MAGMATIC IMPACTS ON THE THERMAL AND RHEOLOGICAL PROFILES OF THE UPPER CRUST: AN EXAMPLE FROM THE BEAR MOUNTAIN FAULT ZONE IN THE SIERRA NEVADA FOOTHILLS, CALIFORNIA

The Bear Mountain Fault Zone (BMFZ), located in the central Sierra Nevada foothills, deforms 149 Ma gabbro of the Guadalupe Igneous Complex (GIC) during its emplacement into the Mariposa sandstone. The BMFZ is a NW-striking, steeply NE-dipping, SE-vergent mylonitic shear zone that provides a natural laboratory to constrain how structures, thermal state, and strength of the upper crust can be affected by active magmatism. A 200-meter-long section of the BMFZ transitions from a subsolidus deformed sheeted sandstone-gabbro in the west, to gabbros with minimal subsolidus deformation in the east. S-folds, asymmetrical clast-tail systems, and quartz c-axis patterns on the XZ-plane indicate mostly east-side-up shearing along the NE-dipping mylonitic plane. The axis of sheath folds on the XY-plane parallel subvertical mylonitic lineation and asymmetrical clast-tail systems on the YZ-plane suggest both right and left-lateral strike-slip motion. In the west and east sections of the outcrop, deformation temperatures are high (>600°C), while the middle is low (400-500°C). Quartz displays dominant Grain Boundary Migration (GBM) recrystallization consistent with prism <a> slip revealed by c-axis patterns. Feldspars exhibit fracturing, Bulging (BLG), Subgrain rotation (SGR), GBM, and static recrystallization. Hornblende shows Shape Preferred Orientation (SPO) parallel to lineation in high-T samples, while in low-T samples show clast-tail-like structures resembling dynamic recrystallization. Clinozoisite appear as small grains or veins in some thin sections suggesting fluid activity which may have assisted mass transfer and micro-fracturing of hornblende to form clast-tail-like structures, even in high-T samples (Lafrance & Vernon, 1993). Based on quartz paleopiezometry and thermal simulation, we infer that high-T subsolidus deformation and differential stress (40-80 MPa) at 10 km depth can be associated with emplacement of the GIC, supporting that it is a syn-tectonic pluton (Vernon et al., 1989). Subsequently, shearing continues in low-T conditions. Our results show that immediately (< 1 Myr) after the emplacement of the gabbro, the strength of the upper crust reduces close to one order of magnitude due to heating from the pluton.