A GEOLOGIC APPROACH TO RESOLVING DISCREPANCIES BETWEEN GEOLOGIC AND GEODETIC SLIP RATES IN THE EASTERN CALIFORNIA SHEAR ZONE

The Eastern California shear zone (ECSZ), a plate boundary dextral deformation zone, has a 2 to 8 times faster fault slip rates inverted from GPS data compared to those from geologic data. The discrepancy may be due to temporal changes with the linked San Andreas fault system (Dolan et al., 2007); unaccounted effects of viscoelastic relaxation (Chuang and Johnson, 2011); or missing or erroneous geologic data.

This study investigates the geologic component of the slip rate discrepancy. We examine fault slip constraints of the Blackwater fault and reinterpret slip amounts of two nearby faults to investigate long-term (10⁶ years) slip rates. We interpret two additional slip constraints to add to the four published geologic slip constraints along the Blackwater fault (Oskin and Iriondo, 2004; Andrew et al., 2014). We also reinterpret slip constraints on the Harper Lake-Gravel Hills and Mt General faults (Bartley et al. (1992) using geologic maps of (Fletcher and Martin, 1998; Fletcher, 1999). The new data show that a consistent 2 km of right-lateral slip occurred on the Blackwater fault along 50 km of strike length. There is no apparent gradual loss of slip as the fault approaches the sinistral Garlock fault as inferred by Oskin and Iriondo (2004). We redefine post-early Miocene slip on the Mt General and Gravel Hills-Harper Lake faults to be 2.8 ±0.2 km and 6.4 ±0.2 km, respectively.

We combine these new slip amounts to published offsets for faults to the south of our study area to examine the behavior of dextral slip in the western portion of the ECSZ. We find that this western portion of the ECSZ accommodates ~18 km of dextral slip. This amount is consistent from south to north, within error. The age of initiation of this dextral faulting in the ECSZ is estimated to be at or younger than 3.8 Ma. The initiation age is based on the age of offset basalts along the Blackwater fault (Oskin and Iriondo, 2004), and it yields a minimum long-term geologic slip rate of 4.7 mm/yr for the western ECSZ. This rate is more compatible, but still smaller than the 8-12 mm/yr interpreted by Meade and Hager (2005) and Savage and Simpson (2013) for the western portion of the ECSZ. These results may indicate that on a province-wide scale there may not be a large discrepancy between geologic and geodetically modeled rates.