Magnetic fabric and microstructure of a mylonite: example from the Bitterroot shear zone, western Montana D. SIDMAN 1 , E. C. FERRE ´ 2 , C. TEYSSIER 1 & M. JACKSON 3 1 Department of Geology and Geophysics, University of Minnesota, MN 55455, USA (e-mail: sidm0001@umn.edu) 2 Department of Geology, Southern Illinois University, Carbondale, IL 62901, USA 3 Institute for Rock Magnetism, University of Minnesota, MN 55455, USA Abstract: The Bitterroot shear zone, SW Montana, is a mylonitic detachment that devel- oped 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 con- tinuity between the low to high-temperature solid-state fabric in the shear zone and the mag- matic 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. The anisotropy of magnetic susceptibility (AMS) is a versatile and fast method for analysing both quantitatively and qualitatively the magmatic and solid-state fabric of granitic rocks (Hrouda 1982; Rochette et al. 1992; Borradaile & Henry 1997; Bouchez 1997). Much work has been carried out to understand the complex relation- ships between the several magnetic carriers com- monly present in granitic rocks and the resulting magnetic fabric (e.g. Borradaile 1991; Rochette et al. 1992; Housen et al. 1995; Gre ´goire et al. 1995; Archanjo et al. 1995). These studies reveal that: (1) the magnetic contribution of ferrimagnetic minerals, such as magnetite, can result from both shape anisotropy and distri- bution anisotropy, the latter of which can be constructive or destructive, depending on the spatial relationships of one or more grains; (2) because para- and ferrimagnetic minerals, such as biotite and magnetite, respectively, crystallize before diamagnetic minerals, such as quartz and feldspar, the magnetic fabric commonly reveals the overall magmatic fabric in igneous rocks; and (3) the variation of magnetic fabric with strain magnitude is poorly understood; however, AMS may track, at least qualitatively, the approximate shape of the finite-strain ellipsoid, although these comparisons are certainly rather complex (e.g. Lu ¨neberg et al. 1999). Quantifying finite strain is an important step in determining the deformation history of a region; however, to use AMS as a method for determining finite strain, it is first necessary to understand the precise relationship between magnetic fabric and mineral fabric. The Bitterroot shear zone (BSZ) was chosen for this study because: (1) its kinematic history and fabric are relatively well defined; (2) a wide range of deformation fabrics (from low- temperature solid state to magmatic) are pre- served in a relatively short distance, so the relationship between deformation and AMS can be easily studied; and (3) both paramagnetic and ferromagnetic (sensu lato) minerals are present, which makes it possible to compare microstructural data with AMS. In this paper, we present the results of this AMS analysis and its implications for the mineral fabric in the BSZ. Geological history The BSZ, western Montana, forms the western- most edge of the Bitterroot metamorphic core complex, a N–S-trending, asymmetric massif approximately 50 km wide and 100 km long (Foster et al. 2001) (Fig. 1). Its asymmetry is due to a larger amount of unroofing in the east than in the west. The core complex is composed From:BRUHN, D. & BURLINI, L. (eds) 2005. High-Strain Zones: Structure and Physical Properties. Geological Society, London, Special Publications, 245, 143–163. 0305-8719/05/$15.00 # The Geological Society of London 2005.