CONTRASTING FLUID SYSTEMS RELATED TO THE BRITTLE-DUCTILE TRANSITION AND SHALLOW BRITTLE CRUST IN TWO STRIKE-SLIP FAULTS

Field, petrographic, IR spectroscopy, and stable isotope data that document hydrothermal alteration and mass transport during faulting in shallow brittle crust at the Miocene-Pliocene San Gabriel Fault (SGF) in southern California and at the brittle-ductile transition at the Cretaceous Sierra Crest Shear Zone (SCSZ) in the Sierra Nevada are presented. These two faults may serve as an analog for the San Andreas Fault.

Occurrences of fracture-controlled clay minerals, and zeolites with minor chlorite and epidote, calcite veins, and calcite and/or zeolite filled slickensides that cut through breccia zones indicate mechanical weakening of the SGF as elevated pore-fluid pressure in response to reduced pore space volume induced failure. Alteration as fluids reacted with fine cataclasites, resulted in weaker phyllosilicate-rich rocks. Non-equilibrium mineral δ¹⁸O and homogeneous δD values indicate a low water/rock ratio system with fluids of meteoric origin at low temperature (<350°).

In contrast, fault rocks from the brittle-ductile transition at the SCSZ display patterns of alteration, mineralogy, and isotopes (H, B, O) indicative of a greater range of temperature (up to 450°C) and water-rock ratio. Fracture-related leached zones with chlorite and sericite in association with quartz+tourmaline veins and epidote-bearing breccia zones occur in metavolcanic and metavolcaniclastic rocks. Ductile calc-silicates display epidote and actinolite alteration with quartz that underwent dynamic recrystallization. These rocks also host multiple generations of quartz±tourmaline veins that record episodic transitions between the ductile and brittle regimes. Biotites are shredded and altered to leucoxene and sericite in ductile rocks. Significant silica mass transfer is evidenced by multiple generations of zoned quartz veins. Isotopic results document a hydrothermal evolution from a regime dominated by near-source fluids (magmatic and metamorphic) to shallow meteoric-hydrothermal fluids as the system transitioned from ductile to brittle. Meteoric-hydrothermal fluids dominated at transtension zones whereas the deep fluids dominated at transpressive zones. Heat provided by the nearby Tuolumne batholith drove fluid circulation, as ductile deformation ceased once the batholith cooled.