DEVELOPMENT OF NARROW BRITTLE-PLASTIC SHEAR ZONES AND “HOT SLICKENSIDES” IN THE WEST-CENTRAL ARIZONA METAMORPHIC CORE COMPLEX BELT

We investigate the microstructural evolution of very narrow (~1–10 mm wide) brittle-plastic shear zones exposed in the footwalls of Miocene metamorphic core complexes across west-central Arizona (the Harcuvar, Harquahala, White Tank, and South Mountains). These shear zones are hosted in granitoids and gneisses near the mylonitic front and commonly record normal-sense shear antithetic to the main top-NE mylonite zone and bounding detachment fault. Many are localized along fractures or within veins and exhibit mixed brittle-plastic behavior and “hot slickensides” with lineations defined by mechanical striations and plastically stretched grains. At the microscale deformation is mylonitic with most strain accommodated by very fine-grained (≤5 µm) polyphase mixtures typically consisting of feldspar+quartz+phyllosilicates and/or within quartz veins or slickenfibers. Relict fractured feldspar clasts with variable sizes within the polyphase matrix and the abundance of secondary phyllosilicates suggest cataclasis and fluid-driven reactions were important processes in developing these layers. The polyphase mixtures lack a crystallographic preferred orientation (CPO), consistent with grain size sensitive deformation mechanisms. Pure quartz layers record heterogeneous subgrain rotation + bulging dynamic recrystallization and a CPO indicative of activity on rhomb <a> and prism <a> slip systems. Mean recrystallized quartz grain sizes range from 8–50 µm, suggesting differential stresses near the brittle-plastic transition varied by ~100 MPa. Assuming these shear zones formed under similar temperatures, this range of stress values suggests strain rates fluctuated by at least 2.5 orders of magnitude. We interpret these narrow shear zones to have developed during flexurally-driven arching and exhumation of the mylonitic front through the brittle-plastic transition. Fluid infiltration associated with brittle deformation played an important role in weakening these rocks, promoting development of fine-grained polyphase mixtures and forming pure quartz veins in feldspar-rich rocks. This study highlights the heterogeneous rheology and deformation conditions of the brittle-plastic transition zone and the significance of fluid-driven weakening during extension in quartzofeldspathic rocks.