VORTICITY ANALYSIS OF THE WESTERN IDAHO SHEAR ZONE

The western Idaho shear zone is a crustal scale, intra-arc zone that was deformed on the western margin of North America in the Late Cretaceous. Though it is believed this shear zone is transpressional in nature, the relative amount of convergence verses trancurrence is not understood. In this study we first attempt to further constrain the shear zone kimimatics by quantifying vorticity and then determine the orientation of the vorticity vector. The western Idaho shear zone has two Cretaceous granitic complexes of interest: the Little Goose Creek Complex and the Payette River tonalite. The Little Goose Creek Complex is a megacrystic orthogneiss characterized by oblate feldspar porphyroclasts. An analysis using a modified porphyroclastic hyperbolic distribution method demonstrates a large component of pure shear associated with deformation with an angle of oblique convergence in the range of 60-85 degrees. The Payette River tonalite is the eastern most rock unit in the shear zone and contains a strain gradient that increases from east to west as indicated by the shape of the quartz fabrics. The application of the RF/Theta method to these quartz fabrics gives an angle of oblique convergence of 40-70 degrees, which also illustrates that deformation had a large component of pure shear. The shape preferred orientation of the hornblende in the Payette River tonalite was used to constrain the orientation of the vorticity vector. Numerical modeling suggests prolate clasts will rotate into parallelism with the vorticity vector during deformation. The orientation of prolate hornblende clasts in the Payette River tonolite indicate a sub vertical vorticity vector, which is consistent with transpressional kinematics for the western Idaho shear zone.