Persistent slip rate discrepancies in the eastern California (USA) shear zone: Comment

The slip rate discrepancy on the Garlock fault can be explained by a combination of left slip on the fault and clockwise rotation. This is kinematically equivalent to dextral shear, and renders the Garlock fault geodetically invisible relative to the surrounding eastern California shear zone. The paper by Evans et al. (2016) is a perceptive and innovative analysis of the discrepancies between geodetic and geologic estimates of slip rates on the faults in the eastern California shear zone (ECSZ), and will inform future discussions of this problematic issue. This Comment concerns the most egregious example of the problem, which is the Garlock fault (GF), as it is quite different from the other faults, and illustrates a simple physical process that can cause apparently large discrepancies between geodetic and geologic slip rate estimates (Platt and Becker, 2013). Unlike the other examples of discrepant slip rates, the geodetic estimate of the slip rate on the GF is much less (almost zero) than the geologic estimates, which average ~5 mm/yr (Ganev et al., 2012). It is also a left slip fault, unlike all the others, which are right slip. The central section of the GF, which is ~90 km long, trends close to normal to the ECSZ, and cuts across an array of northwest-trending faults that accommodate at least 9 mm/yr of right-lateral shear. If these faults cut the GF, as suggested by Evans et al., then the GF should be being progressively offset and disrupted. Given the ~11 m.y. life span of the faults in the ECSZ, these offsets should individually be on the order of 10 km or more. This is clearly not the case: the GF forms a conspicuous and continuous morphological feature, without significant offsets. Given the well-documented slip rate on the GF over this same period (Monastero et al., 1997), the simplest solution to the problem is that the GF is accommodating right-lateral shear in the ECSZ by a combination of left slip and clockwise rotation, both of the fault itself and the rocks surrounding it. The equivalence of these two modes of deformation can be simply expressed in terms of the velocity gradient tensors, taking the trend of the ECSZ as the x1 direction, with dextral shear at rate , and assuming that the GF trends normal to the ECSZ: