RIGID GRAIN NET VORTICITY ANALYSIS OF THE WESTERN IDAHO SHEAR ZONE, McCall, IDAHO

The western Idaho shear zone (WISZ) is a crustal scale zone of Late Cretaceous deformation. The WISZ reactivated the Salmon River suture zone, which represents the arc-craton boundary in that region of west Idaho, as recorded by a change in 87Sr/86Sr ratios. Due to the steep dip of the shear zone, which is nearly vertical after removal of the effects of Miocene to recent extension, it was long thought it represented strike-slip motion, however previous structural analyses indicate transpressional kinematics better describe the WISZ. Additionally, the relatively narrow width of the 87Sr/86Sr transition zone suggests that a significant amount of shortening must have taken place. Shear sense indicators indicate a dextral sense of shear. Past work utilizing the Rf/θ method in the Payette River tonalite suggested an angle of oblique convergence of 40° to 75°, while work utilizing the Porphyroclast Hyperbolic Distribution (PHD) method suggested an angle of 60°-85° in the Little Goose Creek Complex. We apply the Rigid Grain Net (RGN) method to the feldspar megacrystic Little Goose Creek Complex to estimate the mean kinematic vorticity number (Wm) of the WISZ. This method uses the aspect ratio and orientation measurements of feldspar porphyroclasts from 16 sites to estimate the vorticity number. Our data suggests a mean vorticity number between 0.65 to 0.71. This correlates to an angle of oblique convergence of 30° to 35°. The RGN method suggests lower angle of oblique convergence values than the PHD method, both of which were applied to the same unit within the WISZ. We combined the vorticity estimates from the various methods with rigid clast rotation model results to constrain the overall strain in the WISZ and examine how sensitive these strain estimates are to input vorticities.