FRACTURE DEVELOPMENT AND FLUID EVOLUTION ACROSS A DUCTILE-TO-BRITTLE TRANSITION IN A TRANSPRESSIONAL SHEAR ZONE, EASTERN CENTRAL SIERRA NEVADA, CALIFORNIA

Changes in fluid source and flux within fractures across the ductile-to-brittle transition may have an important effect on the seismogenic and rheological behavior of strike-slip shear zones. New structural and stable isotopic data in a Late Cretaceous, dextral, transpressional shear zone in the central eastern Sierra Nevada, California, suggest that ductile folding may have played a key role in the development of fractures that brittle faults subsequently utilized, and that fluid sources for these fractures evolved over time to eventually incorporate meteoric waters. In the ductile regime, shear failure commonly incorporated steeply plunging z-folds of both quartz veins and associated host rock. Inflection points and kinked hinges along these folds were apparently weak zones along which mechanical discontinuities developed, forming sinks for younger hydrothermal fluids. Ductile overprinting of brittle fabrics suggests that these structures formed during the ductile-to-brittle transition. Furthermore, crack-seal structures in major quartz veins suggest incremental growth over time owing to porosity increase and coeval ingress of siliceous hydrothermal fluids. Oxygen and hydrogen stable isotopic values across a >10 m wide vein establish that: 1) δ¹⁸O values range from > +10‰ to < 0‰ and δD range from < -80‰ to < -130‰ and together form a linear trend toward the meteoric water line (MWL), suggesting gradual mixing that evolved from dominantly deep crustal fluids to dominantly meteoric fluids; 2) This vein can be divided into at least four distinct compositional domains representing various points in time during mixing, characterized by discrete isotopic values and color; 3) Isotopic values trend from highest on the margins of the vein to lowest along the median plane, suggesting a syntaxial-style of incremental growth that progressed from deep crustal to meteoric fluids. Thus we propose that mechanically discontinuous shear failures developed during the ductile-to-brittle transition forming conduits that connect an increasingly large fluid reservoir to the seismogenic base of the crust.