COMPOSITION AND STRENGTH OF ULTRAMAFIC-RICH GOUGE FROM THE BARTLETT SPRINGS FAULT, CALIFORNIA COMPARED TO SAFOD GOUGES: IMPLICATIONS FOR FAULT CREEP

Creeping traces of the Bartlett Springs and San Andreas Faults in northern and central California, respectively, consist of meters-wide zones of ultramafic-rich fault gouge sandwiched between sedimentary wall rocks. The gouge zones are interpreted to have been tectonically entrained into the faults from deeply buried serpentinite sources. Although the mineral assemblages differ, the whole-rock compositions and frictional strengths of gouge samples from the Bartlett Springs Fault (BSF) at Lake Pillsbury are closely comparable to those of the foliated gouges recovered in core from the San Andreas Fault Observatory at Depth (SAFOD). Gouge from both faults has high MgO (22–25 wt%, BSF; 17–24%, SAFOD) and low SiO₂ (44–46 wt%, BSF; 42–51%, SAFOD) contents compared to the crustal wall rocks, along with elevated Cr and Ni contents characteristic of ultramafic rocks. The SAFOD gouges have been extensively altered to the low-T (≤125°C) Mg-rich smectite clay, saponite, and the remaining serpentine minerals are lizardite and chrysotile. The BSF mineral assemblage is the higher-T (≥250°C) equivalent of the SAFOD assemblage, consisting of the serpentine mineral antigorite and the metasomatic alteration minerals chlorite, talc, and tremolite-actinolite, with only a minor saponitic overprint. At 100–110°C, close to the temperature at the depth of SAFOD core recovery, gouge samples from both faults are weak; the coefficient of friction, µ = 0.14–0.22 for the BSF and 0.13–0.17 for the SAFOD gouges. They also show velocity-strengthening behavior that promotes stable slip (creep). At 290°C and 140 MPa effective normal stress, corresponding to ~9 km depth, BSF gouge lacking saponite is slightly stronger (µ ~ 0.25–0.30) and remains velocity strengthening. The combined frictional properties of the BSF and SAFOD gouges demonstrate that faults composed of metasomatically altered serpentinite can be weak and slip aseismically (creep) throughout the seismogenic zone.