The Frictional Behavior of San Andreas Fault Materials Formed by Carbonation of Serpentinite

The exposure of serpentinite to CO₂-rich fluids leads to a sequence of carbonation reactions involving the formation of soapstone and listvenite. Since CO₂-rich springs, serpentinite, and listvenite outcrops co-occur along the San Andreas Fault between Cholame Valley and San Juan Bautista, we hypothesize that this sequence explains both the high creeping rates and frequent micro-earthquakes along this section. To test this hypothesis we studied the frictional behaviors of powdered magnesite and mineral assemblages representative of soapstone (magnesite:talc = 1:1, 1:9, and 9:1 by weight) and listvenite (magnesite:quartz = 1:1), and their implications for fault stability, in a suite of experiments using a rotary-shear apparatus. Dry powders were sheared between sandstone forcing blocks at a normal stress of 60 MPa, and subjected to order-of-magnitude step changes in slip rate in the range of 0.1 to 10 μm/s, over sliding displacements of 0.15-0.5 m.

Experiments on magnesite reveal velocity-strengthening friction at all sliding displacements up to ~200 mm. Intriguingly, mixtures of magnesite and talc reveal an evolution of frictional behavior, from velocity-strengthening friction at the onset of sliding, to velocity-weakening friction and subsequently to velocity-neutral behavior with increasing slip. The transition to velocity-weakening friction is unexpected, given the velocity-strengthening behavior of end-member powders (talc, V. Scruggs, unpubl.). We also observe that magnesite:talc mixtures yield peak values of the friction coefficient µ ≥0.8, well in excess of values of µ for magnesite, ~0.6, and talc, <<0.4 (Moore and Lockner, 2007). Tests on the quartz-magnesite mixture reveal a transition from velocity-strengthening to velocity-weakening friction over a displacement of ~100 mm, followed by a transition to velocity-strengthening behavior at larger displacements. This behavior contrasts with that of pure quartz gouge, which exhibits velocity-weakening friction over the entire displacement range, following the initial velocity-strengthening behavior at the onset of sliding (D. Goldsby, unpubl.). The friction data suggest that for sufficiently mature slip zones comprised of the mineral assemblages tested to date, sliding would be intrinsically stable.