THREE-DIMENSIONAL STRAIN AND KINEMATIC VORTICITY ANALYSIS OF MYLONITES FROM THE BITTERROOT LOBE DETACHMENT, NORTH AMERICAN CORDILLERA

Understanding the kinematics and evolution of large-scale, normal-detachment zones associated with the gravitational collapse of over-thickened continental crust is essential in order to improve our fundamental understanding of the mechanisms of orogenesis. Despite the importance of its role in the unloading and exhumation of continental crust, comparatively little is understood about the evolution and kinematics of these low-angle detachment systems. The analysis of quartz crystallographic textures via electron backscatter diffraction (EBSD) combined with three-dimensional strain analysis allows us to develop a more thorough understanding of the material paths within these detachment zones. In particular, we are interested in investigating the role of pure shear driven vertical shortening as it is related to the mechanics and rates of exhumation.

Within the North American Cordillera, the Bitterroots Lobe Detachment (BLD) is located on the eastern edge of the Bitterroots metamorphic core complex (MCC) of eastern Montana. Here, orogenesis has produced an over-thickened and gravitationally unstable continental crust that has collapsed via a low-angle detachment, producing the BLD and a kilometer thick mylonitic shear zone. We have calculated the kinematic vorticity number (Wk) in order to compare the relative amounts of pure and simple shear that the shear zone has undergone. We have measured quartz crystallographic axes textures of mylonitized granodiorites collected across a transect through the BLD footwall. These data have been integrated over the entire deforming zone in order to estimate the total amount of vertical shortening. The integration of our three-dimensional strain analysis, with EBSD crystallographic texture data yields a vorticity number that allows us to calculate the amount of mylonite zone-perpendicular thinning within the BLD. Data for the BLD yields a mean shortening of 26% and mean vorticity number of 0.7, which implies nearly equal amounts of pure and simple shear components were active during deformation.