VORTICITY ANALYSIS OF THE WESTERN IDAHO SHEAR ZONE

The western Idaho shear zone is a crustal scale, intra-arc shear zone that deformed the western margin of North America in the Late Cretaceous. Recent structural analysis suggests a significant contractional component to deformation in this shear zone. Vorticity analysis of mylonites from the Little Goose Creek Complex and Payette River tonalite units in the western Idaho shear zone provide constraints on both the relative amount of pure vs. simple shear and the orientation of the vorticity vector. The Little Goose Creek Complex is a feldspar megacrystic orthogneiss that contains the majority of the western Idaho shear zone. Analysis of this unit using two vorticity gages – the pophyroclast hyperbolic distribution method and the Wallis method – suggest that pure shear kinematics dominate deformation. Preservation of a strain gradient in the Payette River Tonalite, a tonalite-granodiorite complex on the eastern edge of the western Idaho shear zone, allows us to apply the Rf-phi method of vorticity analysis. Preliminary results from this analysis also suggest the predominance of pure shear. Previous work based on numerical modeling of rigid clast rotation suggests that populations of prolate or oblate clasts will align themselves with respect to the vorticity vector. The short axes of the fabric ellipsoids defined by a groups of oblate clasts will gather in the plane normal to the vorticity vector. Conversely, the long axes of the fabric ellipsoids of prolate objects will align themselves parallel to the vorticity vector. Analysis of populations of potassium feldspar megacrysts (obate clasts) from the Little Goose Creek Complex are most consistent with a subvertical vorticity vector for the western Idaho shear zone. Similarly, preliminary results based on fabric analysis of hornblendes (prolate clasts) from the Payette River tonalite also suggest a subvertical vorticity vector. A vertical vorticity vector, parallel to the maximum stretching direction as defined by the mineral stretching lineation, is most consistent with pure shear dominated transpressional kinematics for the western Idaho shear zone.