Evolving Fluid Source During the Growth of a Trench-Parallel Seismogenic Fault System

Abstract

Fluid infiltration along seismically-active faults and fluid-rock interaction influence the mechanical behavior of faults. Nevertheless, how fluid infiltration and fluid-rock interactions evolve at seismogenic depths with fault slip accumulation remain poorly constrained in the geological record. We used hydrogen and oxygen isotope geochemistry to determine the origin of hydrous fluids that percolated within the exhumed Bolfin Fault Zone (BFZ)-a segment of the Early Cretaceous intra-arc Atacama Fault System (Northern Chile)-during progressive fault evolution at seismogenic depth. The BFZ consists of D1 pseudotachylyte-bearing cataclastic strands linked by D2 extensional to hybrid extensional-shear, epidote-rich fault-vein systems that formed in a fluid-rich, seismically active environment at 3-7 km depth and 200-300 degrees C. The D1 pseudotachylytes and cataclasites have delta D values similar to, or slightly higher than, those of unaltered hydrogen-bearing magmatic minerals (-78 parts per thousand <= delta D <= -56 parts per thousand). This similarity indicates that seismic faulting occurred in a rock-buffered environment with limited circulation of external fluids at early stages of fault evolution. Conversely, the epidote of the D2 fault-vein systems has much heavier delta D compositions (-47 parts per thousand <= delta D <= -9 parts per thousand) and delta 18O values ranging from 3.77 to 6.71 parts per thousand, suggesting infiltration of shallow fluids, likely sourced from closed, marine-connected basins. Epidote-quartz oxygen isotope thermometry indicates equilibration at 200-220 degrees C for this stage of fluid infiltration. The influx of external, basin-derived fluids within the BFZ is interpreted to indicate the increased hydraulic connectivity during slip accumulation and fault network growth.