TEMPERATURE-DEPENDENCE OF QUARTZ AND FELDSPAR DEFORMATION MECHANISMS IN TWO CONSTRASTING SHEAR ZONES: THE TOBACCO ROOT MOUNTAINS, MONTANA, AND THE SCHLINIG FAULT, EASTERN ALPS

Electron backscatter diffraction (EBSD) analysis of quartz and feldspar in deformed rocks has yielded lattice patterns that can be used to correlate deformation mechanism to temperature regime, providing in-depth information as to the temperature dependence of quartz and feldspar deformation. Feldspar grains produce a shape preferred orientation and retain that orientation, while crystallographic slip systems in quartz grains produce crystallographic preferred orientation patterns that change with increasing temperature. Samples studied are quartzofeldspathic gneisses and mylonites from the Tobacco Root Mountains, Montana, and the Schlinig fault in the eastern Alps. Both localities record intense non-coaxial deformation under a variety of metamorphic conditions. The Precambrian rocks in the Tobacco Root Mountains have undergone substantial simple shear at the thermal peak of an upper amphibolite facies (>700°C, ~1 GPa) metamorphic event ca. 1.78-1.72 Ga. The Schlinig fault is a Cretaceous thrust fault that records a metamorphic gradient from greenschist to upper amphibolite facies across the length of the fault. It was subsequently reactivated as a normal fault. Quartz and feldspar grains in samples from both regions record substantial shear deformation at the microscopic level in quartz ribbons, serrate grain boundaries, pressure shadows, and subgrain and deformation bands. Presence of these different deformation textures reflect variations in material behavior, and as temperature increases a transition is seen between deformation mechanisms, including bulging recrystallization, subgrain rotation, and grain boundary migration. The thermal activation of slip systems in both quartz and feldspar grains is examined in order to understand these mechanism transitions.