Mineralogy of Faults in the San Andreas System That are Characterized by Creep

The San Andreas Fault Observatory at Depth (SAFOD) is a deep-drilling program sited in the central creeping section of the San Andreas Fault (SAF) near Parkfield, California. Core was recovered from two locations at ~2.7 km vertical depth that correspond to the places where the well casing is being deformed in response to fault creep. The two creeping strands are narrow zones of fault gouge, 1.6 and 2.6 m in width, respectively, that are the products of shear-enhanced metasomatic reactions between serpentinite tectonically entrained in the fault and adjoining sedimentary wall rocks. Both gouge zones consist of porphyroclasts of serpentinite and sedimentary rock dispersed in a foliated matrix of Mg-rich, saponitic ± corrensitic clays, and porphyroclasts of all types are variably altered to the same Mg-rich clays as the gouge matrix. Some serpentinite porphyroclasts also contain the assemblage talc + actinolite + chlorite + andradite garnet, which is characteristic of reaction zones developed between ultramafic and crustal rocks at greenschist- to subgreenschist-facies conditions. The presence of this higher-temperature assemblage raises the possibility that the serpentinite and its alteration products may extend to significantly greater depths in the fault. Similar fault gouge has also been identified in a serpentinite outcrop near the drill site that forms part of a sheared serpentinite body mapped for several kilometers within the creeping section of the SAF. The SAFOD core thus supports the long-held view that serpentinite is implicated in the origin of creep, as does at least one other creeping fault of the San Andreas System. The Bartlett Springs Fault (BSF) is a right-lateral strike-slip fault located north of San Francisco, California. Its slip rate currently is estimated to be 6 +/- 2 mm/yr, and along a segment that crosses Lake Pillsbury half the surface slip rate is taken up by creep. An exposure of this fault segment near Lake Pillsbury consists of sheared serpentinite that has risen buoyantly through late Pleistocene to Holocene (?) fluvial deposits and extruded onto the ground surface. Lighter-colored portions of the sheared body are dominated by antigorite serpentinite, with some retrograde recrystallization to chrysotile and lizardite. Darker zones are rich in porphyroclasts containing one or more of the minerals talc, chlorite, and actinolite in a sheared matrix of the same minerals. Incipient low-temperature alteration accompanying shear has produced Mg-rich smectitic clays, with local development of a foliated, clay-rich gouge that corresponds texturally and mineralogically to the SAFOD gouge zones. This outcrop of the BSF may thus illustrate the early stages in the development of the SAFOD gouge zones.