EXPOSING THE BRITTLE-PLASTIC TRANSITION IN A DEEPLY-ROOTED DETACHMENT SYSTEM, ANACONDA EXTENSIONAL TERRANE, WESTERN MONTANA

The Anaconda extensional terrane of western Montana contains exhumed mid-crustal rocks bound by a low-angle detachment. Unique exposure levels near Anaconda, MT display the transition from brittle to plastic deformation mechanisms in this deeply rooted detachment system. The ~130 km long Anaconda detachment is a multi-tiered zone marked by a 100-800 m thick mylonite that is overprinted by an array of low-angle normal faults. In the studied segment mylonitic foliations in granitoid host rocks strike N-NE and dip moderately (35-45°) to the east. Mineral stretching lineations marked by quartz and feldspar ribbons have a consistent trend of 102-108°. Kinematic indicators (S-C fabrics, rotated dikes and boudins, and asymmetric porphyroclasts) show a consistent top-to-the-east sense of displacement. In northern parts of the study area the fault curves abruptly westward forming a large recess with a northerly dip. In this region the mineral stretching direction and sense shear remain consistent with the southern section. Microstructures show broken feldspar porphyroclasts in a matrix of plastically deformed quartz ribbons and fine-grained quartz neoblasts. This indicates a record of plastic to brittle behavior in the mylonite zone. The transition is also marked by a series of low-angle, listric normal faults that overprint the mylonitic fabrics. These cataclastic faults show angular to subrounded blocks of mylonitic granitoid bounded by fractures filled with brecciated rock ranging in size from centimeters to submicroscopic. Clay gouge zones stained by red oxides are also common. A lack of offset markers makes amount of offset difficult to determine. However, slip lineations are consistent (~100-110°) with those in the mylonite zone. Kinematic indicators including asymmetric macrolithons, shear bands foliated gouge, and shear folds indicate normal movement sense in the southern segment and oblique dextral shear in the northern recess. 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 or other sub-planar features in the mylonite. Relative age of faulting indicates a general younging from west to east. These faults clearly represent the transfer of footwall rocks into the hanging wall along the brittle-plastic transition.