Shear zone development during magmatic arc construction: The Bench Canyon shear zone, central Sierra Nevada, California

Abstract

The Bench Canyon shear zone is a 20-km-long, moderately to steeply dipping zone of ductile to brittle strain that cuts Cretaceous plutonic and volcanogenic rocks located in the central Sierra Nevada, California. This zone is one of several Cretaceous-age ductile shear zones that record fluctuating strain fields associated with the tectonic/magmatic evolution of the central Sierra Nevadan magmatic arc. As determined by field relations, fabric analysis, and U-Pb and 40 Ar/ 39 Ar geochronology, the Bench Canyon shear zone underwent an episodic deformation history involving both contractional and extensional strain over a period of ∼17 m.y. or longer. Deformation is divided into early (ca. 101?–95 Ma), main (ca. 95–90 Ma), and late (ca. 90–78 Ma) phases. Temperature during main-phase deformation followed a retrograde T - t path of ∼600–300 °C after emplacement of multiple plutons. Main-phase deformation involved ductile thrust movement in both the 95 ± 1 Ma Red Devil Lake pluton and Lower to mid-Cretaceous volcanic rocks, where thrusting was facilitated by heat and fluids associated with plutonism. Fingerlike sills associated with the Red Devil Lake pluton were emplaced syntectonically with respect to main-phase deformation, but cooled rapidly, undergoing heterogeneous strain more typical of “pre-tectonic” emplacement. The ca. 90 Ma Mount Givens pluton cuts the shear zone fabric in the wall rock, limiting the bulk of deformation to have occurred prior to that time. Domainal and diachronous late-phase reactivation(s) along the zone involved weak, fluid-enhanced ductile (∼500 > T > 300 °C) deformation, and generation of abundant cataclasite and pseudotachylyte. The long-lived, episodic and diachronous deformation history of the Bench Canyon shear zone illustrates the complexities to be expected within shear zones during continental magmatic arc evolution.