A MULTI-STAGE FLUID EVOLUTION OF A DUCTILE-TO-BRITTLE DEXTRAL SHEAR ZONE IN THE CENTRAL SIERRA NEVADA, CALIFORNIA

A complex fluid history for the dextral Sierra Crest Shear Zone (SCSZ) as the system transitioned from ductile-to-brittle behavior at shallow crustal depth in response to cooling of the adjacent Tuolumne Batholith (TB) at ~85 Ma is indicated by stable isotope geochemistry done in conjunction with detailed mapping. This system has a multi-stage alteration history that includes: (1) ductiley deformed tourmaline-bearing leucogranite dikes, (2) ductile-to-brittle quartz+tourmaline veins, (3) fracture-controlled calc-silicate-rich leach zones, and (4) 1-10-m-thick quartz veins with crack-seal textures. High and variable quartz d¹⁸O values (> +13‰) from greenschist facies metasedimentary rocks belonging to the Saddlebag Lake pendant outside the SCSZ indicate metamorphic fluids at low water/rock ratio. Leucogranite dikes associated with the TB have d¹⁸O (plagioclase = +9.5±0.3‰) and dD (~ –80‰) values consistent with magmatic fluids. Quartz+tourmaline veins that record the ductile-to-brittle transition display a range of d¹⁸O values (quartz: +3.4 to +16.4‰; tourmaline: +8.0‰) that indicate a complex fluid system involving multiple fluids with the system opening to more meteoric fluids with time. Variable mineral d¹⁸O (–1.5 to +11.3‰) and dD (–140 to –77‰) values from Sawmill Sequence calc-silicates and Koip Sequence metavolcanics that display tour±epi±chl±ser alteration adjacent to brittle fractures are the product of a fluid system with magmatic (TB) and meteoric end-member sources, with lowest values found within a pull-apart zone and the highest values are outside this zone. Quartz d¹⁸O (–3.2 to +14.5‰) and fluid inclusion dD values (–137 to –79‰) from thick crack-seal veins define two mixing lines with magmatic (TB) and two meteoric water end members. These separate mixing lines may be the product of fluid separation by boiling as the system abruptly went from lithostatic to hydrostatic pressures during earthquake events. Detailed studies of one 10-m-thick vein reveal a trend of decreasing isotope values from the margin to the median plane. This study documents the opening up of a strike-slip fault system to surface-derived fluids as it evolved through the ductile-to-brittle transition, providing insight into the role of such fluids in the earthquake process.