We present a 3D model of the rotation of populations of rigid clasts in a transpressional setting is based on the Jeffery model rotation equations of Jezek et al. (1994, 1996) and the fabric ellipsoid equations of Robin (2002). Preliminary results indicate that the 3D preferred orientation of populations of rigid ellipses rotating in transpression is a function of: 1) vorticity; 2) average dimensions of the ellipsoids; and 3) amount of finite strain accumulated in the system. The average dimension of the population of feldspars is estimated using elliptical data collected on various sections. Vorticity is constrained on plots of mean orientation vs. degree of preferred orientation of the population. In a monoclinic transpressional setting these vorticity estimates can be compared to sectional estimates from the horizontal plane calculated by other methods. Using this value of vorticity, the magnitude and orientation of finite strain can be calculated.

This model is compaired to 3D field data collected in the western Idaho shear zone, a crustal-scale shear zone that separates the western margin of the North American craton from the accreted terranes of the Cordillera. The shear zone is ~5 km in width and is exposed mostly within a feldspar megacrystic orthogneiss. Excellent exposure allows for feldspar orientation data to be collected on three nearly orthogonal faces, permiting us to calculate a 3D fabric ellipsoid of feldspar at 30 stations across the shear zone. Comparison of these data to the results of the numerical model allow us to constrain the vorticity and estimate a minimum amount of 3D finite strain across the western Idaho shear zone. Results are consistent with the interpretation of the western Idaho shear zone as a dextral transpressional zone. Furthermore, numerical modeling suggests high angles of oblique convergence (i.e. predomiantly contractional), an idea which is inconsistent with classical interpretations of subvertical tectonic boundaries