key: cord-1036932-zzwbsm90
authors: Masoch, Simone; Gomila, Rodrigo; Fondriest, Michele; Jensen, Erik; Mitchell, Thomas; Pennacchioni, Giorgio; Cembrano, José; Di Toro, Giulio
title: Structural Evolution of a Crustal‐Scale Seismogenic Fault in a Magmatic Arc: The Bolfin Fault Zone (Atacama Fault System)
date: 2021-08-09
journal: Tectonics
DOI: 10.1029/2021tc006818
sha: 60e15ad74ba3bafa30c3e217696908029dd24197
doc_id: 1036932
cord_uid: zzwbsm90

How major crustal‐scale seismogenic faults nucleate and evolve in crystalline basements 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 ≤ 300°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 geologic 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 optimal to 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 the hard linkage of these anisotropy‐pinned segments during fault growth.

. Indeed, moderate to large in magnitude (M > 6) earthquakes rupture faults extending for >15 km in length, but such large faults are rarely well-exposed along their whole length at the surface due to weathering and vegetation or Quaternary cover. Major mature faults typically record a long, polyphase deformation history, which might obliterate the incipient stages of nucleation and growth (e.g., Rizza et al., 2019) . Thus, the field geologists' challenge in studying ancient, crustal-scale seismogenic fault systems is to find large areas which meet the following criteria:

• excellent preservation over kilometer-scale exposures of the spatial arrangement of structures (e.g., joints, dykes, and faults) related to multiple deformation stages; • faults exhumed from depths (i.e., 5-15 km depending on tectonic regime, rock composition, temperature gradient, etc.; Scholz, 2019) , where moderate to large in magnitude earthquakes nucleate in the continental crust • presence of tectonic pseudotachylytes (i.e., solidified frictional melts), unambiguous evidence of seismic slip in the rock record (Cowan, 1999; Rowe & Griffith, 2015; Sibson, 1975) .

The sinistral strike-slip Atacama Fault System (AFS) in the Coastal Cordillera (Northern Chile; Figure 1 ) (Arabasz, 1971; Cembrano et al., 2005; Scheuber & González, 1999) , associated with the ancient subduction of the Aluk (Phoenix) plate beneath the South America plate, is well exposed along strike over more than 1,000 km. The exceptional outcrop conditions result from the hyper-arid climate since 25-22 Ma (Dunai et al., 2005) and the slow erosion rates in the Atacama Desert. This makes the AFS an outstanding setting for studying the structural evolution of major faults hosted in the continental crust. Here, we consider the Middle-Late Jurassic-Early Cretaceous sequence of magmatic, solid-state, and brittle deformation that developed along the Northern Paposo segment of the AFS. Specifically, we consider the evolution of the >40-km-long seismogenic Bolfin Fault Zone (BFZ) and the large-scale syn-magmatic to post-magmatic Cerro Cristales Shear Zone (CCSZ) (Figures 1 and 2 ). The study integrates (a) geologic field mapping, (b) analysis of satellite and drone images, and (c) microstructural investigations of fault zone rocks and host rocks. We show that the large-scale sinuous geometry of the seismogenic BFZ is imposed by the local pinning of fault orientation on magmatic structures related to the precursory history of the magmatic arc. The BFZ was seismogenic, as attested by widespread occurrence of pseudotachylytes, and active at ambient temperatures of ≤ 300°C and depths of 5-7 km in a fluid-rich environment. We conclude that magmatic-related structures, such as foliated plutons and dyke swarms, may partly control the nucleation, evolution, and geometry of crustal-scale seismogenic faults. (a) Crustal-scale geometry of the AFS with its three concave-shaped main segments. Shaded-relief image modified from Cembrano et al. (2005) and Veloso et al. (2015) . Red box indicates the area shown in (b). The inset map shows the approximate plate configuration coeval with the Mesozoic sinistral strike-slip deformation along the AFS. Redrawn from Jaillard et al. (1990) . (b) Simplified geologic map of the Coastal Cordillera along the Paposo segment. Igneous lithologies are mapped with color coding by age. Unmapped areas represent metamorphic units and sedimentary covers. Data compiled and simplified from Cembrano et al. (2005) , Domagala et al. (2016) , González and Niemeyer (2005) , and SERNAGEOMIN (2003) . DTM base layer elaborated from USGS Aster GDEM database (https:// earthexplorer.usgs.gov/) as base map. Red box indicates the studied area (Figures 2 and 3 ).

The 1000-km-long AFS is the major crustal-scale, strike-slip fault system within the present-day forearc of the Central Andes ( Figure 1 ) (Arabasz, 1971; Brown et al., 1993; Cembrano et al., 2005; Scheuber & González, 1999) . The AFS includes three main, curved segments (from north to south): (i) Salar del Carmen, The AFS developed in the Early Cretaceous accommodating intra-arc sinistral and sinistral transtensional deformation, once the axis of arc magmatism migrated eastwards (Scheuber & González, 1999; Scheuber et al., 1995) . Brittle faults overprinted mylonites of similar kinematics and the age of ductile and brittle deformation varies along strike (Brown et al., 1993; Scheuber & González, 1999; Scheuber et al., 1995; Seymour et al., 2020 Seymour et al., , 2021 . Along the Paposo segment, in the outer shell of the Cerro Paranal Pluton, syn-mylonitic hornblende and biotite yield 40 Ar/ 39 Ar and Rb-Sr ages in the range between 138 and 125 Ma (Scheuber et al., 1995) . Similar zircon U-Pb ages, referred to the onset of ductile deformation, were reported in the southern Paposo segment (∼139 Ma; Ruthven et al., 2020) . Brittle faulting was constrained between 125 and 118 Ma (Olivares et al., 2010; Scheuber & Andriessen, 1990) . Extensional faulting along the AFS was reported during Miocene to post-Miocene in response to large magnitude subduction earthquakes (e.g., González et al., 2003 González et al., , 2006 .

The crystalline basement of the Bolfin area consists of (a) Early Middle Jurassic meta-igneous Bolfin Complex, (b) Late Jurassic plutons and (c) Late Jurassic to Early Cretaceous Cerro Cristales Pluton, and (d) volcanoclastic rocks of the La Negra Formation (Figures 1b-3) . The Bolfin Complex consists of diorites and gabbros, partially to completely recrystallized at amphibolite-granulite facies, metamorphic conditions (González & Niemeyer, 2005; Lucassen & Franz, 1994; Lucassen & Thirlwall, 1998) .

The Cerro Cristales Pluton, formed by tonalite-granodiorite and diorite-quartz-diorite units (Domagala et al., 2016; González, 1999; González & Niemeyer, 2005) , is an NNE-elongated body showing an outer shell of strongly foliated rocks (Figures 2 and 3) . The eastern and southern contact between the pluton and host rocks is marked by the large-scale CCSZ (González, 1999) (Figure 2 ). According to González (1999) , the CCSZ is a sinistral strike-slip ductile shear zone active at amphibolite-facies conditions, favoring and controlling the emplacement of the pluton. NW-striking syn-kinematic diorite and andesite dykes cut both the plutonic and volcanic rocks. These dykes represent the last magmatic event coeval with the formation of the AFS (Olivares et al., 2010; Scheuber & González, 1999) .

Large-scale, sinistral strike-slip faults of the AFS cut through the crystalline rocks (Figures 1-3) . Some of the N-striking major faults and NW-striking to NNW-striking splay faults are hierarchically organized into strike-slip duplexes over a wide range of scales Jensen et al., 2011; Veloso et al., 2015) . Most of the splay faults accommodated displacements up to a few kilometers Gomila et al., 2016; Mitchell & Faulkner, 2009 ). Brittle faulting occurred at 5-7 km depth at greenschist-facies to sub-greenschist-facies conditions (280-350°C: Arancibia et al., 2014; Cembrano et al., 2005) . Faulting developed chlorite-rich cataclasites, associated with pervasive syn-kinematic hydrothermal activity attested by the widespread occurrence of epidote-rich and chlorite-rich faults and veins (Arancibia et al., 2014; Cembrano et al., 2005; Herrera et al., 2005; Olivares et al., 2010) . In our study, we focus on chlorite-rich cataclastic rocks.

The BFZ is a third-order fault of the AFS bounding the kilometer-scale Caleta Coloso Duplex (Figures 1b-3 ) Herrera et al., 2005; Olivares et al., 2010) . The Early Cretaceous strike-slip structure of the BFZ was overprinted by Late Cenozoic extensional faulting. During this later stage, Miocene-to-Pliocene continental deposits were juxtaposed with chlorite-rich cataclasites of the BFZ fault core.

Original field structural surveys along with remote sensing analysis were performed to characterize the regional-scale pattern of tectonic lineaments (i.e., faults and shear zones) and dykes in the study area (20-km wide, 50-km long). Remote sensing analysis was performed using satellite images (i.e., Sentinel-2, Google Earth and Bing) as reference maps coupled with published geologic and structural maps Domagala et al., 2016; González & Niemeyer, 2005) . Six representative localities along the BFZ and two along the CCSZ were selected for a detailed analysis (Figures 2 and 3) . At each locality, we used a DJI Phantom 4 Pro drone to take nadir-directed aereophotographs. The images were processed in Agisoft Metashape Professional software to generate high-resolution georeferenced orthomosaics (spatial resolution of ∼10 cm/pixel) used as base maps for the surveys at 1:300, 1:500, or 1:1,000 scale. The orientation and kinematics of the different structural elements (magmatic foliations, dykes, joints, faults, and ductile shear zones) were systematically measured and digitalized using ArcGIS 10.6 software. Structural measurements (n = 2,716) were plotted onto stereonets (equal area, low hemisphere) using Stereonet 10 (Allmendinger et al., 2011; Cardozo & Allmendinger, 2013) .

Oriented rock samples (n = 178) were collected for microanalytical investigations. Microstructural observations were conducted on polished thin sections (n = 60) oriented parallel to the X kinematic direction (stretching lineation and slickenline in shear zones and faults, respectively) and orthogonal to the X-Y plane (shear zone boundary and fault plane). Transmitted-light optical microscopy (OM) was used to determine microstructural features at thin-section scale and to identify areas suitable for microanalytical investigations. Scanning electron microscopy (SEM) was used to acquire high-resolution backscattered electron (BSE) images coupled with semiquantitative energy dispersion spectroscopy (EDS) elemental analysis.

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10.1029/2021TC006818 6 of 29 SEM and field-emission SEM investigations were performed with a CamScan MX3000 operating at 25 kV at the Department of Geosciences at Università Degli Studi di Padova and a JOEL JSM-6500F operating at 15 kV at HP-HT laboratories of Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Rome, respectively. Bulk mineralogy of rock samples was retrieved through X-ray powder diffraction (XRPD), and semiquantitative mineralogical composition was retrieved through Reference Intensity Ratio (XRPD-RIR) method. XRPD analyses were performed with a PANalytical X'Pert Pro diffractometer equipped with a Co radiation source, operating at 40 mA and 40 kV in the angular range 3° < 2θ < 85°, installed at the Department of Geosciences (Padova). Mineral composition of main mineral phases was obtained by electron wavelength-dispersive microprobe analysis (EMPA). EMPA investigations were performed with a Joel-JXA8200 microprobe equipped with EDS-WDS (5 spectrometers with 12 crystals), installed at INGV-Rome, and a Cameca SX50 microprobe, installed at the Department of Geosciences (Padova). Data were collected using 15 kV as accelerating voltage and 7.5 nA as beam current. A slightly defocused electron beam with a size of 5 μm was used, with a counting time of 5 s on background and 10 s on peak. Albite (Si, Al, and Na), forsterite (Mg), pyrite (Fe), rutile (Ti), orthoclase (K), and apatite (Ca and P) were used as standards. Sodium and potassium were analyzed first to prevent alkali migration effects. The precision of the microprobes was measured through the analysis of well-characterized synthetic oxide and mineral secondary standards. Based on counting statistics, analytical uncertainties relative to their reported concentrations indicate that precision was better than 5% for all cations.

We describe the spatial distribution and attitude data for magmatic, solid-state, and brittle deformation structures for eight localities. Two localities are along the CCSZ, at the contact between the Cerro Cristales Pluton and either the meta-diorite of the Bolfin Complex (Quebrada Museo) or the Late Jurassic gabbro of the Cerro Mulato (CCSZ-North) (see locations in Figure 2 ). Six localities are along the BFZ and are referred to as Playa Escondida, Sand Quarry, Fault Bend, Quebrada Corta (within the Bolfin Complex), Ni Miedo, and Quebrada Larga (within the Cerro Cristales tonalite-granodiorite) (see locations in Figure 3 ).

Four generations of dykes were recognized based on their composition and crosscutting relationships (Figure 4) . From the oldest to the youngest, they include:

1. Amphibolitic dykes are composed of amphibole and minor plagioclase with grain size of up to 5 mm. The dykes, up to 50-cm-thick, have wavy boundaries or magma-mingling structures with the host rocks ( Figure 4a ) and are commonly foliated. They strike preferentially NE-SW to NW-SE with moderate to sub-vertical dip angles (>45°) ( Figure 5a ) and intrude both the plutonic rocks of the Bolfin Complex and of the Cerro Cristales Pluton, and the fault rocks of the CCSZ. 2. Leucocratic dykes are composed of plagioclase with minor amphibole with grain size up to 15 mm. The dykes, up to 50-cm-thick, exhibit sharp boundaries with the host rocks and commonly localized ductile (solid-state) shearing along their boundaries ( Figure 4b ). The dykes are steeply dipping (>80°) and strike preferentially E-W; instead, a minor set strikes N-S ( Figure 5b ). 3. Pegmatite and aplite dykes are widespread along both the CCSZ and the BFZ. These dykes are zoned (with K-feldspar margins and quartz-plagioclase-muscovite cores), have sharp boundaries with the host rocks, and do not show an internal ductile fabric (Figures 4c and 4d) . Pegmatites, up to ∼50-cm-thick (Figure 4c ), are arranged into two sets striking to NE and NW (the latter set is more pervasive) (Figure 5d ). Steeply dipping (>70°) pegmatites cut the CCSZ (Figures 4c and 7) . Aplites, up to 5-cm-thick (Figure 4d ), are arranged into three main sets, moderately to shallowly dipping toward NE, SW, and SE (Figures 4d and 5d ). Pegmatites and aplites cut each other and cut the amphibolitic and leucocratic dykes (Figures 4a, and 4c-4d). 4. Andesitic and tonalitic dykes have sharp contacts with the host rocks and do not show ductile internal deformation. The dyke contacts are locally exploited by brittle faults. The andesitic dykes, the most common dykes in the Bolfin area, have lengths up to several kilometers and widths up to ∼3 m (Figures 6-8 ).

The few tonalitic dykes have lengths up to hundreds of meters and widths up to 4 m ( Figures 6 and 7) . The andesitic dykes cut the CCSZ and are cut by the BFZ (Figures 6 and 7) . There are several sets of an- 

The CCSZ is a 30-km-long and up to ∼600-m-thick shear zone bounding the eastern and southern sides of the ellipse-shaped Cerro Cristales Pluton ( Figure 2 ). The CCSZ mainly strikes NNE-SSW, bending toward E-W in its southern end (Figure 2 ). At Quebrada Museo locality (Figures 2 and 6) , the meta-diorite shows a steep (>75°) NNE-striking magmatic foliation defined by alignment of feldspar and amphibole crystals. The contact between the CCSZ and the Bolfin Complex is transitional and highlighted by a swarm of elongate mafic microgranular enclaves sub-parallel to the magmatic foliation ( Figure 4e ). The CCSZ consists of strongly foliated and lineated high-grade rocks (hereafter referred to as mafic tectonites). The foliation of the mafic tectonites is steeply dipping (>80°) and parallel to the magmatic foliation with a prominent stretching lineation, marked by amphibole, plunging shallowly (<30°) to NNE ( Figure 2 ). Kinematic indicators indicate dextral sense of shear (Figure 4f ).

At CCSZ-North locality (Figures 2 and 7) , the Cerro Cristales tonalite is strongly foliated (Figure 4g ). The sub-vertical magmatic foliation, marked by alignment of euhedral plagioclase and amphibole crystals, strikes sub-vertically (>80°) NNE-SSW and is associated with a mineral lineation, marked by quartz aggregates, plunging shallowly toward NNE ( Figure 2 ). Close to the contact with the CCSZ, the presence of asymmetric mafic microgranular enclaves within the Cerro Cristales tonalite indicate a dextral sense of shear ( Figure 4g ). The contact between the Cerro Cristales tonalite and the mafic tectonites of the CCSZ is sharp (Figure 7 ). The mafic tectonites show the same structural features as observed at Quebrada Museo locality (Figures 2 and 4h ).

The rocks of the Bolfin Complex and Cerro Cristales Pluton are foliated along most of the BFZ. The magmatic foliation is defined by the preferred alignment of plagioclase and amphibole crystals (Figure 8 ), and by elongated mafic enclaves. From north to south (localities, geologic maps, and structural data in Figure 3 ), the magmatic foliation:

• strikes N-S and dips steeply to the E (Bolfin Complex, Playa Escondida and Sand Quarry localities); • strikes N-S and is sub-vertical (Bolfin Complex, Fault Bend locality); • strikes N-S, is sub-vertical and ∼4-km-long and ∼300-m-wide (Bolfin Complex, Quebrada Corta area; Figures 6 and 8); • strikes NW-SE to NNW-SSE and is sub-vertical (Cerro Cristales Pluton, Ni Miedo locality); • is weak and scattered (the plutonic rocks are mainly isotropic), and strikes ENE-WSW with moderate to shallow dip angles (<40°) toward S or NNW (Cerro Cristales Pluton, Quebrada Larga locality).

Small-scale (cm-dm thick) ductile shear zones are common in the studied area. Paired shear zones ( These strike-slip shear zones accommodated either dextral (E-striking set) or sinistral (N-striking to NW-striking set) displacement ( Figure 5c ) of as much as 1 m. Some of the N-striking to NW-striking amphibolitic dykes localized sinistral, strike-slip ductile shearing ( Figure 5c ) with development of internal S-C foliation and sigmoidal amphiboles.

At CCZS-North locality (Figures 2 and 7) , brittle faults occur in two sets striking NNE and W-to-NW, respectively. The NNE-striking faults overprint the foliation of both the Cerro Cristales Pluton and the mafic tectonites of the CCSZ, and crosscut the NW-striking dykes ( Figure 7 ). The fault rocks consist of dark green, massive cataclasites bounding light green fault gouges, up to tens of centimeters thick. The few measured chlorite-bearing slickenlines in cataclasites are shallowly plunging to NNE. The presence of dykes offset by cataclasites indicates dextral strikeslip kinematics.

The W-to-NW-striking fault set dips gently (>45°) toward N to NE. This set consists of (i) dark green cataclasites and (ii) lineated fault surfaces. The cataclasites are commonly associated with brownish-colored pseudotachylytes, similar to those found along the BFZ s.s (Section 4.3.2). Riedel-type structures indicate dextral strike-slip kinematics. In contrast, the lineated fault surfaces show well-developed epidote-bearing slickenfibers indicating mainly normal dip-slip kinematics, with measured displacement of as much as 1 m. Locally, red-colored fault gouges exploit the W-striking to NW-striking faults. The fault gouges consist of palygorskite, calcite, gypsum, and hematite in variable modal proportions (XRPD-RIR analysis; Section 5.3) and are associated with the Late Cenozoic extensional reactivation (Section 4.3.3).

The BFZ includes multiple fault core strands, up to 5-m-thick each, over a zone as wide as 75 m (Figures 6 and 8 ). The fault core strands (highstrain cataclastic domains) consist of dark green to black cataclasites and ultracataclasites (Figures 6 and 9a-c) , transitionally or sharply bounding low-strain cataclastic domains of dark green protobreccias to protocataclasites, where the original magmatic fabric of the host rocks is still recognizable. Thin anastomosing bands of cataclasites are commonly observed within the low-strain domains. The cataclastic rocks are cemented by chlorite and minor epidote (Section 5.2). The cataclasites and ultracataclasites are either massive or foliated with an S-C fabric indicating sinistral strike-slip kinematics (Figures 9a-9c ). Exposed slickenlines are rare and plunge shallowly toward NNW to N. From north to south (localities, geologic map, and structural data in Figure 3 ), the sinuous fault core of the BFZ:

• strikes ∼NW-SE and dips toward SW sub-parallel to the magmatic foliation (Bolfin Complex, Playa Escondida locality); • strikes N-S and dips toward W sub-parallel to the magmatic foliation (Bolfin Complex, Sand Quarry locality); • bends from NNW-SSE to N-S, is sub-vertical (>80°) and partially overprints the magmatic foliation (Bolfin Complex, Fault Bend locality); • strikes N-S, is sub-vertical and exploits the ∼4-km-long magmatic foliation (Bolfin Complex, Quebrada Corta locality; Figures 6 and 8); • strikes N-S, is sub-vertical and exploits the magmatic foliation (Cerro Cristales Pluton, Ni Miedo locality); • is poorly exposed toward the fault linkage with the Caleta Coloso Fault (Cerro Cristales Pluton, Quebrada Larga locality).

Brittle deformation within the damage zone is accommodated by dark green cataclasites and sharp chlorite-bearing lineated slip surfaces. These structures are either oriented sub-parallel to the fault core or strike NW-SE. The latter corresponds to Riedel-type and splay faults of the BFZ related to the large-scale Caleta Coloso Duplex . Steeply dipping brittle-ductile shear zones with a composite S-C and S-C′ foliation, indicating sinistral strike-slip kinematics, are discontinuously present within the BFZ fault core (Figure 9e ). These foliated fault rocks are up to 1-m-thick and make the transition to both the green cataclasites of the fault core strands and the foliated damaged host rocks.

Pseudotachylyte veins (brownish to black in color and up to 2 cm in thickness) occur along the BFZ (Figures 9b-9d) , especially in the fault core. The pseudotachylytes are polyphase and dismembered (Figure 9d ). In the Quebrada Larga locality, pseudotachylytes are common in subsidiary faults across the damage zone and decorate the contact between green cataclasites and NE-to E-dipping andesitic dykes ( Figure 9d ). Here, pseudotachylyte reactivation is rare.

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10.1029/2021TC006818 11 of 29 Figure 6 . Detailed geologic map of Quebrada Corta and Quebrada Museo localities along the Bolfin Fault Zone (BFZ) and the Cerro Cristales Shear Zone, respectively, and geologic cross-section across the BFZ. The cross-section is oriented N64°E (i.e., perpendicular to the fault core strike). The ∼75-m-wide cataclastic fault core (i.e., low-and high-strain domains) of the BFZ overprints the well-developed sub-vertical magmatic foliation of the Jurassic metadiorites belonging to the Bolfin Complex. The axes are in scale X:Y = 1:2 (i.e., vertical exaggeration of 2×). Google Earth imagery is used as base reference map. Coordinate reference system and projection are WGS84 UTM Zone 19S and Transverse Mercator, respectively. Unmapped areas represent Miocene to Quaternary continental deposits.

Fault gouges, discrete faults, and calcite-bearing veins either exploit or cut the cataclasites both in the fault core and in the damage zone of the BFZ (Figures 3, 9a , and 9f) (see also Olivares et al., 2010) . The fault gouges consist of palygorskite, calcite, halite, gypsum, and hematite in variable modal proportions (Table 1) , and show S-C composite foliations consistent with an extensional kinematics ( Figure 9f ). The discrete faults have calcite-bearing and hematite-bearing slickenlines and slickenfibers on the fault surface and occur in two sets: (i) an NW-striking to NNE-striking extensional to dextral-transtensional set, and (ii) an E-striking to ESE-striking, extensional to sinistral-transtensional set ( Figure 3 ).

We describe the microstructures of the ductile and brittle features pertaining to the CCSZ and BFZ s.s. The XRPD-RIR and EMPA analyses of the fault zone rocks and the mineral phases are reported in Tables 1 and 2, respectively.

Mafic tectonites forming the CCSZ consist of equigranular, polygonal aggregates of plagioclase and amphibole of ∼150-250 μm grain size (Figure 10a ). Plagioclase ranges in composition between Ab 50 An 50 Or 0 and, more commonly, Ab 43 An 56 Or 1 ( at triple grain junctions and along grain boundaries (Figure 10a ). Plagioclase is locally replaced by oligoclase + calcite + sericite. Amphibole, mostly hornblende and minor edenite (Table 2) , is locally replaced by chlorite. Plagioclase-amphibole geothermometry (Holland & Blundy, 1994; Molina et al., 2015) yields T = 788 ± 50°C and P = 185 ± 150 MPa for recrystallization.

Localized ductile shear zones bounding leucocratic dykes ( Figure 4b ) have a homogeneous recrystallized polygonal matrix (∼50 μm grain size) of plagioclase (Ab 67 An 32 Or 1 ), amphibole and magnetite wrapping around mm-sized amphibole and plagioclase (Ab 52 An 47 Or 1 ) porphyroclasts ( Figure 10b and Table 2 ).

Damaged host rocks contain pervasive microfractures and veins whose spatial density increases toward the fault core (see Gomila et al., 2016; Jensen et al., 2011; Mitchell & Faulkner, 2009 for description of nearby faults). Magmatic minerals present intense fluid-induced alteration. Plagioclase is either altered to fine-grained sericite + calcite ± epidote or replaced by albite (Table 2 ). Amphibole is replaced by either (Fe-)actinolite or chlorite and opaques. Biotite is replaced by chlorite and opaques. Quartz shows undulose extinction and K-feldspar is fractured and micro-faulted. Micro-stylolite seams are common in the damaged host rocks and are sub-parallel to the cataclasites.

Brittle-ductile shear zones consist of (i) microlithons of plagioclase (altered to fine-grained white mica + calcite or replaced by albite), (ii) high-strain horizons of quartz porphyroclasts immersed in a fine-grained (<20 μm grain size) recrystallized calcite matrix, and (iii) calcite antitaxial extensional veins (Figures 10c  and 10d ). Altered plagioclase microlithons are defined by micro-stylolite seams delineating a composite S-C and S-C′ foliation (Figure 10c ). Quartz porphyroclasts (i) show undulose extinction, (ii) are locally recrystallized into fine-grained aggregates along grain boundaries and microfractures, and (iii) are surrounded by pressure shadows of fibrous calcite (i.e., strain fringe) (Figure 10d ). Antitaxial veins consist of fibrous calcite, which cut the plagioclase microlithons and the high-strain horizons. The veins are orthogonal to the micro-stylolite seams and their spatial arrangement is consistent with sinistral strike-slip kinematics (Figure 10c ).

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13 of 29 Cataclasites consist of a fine-grained matrix of chlorite + epidote + quartz + albite + K-feldspar, including angular clasts of altered plagioclase, quartz, and earlier cataclasites (Figure 10e ). Cataclasites are locally foliated and, in the thickest horizons, layered for variable matrix/clasts ratios.

Pseudotachylytes show typical features of quenched melts: chilled margins, flow structures, and presence of microlites and spherulites (e.g., Di Toro et al., 2009; Swanson, 1992) . Alteration variably affected the pseudotachylytes. The most pristine pseudotachylytes have a homogeneous cryptocrystalline matrix with a "K-feldspar-rich" composition which contains (i) albite microlites, intergrown with amphibole and titanite (Figure 10f ), locally arranged into spherulitic aggregates and (ii) quartz and plagioclase clasts (Figure 10f) . Altered pseudotachylytes consist of fine-grained (∼20-30 μm grain size) albite + chlorite + epidote ± K-feldspar association (Figure 10g ).

In the reddish foliated fault gouges (Figure 9f ), the S-C fabric is marked by fine-grained palygorskite, clays and hematite, which wraps halite and gypsum mantled clasts (Figure 10h ). Halite mantled clasts are up to 1 mm in size and commonly fractured along cleavage planes. Gypsum clasts are up to ∼200 μm in size and show undulose extension (Figure 10h ). Veins consist of either (i) blocky-shaped calcite grains or ( 

First (Section 6.1), we discuss the field and microstructural observations (Sections 4 and 5) that allow us to constrain the P-T deformation conditions and to recognize a sequence of five main deformation stages. This information is required to interpret the formation of the seismogenic BFZ sensu strictu . Then (Section 6.2), we discuss the role of precursory structures on the evolution of the BFZ s.s. and we propose a more general model of fault growth within a heterogeneous magmatic arc.

The BFZ experienced a polyphase evolution that includes magmatic and solid-state deformation episodes (stages 1-2), followed by the emplacement of multiple generations of dykes (stage 3). This predated the Early Cretaceous brittle seismogenic strike-slip faulting (stage 4) and the late Cenozoic extensional faulting (stage 5). The whole evolution is summarized in the block diagrams of Figure 11 .

Stage 1 (Syn-magmatic to late-magmatic deformation). Along the CCSZ, the orientation of the magmatic and solid-state foliations and of the stretching lineations are similar ( Figure 2 ). These structural features are characteristic of syn-magmatic thermal aureoles related to pluton emplacement (Clemens, 1998; Miller & Paterson, 1999; Paterson & Vernon, 1995 and references therein) . At CCSZ-North locality, dextral strike-slip shearing in the mafic tectonites is spatially associated with melt segregation structures (Figure 4h ) (e.g., Sawyer, 2000; Weinberg, 2006) . This indicates that shearing accommodated by the CCSZ initiated during crystallization of the Cerro Cristales Pluton (T > 700°C) as also supported by the high-temperature conditions (788 ± 50°C) estimated for the recrystallized matrix of the mafic tectonites.

Based on our observations, we interpret the CCSZ as a large-scale syn-magmatic to post-magmatic shear zone related to the emplacement of the Cerro Cristales Pluton and the pervasive magmatic foliation of the outer shell of the pluton related to magma inflation/ballooning. Additionally, the dextral slip of the CCSZ may result from the local stress perturbation induced by the pluton emplacement, which possibly perturbed, at least locally, the regional far-stress field associated with the ancient SE-directed oblique subduction (Scheuber & González, 1999) . González (1999) estimated an emplacement depth of ∼13 km (400 MPa) for the Cerro Cristales Pluton, based on hornblende geobarometry and assuming a geothermal gradient of 30 °C/km. Instead, assuming a geothermal gradient of ∼50°C/km, typical of an active magmatic arc (e.g., the Southern Andes Volcanic Zone: Pearce et al., 2020; Sielfeld et al., 2019) and considering the estimated pressure (185 ± 150 MPa) for recrystallized matrix of the mafic tectonites, the emplacement depth of the pluton results at <10 km depth, as for plutons of similar age (140-155 Ma) emplaced along the El Salado segment of the AFS (Espinoza et al., 2014; Grocott & Taylor, 2002; Seymour et al., 2020) . Moreover, the emplacement of several plutons in a short time span thermally weakens the crust facilitating pluton emplacement at shallow depths (Cao et al., 2016) . We conclude that the CCSZ started forming at >700°C and < 10 km depth during the emplacement of the Cerro Cristales Pluton (Figures 11a) . The Bolfin Complex and the Cerro Cristales Pluton are intruded by the amphibolitic dykes, which cut also the CCSZ. These dykes show mingling structures and evidence of remelting, indicating that they intruded in a still partly molten material during a late-magmatic deformation stage (Figures 4a and 11a) . The dextral, E-striking ductile shear zones and the sinistral, N-to-NE-striking sheared amphibolitic dykes are arranged to form a conjugate set (Figure 5c ) associated with a sub-horizontal NW-SE compressional direction (i.e., σ 1 ) (Figure 11b ). This σ 1 is consistent with the SE-directed oblique subduction recorded in the Coastal Cordillera (Scheuber & González, 1999; Veloso et al., 2015) . Scheuber and González (1999) reported localized ductile shear zones formed at greenschist-facies metamorphic conditions, which is not consistent with the high-temperature conditions determined for the localized shear zones described here. The absence (or very scarce occurrence) of greenschist-facies localized ductile shear zones can be explained by the fast eastward migration of the magmatic arc, rapid regional-scale exhumation, and the shallow emplacement depth of plutons (<10 km depth). This likely promoted a sharp transition from high-temperature, ductile deformation to low-temperature, brittle faulting.

Several generations of dykes intruded the Coastal Cordillera after most of the plutons crystallization (≤147 Ma, U-Pb zircon age from the Cerro Cristales granodiorite; Domagala et al., 2016) . Based on orientation and crosscutting relationships of pegmatite and andesitic dykes (Figures 4 and 5) , σ 1 and σ 3 directions should have cyclically switched their orientation, from NW-SE to NE-SW (stage 3 in Figures 11c) . The σ 1 -σ 3 cyclic switching might be related to either (i) several intra-arc sinistral (i.e., NW-SE directed σ 1 ) and dextral (i.e., NE-SW directed σ 1 ) deformation stages, as much as the different dyke sets crosscut each other, as proposed by Scheuber and González (1999) or (ii) intermittent transient stress rotations (i.e., switch of principal stress axes) in the upper plate induced by megathrust earthquakes (Acocella et al., 2018; Becker et al., 2018; Hardebeck & Okada, 2018; Lara et al., 2004; Lupi & Miller, 2014; Lupi et al., 2020; Mancktelow & Pennacchioni, 2020; Pérez-Flores et al., 2016) . The latter interpretation is also supported by the NNE-striking strike-slip faults exploiting the foliation of the CCSZ and the Cerro Cristales tonalite (Figure 7) . Indeed, these foliations are well-oriented for reactivation as dextral strike-slip faults during the transient tectonic regime with NE-directed σ 1 . However, the hypothesis of megathrust earthquakes-related transient stress rotations requires further work to be tested. Finally, the moderately to shallowly dipping aplite dykes are interpreted as related to exhumation occurring during Late Jurassic and Early Cretaceous.

MASOCH ET AL.

10.1029/2021TC006818 20 of 29 

The BFZ fault core is spatially associated and kinematically (sinistral sense of shear) consistent with the brittle-ductile shear zones (Figures 8, 9e, and 10c and 10d) . The latter structures accommodated deformation by combined pressure-solution mechanism, incipient low-temperature crystal plasticity and fragmentation of quartz (Figures 10c and 10d) , suggesting a deformation temperature ≤ 300°C (Stipp et al., 2002) , consistently with their mineral assemblage. The mutual crosscutting relationship between the calcite crack-seal extensional veins and the composite S-C and S-C′ fabric indicates cyclic, transient syn-kinematic extensional fracturing, triggered by cyclic increases of pore fluid pressure, during viscous deformation. This combined diffusive to crystal-plastic and cataclastic deformation is typical of the viscous-plastic to elasto-frictional transition in presence of fluids (e.g., Snoke et al., 1998) . The spatial association of the brittle-ductile shear zones with the fault core is interpreted as the result of the transition from viscous-plastic to elasto-frictional rheology of the BFZ, as for other fault segments of the AFS (e.g., Grocott & Taylor, 2002; Scheuber & González, 1999; Scheuber et al., 1995; Seymour et al., 2020) . The transition may have resulted from (a) different P-T deformation conditions, also during passive exhumation, (b) variations of strain rate experienced by the BFZ, or (c) a combination of (a) and (b). However, the brittle-ductile shear zones are found discontinuously along the BFZ. This could either reflect (a) a change of P-T deformation conditions or strain rate along the fault or (b) their local obliteration due to pervasive fluid-rock interaction and cataclasis.

Indeed, hydrothermal alteration was pervasive during brittle faulting as recorded by chloritization of amphibole and biotite, and albitization/saussuritization of plagioclase. This greenschist-to sub-greenschist-facies alteration indicates temperatures of 250-350°C as well as the stable mineral assemblage of the green cataclasites, including chlorite + epidote + albite + quartz (Figures 10 and Tables 1  and 2) , consistent with the observations from the Caleta Coloso Fault Zone (Arancibia et al., 2014) .

The widespread occurrence of pseudotachylytes documents the ancient seismicity of the BFZ as well as of the strike-slip NW-striking faults cutting the CCSZ (Figures 3, 7 , 9b-9d, and 10f and 10g). Pseudotachylytes are either pristine or strongly altered and spatially associated with epidote-chlorite-bearing veins . This indicates that seismic faulting occurred in presence of fluids . Brittle faulting along the AFS developed once the magmatism waned (Scheuber & Andriessen, 1990; Scheuber & González, 1999) . However, the geothermal gradient remained elevated (∼50°C/km) within the abandoned magmatic arc till ∼100 Ma as documented by the cooling evolution of plutons along the El Salado segment . Thus, such elevated geothermal gradient rose the brittle-ductile transition at 5-7 km depth (Arancibia et al., 2014; Cembrano et al., 2005; Seymour et al., 2020) . As a result, we infer that the ambient conditions for seismogenic faulting were ≤ 300°C and 5-7 km depth in a fluid-rich environment (Figures 11d).

The red-to-purple-colored fault gouges, and the calcite-bearing and hematite-bearing lineated fault surfaces overprint the BFZ s.s (Figures 9a and 9f) . These late Cenozoic faults accommodated normal dip-slip to oblique strike-slip displacement associated with reactivation of the AFS (Figure 11e ). Fault reactivation was associated with a change of the plate convergence from the SE-directed oblique subduction during Jurassic and Cretaceous to the first NE-directed and the latter ENE-directed oblique subduction during Cenozoic (Pardo-Casas & Molnar, 1987; Scheuber & González, 1999 (e.g., González et al., 2003 González et al., , 2006 , associated with the ENE-trending subduction of the Nazca oceanic plate (Veloso et al., 2015 , and references therein). The fault mineral assemblage, including calcite + palygorskite + halite + gypsum + hematite, indicates temperatures < 150°C (e.g., Bradbury et al., 2015; Morton et al., 2012) . The local occurrence of type II twins in calcite grains and clasts within veins suggests however that the deformation temperatures were locally ≥200°C (Ferrill et al., 2004) . Cenozoic faulting occurred at shallow crustal levels (<2-3 km depth), consistently with the stratigraphic constraints, as indicated by (a) well-developed S-C foliation within fault gouges associated with plastic deformation of gypsum (Figure 10h) (b) and low-temperature twinning of calcite within the veins (Ferrill et al., 2004) .

The BFZ has a sinuous fault trace and, although being mostly sub-vertical, the BFZ dip changes from SW to W (northern segment: Playa Escondida and Sand Quarry localities) and NE (southern segment: Quebrada Larga locality) (Figure 3 ). This change in dip depends on the control on the BFZ orientation by precursory anisotropies as observed for several mesoscale faults hosted in crystalline basement rocks elsewhere (e.g., d'Alessio & Martel, 2005; Di Toro & Pennacchioni, 2005; Griffith et al., 2008) . Indeed, the BFZ exploited the magmatic foliation of the Bolfin Complex and the Cerro Cristales Pluton along its northern (Playa Escondida and Sand Quarry localities) and central segments (Quebrada Corta and Ni Miedo localities) (Figures 3,  6 , and 8). The NW-striking subsidiary cataclasites and associated pseudotachylytes within the damage zone nucleated on NE-dipping andesitic dykes within the isotropic tonalites and granodiorites of the Cerro Cristales Pluton along the southernmost segment (Quebrada Larga locality) (Figures 3 and 9d) .

The reactivation of a precursory structure is controlled by its orientation with respect to the local stress field. In the Bolfin area, the brittle faults of the AFS are organized in strike-slip duplexes, which partitioned deformation into hierarchically arranged faults, and the BFZ is a third-order fault splaying out from the second-order Caleta Coloso Fault Zone . Thus, in the framework of sinistral intra-arc deformation imposed by the ancient subduction of the Aluk (Phoenix) plate, that is, NW-SE sub-horizontal σ 1 (Figures 11d) (Brown et al., 1993; Cembrano et al., 2005; Scheuber & González, 1999; Veloso et al., 2015) , the optimal direction for third-order splay faults to accommodate sinistral strike-slip shearing should be ∼NNW-SSE. As a result, the magmatic foliation of the foliated meta-diorites, tonalites, and granodiorites was optimally oriented to be reactivated as a sinistral strike-slip fault in the Early Cretaceous tectonic framework ( Figure 12a) . Instead, the NE-dipping andesitic dykes were favorably oriented for NNW-striking sinistral strike-slip fault reactivation related to a NW-SE sub-horizontal σ 1 . Several studies however pointed out that faults interaction perturb the regional stress field at fault tip and linkage causing a local stress reorientation (e.g., d'Alessio & Martel, 2004; Kim et al., 2003 Kim et al., , 2004 Pachell & Evans, 2002; Segall & Pollard, 1983) . Thus, the exploitation of the andesitic dykes may also be partly related to local stress reorientation induced by the interaction between the southernmost proto-segment of the BFZ and the central proto-segment of the Caleta Coloso Fault Zone (Figure 12b ). On the contrary, where misoriented, the precursory structures are cut by the BFZ, which, for instance, displace the CCSZ of 1 km (between Quebrada Museo and CCSZ-North localities).

We propose that the nucleation of the BFZ occurred through the exploitation of favorably oriented precursory geometrical anisotropies (i.e., magmatic foliations and dykes). Thus, the BFZ formed as a series of overstepping anisotropy-pinned fault segments (Figure 12b ). During fault growth, NW-striking splay and horsetail linkage faults developed at the tip of these fault segments (e.g., Fault Bend locality) (Figure 12c ). The progressive growth of the BFZ occurred through hard linkages of anisotropy-pinned fault segments related to the precursory evolution of the magmatic arc and explained the complex and sinuous geometry of the BFZ (Figure 12 ).

Based on this model, we propose that magmatic-related structures, such as foliated plutons whose magmatic foliation can extend for several kilometers and dyke swarms, play a pivotal role in controlling the geometry of crustal-scale faults within magmatic arcs, as do cooling joints at the scale of mesoscale faults within a single pluton (e.g., Di Toro & Pennacchioni, 2005; Pennacchioni et al., 2006; Segall & Pollard, 1983; Smith et al., 2013) . Indeed, the exploitation of km-long foliated plutons and dyke swarms (fault nucleation stage) and consequent linkage of anisotropy-controlled segments (fault growth stage) could lead to the formation of non-planar faults with either sinuous trace, as the case of the BFZ, and concave-shaped trace, such as the first-order faults of the AFS. The latter was partially documented along the El Salado segment (Figure 1a) , where the main fault branch exploited the mylonitic foliation of syn-magmatic thermal aureoles bounding several Late Jurassic to Early Cretaceous plutons (Brown et al., 1993; Espinoza et al., 2014; Grocott & Taylor, 2002; Seymour et al., 2020) . Fault localization along these anisotropies might be promoted by the syn-kinematic emplacement of both the Late Jurassic-Early Cretaceous plutons, which are ∼N-S-elongated, and dyke swarms, controlled by the same far-stress field associated with brittle faulting along the AFS.

We described the spatial and temporal distribution of dykes, magmatic and solid-state foliations, and brittle faults along the seismogenic BFZ and the syn-to post-magmatic CCSZ in the Coastal Cordillera in northern Chile (Figures 1-3) . By combining field geologic surveys, analysis of satellite and drone images, and microstructural and microanalytical observations, we reconstructed the spatio-temporal evolution of the BFZ, a >40-km-long seismogenic splay fault of the 1000-km-long strike-slip AFS. The structural evolution of the BFZ includes five main deformation stages ( Figure 11 The crustal-scale BFZ has a sinuous geometry, which is controlled by precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and dyke swarms (southern segments) (Figure 3 ). These precursory structures were favorably oriented to be reactivated with respect to the inferred long-term stress field associated with the ancient oblique subduction. We propose a conceptual model of fault growth including (i) the exploitation of these favorably oriented precursory anisotropies during fault nucleation and (ii) hard linkage of these anisotropy-pinned fault segments during fault growth, leading to the formation of the sinuous geometry of the BFZ (Figure 12 ). The fault evolution proposed for the BFZ may be possibly extended to the formation of the AFS and applied to other crustal-scale faults in the crystalline basement associated with widespread magmatism.

All the structural data are available at the repository with the following link: http://researchdata.cab.unipd. it/id/eprint/443.

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This study was funded by the European Research Council Project (NOFEAR) 614705 (Simone Masoch, Rodrigo Gomila, Michele Fondriest, and Giulio Di Toro)