VARIATION WITH CREEP RATE IN THE MINERALOGY AND TEXTURES OF FAULT GOUGE FROM THE SAN ANDREAS FAULT OBSERVATORY AT DEPTH (SAFOD)

The SAFOD deep drillhole, located 14 km northwest of Parkfield, CA, crosses the central creeping section of the San Andreas Fault where measured creep rates are ~25 mm/yr. Coring at 2.65–2.70 km vertical depth 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 CDZ takes up most of the creep, as evidenced by the more pronounced well-casing deformation associated with it. The gouge zones are the product of shear-enhanced metasomatic reactions between sedimentary wall rocks of the Great Valley Group and serpentinite that was tectonically entrained in the fault from a source in the Coast Range ophiolite. Although the CDZ and SDZ are closely similar in character, consisting of porphyroclasts of serpentinite and sedimentary rock dispersed in a foliated matrix of Mg-rich clay minerals, they exhibit differences in texture and mineralogy that can be attributed to different shearing rates. In addition, a ~0.2-m-wide sector of the CDZ located at its northeastern margin (NE-CDZ) is essentially identical in texture and Mg-clay chemistry to the SDZ, possibly due to a gradient in creep rate across the CDZ. The SDZ and NE-CDZ core samples contain a larger proportion of porphyroclasts than the majority of the CDZ, and average porphyroclast sizes are also larger. Narrow veinlets of calcite up to 5 mm in length and oriented parallel to the foliation were observed only in the matrices of the SDZ and NE-CDZ gouges. The gouge matrix clays in the SDZ and NE-CDZ consist of saponite (trioctahedral, Mg-rich smectite) and corrensite (1:1 ordered, interstratified saponite-chlorite), whereas those in the rest of the CDZ are largely saponite. Where age relations can be determined, saponite is always later than corrensite. Larger sizes and greater abundances of serpentinite and sedimentary clasts in the SDZ and NE-CDZ are consistent with a lesser degree of shear-enhanced reaction to form the Mg-rich clays. The temporal differences in clay-mineral compositions may reflect a change in physico-chemical conditions such as temperature or fluid chemistry, with clays in the more actively deforming portions adjusting more completely to the new conditions than those in more slowly creeping portions.