GENERATION AND ROTATION OF MAGMATIC FABRIC PROVED BY AMS DATA, BITTERROOT CORE COMPLEX, MONTANA

The Bitterroot shear zone is a 100 km, nearly linear, north-south trending feature which formed due to localization of strain during Paleocene-Eocene exhumation of the Bitterroot igneous-metamorphic core complex. Kappa-bridge AMS measurements of K1, K2, and K3 (K1-K2=magnetic foliation plane, K1=magnetic lineation) of samples from two transects (Lost Horse transect and Sweathouse transect) across the shear zone correlate well with field data from the shear zone; furthermore, these data clearly show the fabric continuity between the low-temperature solid-state shear zone and the protolithic granites of the Idaho-Bitterroot Batholith. This fabric gradually rotates from east-dipping in the shear zone to west-dipping in the magmatically deformed granites. Two alternative hypotheses are suggested to explain the fabric rotation: (1) fabric developed under magmatic to solid-state conditions in the footwall of the Bitterroot shear zone under bulk simple shear, or (2) the magmatic fabric developed first during incipient gravitational collapse of the molten crust and was later arched by localization of strain along the extensional Bitterroot shear zone.