TRANSITIONAL DUCTILE TO BRITTLE STRIKE-SLIP RHEOLOGY: AN EXHUMED LABORATORY OF THE SEISMOGENIC ZONE IN THE EASTERN CENTRAL SIERRA NEVADA, CALIFORNIA
New age constraints from along this fault, including U/Pb detrital zircons from sedimentary units and autocrystic plutonic and volcanic zircons, field relationships, and new PTt results place an age gap of ~40 My across this segment and constrain the BDT to ~84-80 Ma. Triassic volcanics (235-219 Ma) to the east are juxtaposed with Jurassic marine sediments (~180-170 Ma) and Late Cretaceous volcanics (113-95 Ma) to the west. Temperatures and pressures during shearing range from ~715-400 °C and 3.1-2.4 kbars, respectively. The BDT in this slip system occurred during rapid cooling due to the termination of arc magmatism and increased exhumation rates (Cecil et al., 2012).
The ductile shear zone is ~2 km wide with increased partitioning of strike-slip and contractional deformation away from the main shear zone. Transitional structures include faults that locally switch from brittle quartz-sealed veins to mylonites, and faults with gently folded host rock. The brittle fault network is at least the same width as the ductile shear zone, and contains quartz-sealed breccias, microbreccias, psuedotachylites, low T microstructures and low T oblique dextral kinematics. ~93-86 Ma dikes extending out from the TIC were deformed by shearing under magmatic and high T subsolidus conditions, were subsequently truncated by brittle faults, and are not found east of the main brittle fault. These dikes, as well as large (100 m long and up to >5 m wide) crack-sealed quartz veins, cm-scale leach zones, and tourmaline-rich veins indicate a complex fluid history that may provide invaluable insights into the role and evolution of fluids across the BDT.