STAUROLITE-BASED KINEMATIC VORTICITY IN THE SNAKE RANGE DETACHMENT SHEAR ZONE, NEVADA

Much can be learned about strain and kinematic parameters from the deformation of material lines and rigid objects that are embedded in a rock. This study investigates the behavior of large staurolite porphyroclasts that were inherited from Cretaceous metamorphism and deformed to various degrees during Oligo-Miocene extension in the Northern Snake Range detachment shear zone. This shear zone makes up the footwall of the Snake Range detachment fault that separates unmetamorphosed Lower Cambrian carbonate units from greenschist-amphibolite facies core complex metamorphic rocks. The exposed shear zone is approximately 300 m thick in Hendry’s Creek and Hampton Creek on the western flank of the core complex and consists of interlayered mica schist and quartzite units (Upper Proterozoic McCoy Creek Group and Lower Cambrian Prospect Mountain Quartzite) that have registered top-to-east shear during Cenozoic extension. Staurolite porphyroclasts (3-15 cm long) from the lowest schist unit were traced in the field on faces that are normal to foliation and parallel and perpendicular to lineation. The shape preferred orientation of staurolite was investigated from these 2D tracings and also from 3D X-ray computed tomography (XRCT). Analysis of tracings reveals that deformation of staurolite, if considered a rigid object, was achieved by a larger contribution of pure shear than simple shear. This is consistent with relatively symmetric quartz crystallographic fabric patterns obtained from the quartzite units that directly overlie the staurolite schist. XRCT analysis demonstrates that staurolite is stretched and/or boudinaged in the high strain zones of the schist. High aspect ratio staurolite crystals first stretched uniformly and their behavior was close to that of material lines; once staurolite was boudinaged, the boudin segments behaved more like rigid objects and rotated according to general shear kinematics. The deformation of both schist and quartzite units in the Snake Range shear zone indicates that considerable thinning, as well as top to east shear, participated in unroofing the core complex.