AN INVERSE APPROACH TO EXTRACTING STRAIN AND VORTICITY DATA FROM PORPHYROCLAST POPULATIONS

Major crustal scale shear zones in granitic batholiths outcrop all along the western margin of the North American Cordillera. While granitic rocks lack traditional strain markers, they are often rich in porphyroclasts. Porphyroclasts have been used widely to constrain kinematic vorticity (e.g., the critical aspect ratio vorticity gauge). Attempts to quantify strain using porphyroclasts requires an assumption that the initial porphyroclast population was randomly oriented. In contrast with earlier methods, our approach allows us to avoid the 2D vorticity analysis assumptions. Moreover, we are able to constrain the extent to which the initial population of porphyroclasts were randomly oriented. We apply a 3D inverse numerical model of rigid clast rotation to feldspar shape preferred orientation data from the western Idaho shear zone (WISZ). We compute the deformation that retro-deforms these clasts to a maximally unaligned initial state. By considering a variety of large-scale deformation models and searching for the best-fit model, we constrain the strain and kinematics of the WISZ in 3D without assuming the initial state was random and isotropic. Model output includes specific predictions about the shear zone dip, strain, vorticity, and fabric orientation. We discuss the model results in the context of field measurements, vorticity estimates, and geochemical data, all of which provide and independent opportunity to determine the geological appropriateness of the model results. Preliminary comparison yields mixed results. Among inclined transpression models, the best-fit model is vertically bounded transpression, which is consistent with geochemical data that suggests the WISZ is nearly vertical. The model also suggests a strong simple shear component to transpression, consistent with application of the critical aspect ratio method to our data. However, the model predicts less shortening across the shear zone than strain analysis of the 87Sr/86Sr gradient suggests. Additionally, the model indicates a nearly horizontal lineation in direct contrast to the consistently subvertical field lineation. Both of these discrepancies suggest that our inverse modeling approach underestimates the pure shear component of deformation.