Atacama Fault Research Articles

Exploring frictional properties of upper plate fault reactivation in subduction zones: The Atacama Fault System in northern Chile

The Maule 2010 and Tohoku-Oki 2011 earthquakes demonstrated how dormant upper plate faults can be reactivated as normal faults by plate margin relaxation following megathrust slip. However, the reactivation mechanisms of these types of faults are yet unexplored. To provide a better understanding of these mechanisms, we collected fault core samples from fault segments of the Atacama Fault System in northern Chile. The sampled fault segments have clear morphological evidence of Quaternary reactivation as normal fault. We performed laboratory experiments to measure the fault strength, stability and dynamic weakening. We consider low-velocity tests for exploring the frictional strength and velocity dependence of friction via a double-direct shear apparatus and ii) high-velocity tests for investigating the frictional properties at seismic velocities via a rotary shear apparatus. The experiments revealed that fault cores have low frictional strength, velocity-strengthening behaviour and strong dynamic weakening. Additionally, a novel experimental procedure that simulates stress relaxation by stepwise reducing of the normal stress on the sample assembly showed: 1. Accelerating creep towards dynamic weakening in chlorite-rich gouges and 2. oscillatory sliding in fault gouges enriched in illite. By extrapolating our experimental observations to natural conditions, we conclude that stable sliding is favoured during the interseismic phase of the subduction earthquake cycle, whereas unstable sliding is favoured during the coseismic and postseismic phases. The latter occurs via normal stress reduction during the shift from interseismic compression to co- and postseismic tension at the plate margin.

Lithological control of the active Mejillones fault from Triassic to Quaternary, northern Chile

Understanding the lithological control of crustal faults near cities is important for assessing the seismic hazard and characterising complex fault systems. We studied the Mejillones Fault (MF), located near one of the strategic ports in northern Chile. The MF is an active geological fault that extends 40 km in the N-S direction and is part of an extensive complex system of structures known as the Atacama Fault System (AFS). We built an S-wave tomography model using seismic ambient noise data that was recorded continuously in a dense network of stations from 2013 to 2015. The tomographic maps exhibited velocity anomalies that correlate with surface geology. We found lower velocities in the western part of the MF than in the eastern zone, with a shear wave velocity (Vs) of about 2.4 km/s, associated to fractured metamorphic rocks (Triassic) and marine sediments (Neogene-Quaternary). Higher velocities (Vs > 3.0 km/s) in the eastern part of the fault are related to igneous rocks from the Upper Jurassic. The difference in velocities between the western and eastern zones of the MF remained persistent in depth. The potential depth of the MF could be >5 km and delimits the boundary between 2 different geological zones. Intermediate Vs (∼2.8 km/s) at −23.25°, observed up to 4 km depth, can be associated with andesitic rocks deposited in an intra-arc basin (Upper Triassic) and accommodated by the MF over the last ∼200 Myr, evidencing the activity of this fault from Triassic to Quaternary.

Structural Characterization of the Taltal Segment in Northern Chile Between 22°S and 26°S Using Local Earthquake Tomography

AbstractRecordings of earthquakes by a temporary deployment of 84 short period seismometers in northern Chile were used to derive regional 3D seismic velocity models for the Taltal segment. We used the Regressive ESTimator (REST) package for event detection and automatic onset estimation of P‐ and S‐wave arrival times to create an earthquake catalog with 23,985 hypocenters. We followed standard acceptability criteria (i.e., azimuthal gap and residual cutoff) to create a high‐quality data set and inverted for 3D Vp, Vs and Vp/Vs models using local earthquake tomography. Plots of hypocenters from the catalog, comprising 16,349 earthquakes, reveal active structures in the upper crust, dip changes along the slab and fracturing within the oceanic crust. Moreover, the wavespeed models illuminate anomalies in both the Nazca and South American plates that correlate with the observed seismicity distribution, including variations from low (1.75) to high (>1.80) Vp/Vs near the Atacama fault system on the coastline and the Domeyko Fault System in the forearc. The seismic velocity models also provide evidence for fluid circulation caused by the subducting Taltal ridge on the coast and partial melting feeding a volcanic complex close to the Andes. Finally, the observed low Vp/Vs ratios (∼1.75) are associated with copper mining operations in the area, suggesting that this kind of imaging can be used to characterize the distribution of potential ore deposits in the area.

Sodic-calcic alteration and transpressional shear along the Atacama fault system during IOCG mineralization, Copiapó, Chile

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.

Tectonic setting, structures, and Au–Cu mineralization age of the Indiana deposit: An example of ore deposit formation controlled by Andean transverse faults, Atacama region, Chile

The Indiana Deposit corresponds to a Cu–Au (Mo–Co) fault-vein deposit located in the Central Andes Coastal Cordillera Belt (∼27°S). It is hosted by strongly altered volcanic and intrusive rocks located between the NNE–striking central and principal branches of the Atacama Fault System (AFS) in northern Chile. The Indiana Deposit is part of a 10 km-long and 5 km-wide mine district that includes the Iron–Copper bearing Cerro Negro Norte (CNN) deposits and the Cu–Au Galleguillos vein system, termed the CNN-Indiana District in this study. The district provides insights into the mid-Cretaceous structural and lithological control of iron, copper and gold mineralization in northern Chile.Mineralization in the Indiana Deposit is spatially and temporally related to NW- and ENE-striking fault-vein systems, which are kinematically unrelated to the AFS. Three main mineralized ENE-striking, hundred-meter-long structures have been identified in Indiana. Structural and kinematic data suggest that mineral precipitation took place at dilation zones (e.g. extensional jogs, transtensional fault terminations) generating steeply-plunging ore shoots, and reactivating pre-existing structures (NW fault-vein system).Sinistral transtensional kinematics observed in ENE-striking fault-vein structures postdate the activity of the margin-parallel Atacama fault system, which has for long been regarded as accommodating the strike-slip component of plate convergence during the Jurassic to middle Cretaceous neglecting the role of Andean Transverse Faults (ATF), such as the ones recognized in this study. Sinistral transtension along ENE-striking faults gave rise to a local NE-striking maximum principal stress, different from that arising from displacement along the main margin-parallel faults, which in turn is consistent with far field stresses at that time.The CNN-Indiana district evolved from an early Ca–Na high temperature system related to Iron-bearing deposits to a brittle mineralized fault-vein copper-gold-bearing quartz-sericite (k-feldspar-calcite) corridor associated with a lower temperature environment along the Costal belt.Re–Os ages yielded 109-108 Ma obtained along the mineralized ENE-striking structures in Indiana Deposit and are slightly younger than those obtained from similar deposits associated with deformation along the AFS, which is consistent with our field observations indicating that at least part of the strike slip displacement on transverse faults hosting mineralization outlasts that of the AFS.

The Montecristo mining district, northern Chile: the relationship between vein-like magnetite-(apatite) and iron oxide-copper–gold deposits

The Montecristo district, northern Chile, is one of the few places worldwide where there is a direct relationship between magnetite-(apatite) (MtAp) mineralization and iron oxide-copper–gold (IOCG) mineralization. The MtAp mineralization includes Ti-poor magnetite, fluorapatite, and actinolite and is crosscut and partially replaced by a younger IOCG mineralization that includes a second generation of actinolite and magnetite with quartz, chalcopyrite, pyrite, and molybdenite. The MtAp stage at Montecristo is interpreted as the crystallized iron-rich melts that used the pre-existing structures of the Atacama Fault System as conduits. These rocks later acted as a trap for hydrothermal IOCG mineralization. Geochronology data at Montecristo indicate that the host diorite (U–Pb zircon 153.3 ± 1.8 Ma, 2-sigma), MtAp mineralization (40Ar-39Ar in actinolite, 154 ± 2 Ma and 153 ± 4 Ma, 2-sigma), and the IOCG event (Re-Os on molybdenite, 151.8 ± 0.6 Ma, 2-sigma) are coeval within error and took place in a time span of less than 3.4 Ma. The εHfi and εNdi values of the host diorite are + 8.0 to + 9.8 and + 4.3 to + 5.4, respectively. The whole-rock 87Sr/86Sri values of the IOCG mineralization (0.70425 to 0.70442) are in the lower end of those of the MtAp mineralization (0.70426–0.70629). In contrast, εNdi values for the IOCG mineralization (+ 5.4 and + 5.7) fall between those of the MtAp rocks (+ 6.6 to + 7.2) and the host diorite, which suggests that the IOCG event was related to fluids having a more crustal Nd (εNdi < + 5.7) composition than the MtAp mineralization. This likely reflects the mixing of Nd from the MtAp protolith and a deep magmatic-hydrothermal source, very likely an unexposed intrusion equivalent to the host diorite. Sulfur isotope compositions (δ34S, + 0.3 to + 3.4‰) are consistent with a magmatic source.

Along-strike architectural variability of an exhumed crustal-scale seismogenic fault (Bolfin Fault Zone, Atacama Fault System, Chile)

Fault zone architecture and its internal structural variability play a pivotal role in earthquake mechanics, by controlling, for instance, the nucleation, propagation and arrest of individual seismic ruptures and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the along-strike architectural variability of crustal-scale seismogenic sources over regional distances is still poorly investigated. Here, we describe the architectural variability of the >40-km-long exhumed, seismogenic Bolfin Fault Zone (BFZ) of the intra-arc Atacama Fault System (Northern Chile). The BFZ cuts through plutonic rocks of the Mesozoic Coastal Cordillera and was seismically active at 5–7 km depth and ≤ 300 °C in a fluid-rich environment. The BFZ includes multiple altered fault core strands, consisting of chlorite-rich cataclasites-ultracataclasites and pseudotachylytes, surrounded by chlorite-rich protobreccias to protocataclasites over a zone up to 60-m-thick. These fault rocks are embedded within a low-strain damage zone, up to 150-m-thick, which includes strongly altered volumes of dilatational hydrothermal breccias and clusters of epidote-rich fault-vein networks at the linkage of the BFZ with subsidiary faults. The strong hydrothermal alteration of rocks along both the fault core and the damage zone attests to an extensive percolation of fluids across all the elements of the structural network during the activity of the entire fault zone. In particular, we interpret the epidote-rich fault-vein networks and associated breccias as an exhumed example of upper-crustal fluid-driven earthquake swarms, similar to the presently active intra-arc Liquiñe-Ofqui Fault System (Southern Andean Volcanic Zone, Chile).

The Effect of Fault Roughness and Earthquake Ruptures on the Evolution and Scaling of Fault Damage Zones

AbstractFault damage zones dominate the mechanical, hydraulic and seismological properties of faults yet the relative contributions from processes leading to their development and growth is obscure. In this study, we investigate the damage development related to slip on rough faults and passage of earthquake ruptures. We compare the cumulative damage with slip and damage distribution from numerical models against field data from exhumed faults with slip less than 3.5 m within the Atacama Fault Zone in northern Chile. Models are constrained by experimentally determined mechanical properties of the host rock. We perform simulations of damage accumulation during quasistatic slip on rough faults and during sequences of earthquakes on planar and rough faults governed by rate and state friction laws. Both sets of simulations include Drucker–Prager rheology of the bulk to identify off‐fault damage where the yield stress is exceeded. Our results indicate that the extent and distribution of damage depend on the characteristics of fault roughness, amount of slip, and, when present, the intensity and variability of dynamic ruptures. When typical values for fault roughness are used, the scaling of damage zone width versus slip during quasistatic slip is comparable to that observed in the field data. Earthquake rupture on smooth faults by itself does not explain the field data. Simulations of earthquake sequences on rough faults leads to significantly larger damage zone widths with slip than that observed in the field data, suggesting the development of damage for small displacement is dominated by quasistatic slip on rough faults.

Geology and mineralogy of sapphire-rich metasomatites (sapphirites) deposit at the Portezuelo de Pajas Blancas, northern Chile: Genetic implications for unusual metasomatic processes in the Central Andes

The Portezuelo de Pajas Blancas deposit, located in northern Chile, represents a unique opportunity to constrain unusual geological processes involved in the genesis of sapphire-rich metasomatites in the central Andes. For this purpose, geological mapping was combined with conventional and high-resolution petrographic and mineralogical analysis (light microscope, SEM-EDS and automated mineralogy). Results demonstrate that this uncommon occurrence is genetically related to eight main lithological units that host variable proportions of corundum (sapphire), sillimanite, andalusite, diaspore, böhmite, dravite, magnesiofoitite, olenite, alunite, and calcite. Microtextural relationships between mineralogical assemblages demonstrate that the origin of this deposit was associated with two main genetic stages. The early stage is linked to the initial crystallisation of abundant sapphire, promoted by an extreme desilication during high-temperature prograde metasomatic processes due to the intrusion of a Cretaceous granodioritic magma body (Si-Al-rich) into basaltic rocks from the La Negra Formation (Al-rich Si-poor) through the Atacama Fault System. The late-stage corresponds to hydrothermal alteration of sapphires to böhmite and diaspore in the deep and shallow zones, respectively, where dravite and alunite were precipitated directly from these fluids. The formation of this late hydrothermal assemblage can be interpreted as an overprinting of a metasomatic retrograde system. This study demonstrates that the sapphire-bearing rocks at Portezuelo de Pajas Blancas correspond to primary metasomatic sapphirites related to middle- to high-temperature contact metasomatism at estimated pressures of ∼ 2 kbar.

The role of interplate locking on the seismic reactivation of upper plate faults on the subduction margin of northern Chile

Quaternary deformation in the northern Chile forearc is controlled by trench parallel shortening along reactivated Mesozoic faults. Dextral strikes-slip is expressed in NW–SE striking faults of the Atacama Fault System, and reverse displacement dominates in E–W faults. This deformation results of the convergence in a concave-seaward continental margin. On September 11th, 2020, a Mw 6.3 earthquake and its subsequent aftershocks took place in the coastal region of northern Chile, revealing the reactivation of the deepest segment of a WNW–ESE striking upper plate fault. The reactivation of this fault occurred after the Mw 8.1 Iquique earthquake, and it seems to be connected to a N–S interplate locking segmentation of the plate margin, which is clearly shown by the locking pattern before the Iquique earthquake. This poses the question of how heterogeneous locking influences upper plate seismicity and how it relates to trench-parallel shortening.

Relation between intrusive and deformational processes in oblique subductive margins. The case of the zoned Flamenco pluton in northern Chile

Oblique convergent margins, like the subduction of Phoenix beneath the South American plate during Jurassic and Early Cretaceous times, are characterized by strain partitioning and a positive feedback loop between strike-slip deformation and magma ascent along the magmatic arc.Located in the Coastal batholith of northern Chile, the Flamenco pluton is one of the youngest Andean intrusives emplaced in the western active margin of South America. Besides an older SW domain of granodioritic rocks (c.a. 213 Ma), the NW and E domains of the Flamenco pluton were emplaced between 194 and 186 Ma. They present a normally zoned structure constituted by external gabbroic to Qtz-dioritic magmatic facies and tonalites and granodiorites located in inner areas of the intrusive body. These domains are separated by a central strip of stretched coarse-grained Crd-schists that presents ductile asymmetrical folding and S–C structures that point to the NW-directed displacement of the E domain of the pluton. This syn-emplacement shear zone shows kinematic compatibility and continuity to the north and south with folded and mylonitic metasediments out of the contact aureole of the pluton. Together, these segments constitute a large, steeply dipping sigmoidal structure of average N–S direction; the called here Chañaral transcurrent shear zone.Contemporary to the emplacement of the Flamenco pluton, slight variations in the trend of the crustal-scale structure generated strike-slip and transpressive sectors along the Chañaral shear zone, which favored the access of intruding magmas to the final emplacement level. As a paradigmatic example, the curviplanar Flamenco shear zone, an internal, magmatic branch of the main structure that traverses the E domain of the pluton, is defined by the sinistral and reverse shearing under magmatic conditions of the previously mingled mafic and felsic batches. Consequently, the transpressive Flamenco shear zone is interpreted as an ascent conduit where gabbroic and granodioritic liquids interacted during the building of the intrusive body. In addition, these sheared rocks were affected by late textural coarsening processes that evidence the slow and cyclical cooling of the growing magma reservoir. In contrast with the steeply dipping contacts and structures found to the east, the NW domain of the pluton shows sharp and gently dipping contacts between almost horizontal magmatic layers. We suggest that the western block of the Chañaral shear zone was a relatively passive footwall dominated by horizontal flow trajectories and lower replenishment rates according to the inverse emplacement sequence, i.e., late external mafic batches intruded along the margins of the felsic core. The variable structural arrangement of the crustal rocks that hosted the Flamenco pluton was the result of the complex interaction between far-field and local, magmatic forces during the emplacement process, besides the interference with pre-Andean structures.The presence of the Chañaral shear zone favoring the emplacement of the Flamenco pluton demonstrates that the Late Jurassic to latest Early Cretaceous Atacama Fault System had earlier precursors and both the magmatic arc axis and the transcurrent shear zones migrated landward during Jurassic times.

Structural Evolution of a Crustal-Scale Seismogenic Fault in a Magmatic Arc: The Bolfin Fault Zone (Atacama Fault System).

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.

Frictional Melting in Hydrothermal Fluid-Rich Faults: Field and Experimental Evidence From the Bolfín Fault Zone (Chile).

Tectonic pseudotachylytes are thought to be unique to certain water‐deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid‐rich faults hosted in the continental crust. Pseudotachylytes were found in the >40 km‐long Bolfín Fault Zone of the Atacama Fault System, within two ca. 1 m‐thick (ultra)cataclastic strands hosted in a damage‐zone made of chlorite‐epidote‐rich hydrothermally altered tonalite. This alteration state indicates that hydrothermal fluids were active during the fault development. Pseudotachylytes, characterized by presenting amygdales, cut and are cut by chlorite‐, epidote‐ and calcite‐bearing veins. In turn, crosscutting relationship with the hydrothermal veins indicates pseudotachylytes were formed during this period of fluid activity. Rotary shear experiments conducted on bare surfaces of hydrothermally altered rocks at seismic slip velocities (3 m s−1) resulted in the production of vesiculated pseudotachylytes both at dry and water‐pressurized conditions, with melt lubrication as the primary mechanism for fault dynamic weakening. The presented evidence challenges the common hypothesis that pseudotachylytes are limited to fluid‐deficient environments, and gives insights into the ancient seismic activity of the system. Both field observations and experimental evidence, indicate that pseudotachylytes may easily be produced in hydrothermal environments, and could be a common co‐seismic fault product. Consequently, melt lubrication could be considered one of the most efficient seismic dynamic weakening mechanisms in crystalline basement rocks of the continental crust.

Quantitative anisotropies of palaeopermeability in a strike-slip fault damage zone: Insights from micro-CT analysis and numerical simulations

Fracturing and damage around faults related to seismogenesis can enhance hydrothermal fluid percolation, causing mineral precipitation. This study uses hydrothermally sealed microfractures across an ancient exhumed fault to unravel the 3D-spatial distribution of fault damage and related anisotropy in permeability. We studied the fault damage zone of the Jorgillo Fault, a left-lateral strike-slip fault, exposed by ca. 20 km in the Atacama Fault System, northern Chile. The study was conducted by addressing the 3D-spatial distribution of the microfracture network through X-ray micro-computed tomography and palaeopermeability modeling using a computational fluid dynamic approach, thus assessing mm-scale fault-related permeability tensors. 3D modeled fault-directed permeability ellipsoids on both sides of the fault core are transverse anisotropic, where palaeopermeability (matrix permeability) in the fault-parallel plane is higher than across-strike of the Jorgillo Fault (2.4 and 1.9 times in the eastern and western block of the fault, respectively). Modeled 3D permeability values (ca. 10−11 to 10−15 m2) show a mean overestimation factor of 8.4 of the estimated 2D permeability (ca. 10−9 to 10−12 m2). Permeability anisotropy distribution in the damage zone is related to off-fault damage generation, and could be explained by tip propagation fault growth and dynamic rupture due to earthquakes under the fault-valve mechanism. Whereas the fault would act as an impermeable seal except for post-failure, when it became highly permeable for fluids.

Magnitude, timing, and rate of slip along the Atacama fault system, northern Chile: implications for Early Cretaceous slip partitioning and plate convergence

Displacement estimates along the Atacama fault system (AFS), a crustal-scale sinistral structure that accommodated oblique convergence in the Mesozoic Coastal Cordillera arc, vary widely due to a lack of piercing points. We map the distribution of plutons and mylonitic deformation along the northern c. 70 km of the El Salado segment and use U–Pb geochronology to establish the slip history of the AFS. Along the eastern branch, mylonitic fabrics associated with the synkinematic c. 134–132 Ma Cerro del Pingo Complex are separated by 34–38 km, and mylonites associated with a synkinematic c. 120–119 Ma tonalite are separated by 20.5–26 km. We interpret leucocratic intrusions to be separated across the western branch by c. 16–20 km, giving a total slip magnitude of c. 54 ± 6 km across the El Salado segment. Kinematic indicators consistently record sinistral shear, and zircon (U–Th)/He data suggest dip-slip motion was insignificant. Displacement occurred between c. 133–110 Ma at a slip rate of c. 2.1–2.6 km Myr –1 . This slip rate is low compared to modern intra-arc strike-slip faults, suggesting (1) the majority of lateral slip was accommodated along the slab interface or distributed through the forearc or (2) plate convergence rates/obliquity were significantly lower than previously modeled. Supplementary material: Full U-Pb, (U-Th)/He, petrographic, and structural data with locations is available at https://doi.org/10.6084/m9.figshare.c.5262177 Thematic collection: This article is part of the Isotopic dating of deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

Neotectonic evidence for Late Quaternary reverse faulting in the northern Chile outer forearc (22.5°S-23°S): Implications for seismic hazard

In the northern Chile outer forearc (19°S to 23°S), extension is the dominant deformation style conducted by crustal faults during the Neogene-Quaternary. Most of this extension has been produced by normal faulting along submeridian branches of the Atacama Fault System (AFS). During the Late Quaternary, several of these faults have built conspicuous metric-scale normal faulting scarps in alluvial deposits; the construction of these scarps has been related, in some cases, to M~7 normal faulting paleoearthquakes. On the contrary, shortening during this time span has only been locally reported. Until now, no metric-scale reverse faulting scarps have been documented along submeridian strands of the AFS. This study presents novel evidence corresponding to reverse fault scarps produced during the Late Quaternary by three NNE and NNW trending faults in the outer forearc between 22.5°S and 23°S: the El Toro (ETF), Caliche (CF), and Sierra Valenzuela (SVF) faults. These scarps, with up to 2.5 m height, developed in basement rocks, pre-Quaternary, and/or Quaternary alluvial deposits. Based on their characteristics and the evidence suggesting that similar fault scarps in the Andean forearc have a seismogenic origin, we interpret that these neotectonic landforms were produced by single reverse faulting paleoearthquakes. Considering the maximum scarp height as a proxy for the coseismic slip and fault length as a proxy for surface rupture length, we have estimated magnitudes between M6.2 and M6.9 for the causative rupture events. To discuss how reverse crustal faulting may be related to the interseismic and/or coseismic stages of the subduction cycle, we perform elastic dislocation models to calculate the horizontal displacement fields (at 0 and 20 km depth) induced by the interplate locking distribution in the area (over a seismic cycle) and three megathrust earthquakes (Antofagasta, 1995, Tocopilla, 2007, and a synthetic rupture). Modeling results suggest that the studied faults may experience reverse slip during both stages of the subduction cycle. We propose that the ETF, CF and SVF are faults capable of producing moderate-to-large earthquakes in the future. Finally, we reinforce the need to address the earthquake potential of the studied faults, and others similar to them, to acquire a complete assessment of the seismic hazard in the northern Chile outer forearc.

Tectonic setting of Cretaceous porphyry copper deposits of northern Chile (28°-30° S) and its relations with magmatic evolution and metallogeny

Cretaceous porphyry copper deposits of northern Chile (28º-29º30’ S) are genetically related with dacitic to dioritic porphyries and they represent a still poorly-explored target for Cu resources. The porphyries correspond to stocks distributed into two separated discontinuous NS trending belts of different age. The location of these porphyries is generally adjacent to orogen-parallel major fault systems that extend along the studied segment and also have a marked temporal relationship with deformation events registered along these structures. A first episode of Cu-bearing porphyry emplacement took place between 116 and 104 Ma (Mina Unión or Frontera, Cachiyuyo, Punta Colorada, Dos Amigos, Tricolor porphyries). These Early Cretaceous dacite to diorite porphyries are spatially associated with the eastern segments of the Atacama Fault System, which records sinistral transpression that started at 121 Ma producing ground uplift, consequent denudation and exhumation of the Early Cretaceous magmatic arc. This resulted in a change from marine to continental deposition with an angular unconformity in the site of the back-arc basin after of eastward migration of the deformation around 112-110 Ma. At the scale of the continental margin, this deformation is correlated with early stage of the Mochica Orogenic event described in Perú. A second episode of Cu-bearing porphyry emplacement occurred between 92 and 87 Ma (Elisa, Johana, Las Campanas and La Verde deposits), which are spatially and temporally associated with the regional-scale Las Cañas-El Torito reverse fault, active between 89 and 84 Ma, during the Peruvian Orogenic Phase. This fault up thrust to the west part of the Chañarcillo Group rocks (Lower Cretaceous) over the younger upper levels of the Cerrillos Formation (Upper Cretaceous). The integrated geological mapping and geochemical data of the Early to Late Cretaceous volcanic rocks indicates that both Early Cretaceous sinistral transpression and Late Cretaceous east-west compression were not significant in promote changes in magma genesis, except for slight changes in trace element ratios (increase in Th/Ta, Nb/Ta and La/Yb) suggesting that the Late Cretaceous deformation event produced only slightly increase in crustal thickness (&gt;40 km), but far from being comparable to major Cenozoic orogenic phases, at least along the magmatic arc to back-arc domains in the study area. Finally, our study give insights about regional geological parameters that can be used as a first order guide for exploration of Cu resources along Cretaceous magmatic belts of northern Chile, where both Early and Late Cretaceous Cu-bearing porphyry intrusions are restricted to a large structural block bounded to the west and east by Cretaceous fault systems.

The role of the subducting slab and melt crystallization in the formation of magnetite-(apatite) systems, Coastal Cordillera of Chile

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 eNdi 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 eNdi 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.

The Relationship Between Magmatism and Deformation Along the Intra‐arc Strike‐Slip Atacama Fault System, Northern Chile

AbstractMagmatic arcs may play a major role in the initiation, behavior, and abandonment of intra‐arc strike‐slip systems. Here we present zircon U‐Pb and (U‐Th)/He geochronology/thermochronology with new mapping to relate Coastal Cordillera arc magmatism to sinistral shear along the Atacama fault system (AFS) in northern Chile. New dates from 18 intrusions along the AFS between 24.6°S and 27°S compiled with published data record a minor Early Jurassic magmatic pulse (195–175 Ma), broad latest Jurassic to Early Cretaceous (150–120 Ma) pulse, and a minor younger (120–105 Ma) pulse. Mylonitization occurred only along the margins of Early Cretaceous plutons and surrounding Paleozoic metasedimentary rock, whereas Jurassic plutons and metasedimentary rocks away from Early Cretaceous plutons lack mylonitic fabrics. Early Cretaceous magmatism facilitated AFS deformation by thermally weakening the crust with elevated geothermal gradients that enabled mylonitization to take place at ~5‐ to 7‐km depths and low stresses. Spatial variability of pluton emplacement produced significant rheological heterogeneity, giving rise to a highly segmented fault system that did not originate as a regional‐scale shear zone. Synkinematic dikes (~120–117 Ma) cut mylonitic fabrics, and a postkinematic dike (~110 Ma) records the end stages of slip. The cessation of slip coincided with cooling below ~180 °C at ~116–99 Ma as arc magmatism migrated eastward and geothermal gradients relaxed, coeval with a major reorganization in plate motion and the onset of seafloor spreading in the south Atlantic.

Timing, Kinematics, and Displacement of the Taltal Fault System, Northern Chile: Implications for the Cretaceous Tectonic Evolution of the Andean Margin

AbstractAn extensive system of NW striking faults constitutes a major tectonic feature of the Coastal Cordillera in northern Chile, but fundamental questions remain about timing and kinematics of these structures. We present new geologic mapping and geochronology that provide insight into the structural evolution and tectonic significance of the Taltal fault system (TFS). The TFS displaces the Early Cretaceous arc‐parallel Atacama fault system (AFS) with ~10.6 km cumulative offset across a ~15 km wide zone. Brittle fault data demonstrate that the TFS is vertical to steeply NE dipping with an average sinistral slip vector plunging 11° from the NW, compatible with E‐W shortening. Two late Early Cretaceous dikes cut the AFS but are cut by TFS faults, and synkinematic calcite on a TFS strand yielded a U‐Pb calcite date of 114.1 ± 7.0 Ma. These data demonstrate that the AFS was abandoned and deformation (re) initiated on the TFS between ~114–107 Ma, with continued slip after intrusion of the Tropezón (~110 Ma) and Librillo (106–101 Ma) plutonic complexes. Emplacement of a ~146 Ma rhyolite dike along the main Taltal fault and 141 ± 11 Ma calcite mineralization in the fault core suggests that a precursor structure influenced magma emplacement and fluid flow in the Late Jurassic/Early Cretaceous, supporting the hypothesis that the TFS reactivated long‐lived inherited crustal weaknesses. The Early Cretaceous shift from arc‐parallel shear to slip on the TFS and E‐W shortening shortly preceded migration of the magmatic arc and records a change in the Chilean margin subduction dynamics.