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

Abstract:

The application of stable isotopes in conjunction with detailed mapping of the dextral Steelhead Lake shear zone (SLSZ) in the central Sierra Nevada reveals a complex fluid history 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. 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) brittle calc-silicate-rich leach zones, and (4) 1-10-m-thick quartz veins with crack-seal textures. High and variable quartz δ¹⁸O values (> +13‰) from metasedimentary rocks belonging to the Saddlebag pendant outside the SLSZ indicate metamorphic fluids at low water/rock ratio. Leucogranite dikes associated with the TB have δ¹⁸O (plagioclase = +9.5±0.3‰) and δD (~ –80‰) values consistent with magmatic fluids. Quartz+tourmaline veins that record the ductile-to-brittle transition display a range of δ¹⁸O values (quartz: +3.4 to +16.4‰; tourmaline: +8.0‰) that indicate a complex fluid system involving magmatic, metamorphic, and meteoric-hydrothermal fluids. Variable mineral δ¹⁸O (–1.5 to +11.3‰) and δD (–140 to –77‰) values from the Sawmill Sequence calc-silicates and Koip Sequence metavolcanics are the product of a fluid system with magmatic (TB) and meteoric-hydrothermal end-member sources, with lowest values found within a pull-apart zone and the highest values are outside this zone. Quartz δ¹⁸O (–3.2 to +14.5‰) and fluid inclusion δD values (–137 to –79‰) define a mixing line with magmatic (TB) and meteoric-hydrothermal water end members for the crack-seal veins. Detailed studies of one 10-m-thick vein reveal a trend of decreasing isotope values from the margin to the median plane. Most mineral pairs failed to reach isotopic equilibrium, indicating a very complex and short-lived fluid evolution for this system. This study documents the opening up of a strike-slip fault system to surface-derived fluids as the SLSZ evolved through the ductile-to-brittle transition, providing insight into the role of such fluids in the earthquake process.