CHEMOMECHANICAL CONTROLS ON CREEP PROCESSES IN THE SAN ANDREAS FAULT REVEALED IN SAFOD CORE

One of the main goals of the San Andreas Fault Observatory at Depth (SAFOD), a key component of Earthscope, was the recovery of core to allow petrographic, chemical, and physical examination of an active, plate-boundary fault at depth. The SAFOD deep drillhole, located 14 km northwest of Parkfield, CA, crosses the central creeping section of the San Andreas Fault (SAF) where measured creep rates are ~25 mm/yr. Coring at 2.65–2.70 km vertical depth (~112°C) successfully sampled two zones of foliated gouge where creep is localized: the 2.6-m-wide central deforming zone (CDZ) and the 1.6-m-wide southwest deforming zone (SDZ). The two gouge zones are closely similar in character, consisting of porphyroclasts of serpentinite and sedimentary rock dispersed in a fine-grained, foliated matrix of Mg-rich smectitic clays (saponite ± corrensite). The boundaries of the CDZ and SDZ with adjoining rock units of the Great Valley Group are mineralogically, chemically, and texturally sharp. The Mg-rich clay minerals in the gouge zones are interpreted to be the product of fluid-assisted, shear-enhanced metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite that was tectonically entrained in the SAF from a source in the Coast Range ophiolite. At the surface, similar rocks are found in the active trace of the SF to distances of at least 4 km from SAFOD, and the gouge in the surface outcrops is inferred to connect to one or both of the gouge zones at depth. The metasomatic reactions identified in the CDZ, SDZ and surface outcrops were duplicated in recent laboratory friction tests at hydrothermal conditions, in which serpentinite gouge was sheared slowly for ~15 days between quartz-bearing rocks. The serpentinite gouges in those experiments showed an immediate weakening and stabilization of shear, in contrast to the relatively strong and potentially unstable behavior of serpentinite in an ultramafic chemical system. This suggests that fault creep along the SAF may have initiated as soon as serpentinite was juxtaposed against crustal rocks in the fault. Long-term shear of the serpentinite has resulted in even more significant, reaction-induced weakening accompanying creep, as extremely weak Mg-rich clays progressively dominate the mechanical behavior of the fault.