The relationship between slickenside surfaces in fine-grained quartz and the seismic cycle

Abstract

Well polished or reflective fault surfaces, with or without linear striae indicating the sliding direction, are commonly referred to as slickensides. This study examines the development of slickensides which are now exposed along the surface trace of a large, seismically active normal fault zone in Dixie Valley, Nevada, U.S.A. Geologic and mineralogic constraints indicate the slickenside surfaces formed at depths of less than 2 km and temperatures less than 270°C. The slickenside surface material is composed of greater than 98% quartz, with less than 2% kaolinite and iron oxide. Transmission electron microscope (TEM) observations reveal that the slickenside surface material has an extremely fine yet variable grain size (0.01–1 μm), and an unusual, nonequilibrium texture characterized by irregular grain boundaries and low dislocation density. Angular fragments in the cataclasite to either side of the slickensides provide clear evidence of cataclasis. Crystallographic preferred orientation in the slickenside surface material indicates that non-brittle, continuous deformation occurred within 0.1–10 mm of the fault surfaces in many areas. Non-brittle, continuous deformation must have alternated with cataclasis, because some fragments in the cataclasite have strong preferred orientations. We suggest that in the Dixie Valley slickensides, continuous deformation and the development of crystallographic preferred orientations occurred at relatively low strain rates during the interseismic period, while cataclasis occurred at higher strain rates associated with seismic events. The wide range of strain rates occurring within a fault zone during the earthquake cycle may be an essential element in the formation of fine-grained, glassy slickensides, both at Dixie Valley, Nevada, and many other areas.