rotates ~20º clockwise during this slip episode. We interpret the dynamics of the two deformation episodes of the CGF as (1) left-lateral accommodation of WSW-directed extensional faulting in the WCBR and (2) reactivation of a coherent, but frictionally weak CGF during NW-directed dextral shear of the ECSZ. The slip and rotation of the CGF during the second episode appears to account for all of the dextral shear in the ECSZ, thus allowing an overall through-going dextral shear zone with the embedded CGF as an unbroken discontinuity. Lastly, we agree with previous workers that the Garlock fault may be localized by late Mesozoic lithospheric disruption.