GEOMETRIC AND KINEMATIC RELATIONSHIPS BETWEEN HIGH-TEMPERATURE AND LOW-TEMPERATURE FAULTING IN THE ANACONDA DETACHMENT ZONE, SOUTHWEST MONTANA

The Anaconda Metamorphic Core Complex, SW Montana, contains unique exposures of the plastic-brittle transition in a deep-rooted detachment system. The Anaconda detachment juxtaposes high-temperature metamorphic rocks (~5-6 kbar) and Cretaceous through Tertiary granitoid intrusive rocks with a hanging wall of unmetamorphosed Precambrian through Tertiary units. Along its southern section the studied segment of the fault strikes N-NE and dips moderately (35-45°) to the east. To the north it curves abruptly westward and dips to the north. The composite detachment, well exposed in glaciated canyons, is comprised of three structural domains 1) a domed 500-800 m thick mylonite zone cutting high-temperature footwall rocks, 2) a chlorite breccia zone disrupting rocks structurally above, within and below the mylonite and 3) an array of listric-shaped cataclastic normal faults that cut mylonite, chloritic breccias and also hanging wall rocks. Quartz-feldspar lineations in the mylonite, chlorite bundle and white mica lineations in the chlorite breccia, and slip lineations along brittle faults are consistent (~100-110°). Mylonitic kinematic indicators (S-C fabric, mica fish, asymmetric porphyroclasts) indicate top-to-the-east (normal) sense of movement; those along the north-dipping segment show an oblique dextral shear sense. Kinematic indicators within the chlorite breccia and cataclastic faults including asymmetric host-rock macrolithons, shear bands in chlorite and foliated gouge, and shear folds coincide with normal movement sense in the southern segment and oblique dextral shear to the north. Cataclastic deformation is dominated by an extensive system of concave-up listric faults. High-angle segments are closely spaced at ~.1- 1 km and sharply cross-cut higher temperature fabrics. Lower angle segments are often well developed along high-strain zones mylonites or other sub-planar footwall features. Slip magnitudes are difficult to attain since host rocks are generally granitic and lack offset markers. However, mylonite fabrics, high-temperature foliations and magmatic fabrics show incremental east to west rotation away from the main detachment. We propose that apparent footwall doming was partially created by incremental incision and rotation by this listric normal fault system.