SERPENTINITE AND CREEP ALONG THE RODGERS CREEK FAULT, NORTHERN CALIFORNIA

A direct link between serpentinite and fault creep has been demonstrated in recent years along the central creeping section of the San Andreas Fault (SAF) and a creeping segment of the Bartlett Springs Fault (BSF) in northern California. Serpentinite from deep sources is tectonically entrained into both creeping faults, and the contrast in chemistry between the ultramafic and crustal wall rocks promotes weakening and stable shear (creep) of the serpentinite through solution-transfer-creep and, eventually, reaction-weakening processes. We have now initiated work along the Rodgers Creek Fault (RCF) to determine if serpentinite also explains the distribution of creep along that fault. The right-lateral RCF is a major strand of the SAF system north of San Francisco Bay, and its deep slip rate is estimated at 10-11 mm/yr (Prescott et al., 2001; Xu et al., 2018). The northern half of the ~70-km long fault is partially creeping at spatially varying rates up to 6 mm/yr. Exposures of serpentinite within the fault have been identified at three locations. Two of these are situated near the northern end of the creeping section NW of Healdsburg, while the third is close to its southern limit near Taylor Mountain, ~4 km southeast of Santa Rosa. All three serpentinite localities are directly above recently identified repeating earthquakes (Shakibay Senobari and Funning, 2019; also Xu et al., 2018, for the Taylor Mountain site), which are considered to denote creep at depth within a fault. The RCF repeating earthquakes occur at depths as great as 7 km. Preliminary study of the two northerly serpentinite bodies shows that both contain the high-temperature serpentine mineral, antigorite. Their chrome spinel compositions suggest an origin similar to that of the Coast Range ophiolite, as was also found for the BSF and SAF creeping segments. Future work will explore whether the serpentinite outcropping along the RCF was entrained into the fault from an ~7-km deep ophiolite source.