AN INVERSE APPROACH TO CALCULATING STRAIN, VORTICITY, AND INITIAL FABRIC ORIENTATIONS IN A HIGH STRAIN ZONE

We develop a method for fitting tectonic-scale kinematic rock deformation models to rigid ellipsoidal clast data. At each of 23 field sites in the Western Idaho shear zone, our data include a fabric ellipsoid and an aspect ratio for rigid feldspar porphyroclasts. We randomly generate a synthetic population of spheroids with the observed aspect ratio and fabric ellipsoid. We subject the population to an inverse deformation, simulating the rotation of the rigid spheroids (Jeffery, 1922) back to their initial orientations. We try deformations of various classes: monoclinic transpression, inclined transpression, etc. Within each class, we seek the deformation that returns the spheroids at all 23 sites to a minimally anisotropic initial state. This approach is powerful because it allows the user to simultaneously solve for strain, vorticity, and the initial orientation of fabrics. Moreover, the sizes of the anisotropy minima provide qualitative constraints on how well these values are known. Deformable clasts can be modeled by the same approach.