THE ROSY FINCH SHEAR ZONE: A TRANSPRESSIONAL HIGHLY STRAIN ZONE THAT ACCOMMODATES SIGNIFICANT SHEAR ZONE THINNING AND VERTICAL EXTRUSION

Crystallographic preferred orientations (CPOs) of quartz grains collected via Electron Backscatter Diffraction (EBSD) within the Rosy Finch Shear Zone (RFSZ) combined with three-dimensional grain shape data permit a kinematic analysis to determine the dynamics of the deforming zone. The RFSZ is a NNW trending transpressional zone along the eastern margin of the Sierra Nevada mountain range, and the southernmost shear zone within the Sierra Crest Shear Zone (SCSZ). Oblique subduction along the western coast of the North American Plate, combined with westward-directed compression (ca. 90 Ma), is concentrated within the shear zone. Highly deformed metasedimentary rocks within the zone have a prominent foliation with a mean dip, dip direction of 83°, 236°. These rocks have a relatively steeply plunging and penetrative stretching lineation with a trend and plunge of 165°, 68°. Three-dimensional strain analysis of the RFSZ, in addition to crystallographic texture analysis via EBSD, provide two comparative methods for kinematic analysis and allow the completion of a vorticity study to determine the relative amounts of pure and simple shear within the zone of interest. Strain and vorticity data indicate that there are significant components of both pure and simple shear within the zone, and that the pure shear component is slightly greater than the simple shear component. A mean Flinn’s k-value of 1.189, and Lode’s ratio of 0.058, indicate an approximately nearly plane strain deformation within the zone. The observation of nearly plain strain deformation suggests that material did not flow laterally, i.e., parallel to the strike of the shear zone, and therefore most of the pure shear deformation must be accommodated by vertical extrusion parallel to the sub-vertical lineation. The RFSZ is the product of horizontal shortening and dextral shearing that has resulted in a thinning shear zone where material is leaves the system by traveling vertically. Samples collected along an E-W transect of the field area provide crystallographic axes orientation data from EBSD analysis which are combined with three-dimensional finite strain data to ultimately calculate the amount of across-the-zone thinning. Samples collected from the 0.65 km zone of interest have yielded a mean shortening of nearly 18%, or approximately 119 m.