Kinematics and timing of the Pinto shear zone, New York Mountains, northeastern Mojave Desert, California

New studies of the Pinto shear zone, New York Mountains, California, improve our understanding of the importance of Mesozoic extension in Mojave Desert and also contribute to the growing body of evidence for syncontractional extension in the Mesozoic Sevier orogen. The Pinto shear zone has a Z-shaped map pattern and foliation in the ∼92–97 Ma Mid Hills adamellite of the Pinto shear zone generally dips 20–65° S and SW. Sense of shear indicators demonstrate top-to-the S-SW shearing, parallel to a S-SW plunging lineation, suggesting the Pinto shear zone is a normal fault. Hanging-wall deformation is accommodated by antithetic high-angle faulting localized along porphyry dike margins. Deformation fabric within the shear zone grades structurally upwards from undeformed footwall granitoids beneath the shear zone to protomylonite, mylonite, and ultramylonite near its top. Microstructural studies of mylonite and ultramylonite indicate that deformation occurred over a broad range of decreasing temperatures, from lower amphibolite-uppermost greenschist through lower greenschist facies conditions. The decrease in temperature of deformation probably occurred during progressive deformation. (Abstract shortened by UMI.)...