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

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°C. The D₁ pseudotachylytes and cataclasites have δD values similar to, or slightly higher than, those of unaltered hydrogen-bearing magmatic minerals (−78‰ ≤ δD ≤ −56‰). 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 δD compositions (−47‰ ≤ δD ≤ −9‰) and δ¹⁸O values ranging from 3.77 to 6.71‰, suggesting infiltration of shallow fluids, likely sourced from closed, marine-connected basins. Epidote-quartz oxygen isotope thermometry indicates equilibration at 200–220°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.