TRANSITIONAL DUCTILE TO BRITTLE STRIKE-SLIP RHEOLOGY: AN EXHUMED LABORATORY OF THE SEISMOGENIC ZONE IN THE EASTERN CENTRAL SIERRA NEVADA, CALIFORNIA

The Sierra Crest Shear Zone (SCSZ) is an ~100 km long zone of Cretaceous synmagmatic dextral transpression in the eastern Sierra Nevada, California, including the Rosy Finch and Gem Lake shear zones (Tikoff and Greene, 1997). New mapping as far north as Twin Lakes has confirmed the existence of another ~30 km of this shear zone along the eastern boundary of the Tuolumne intrusive complex (TIC). Our mapping from Tioga Pass north to Sawmill Canyon documents that strike-slip kinematics were continuous across the brittle-ductile transition (BDT). This area is thus a spectacular exhumed laboratory for studying the structural, fluid, and thermal processes in the seismogenic crust of strike-slip systems.

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.