A STABLE ISOTOPE STUDY OF FLUID-ROCK INTERACTIONS IN THE SAN GABRIEL FAULT ZONE AND ITS RELATIONSHIP TO SEISMIC PROCESS

The Plio-Pleistocene uplift and erosion of the San Gabriel Mountains allows us to directly observe the seismogenic paleodepths of San Gabriel Fault and to determine its fluid history using stable isotopes. Integrated petrographic, geochemical and structural studies indicate that fluids have played a significant role in weakening the fault zone by two interrelated processes: 1) Mechanical weakening of the fault zone as rock failure is induced by elevated pore-fluid pressure in response to reduced pore-space volume, resulting in the reduction of friction along the fault. 2) Chemical weakening due to alteration that resulted as fluids reacted with fine-grained cataclasites, resulting in weaker foliated phyllosilicate-rich fault rocks. This process is evidenced by occurrences of fracture-controlled chlorite, epidote, clay minerals and zeolites, veins of carbonate and slickenside surfaces filled with calcite and/or zeolites that cut through brecciated zones. Stable isotopes provide evidence of fluid sources and temperatures of fluid-rock interaction during deformation. δD values of biotite (–67 to –95‰), hornblende (–72 to –81‰), and chlorite (–73 to –76‰) indicate hydrous minerals have been altered with local meteoric water (δD ≈ –40 to –50‰). High temperature oxygen isotope fractionations (Δ¹⁸O_BIO/HBL-H2O = 6) indicate high temperature ¹⁸O/¹⁶O equilibrium (T ~ 475°C) with rock-buffered δ¹⁸O_H2O values (~+8‰), but this also suggests a low water-rock ratio system involving meteoric-hydrothermal waters as some mineral pairs display non-equilibrium ¹⁸O/¹⁶O fractionations (Δ¹⁸O_BIO/HBL-H2O = 2). A reinterpretation of published carbonate vein δ¹³C and δ¹⁸O values (Pili et al, 2011) also indicates meteoric water dominant in the evolution of the San Gabriel Fault Zone. Similar processes that involve weakening by alteration due to circulation of small amounts of external meteoric water may be responsible for the weakening of the San Andreas Fault.