Characterization of Natural Slip Surfaces Relevant to Earthquake Mechanics

The Punchbowl fault, an ancient trace of the San Andreas, displays extreme localization of displacement to a meters-thick fault core containing an ultracataclasite layer with mesoscale slip surfaces. New maps of the Punchbowl fault slip surface and ultracataclasite produced using optical and electron microscopy document that extreme localization of slip also occurred at the microscopic scale. The prominent mesoscopic scale slip surface is a thin (mm thick), texturally distinct layer of ultracataclasite with a crystal-lattice preferred orientation, as evidenced by uniform birefringence, within which distinct microscopic slip surfaces are evident. Thin sinuous seams and possible injection-like structures of opaque, ultrafine material and thin zones of cataclastic flow occur at the microscopic slip surface. Evidence for multiple episodes of synfaulting mineral alteration and cementation in the surrounding ultracataclasite documents chemical processes likely were important to strength recovery. Overall, microstructural features are consistent with the inference that repeated slip occurred on the mesoscopic-scale slip surface, and that older slip-surfaces are present throughout the ultracataclasite layer. Diffraction-contrast TEM imaging, micro-electron diffraction, quantitative elemental mapping and energy dispersive spectroscopy (EDS) are being used to determine relative abundance and types of crystalline phases, glass, or other friction-induced amorphous phases (e.g., silica gel). Preliminary TEM observations indicate that the ultracataclasite consists almost entirely of ultrafine particles (4 to 400 nm diameter), with rounded relict grains of the host rocks, faulted and unfaulted veins, sheared and kinked clay minerals, and new, euhedral grains. Low permeabilities are implied by the fine grain size and corresponding nm-scale pores. Although rapid slip on mesoscopic scale slip surfaces would be expected to have produced thermal transients, we have not, as yet, found any direct evidence of frictional heating. While most of the ultracataclasite is made of nano-particles and specific surface areas of the material are large, micro-diffraction of individual particles reveals their crystalline nature. We have not ruled out the possibility of alteration of glass to zeolites, but to date, amorphous glass has not been imaged, and clays have not been found with textures that suggest devitrification of glass; rather, clays exhibit evidence of shear during and after growth at low temperature. Zeolites locally are abundant and occur in intact, offset and fragmented veins, and as nanometer to micrometer size particles.