Large-Scale Continental Deformation: Fluid-Rock Interaction and Compartmentalization along the San Andreas Fault at SAFOD

The SAFOD project provides a unique view into the San Andreas Fault (SAF) zone. This presentation focuses on characterizations of drill cuttings from SAFOD Phases 1 and 2 determined using XRD, XRF, and IR spectroscopy

Two inactive strands of the SAF system were crossed by SAFOD at ~1360m and ~1926m (all depths are measured depth along the drill hole). These faults are dominated by cataclasites rich in laumontite. There is indication of fluid activity given by an increase in Mn+Fe+Sr in the 1360m fault and an increase in zeolites in the 1926m fault. The fault at 1926 m was apparently a barrier to cross-fault fluid flow, as Pt+Hg are detected below, but not above it.

A fault zone ~150m wide occurs at ~2620m associated with geochemical anomalies, including a broad increase in Rb+Fe+Mn and localized spikes in As+Hg+Au+Pt. This zone is enriched in authigenic fault-related clays.

The active SAF at a depth of ~3300m revealed by anomalously low seismic velocities and resistivity in a zone ~200m wide, containing two currently active traces of the SAF as measured by ongoing casing deformation (Zoback et al., 2005). Published studies of mud gas chemistry indicate that this fault zone is a barrier to cross-fault flow (Wiersberg and Erzinger, 2006). The only occurrence of As above this fault is in association with the fault at 2620 m, although As is common below the 3300m fault. This fault also contains illite-smectite, serpentine and talc. These assemblages allow the fault to be comparatively weak and may explain why it is currently creeping.

These results indicate that the properties of the SAF system have evolved over time, and that this evolution is related in large part to fluid-rock interaction.