AMS FABRIC OF OROGENIC COLLAPSE IN MYLONITIC DETACHMENT AND GRANITE FOOTWALL

The Bitterroot shear zone (BSZ), SW Montana, is a textbook example of a mylonitic detachment developed during the orogenic collapse of the Cordilleran orogen. The BSZ formed by strain localization during the Paleocene-Eocene exhumation of the Bitterroot igneous-metamorphic core complex. At exposure level, the shear zone foliation has a consistent dip of 25°E, lineation trends N.110° on average, and sense of shear is unequivocally top to the E. In its southern region, the footwall of the BSZ is composed of granite plutons of early Tertiary ages, contemporaneous with activity on the shear zone. An anisotropy of magnetic susceptibility (AMS) study was conducted (1) to test the variation of AMS across a strain and metamorphic gradient from magmatic conditions in the footwall to progressively low-temperature, mylonitic and cataclastic deformation upward across the shear zone, and (2) to map the fabrics in the plutonic footwall that displays sub-isotropic structure but records a magnetic foliation and lineation.

Multi-domain magnetite dominates the magnetic susceptibility in most rocks of the BSZ and the footwall. The larger the bulk susceptibility, the greater the degree of anisotropy (P); however, there is no direct correlation between finite strain and P; some samples from the footwall granite have a greater AMS than mylonites. Image analysis shows that the AMS results from the shape anisotropy of magnetite and that the principal axes of the AMS ellipsoid correlate with the mineral fabric axes across the shear zone. The AMS method stands out as a robust indicator of fabric axes across a wide range of deformation conditions.

AMS measurements of samples from two transects across the shear zone and into the footwall demonstrate fabric continuity between the low to high-temperature solid-state shear zone and the footwall granites. This fabric gradually and smoothly rotates from east-dipping in the shear zone to west-dipping in the magmatically deformed footwall granites, forming an arch over a ~10 km half wavelength. AMS analysis demonstrates that the magmatic fabric in the footwall granites and the solid-state fabric in the BSZ were acquired during the same E-W crustal extension. During E-W flow and cooling/crystallization of the molten or partially molten crust, the top-to-the-east shear zone initiated, and exhumed and arched the magmatic fabrics.