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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

Earthquake swarms commonly occur in upper-crustal hydrothermal-magmatic systems and activate mesh-like fault networks. How these networks develop through space and time along seismic faults is poorly constrained in the geological record. Here, we describe a spatially dense array of small-displacement (< 1.5 m) epidote-rich fault veins (i.e., hybrid extensional–shear veins) within granitoids, occurring at the intersections of subsidiary faults with the exhumed seismogenic Bolfin Fault Zone (Atacama Fault System, northern Chile). Epidote hybrid extensional–shear veining occurred at 3–7 km depth and 200–300 °C ambient temperature. At a distance of ≤ 1 cm to fault veins, the magmatic quartz of the wall rock shows (i) thin (< 10 µm thick) interlaced deformation lamellae and (ii) systematically crosscutting veinlets healed by quartz and feldspars, and it appears shattered at the vein contact. Clasts of deformed magmatic quartz, with deformation lamellae and healed veinlets, are included in the epidote-rich fault veins. Deformation of the wall-rock quartz is interpreted to record the transient large stress perturbation associated with the propagation of small earthquakes preceding conspicuous epidote mineralization. Conversely, the epidote-rich fault veins record cyclic events of extensional-to-hybrid veining and either aseismic or seismic shearing. The dilation and shearing behavior of the epidote-rich fault veins are interpreted to record the later development of a mature and hydraulically connected fault–fracture system. In this latter stage, the fault–fracture system cyclically ruptured due to fluid pressure fluctuations, possibly correlated with swarm-like earthquake sequences.

How major crustal-scale seismogenic faults nucleate and evolve in the crystalline basement represents a long-standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long exhumed seismogenic splay fault of the 1000-km-long strike-slip Atacama Fault System. The BFZ has a sinuous fault trace across the Mesozoic magmatic arc of the Coastal Cordillera (Northern Chile) and formed during the oblique subduction of the Aluk plate beneath the South American plate. Seismic faulting occurred at 5-7 km depth and ≤ 310 °C in a fluid-rich environment as recorded by extensive propylitic alteration and epidote-chlorite veining. Ancient (125-118 Ma) seismicity is attested by the widespread occurrence of pseudotachylytes. Field geological surveys indicate nucleation of the BFZ on precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and andesitic dyke swarms (southern segment) within the heterogeneous crystalline basement. Seismic faulting exploited the segments of precursory anisotropies that were favorably oriented with respect to the long-term far-stress field associated with the oblique ancient subduction. The large-scale sinuous geometry of the BFZ resulted from hard linkage of these anisotropy-pinned segments during fault growth.