Pseudotachylyte generated in the semi-brittle and brittle regimes, Bench Canyon shear zone, central Sierra Nevada

Abstract

Pseudotachylyte cuts Cretaceous plutonic and volcanic rocks in the Bench Canyon shear zone, central Sierra Nevada, California. As documented by optical and scanning electron microscopy, the pseudotachylyte displays evidence for a friction melt origin, including vesicles, amygdules, crystallites, flow fabrics and embayed crystal fragments. Based on fault-related rock association, vein morphology, and inferred crustal level of generation, two distinct types of pseudotachylyte are documented in the shear zone. Pseudotachylyte associated with cataclasite was generated at shallow crustal levels (~1–10 km) via unstable, velocity-weakening, abrasive wear friction. Pseudotachylyte associated and interlayered with mylonite was generated at deeper crustal levels (~10–15 km) most likely via stable, velocity-strengthening, adhesive wear friction; this pseudotachylyte underwent ductile deformation in the solid state. Interlayered pseudotachylyte and mylonite records cyclic aseismic/seismic slip at a transitional crustal level between the semi-brittle field of quartzofeldspathic rocks and the base of the seismogenic zone. The cyclicity is attributed to strain rate fluctuation effected by strain-resistant phacoids, pore-fluid pressure variation, ductile instabilities, or downward fault propagation. The mode of deformation within the shear zone progressed from (i) production of mylonite during amphibolite to greenschist facies conditions, to (ii) generation of pseudotachylyte by intermittent seismic faulting within a background of aseismic shear in the upper realm of the semi-brittle field (~300–400 °C), to (iii) generation of pseudotachylyte through seismic events in the brittle (< ~300 °C) during uplift/exhumation and cooling; some of the latter may have been generated at shallow enough crustal levels (< ~4 km) to allow vesicle formation. The exhumation of a crustal level exposing such an assemblage provides a rare opportunity to study the enigmatic nature of the transitional change from semi-brittle flow to seismogenic rupturing.