MICROSTRUCTURAL EVIDENCE OF KINEMATICS IN THE MID-CRETACEOUS COURTRIGHT-WISHON SHEAR ZONE, CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA

The Cretaceous Sierra Nevada batholith contains ductile shear zones that were active during arc magmatism. The interpretation of the kinematics of these shear zones, however, varies from extensional to transpressional. The Courtright-Wishon Shear Zone (CWSZ) is one of a series of mid-Cretaceous shear zones in the central part of the batholith, and is likely originally continuous with the Quartz Mountain and Kaiser Peak shear zones to the north.

The CWSZ is discontinuously exposed for ~12 km along the eastern margin of the 102 Ma granodiorite of Dinky Creek and is intruded by the locally undeformed 89 Ma Mt. Givens granodiorite. Our study, combined with other recent studies, suggests that shearing initiated at ~100 Ma. Foliation in the CWSZ strikes NW-SE, dips steeply eastward, and exhibits steeply plunging stretching lineations recorded by biotite and hornblende. Because of the slight westward tilting of the Sierra Nevada range during the Cenozoic, we assume the fabrics restore to a sub-vertical Cretaceous orientation. Mutually perpendicular thin sections were cut with respect to the solid-state fabric, specifically in the XZ (sub-vertical and NE-SW oriented) and the YZ (sub-horizontal) planes. These thin sections were used to analyze 3D kinematics with microstructural observations and electron backscatter diffraction techniques. Shear sense determined from the thin sections show E-side down kinematic indicators in the XZ sections and dextral kinematic indicators in the YZ sections. Quartz crystallographic vorticity analysis (CVA) indicates a vertical axis of rotation. Dextral kinematic indicators in the YZ section and the vertical CVA are broadly compatible with pure shear dominated transpression. This interpretation is consistent with the kinematics inferred for the Sing Sing shear zone located ~60 km along strike to the NNW. However, the E-side down kinematic indicators suggest an overall triclinic deformation. Work is continuing to quantify the magnitude of the triclinic component of deformation.