Journal Article

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

Di Toro, Giulio, Masoch, Simone, Cembrano, José, Dallai, Luigi, Pennacchioni, Giorgio, Gomila, Rodrigo, Fondriest, Michele, Novella, Davide

2025

Overview

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. Plain Language Summary Fluid infiltration along seismically‐active faults and fluid‐rock interaction influence the aseismic versus seismic behavior of faults. However, little is known about how fluid infiltration evolves with fault slip accumulation. Here we investigate the origin of fluids infiltrating the Early Cretaceous intra‐arc Bolfin Fault Zone, an ancient seismic fault exceptionally well‐exposed in the Atacama Desert. By using hydrogen and oxygen isotope geochemistry, we document that the fault was progressively infiltrated by shallow fluids with increasing slip accumulation. At early stages of fault evolution, the infiltrating fluids maintained a nearly rock‐buffered composition, similar to that one of host‐rocks, due to the low hydraulic connectivity of the fault, which accommodated up to 1 km of cumulative slip. Conversely, at late stages of fault evolution, large volumes of shallow fluids, likely derived from closed, marine‐connected basins, infiltrated the more mature and hydraulically connected fault, which accommodated larger cumulative slip (up to 2–3 km). This exhumed seismic fault provides an outstanding example of how fluid infiltration and fluid‐rock interaction evolve in intra‐arc seismic faults. Key Points Hydrogen and oxygen isotopes constrain fluid‐rock interaction at seismogenic depths and trace the growth of fault hydraulic connectivity Pseudotachylytes‐cataclasites derive from rock‐buffered fluid regimes; instead, epidote‐rich fault‐veins and breccias form from basin‐sourced fluids Fault slip accumulation promotes the infiltration of large volumes of basin‐sourced fluids at 3–7 km depth at late stages of fault growth

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Accumulation / Atacama Fault System / Cretaceous / Depth / Evolution / Fault lines / Fault zones / Fluids / Geochemistry / Hydrogen / hydrothermal systems / Infiltration / intra‐arc fault system / Oceanic trenches / Oxygen / Oxygen isotopes / pseudotachylyte / Rock / Rocks / seismogenic fault / stable isotopes / Thermometry / Veins (geology)