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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

Inland-normal faulting is recognised as an important process following large subduction earthquakes. The lack of data limits the understanding of how normal fault reactivation relates to the subduction earthquake cycle. We characterised the palaeoseismology of the Atacama fault system (AFS) in the Chilean subduction zone. Our results showed that upper plate normal faulting earthquakes with M w 7.0 and recurrence intervals of 35 ± 9 ky generated surface ruptures preserved as fault scarps. The average fault slip rate of 0.07 ± 0.01 m/kyr is three orders of magnitude slower than the convergence velocity, and the recurrence intervals of surface-rupturing earthquakes on the studied faults are much larger than the recurrence of great to giant (M w  > 8.5) subduction earthquakes in the Chilean margin. This demonstrates that the reactivation of an individual fault in the AFS is not always synchronised with this type of subduction earthquake.

The Punta del Cobre district near Copiapó is a center of iron oxide-copper–gold (IOCG) mineralization spatially and temporally associated with regional sodic-calcic hydrothermal alteration, the Atacama fault system (AFS), and two phases of Early Cretaceous magmatism. Here, we investigate the spatiotemporal and geochemical relationships between magmatism, ductile deformation, and hydrothermal alteration along the ~ 200 to 300-m-thick steeply NW-dipping Sierra Chicharra shear zone, interpreted to be the major strand of the AFS. Mylonitic fabrics and oblique sinistral-reverse kinematic indicators together record coaxial flattening in a transpressional regime. Deformation on the AFS took place before, during, and after intrusion of the synkinematic Sierra Chicharra quartz diorite of the Coastal Cordillera arc at ~ 122 Ma and terminated before intrusion of the unstrained ~ 114 Ma Sierra Atacama diorite of the Copiapó batholith. Geochemical data show that the Copiapó batholith was more mafic and more K-rich than the calc-alkaline Coastal Cordillera arc. This time period thus overlaps IOCG mineralization in the Punta del Cobre district (~ 120 to 110 Ma). Multiple phases of sodic-calcic alteration in and around the AFS shear zone are recognized. Textures of altered rock in the shear zone show both synkinematic assemblages and post-kinematic hydrothermal oligoclase. A ~ 775-m-long andradite vein that cuts the shear zone formed broadly at the end of magmatism in the district (~ 95 Ma). Oxygen isotope ratios from the vein indicate that hydrothermal fluids were likely magmatically derived. Together, this work shows the AFS-related shear zone and nearby IOCG mineralization developed in a regional transpressional regime produced by SE-directed oblique convergence across a NE-striking shear zone. IOCG-related magmatic-hydrothermal fluids exploited this transcrustal shear zone to produce multiple episodes of regional sodic-calcic alteration formed from fluids exsolved from magmas or driven by the heat of the Coastal Cordillera arc and Copiapó batholith.

The Mesozoic magnetite-(apatite) deposits of the Coastal Cordillera of Chile are interpreted as the product of the crystallization of oxidized iron-rich melts and subsequent hydrothermal alteration produced by related magmatic-hydrothermal systems. These deposits form a regional-scale mineral system controlled by the Atacama Fault System and where the mineralization spans more than 10 km in vertical extent. Individual sub-vertical bodies of massive magnetite coexist with and evolve vertically into pegmatite-, breccia-, and vein-like apatite-actinolite-magnetite/ilmenite rock. The mineralization is always hosted by a hydrothermal aureole of alkali-calcic-iron alteration that includes stockwork-like to disseminated mineralization. The deposits cluster in two groups. Those located in the northern part are mostly vein-like, and are hosted by Jurassic diorite. They have 87Sr/86Sri and εNdi values of 0.7042–0.7062 and + 5.1 to + 7.2, respectively. The southern group includes shallowly emplaced ore lenses in broadly coeval (sub-)volcanic intermediate rocks. They show similar 87Sr/86Sri signatures (0.7033–0.7065, with one value up to 0.7097) and more variable εNdi values (+ 3.9 to + 8.6). As a whole, the Sr-Nd data do not seem to be influenced by the type of crust intruded, but rather, likely track the mixing between a MORB-like reservoir and another source with elevated 87Sr/86Sri (≥ 0.706). The genetic model proposed involves the dehydration of variably altered subducted oceanic crust, the interaction of fluids released from the mantle wedge, the separation of iron-rich melts, and their ascent along transcrustal faults. The broadly coeval intermediate host rocks show a lesser contribution of subducted crust, something that perhaps excludes a genetic relationship between these rocks and the magnetite-(apatite) mineralization.

Chile is a very important country that forms part of the Andean metallogenic belts. The Atacama and Domeyko fault systems in northern Chile control the tectonic–magmatic activities that migrate eastward and the types of mineral resources. In this paper, we processed and interpreted aeromagnetic data from northern Chile using reduction to pole, upward field continuation, the second derivative calculation in the vertical direction, inclination angle calculation, and analytical signal amplitude analysis. We revealed the locations and planar distribution characteristics of the regional deep faults along the NNE and NS directions. Furthermore, we observed that the major reasons for the formation of the tectonic–magmatic rocks belts were the nearly parallel deep faults distributed from west to east and multiple magmatic activities along these faults. We ascertained the locations of volcanic mechanisms and the relationships between them using these regional deep faults. We deduced the spatial distributions of the basic–intermediate, basic, and acidic igneous rocks, intrusive rocks, and sedimentary sequences. We showed the linear positive magnetic anomalies and magnetic anomaly gradient zones by slowly varying the background, negative magnetic anomaly field, which indicated the presence of strong magmatic activities in these regional deep faults; it also revealed the favorable areas of copper and polymetallic mineralization. This study provides some basic information for further research on the geology, structural characteristics, and mineral resource prospecting in northern Chile.

We describe the geometry and kinematics of a Jurassic to Early Cretaceous transpressive sinistral strike-slip system within a metamorphic basement inlier of the Mesozoic magmatic arc near Bahia Agua Dulce at latitudes 31–32°S in north-central Chile and discuss possible relations with the Atacama Fault System further north. Sinistral transpression overprints structures of an accretionary system that is represented by the metamorphic basement. Sub-vertical semi-ductile NNW-striking strike-slip shear zones are the most conspicuous structures. Chlorite and sericite grew, and white mica and quartz dynamically recrystallized, suggesting low-grade metamorphic conditions during semi-ductile deformation. Folds at the 10–100 metre scale developed before and during strike-slip shearing. The folds are deforming a former sub-horizontal transposition foliation that originated during prior accretion processes. The folds have axes sub-parallel to the strike-slip shear zones and sub-vertical axial surfaces indicating a component of shortening parallel to the shear-zone boundaries, suggesting an overall transpressive deformation regime. Transpressive strike-slip deformation also affects Middle Triassic (Anisian) basal breccias of the El Quereo Formation. 40Ar–39Ar laser ablation ages of synkinematically recrystallized white mica in one of the shear zones provide an age of 174–165 Ma for the waning stages of semi-ductile strike-slip shearing. The semi-ductile shear zones are cut by mafic and rhyolite dykes. Two rhyolite dykes yield 40Ar–39Ar ages of 160.5 ± 1.7 Ma and 131.9 ± 1.7 Ma, respectively. The latter dyke has been affected by brittle faulting. Fault-slip analysis shows that the kinematics of the faulting event is similar to the one of the semi-ductile shearing event, suggesting that sinistral transpression continued after ~130 Ma. Timing, kinematics and geographic position suggest that the shear zones at Bahia Agua Dulce represent a southern continuation of the prominent Atacama Fault System that affected the Jurassic/Early Cretaceous arc over its ~1400 km length.