Structure of the San Andreas Fault Zone at SAFOD

Spot cores from the San Andreas Fault Observatory at Depth (SAFOD) provide a remarkable view of the structure of the San Andreas fault (SAF) at the southern limit of the creeping segment near Parkfield, CA. At SAFOD there is a broad zone of damaged rock cut by localized zones of slip consistent with large cumulative displacement and a history of seismic and aseismic deformation in bedded, clay-bearing, fine-grained host rocks. Active creep has been documented along two narrow zones at 10,480 and 10,830 ft MD. Spot cores spanning these zones each contain a distinct, meters-thick layer of incohesive gouge with a penetrative, micro-scaly, layer-parallel fabric, and matrix-supported, sub-rounded, mesoscale porphyroclasts (up to 15% by volume) of sandstone, siltstone, and serpentinite. The overall structure is consistent with distributed shear, but where the vast majority of shear displacement is confined to the two gouge layers. A marked change in lithology and structure occurs across the 10,480 ft MD gouge layer. West of this layer is a broad zone of fractured rock that grades eastward into foliated cataclasites, analogous to that seen in exhumed traces of the SAF to the south. This damage zone displays extensive microscale fractures and mesoscale conjugate shears that record contraction approximately normal to the SAF, and evidence of pervasive secondary mineralization. In contrast, the low velocity zone northeast of the 10,480 ft MD gouge layer is composed of thinly bedded sandstone, siltstone and shale that has experienced bedding parallel extension via low angle shears, extensional veins, and boudinage. Bedded rock and foliated cataclasites near the 10,480 and 10,830 gouge layers display LS mesoscale fabrics consistent with non-coaxial deformation. Ongoing work is focused on using map and CT data to define deformation fabrics and the geographic orientation of Phase 3 spot cores for kinematic and paleostress analyses.