Preliminary Geologic and Geophysical Analyses of Phase 3 Core, San Andreas Fault Observatory at Depth Borehole (SAFOD), Parkfield, California

The SAFOD borehole provides an excellent opportunity to study the in situ conditions adjacent to a seismically active portion of the San Andreas fault zone via direct sampling of deformed and altered rocks. At least two active slip surfaces were identified in the geophysical logs at 10,480 and 10,830 ft MD during Phase 2 that correspond to creeping movement along the fault (www.earthscope.org/data/safod).

We merge wireline geophysical data with sub-m to m-scale observations of lithologic and facies analysis, structural core mapping, thin section and X-Ray diffraction analyses of Phase 2 cuttings, petrologic and microstructural analysis of Phase 2 sidewall cores within the intervals adjacent to the slip surfaces. We refine predictions of rock and fluid interactions from interpretations of the geophysical data with direct observations on the core.

Initial results suggest rock sequences sampled in Phase 3 represent the lower Great Valley forearc basin sequence and possibly portions of the Franciscan Formation. Greenish-gray to grayish-red sandstones, dark green to gray to black siltstones and mudstones, serpentinite mudstones and clasts, with foliated fault gouge comprised of cataclasite and sheared serpentinite are observed. Facies interpretations shed insight into the unique provenance for these rocks and may define lithologic controls on the deformation behavior of the fault zone at depth.

Geophysical logging and petrologic data suggest complex compositional and structural relationships. Velocity data are variable with depth and show weak correlation to the amount of cataclasis, alteration, and microfractures obtained through the cuttings analyses. However, Vp and Vs are in some cases positively correlated with regions of increased cataclasis and alteration, suggesting that cementation or strain hardening may be occurring at depth.