FLUID FLOW IN A TRANSITIONAL DUCTILE TO BRITTLE SYSTEM IN THE CENTRAL EASTERN SIERRA NEVADA

Fluid flow can dramatically alter rock strengths, metamorphic conditions, and seismic behavior. Mapping, sample collection/analysis in the Saddlebag Lake and northern Ritter Range pendants of the central eastern Sierra Nevada, California, have established a record of complex fluid flow in the exhumed brittle-ductile transition of a Late Cretaceous, transpressive, dextral shear zone. Ductile structures are observed from microscopic to outcrop scales and indicate dextral transpression during the earlier stages of this shear zone. Evidence of brittle deformation is indicated by massive crack-seal quartz veins, breccia, pseudotachylyte, Riedel shear fractures, and strike-slip duplexes. Together, these structures demarcate the >50 km northern extent of the Sierra Crest shear zone. In this study, we evaluate the similarities and differences in fluid flow along the strike of this northern segment. Stable isotope analyses have focused on a range of quartz veins and indicate a two-component mixing transition in which earlier hydrothermal quartz deposits are associated with regional magmatic or metamorphic fluids and later quartz vein deposits are associated with meteoric fluids. Oxygen isotope values are interpreted to represent the oxygen that formed quartz in equilibrium with aqueous fluids, and hydrogen isotope values are interpreted as the preserved fluids. Although both ductile and brittle structures occur for 10’s of km along strike, veins representing the meteoric component have only been observed in the Sawmill Canyon area ± a few km. New hydrogen isotope values from quartz veins collected up to 20 km north and south of Sawmill Canyon were obtained using a Thermocombustion elemental analyzer and a ThermoFinnegan MAT DeltaPlus-XP light-isotope-ratio mass spectrometer. Hydrogen values range from dD=-70.8‰ to -105.3‰. These data do not imply involvement of meteoric waters, indicating that meteoric ingress may have been focused to a small part of the shear zone.