EXHUMATION PROCESSES WITHIN THE BITTERROOT LOBE DETACHMENT, NORTH AMERICAN CORDILLERA, A THREE-DIMENSIONAL STRAIN AND KINEMATIC VORTICITY ANALYSIS

Collisional events form mountain belts and thicken the continental crust. Overthickened crust is gravitationally unstable and collapses in order to regain typical crustal thickness by vertical shortening of up to ~50%. Understanding the kinematics and evolution of large-scale, normal-detachment zones associated with the collapse and exhumation of mountain belts is essential for understanding orogenesis at the first-order. Despite their importance, we know comparatively little about the kinematics and evolution of these detachment zones.

The Bitterroot Lobe Detachment (BLD) marks the eastern edge of the Bitterroot metamorphic core complex, a kilometer-thick mylonitic shear zone created during the orogenic collapse of the North American Cordillera. Using a combination of three-dimensional strain analysis of deformed quartz grains and crystallographic texture analysis via electron backscatter diffraction (EBSD), we estimate how much vertical shortening of this mylonite zone has experienced and whether that thinning has affected the rate of exhumation in the region.

The vorticity number is particularly useful kinematic indicator because it relates the relative amount of pure shear experienced by a deformed material to the relative amount of simple shear. These measurements can be integrated over the entire deforming zone in order to estimate the amount of total vertical shortening. We have measured quartz crystallographic axes orientations from mylonitized granodiorites collected along a transect through the BLD. These data have been combined with measurements of strain geometry, axes orientation, and strain magnitudes to yield a vorticity number and to ultimately be able to calculate the amount of zone-perpendicular thinning incurred within the BLD. Preliminary data for the BLD yield an estimated 67% vertical shortening.