The importance of clay mineral reactions on the seismic behavior of the San Andreas Fault: Part 2-Hydration states and timing of mineralization

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The San Andreas Fault Observatory at Depth (SAFOD) main hole, drilled in Parkfield, California, USA, provides a unique opportunity to study the nature of clay-rich samples collected at depth from an active fault System and to assess their contribution to controlling seismogenic and creeping behavior. During 2005, fresh rock chips were collected from a core and impregnated onsite using low viscosity polymer-based resins. In the first year of research, samples were characterized using high resolution microscopy and X-ray diffractometry. In the second year, the interlayer hydration state will be determined along with the original clay mineral textures. Quantifying the amount of water adsorbed on surface and interlayer sites at circa 3 km depth is critical for understanding the role of swelling clays in the faulting process. Additionally, 40Ar/39Ar dating of fault-related neocrystallized illitic minerals on key fault planes will be used to constrain the timing of mineralization in this fault System. The study will include collection and study of new sample material that will become available during the scheduled 2007 site coring activities.