EBSD AND THREE-DIMENSIONAL STRAIN ANALYSIS OF TRANSPRESSIONAL THINNING WITHIN THE ROSY FINCH SHEAR ZONE

Kinematic analysis of the Rosy Finch Shear Zone (RFSZ) via three-dimensional strain data and Electron Backscatter Diffraction (EBSD) analysis, has allowed for the calculation of the vorticity number, which relates the relative amounts of pure shear and simple shear within the zone and ultimately can be used to estimate the amount of across-the-zone thinning. The RFSZ is a 3.5 km wide 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, is concentrated within the shear zone. Highly deformed metasedimentary rocks within the zone have a prominent foliation with a mean dip, dip direction of 233°, 79°. These rocks also have a relatively steeply plunging and penetrative stretching lineation with a trend and plunge of 63°, 177°. A mean Flinn’s k-value of 0.845, and Lode’s Ratio of 0.058, indicate the predominance of plane strain deformation within this zone. Samples collected along an E-W transect of the field area provide crystallographic axes orientation data from EBSD, which further supplement the three-dimensional strain data in determining the dynamics of the shear zone. The transpressional nature of this obliquely convergent boundary, represented by the RFSZ, has resulted in vertical extrusion of material. Preliminary data, measured from samples that were collected during a 2007 field campaign, have yielded a mean shortening of nearly 21%, or approximately 135 m of the 0.65 km zone of interest.