TRICLINIC DEFORMATION IN THE COURTRIGHT-WISHON SHEAR ZONE, SIERRA NEVADA BATHOLITH, CA: EVIDENCE FOR MID-CRETACEOUS, INTRA-ARC MAGMATISM IN THE NORTH AMERICAN CORDILLERA

The Courtright-Wishon shear zone (CWSZ) is located in the central Sierra Nevada batholith. The shear zone was active in the mid-Cretaceous. It deforms the 102 Ma Dinky Creek granodiorite and is crosscut by the undeformed 89 Ma Mt Givens granodiorite. The shear zone records steeply dipping, NNW-oriented foliations and a vertically plunging lineation. Shear sense indicators are locally present on the XZ plane, with W-side down indicators dominant. In contrast, dextral shear sense indicators are common on the YZ plane. Finite strain analysis of deformed mafic enclaves records primarily prolate strains with the maximum stretching axis parallel to the lineation.

The western margin of the shear zone preserves a finite strain gradient, with fabrics in the Dinky Creek granodiorite becoming increasingly strong toward its eastern margin. The deformational fabric on the eastern margin is abruptly terminated by the Mt Givens granodiorite, which does not contain a solid-state fabric.

Microstructures constrain the bulk deformation conditions. Most samples record microstructures dominated by grain boundary migration recrystallization in quartz and subgrain rotation in feldspar. Electron Backscattered Diffraction (EBSD) analyses were completed to characterize crystallographic preferred orientation (CPO) and crystallographic vorticity analyses (CVA). CPO of quartz c-axes in multiple samples exhibit a Y-maxima indicating prism <a> slip. Together, these data indicate dextral, non-coaxial dominated prism <a> slip. CVA-based bulk vorticity is oblique to the lineation but lies within the plane of foliation.

We interpret that the CWSZ records dextral transpressional kinematics, similar to other mid-Cretaceous shear zones in central Sierra Nevada. There are three issues with this interpretation. First, although the CWSZ records a field fabric dominated by pure shear transpression, it also records a simple shear dominated quartz CPO. Second, both the CPO and the field fabric are consistent with monoclinic transpressional kinematics, yet the bulk crystallographic vorticity axis is oblique to the lineation. Third, the fabrics are prolate, while oblate fabrics are predicted by transpressional strain modeling. We interpret that triclinic, transpressional deformation may explain these anomalous features.