- Webb 1100
Title: The role of serpentinite in promoting creep in the San Andreas Fault System of central and northern California
Abstract: Core recovered from two actively creeping strands of the San Andreas Fault (SAF) at ~2.7 km depth in the San Andreas Fault Observatory at Depth (SAFOD) borehole is rich in serpentinite and its low-T alteration products, Mg-rich smectite clays. The core provided the first direct evidence supporting the long-held view that serpentinite is implicated in the origin of creep. Using the core samples as a guide, similar serpentinite-rich fault gouge has now been identified in outcrops of the SAF near the drill site and, recently, in a creeping strand of the Bartlett Springs Fault (BSF) in northern California. For both the central SAF and BSF occurrences, the serpentinite is confined to the ~1-3- m-wide shear zones where creep is localized, and it is sandwiched between quartzofeldspathic wall rocks. My talk will focus on how shear-enhanced reactions between the ultramafic serpentinite and the quartzose crustal rocks can promote fault creep at otherwise seismogenic depths. I will first describe the mineralogy, textures, and alteration features of the serpentinite-bearing fault gouge samples. I will then review a series of friction experiments at hydrothermal conditions in which serpentinite gouge sheared between quartz-bearing rocks weakens and slides stably, in contrast to the relatively strong and potentially unstable behavior of serpentinite in an ultramafic chemical setting. The Mg-phyllosilicate alteration products of serpentinite (e.g., saponite, talc, chlorite) are also weak and stably sliding over their P-T stability ranges. Finally, I will correlate the geographic distribution of creeping faults in northern California with tectonic models of the occurrence of the Coast Range ophiolite at depth, and discuss the mode(s) of emplacement of serpentinite into those faults.