Magnetic fabric and microstructure of a mylonite: example from the Bitterroot shear zone, western Montana

Abstract

AbstractThe Bitterroot shear zone, SW Montana, is a mylonitic detachment that developed by strain localization during the Palaeocene-Eocene orogenic collapse of this part of the North American Cordillera. Anisotropy of magnetic susceptibility (AMS) data from two transects across the shear zone and into the granitic footwall demonstrate the continuity between the low to high-temperature solid-state fabric in the shear zone and the magmatic fabric developed in the footwall granite. This fabric gradually and smoothly rotates from E-dipping in the shear zone to W-dipping in the footwall granites, forming an arch over 10 km wide. Furthermore, the mineral fabric of both paramagnetic and ferrimagnetic minerals is consistent with the AMS fabric, displaying the same arching, which is interpreted to have developed by a rolling-hinge process in the footwall granites during activation of the Bitterroot shear zone. The AMS method thus stands out as a robust indicator of fabric over a wide range of deformation conditions.