THE POTENTIAL AND ROLE OF MINERAL AND ROCK TRANSFORMATIONS IN THE SAN ANDREAS FAULT

There is growing evidence that chemical and state transformations in shallow rocks can play an important, perhaps even a key role in shallow fault processes. The study of recently drilled SAFOD samples shows ample evidence for secondary coatings on grain boundaries and fracture surfaces, while exhumed fault rocks from an analogous setting in New Zealand show that friction melting may occur at similarly shallow levels in continental strike-slip fault zones. X-ray diffraction and electron microscopy of Phase 1 SAFOD core samples from a fault segment at 3066 m depth show abundant clay mineral phases on grain boundary and fracture surfaces. In the <2 mu fraction, mixtures of detrital 2M1 and authigenic 1M/1Md illite polytypes with irregular shapes and strongly dissolved rims are abundant. In coarser grain sizes, the detrital 2M1 illite polytype is most abundant. The predominant clay minerals in the finer grain sizes are 20-30 nm illite packets of 1Md polytype. Encapsulated Ar dating of illite grain size fractions produces a preliminary age of ~2.6 Ma for these coatings and a detrital (host rock) age of ~61 Ma. These authigenic mineral phases are likely formed during the movement of mineralizing fluids along newly-formed grain boundaries and fractures, partly at the expense of larger (~150 nm thick) detrital packets. Laser-ablation Ar analysis of pseudotachylyte veins along the exhumed central portion of the Alpine Fault (AF) of New Zealand yielded total gas ages that exponentially decrease with increasing proportion of melt matrix. Calculation of intercepts for all-melted matrix end-member component indicates a ~570 ka friction-melting age. The average exhumation rate of 6-9 mm/yr along the AF implies that friction melts were generated during major slip episodes at 3.5–5 km crustal depth, just below the neomineralized cataclasite of the SAF. From these studies a picture emerges where cataclasis creates nucleation sites for neomineralization on grain surfaces during creep. During sudden, large displacements, the energy is sufficient to produce friction melts that are similarly generated at slip surfaces. Whereas volumetrically limited, the localization and kinetics of these transformations may ultimately define mechanical properties of faults.