QUANTIFYING THE AMOUNT OF THINNING IN A ZONE OF TRANSPRESSIONAL SHEARING: A KINEMATIC ANALYSIS OF A SECTION OF THE ROSY FINCH SHEAR ZONE

The Rosy Finch Shear Zone (RFSZ) is a zone of transpressional shearing belonging to the Sierra Crest Shear Zone extending along the eastern margin of the Sierra Nevada mountain range. The RFSZ trends roughly NNW and is approximately 3.5 km wide. Foliation in the RFSZ is NNW-trending and sub-vertical, with a dip, dip direction of 78, 241. The orientation of the mineral stretching lineations have a bimodal distribution, with the stronger of the two trending and plunging at 156, 60 and the weaker at 295, 71. Data for this study were collected from Mesozoic-age plutonic rocks and Paleozoic-age meta-sedimentary rocks. The zone of highest deformation is within the meta-sedimentary rocks, a heterogeneous suite of deformed pelitic schists, quartz-rich meta-sedimentary rocks and limestones. Weakly foliated granodiorite rocks form the western margin of the shear zone.

Seven representative samples were selected from a 0.65 km-wide section of the RFSZ for strain analysis. We determined the mean octahedral shear strain to be 0.245. Flinn's K-value and Lode's Ratio values were determined to be 1.332, and -0.0436, respectively, suggesting that the deformation closely approximates plane strain within this area. The mean long axis for the strain ellipsoids generated has a trend and plunge of 282,75 and the mean intermediate axes is sub-horizontal at 145, 16. The samples also yield vorticity numbers ranging from 0.18 to 0.71. From the vorticity data, the mean percentage of shortening perpendicular to the flow plane is 26%. This represents ca. 230m of thinning of our 0.65km zone of interest.