THE INFLUENCE OF METASOMATIC ALTERATION ON THE FRICTIONAL PROPERTIES OF SERPENTINITE-BEARING GOUGE IN THE BARTLETT SPRINGS FAULT, NORTHERN CALIFORNIA

The Bartlett Springs Fault (BSF) is a right-lateral strike-slip fault that is part of the San Andreas Fault System in Northern California with an estimated slip rate of ~7 mm/yr. Near Lake Pillsbury, the BSF slip rate is expressed as fault creep at a rate of 3.4 ± 0.2 mm/yr, measured at an alinement array (site: BSLP). An exposure of the fault near the alinement array reveals a ~1.5 m-wide zone of serpentinite-bearing gouge that has risen buoyantly to the surface through Pleistocene fluvial deposits in a manner similar to that documented for the creeping segment of the San Andreas Fault at SAFOD. The gouge is a heterogeneous mixture of the high-temperature serpentine mineral antigorite and the greenschist facies alteration assemblage talc + chlorite + tremolite, all of which are stable at temperatures >250°C, indicating that the gouge was tectonically entrained in the fault from depths at or near the base of the seismogenic zone. Antigorite has been shown to promote fault creep when sheared between crustal rocks at hydrothermal conditions. However, the effect of thorough metasomatism of antigorite on sliding stability and strength are unknown. We conducted velocity-stepping strength experiments to explore the effect on the frictional properties if the serpentinite is completely replaced by the talc-chlorite-tremolite assemblage. The experiments were conducted at 290°C, 140 MPa effective normal stress, and 90 MPa fluid pressure to simulate conditions at ~9 km depth. We tested mixtures of the three minerals in varying proportions (ternary mixing-law), using porphyroclasts from the BSF gouge for the chlorite and tremolite end members and talc from the stock used in previously published studies. The end-member samples show a four-fold variation in frictional strength, talc being the weakest (µ ~ 0.12), tremolite the strongest (µ ~ 0.55), and chlorite intermediate (µ ~ 0.30). Talc and chlorite are velocity strengthening (a-b > 0) and tremolite velocity weakening (a-b < 0) over the conditions tested. Talc is much more effective than chlorite in weakening and promoting stable slip in tremolite-bearing gouge, and mixtures containing >50% talc have coefficients of friction <0.2 with (a-b) ≥ 0. Talc would thus need to be concentrated in the sheared gouge matrix to promote creep in thoroughly altered serpentinite at depth.