The Rosy Finch Shear Zone (RFSZ) is a zone of transpression belonging to the larger Sierra Crest Shear Zone (SCSZ) and extends along the eastern margin of the Sierra Nevada mountain range. The SCSZ represents synmagmatic deformation along an oblique subduction zone. The RFSZ trends roughly NNW and is approximately 3.5 km wide. Foliation in the RFSZ trends NNW and is sub-vertical, with a mean dip, dip direction of 78º, 241º. The metasedimentary and plutonic rocks in the RFSZ have a strong, steeply plunging, and penetrative mineral stretching lineation. The orientation of the mineral stretching lineations have a bimodal distribution, with the stronger of the two trending and plunging at 156º, 60º and the weaker at 295º, 71º. The zone of highest deformation is concentrated in a heterogeneous suite of meta-sedimentary rocks including garnet-bearing pelitic schists, quartz arenites, and limestone. Weakly foliated granodiorite rocks form the western margin of the shear zone.
Eight representative samples were selected for strain analysis from a 0.65 km-wide section of the RFSZ. The mean strain magnitude is an octahedral shear strain of 0.232. We determined the mean Flinn’s k-value and Lode’s Ratio values to be 0.953, and 0.864, respectively, suggesting that the deformation closely approximates plane strain within this zone. The mean long axis orientation for the strain ellipsoids has a trend and plunge of 268º, 61º, and the mean short axis orientation is 044º, 08º. The samples yield a large range of vorticity numbers ranging from approximately zero to 0.94. Using these vorticity data, we calculated the mean percentage of shortening perpendicular to the shear zone to be nearly 21%. This suggests that ca. 135m of thinning has occurred across this 0.65km zone of interest.