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Bureau Seminar, February 27, 2009

Clay minerals and their role in fault-creep activity in the San Andreas Fault

Anja Schleicher
University of Michigan

The San Andreas Fault Observatory at Depth (SAFOD) drill-site situated near Parkfield, California offers the opportunity to investigate the role of in-situ, fault-related mineralization along an active strike-slip plate boundary. Many investigations have shown that phyllosilicates contribute to a mechanically weak San Andreas Fault, but there are contrasting opinions as to which minerals are responsible for low frictional coefficients and fault creep behavior along the active section of the fault. A detailed mineralogical study of two shear zones at ~3066 m and ~3300 m measured depth show abundant illite and illite-smectite (I-S) clay minerals, with additional chlorite and chlorite-smectite (C-S) in the deeper sample. The characteristics of some authigenic I-S and C-S indicate a deep diagenetic origin, with the most smectite-rich mixed-layered assemblage and the highest water content in the actively deforming creep zone at ca. 3300-3350 m measured depth. Other localized precipitations of hydrous mixed-layered clay minerals grow preferentially on fractured clasts with polished and occasionally slickensided displacement surfaces. Such sub-micron thin films (nano-coatings) are of particular interest due to their localization on fracture surfaces, large surface areas, cation exchange capacities, clay hydration state, and preferred orientation produced by substrate-controlled growth. 40Ar/39Ar dating of the illitic coatings reveal an “older” fault strand (8 Ma) at 3066 m measured depth, and a “younger” fault strand (5 Ma) at 3295 m measured depth. These ages imply that the fault zone initiation is at least 5 Ma and 8 Ma old and that the recent creep activity at ca. 3300 m measured depth reflects a stage of fault reactivation. We suggest that the majority of slow creep occurs along these heterogeneously distributed, micron-scale thin films (or nano-coatings), accommodated by a combination of 1) slip along particles surfaces, 2) displacement along hydrated interlayers and 3) intracrystalline deformation of the clay lattice, possibly associated with repeated nucleation and growth. We emphasize that it is the nature of minerals occurring along slip surfaces of the San Andreas Fault that contribute most to the creep behavior, rather than matrix minerals that do not otherwise appear to have a genetic link to the faulting process. From these studies, a picture emerges where cataclasis creates nucleation sites for neomineralization on grain surfaces during creep. Although volumetrically limited, the localization and kinetics of the mineralization may ultimately define the mechanical properties of faults.


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