Atacama Fault Research Articles

On the structure and mechanical properties of large strike-slip faults

Abstract Elucidation of the internal structure of fault zones is paramount for understanding their mechanical, seismological and hydraulic properties. In order to observe representative brittle fault zone structures, it is preferable that the fault be passively exhumed from seismogenic depths and the exposure must be in arid or semi-arid environments where the fragile rocks are not subject to extensive weathering. Field observations of two such faults are used to constrain their likely mechanical properties. One fault is the Carboneras fault in southeastern Spain, where the predominant country rocks are phyllosilicate-rich lithologies, and the other is part of the Atacama fault system in northern Chile, where faults pass through crystalline rocks of acidic to intermediate composition. The Carboneras fault is a left lateral fault with several tens of kilometres offset exhumed from approximately 4 km depth, and displays multiple strands of clay-bearing fault gouge, each several metres wide, that contain variably fractured lenses of protolithic mica schists. The strain is evenly distributed across the gouge layers, in accordance with the measured laboratory mechanical behaviour which shows predominantly strain hardening characteristics. The overall width of the fault zone is several hundred metres. Additionally, there are blocks of dolomitic material that are contained within the fault zones that show extremely localized deformation in the form of faults several centimetres wide. These are typically arranged at an angle of c . 20° to the overall fault plane. These differing types of fault rock products allow for the possibility of ‘mixed mode’ seismicity, with fault creep occurring along the strands of velocity strengthening clay-rich gouge, punctuated by small seismic events that nucleate on the velocity weakening localized faults within the dolomite blocks. The Caleta Coloso fault in northern Chile has a left-lateral offset of at least 5 km and was exhumed from 5–10 km depth. The fault core is represented by a 200–300 m wide zone of hydrothermally altered protocataclasite and ultracataclasite. This is surrounded by a zone of micro and macro-fractures on the order of 150 m thick. The fault core shows a heterogeneous distribution of strain, with alternate layers of ultracataclasite and lower strain material. The strain-weakening behaviour of crystalline rocks might be expected to produce highly localized zones of deformation, and thus the wide core zone must be a result of additional process such as precipitation strengthening or geometric irregularities along the fault plane.

Hydrothermal alteration and magnetic properties of rocks in the Carolina de Michilla stratabound copper district, northern Chile

In the Carolina de Michilla district, northern Chile, stratabound copper mineralization is hosted by Jurassic volcanic rocks along the trace of the Atacama fault system. In this study, we present the overall effects of hydrothermal alteration on the magnetic properties of rocks in this district. Two types of metasomatic alteration associations occur, one of regional extent and the other of local hydrothermal alteration associated with copper mineralization (e.g., Lince–Estefania–Susana). Regional alteration is interpreted as a low-grade “propylitic association” characterized by an epidote–chlorite–smectite–titanite–albite–quartz–calcite association. The local hydrothermal alteration is characterized broadly by a quartz–albite–epidote–chlorite–calcite mineral assemblage. The most pervasive alteration mineral is albite, followed by epidote and, locally, actinolite. These minerals contrast sharply against host rock minerals such as chlorite, calcite, zeolite, prehnite, and pumpellyite, but alteration is constrained to mineralized bodies as narrow and low contrast alteration halos that go outwards from actinolite–albite to epidote–albite, to epidote–chlorite, and finally to chlorite. Hydrothermal alteration minerals, compared to regional alteration minerals, show iron-rich epidotes, a lower chlorite content of the chlorite–smectite series, and a nearly total albite replacement of plagioclase in the mineralized zones. Opaque minerals associated with regional alteration are magnetite and maghemite, and those associated to hydrothermal alteration are magnetite, hematite, and copper sulphides. We present paleomagnetic results from nine sites in the Michilla district and from drill cores from two mines. Local effects of hydrothermal alteration on the original magnetic mineralogy indicate similar characteristics and mineralogy, except for an increase of hematite that is spatially associated with the Cu–sulphide breccias with low magnetic susceptibilities. Results indicate that it is impossible to magnetically differentiate mineralized bodies from unmineralized lavas, except for pyrite-rich hydrothermal breccias. In conclusion, for stratabound copper deposits of the Michilla type, the overall effect of hydrothermal alteration on the paleomagnetic properties of rocks is of low contrast, not clearly discernable even at a small scale. From an exploration point of view, magnetic exploration surveys should not discern mineralized bodies of Cu–sulphide breccias except in detailed ground surveys due to the small size of contrasting bodies. Unoriented drill cores with primary ore mineralization record a characteristic remanent magnetization of reverse polarity. Taking into account the azimuth and dip of the drill cores, we were able to compare the magnetization of the mineralized bodies with the characteristic directions from sites drilled in situ from Late Jurassic–Early Cretaceous intrusives mostly. The characteristic direction recorded by the Pluton Viera is similar to the magnetization of the ore bodies of the Estefania mine. If copper mineralization mostly postdates the tilt of the volcanic flows, the low paleomagnetic inclinations suggest an age for the mineralization near 145 Ma, the time of the lowest paleolatitude for the South American plate during the Mesozoic.

The Mantoverde Iron Oxide-Copper-Gold District, III Region, Chile: The Role of Regionally Derived, Nonmagmatic Fluids in Chalcopyrite Mineralization

Located in the Cordillera de la Costa of northern Chile, the mines of the Mantoverde district exploit super-gene oxide ore developed over several Lower Cretaceous, hematite-rich, iron oxide-Cu-Au (IOCG) deposits with an average protore grade of 0.52 percent Cu and 0.11 g/t Au (e.g., Mantoverde proper, Manto Ruso). The geologic setting and genesis of this productive IOCG district are clarified herein through regional petrologic and lithogeochemical study and light stable isotope analysis of paragenetically constrained samples from Mantoverde and its satellite deposits. Together with chalcopyrite-bearing, but subeconomic, bodies of metasomatic magnetite (e.g., Montecristo and Franco) and Cu-barren magnetite-fluorapatite-pyrite bodies (e.g., Ferrifera), the deposits of the Mantoverde district were emplaced along the main and, more commonly, subsidiary segments of the plate boundary-parallel Atacama fault system. They are hosted by Middle to Upper Jurassic andesites of the La Negra Formation and diorites and monzodiorites assigned to the Lower Cretaceous Sierra Dieciocho plutonic complex. Prior to mineralization, the Jurassic and Neocomian igneous rocks of this Andean transect were subjected to moderate albitization (spilitization) and hydrolytic alteration and, subsequently, to regional, nondeformational metamorphism, which locally attained the lower greenschist facies. Both processes, however, were focused along the western margin of a Neocomian marginal basin, 25 to 30 km east of the Atacam fault system, and there is no evidence of widespread albitization in the vicinity of the major IOCG centers. An extensively revised paragenetic model for Mantoverde and its satellite deposits incorporates four stages. Stage I was dominated by widespread potassium and iron metasomatism which converted granitoid and volcanic rocks to orthoclase and magnetite, respectively. Stage II comprises chloritic and sericitic alteration and veining. The deposition, early in stage II, of marialitic scapolite, subsequently largely replaced by chlorite, was probably contemporaneous with regional scapolitization in the area between the Atacama fault system and the marginal basin. Chalcopyrite deposition was restricted to the ensuing stage III, hosted by calcite veins and, particularly, specular hematite-dominated hydrothermal breccias and stockworks. Stage IV barren calcite-quartz vein swarms record the terminal hydrothermal activity. Stable isotope fractionation relationships and published fluid inclusion microthermometry define a retrograde thermal evolution, from above ~460°C in stage I, through ~350°C in stage II, to ~210° to 280°C in ore stage III, and ~110° to 240°C in stage IV. The δ34S values of chalcopyrite and pyrite from Mantoverde and its associated orebodies and prospects range overall from −6.8 to +11.2 per mil, overlapping extensively. However, the narrow range, −0.6 to +2 per mil, of δ34S values of pyrite associated with stage I magnetite contrasts with the much wider range, −1.2 to +9.1 per mil, of that deposited in stage II. The compositional variability increases from +1.4 to −11.2 per mil in the mineralized assemblages of stage III, chalcopyrite generally having higher values than pyrite. The iron oxides in the district have δ18O values that vary overall from −1.9 to +4.1 per mil, the highest values, +1.4 to +4.1 per mil, occurring in stage I metasomatic magnetite, whereas stage III hematite has lower values of −2.0 to +1.7 per mil. Estimated equilibrium δ34Sfluid values increased dramatically with time, from +0.4 to +4 per mil in stage I, through +9.1 to +14.9 per mil during stage II, to +26.4 to +36.2 per mil for the most richly mineralized hematitic breccias. Stage III hematite equilibrated with a fluid with δ18O values of +3.0 to +8.0 per mil, significantly lower than those of fluids from which stage I magnetite crystallized (i.e., +7.3 to +9.9‰). The fluids responsible for barren stage I magnetite-pyrite assemblages, with δ34S and δ18O values close to 0 and +8 per mil, respectively, may have been products of the second boiling of granitoid magmas, possibly of the Sierra Dieciocho complex. Markedly higher δ34S and lower δ18O values in stages II and, particularly, stage III, in which all significant chalcopyrite and gold were deposited, are interpreted as evidence for the incursion of modified seawater, possibly via evaporitic sediments. Such externally derived fluids, probably mobilized by marginal basin inversion and recorded by the district-wide scapolitization (Na-Cl metasomatism), may have been a prerequisite for hypogene Cu(-Au) mineralization in the Mantoverde district.

Late Cenozoic geomorphologic signal of Andean forearc deformation and tilting associated with the uplift and climate changes of the Southern Atacama Desert (26°S–28°S)

We analyze remarkable examples of the large (∼ 10,000 km 2) and local-scale (∼ 100 km 2) landscape forms related to Late Cenozoic geomorphologic evolution of the Andean forearc region in the Southern Atacama Desert. We also consider the continental sedimentary deposits, so-called “Atacama Gravels”, which are related to the degradation of the landscape during the Neogene. Our analysis integrates 1:50,000 field cartography, Landsat TM images observations, ∼ 1:1000 sedimentary logging data, and 50 m horizontal resolution topographic data to reconstruct the Late Cenozoic geomorphologic evolution of this region and discuss the factors that control it, i.e., Miocene aridification of the climate and Neogene Central Andean uplift. We determine that the Precordillera was already formed in the Oligocene and most of the present-day altitude of the Precordillera was reached before that time. Afterward, five episodes of geomorphologic evolution can be differentiated: (1) the development of an Oligocene deep incised drainage system cutting the uplifted Precordillera (up to 2000 m of vertical incision) and connecting it to the Ocean; followed by (2) the infilling of deep incised valleys by up to 400 m of Atacama Gravels. This infill started in the Early Miocene with the development of fluvial deposition and finished in the Middle Miocene with playa and playa lake depositions. We propose that playa-related deposition occurs in an endorheic context related to tectonic activity of the Atacama Fault System and Coastal Cordillera uplift. However, the upward sedimentologic variation in the Atacama Gravels evidences a progressive aridification of the climate. Subsequently, we have identified the effects of the Middle–Upper Miocene slow tectonic deformation: the Neogene Andean uplift is accommodated by a tilting or flexuring of the inner-forearc (Central Depression and Precordillera) related to some hundreds of meters of uplift in the Precordillera. This tilting or flexuring results in (3) the Middle Miocene re-opening of the valley network to the Pacific Ocean. Upper Miocene aridification, from arid to hyperarid, induces alluvial fans backfilling in the Central Depression (4) resulting in up to 50 m of Atacama Gravel deposition. Finally, in response to an increase in the rate of tilting, a new phase of vertical incision (up to 800 m in the Precordillera) allows the development of the canyon that crosses the forearc (5).

SHRIMP U–Pb dating of the Antucoya porphyry copper deposit: new evidence for an Early Cretaceous porphyry-related metallogenic epoch in the Coastal Cordillera of northern Chile

The Antucoya porphyry copper deposit (300 Mt at 0.45% total Cu) is one of the largest deposits of a poorly known Early Cretaceous porphyry belt in the Coastal Cordillera of northern Chile. It is related to a succession of granodioritic and tonalitic porphyritic stocks and dikes that were emplaced within Jurassic andesitic rocks of the La Negra Formation immediately west of the N–S trending sinistral strike-slip Atacama Fault Zone. New zircon SHRIMP U–Pb data indicate that the porphyries of Antucoya crystallized within the time span from 142.7 ± 1.6 to 140.6 ± 1.5 Ma (±2 σ), and late, unmineralized, NW–SE trending dacite dikes with potassic alteration and internal deformation crystallized at 141.9 ± 1.4 Ma. The Antucoya porphyry copper system appears to be formed after a change of stress conditions along the magmatic arc from extensional in the Late Jurassic to transpressive during the Early Cretaceous and provides support for an Early Cretaceous metallogenic episode of porphyry-type mineralization along the Coastal Cordillera of northern Chile.

Young displacements on the Atacama Fault System, northern Chile from field observations and cosmogenic 21Ne concentrations

We present the first numerical age constraint for young deformation of the Atacama Fault System (AFS) in northern Chile. The young activity of the AFS is expressed by several fault scarps which affects alluvial fan sediments of the eastern side of the Coastal Cordillera (23°30′–23°42′S). Detailed mapping of alluvial fans reveals a complex relationship between fault motion, erosion and alluvial fan development. An older group of alluvial fans became inactive prior to the scarp formation. Younger alluvial fans, arising directly from feeder channels and entrenched in the fault scarps, posts date the scarp formation. The youngest slip on the AFS is recorded by headward eroding channels entrenched across the scarp which are in turn displaced vertically 0.3–0.5 m by the fault. Quartz fragments in four sites on the older inactive fan group were analyzed for cosmogenic 21Ne concentrations yielding an average age of 424 ± 151 ka, the upper limit for the recent activity of the fault. Combined with the height of fault scarp, we calculate a 0.01 mm/yr minimum vertical fault slip rate. Thus young displacement on the AFS is Quaternary in age and confined to the late Pleistocene.

Geometry and kinematics of adhesive wear in brittle strike-slip fault zones

Detailed outcrop surface mapping in Late Paleozoic cataclastic strike-slip faults of coastal Maine shows that asymmetric sidewall ripouts, 0.1–200 m in length, are a significant component of many mapped faults and an important wall rock deformation mechanism during faulting. The geometry of these structures ranges from simple lenses to elongate slabs cut out of the sidewalls of strike-slip faults by a lateral jump of the active zone of slip during adhesion along a section of the main fault. The new irregular trace of the active fault after this jump creates an indenting asperity that is forced to plow through the adjoining wall rock during continued adhesion or be cut off by renewed motion along the main section of the fault. Ripout translation during adhesion sets up the structural asymmetry with trailing extensional and leading contractional ends to the ripout block. The inactive section of the main fault trace at the trailing end can develop a ‘sag’ or ‘half-graben’ type geometry due to block movement along the scallop-shaped connecting ramp to the flanking ripout fault. Leading contractional ramps can develop ‘thrust’ type imbrication and forces the ‘humpback’ geometry to the ripout slab due to distortion of the inactive main fault surface by ripout translation. Similar asymmetric ripout geometries are recognized in many other major crustal scale strike-slip fault zones worldwide. Ripout structures in the 5–500 km length range can be found on the Atacama fault system of northern Chile, the Qujiang and Xiaojiang fault zones in western China, the Yalakom–Hozameen fault zone in British Columbia and the San Andreas fault system in southern California. For active crustal-scale faults the surface expression of ripout translation includes a coupled system of extensional trailing ramps as normal oblique-slip faults with pull-apart basin sedimentation and contractional leading ramps as oblique thrust or high angle reverse faults with associated uplift and erosion. The sidewall ripout model, as a mechanism for adhesive wear during fault zone deformation, can be useful in studies of fault zone geometry, kinematics and evolution from outcrop- to crustal-scales.

Fault zone development and strain partitioning in an extensional strike-slip duplex: A case study from the Mesozoic Atacama fault system, Northern Chile

Upper crustal strike-slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system. Detailed field mapping of the Mesozoic Atacama fault system in the Coastal Cordillera of Northern Chile documents the progressive development of second- and third-order faults forming a duplex at a dilational jog between two overstepping master faults: the sinistral strike-slip, NNW-striking, Jorgillo and Bolfin faults. These are constituted by a meter-wide core of foliated S-C ultracataclasite and cataclasite, flanked by a damage zone of protocataclasite, splay faults and veins. Lateral separation of markers along master faults is on the order of a few kilometers. Second-order, NW-striking, oblique-slip subsidiary fault zones do not show foliated ultracataclasite; lateral sinistral separations are in the range of ∼ 10 to 200 m with a relatively minor normal dip-slip component. In turn, third-order, east–west striking normal faults exhibit centimetric displacement. Oblique-slip (sinistral–normal) fault zones located at the southern termination of the Bolfin fault form a well-developed imbricate fan structure. They exhibit a relatively simple architecture of extensional and extensional-shear fractures bound by low displacement shear fractures. Kinematic analysis of fault slip data from mesoscopic faults within the duplex area, document that the NW-striking and the EW-striking faults accommodate transtension and extension, respectively. Examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure. Faults started from arrays of en echelon extensional/extensional-shear fractures that then coalesced into throughgoing strike-slip faults. Further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement. We interpret that the duplex formed by progressive linkage of horsetail-like structures at the southern tip of the Bolfin fault that joined splay faults coming from the Jorgillo and Coloso faults. The geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike-slip fault zones in the upper crust.

The Cretaceous iron belt of northern Chile: role of oceanic plates, a superplume event, and a major shear zone

The Cretaceous constitutes a turning point in the tectonic, magmatic, and metallogenic history of Chile. The geological evidence indicates that a major change occurred in late Neocomian time when superplume emplacement (Mid-Pacific Superplume) and plate reorganization processes took place in the Pacific. The superplume event resulted in a major ridge-push force resulting in increased coupling between the subducting and overriding plates. This completely changed the tectonic setting of Chile ending the Early Cretaceous extensional period (aborted rifting in the back-arc basin), and increasing stress at a crustal scale. As a consequence, overpressurized dioritic magmas were pushed up mainly along the best possible structural path in northern Chile, i.e., the Atacama Fault Zone, eventually forming a +500-km-long belt of Kiruna-type iron deposits with reserves of ~2,000 Mt (60% Fe), a unique case in Chile's geological history.

Landsat TM analysis of fracture patterns: a case study from the Coastal Cordillera of northern Chile

Cretaceous rocks on the continental margin of northern Chile record a complex geodynamic evolution. Cycles of transtensional and transpressional deformation and of extrusive and intrusive magmatism are linked to the development of crustal-scale lineaments. The Landsat Thematic Mapper is used here as a tool to define these structural features. Geocorrected data were digitally enhanced and lineaments plotted directly from a hard copy image, thereby excluding artificial or non-geological features that might degrade the subsequent structural analysis. The lineaments were then digitized and analysed using a Weighted Moving Average (WMA) technique to suppress noise and to enhance azimuthal variation. Statistical analysis of the data reveals three lineament populations. The first is a set of NNE-trending lineaments that belong to the margin-parallel, sinistral Atacama Fault System. The second is a series of NW-trending lineaments with a similar orientation to large-scale structures identified across the South American continental plate. The third is a widely spaced set of NE-trending lineaments. The key result of this study is that lineaments identified from remotely sensed data may have orientation patterns that differ considerably from those identified by traditional geological mapping and that full structural analysis of structurally complex crustal regions will likely be incomplete without a comprehensive analysis of remotely sensed data. Although the NW-trending structures are numerically dominant on the Landsat TM image, they are seldom recorded at map scale and are under-represented on published geological maps. Of the 275 faults marked on the published geological map sheets, 89 are N to NNE-trending and only 88 are NW-trending. By contrast, of 841 lineaments identified from the satellite image, 455 are NW-trending and 178 are N- to NNE-trending. The lack of prior recognition of the NW-trending structures means that their importance has been underestimated in reconstructions of the geodynamic evolution of the region. In addition, as major ore deposits in the region are frequently located at intersections between two fracture systems, the recognition here of the NW-trending set of structures should illuminate future mineral exploration programmes.

A geomorphological approach to determining the Neogene to Recent tectonic deformation in the Coastal Cordillera of northern Chile (Atacama)

The large (≈10000 km 2) and local-scale (<400 km 2) geomorphologic, geomorphometric and field evidence indicates that, from the mid-Miocene onwards, the Atacama Fault System (AFS) accommodated the relative uplift of the western side of the Chilean Coastal Cordillera of the Chañaral region (southern Atacama Desert). The mean regional altitude systematically decreases eastwards crossing the AFS, independent of the lithological characteristics of the substratum cut by this system of faults. Topographic analysis reveals a more incised landscape west of the AFS that, at the local scale, is reported by the distribution of the altitudes (hypsometric curves and integrals) of tributary basins and by the presence of terraces. In the Middle and Upper Miocene, a thick (>300 m) sedimentary succession was deposited east of the AFS. The succession fills previously deep paleovalleys. And it consists of gravel, so-called “Atacama Gravels”, which passes laterally into fine-grained playa related deposits near the AFS. We interpret the deposition of this succession as a result of a blocking closure of the valley flowing from the Precordillera due to the activity on AFS. A pedimentation episode followed sediment deposition and is locally strongly re-incised by the main modern-day river valleys draining the Precordillera. Incision may result from either regional uplift of the forearc, and/or from more localized activity on the AFS. Furthermore, Recent (Quaternary?) tectonic activity on the AFS has been observed which is consistent with a localized relative uplift of the crustal block west of the AFS.

Magmatic arc fault systems, deformation partitioning and emplacement of granitic complexes in the Coastal Cordillera, north Chilean Andes (25°30′S to 27°00′S)

Three fault systems were responsible for Permian to Late Cretaceous deformation of the overriding plate of the Andean convergent margin in the Coastal Cordillera of northern Chile (25°30′ to 27°00′S). Displacements were linked to crustal growth expressed by the emplacement of a sequence of magmatic arcs. The Tigrillo Fault System, active from Triassic to Early Cretaceous time, was characterized by arc-normal extension with increasing left-oblique extension (transtension) from Early Jurassic to Early Cretaceous time. Stretching of the crust created space for Triassic, Early Jurassic and Early Cretaceous arc basins where epiclastic, volcaniclastic and volcanic sequences accumulated in continental to shallow marine environments. Tabular plutonic complexes were emplaced by roof uplift–floor subsidence that allowed a vertical transfer of material in the crust without significant horizontal extension. The Atacama Fault System was initiated at c. 132 Ma as a (mainly) left strike-slip fault during left-oblique extension of the margin. Elongate, tabular plutonic complexes were emplaced within the Atacama Fault System between c. 132 and c. 106 Ma, again by roof uplift–floor subsidence mechanisms. Ductile–brittle transitions in synplutonic mylonitic rocks of the Atacama Fault System provided the setting for Kiruna-type Fe-apatite, and Fe oxide (with Cu and/or Au) ores. The Chivato Fault System was active as an extensional fault system at the eastern side of the Coastal Cordillera during displacement on the Tigrillo Fault System and later, between c. 125 and c. 93 Ma, as a partitioned left-oblique extensional fault system. In post-Early Cretaceous time the Chivato Fault System was inverted by left-oblique contraction (transpression) when NW-trending transfer faults, some probably reactivated lateral ramps in the Tigrillo Fault System, accommodated clockwise vertical-axis rotations of 35–45°. Contraction inverted the Atacama Fault System and Tigrillo Fault System and was responsible for west-vergent, thin-skinned, fold–thrust deformation in stratified rocks throughout the margin.

New 40Ar/ 39Ar dates from the Las Tazas complex, northern Chile: Tectonic significance

The Atacama Fault Zone is a major Mesozoic structure that trends along the Coastal Batholith of northern Chile. Part of the fault zone underwent a kinematic change from dip-slip to strike-slip displacement during the Early Cretaceous. The Las Tazas complex intruded the fault zone during this change. New analyses of country rock protomylonites from the edge of the complex firmly constrain the age of the change to 130 Ma and confirm that the complex was emplaced during active displacement along the fault zone. The intrusion heated its immediate country rocks and allowed localised ductile shearing during emplacement. Upper crustal intrusions like the Las Tazas complex are ideal targets for geochronological studies of major shear zones.

On the mechanism of crustal block rotations in the Central Andes — Comment

In this paper Beck (1998)reviews the database of palaeomagnetically observed rotations in the Andean margin of South America, in particular with respect to the well known change in sense of rotation linked to the Arica deflection (Bolivian Orocline) in the orogen. The paper argues firstly for the latitudinal distribution of rotations supporting an interpretation that distributed shear combined with oroclinal bending is the best hypothesis to explain the observed pattern of rotations and secondly that this rotation/deformation is time-dependent. While we essentially agree with the former conclusion we would define it more tightly in that the over-arching control on the pattern of rotations is a combination of differential shortening, as opposed to oroclinal bending, along the orogen coupled with distributed shear across the orogen. Our main concern with this paper is that it fails to provide a convincing demonstration that there is a time-dependent element to the magnitude of rotations. Primarily we will argue that it is far from clear from the palaeomagnetic data presented that there is a case for a continuous rate of rotation applicable to the southern limb of the orocline, at least. Not withstanding this we would also argue that geologically the deformation cannot and should not be described by a continuous deformation rate over time, as postulated. We also make specific points about aspects of the paper dealing with the Atacama Fault Zone and its relation to the observed rotations.

The emplacement of the granitic Las Tazas complex, northern Chile: the relationship between local and regional strain

The Coastal batholith of northern Chile grew under transtensional–extensional conditions that prevailed along the Andean margin during the Mesozoic. The batholith hosts the Atacama Fault Zone, a major arc-parallel fault system which was characterised by sinistral transtensional shearing during the Early Cretaceous. The Las Tazas complex is a composite granitoid intrusion that was emplaced syntectonically along the Atacama Fault Zone at ∼130 Ma. Syntectonic emplacement is indicated by a consistent kinematic history between the complex and its wall rocks, together with synchronous crystallisation and shearing ages. In contrast to regional patterns, the Las Tazas complex was emplaced during a local change from vertical east-side-down to dextral transcurrent displacement along the fault zone. During intrusion, strain was partitioned between non-coaxial simple shearing within country-rock mylonites and a flattening strain across the crystallising complex. This combination indicates that the pluton was emplaced under temporary transpressive conditions that were localised around the pluton, probably induced by magma emplacement. Such a difference between local and regional strain suggests that emplacement-related structures should only be related to regional strain-states with great care.

Tectonics of the Jurassic‐Early Cretaceous magmatic arc of the north Chilean Coastal Cordillera (22°–26°S): A story of crustal deformation along a convergent plate boundary

The tectonic evolution of a continental magmatic arc that was active in the north Chilean Coastal Cordillera in Jurassic‐Early Cretaceous times is described in order to show the relationship between arc deformation and plate convergence. During stage I (circa 195–155 Ma) a variety of structures formed at deep to shallow crustal levels, indicating sinistral arc‐parallel strike‐slip movements. From deep crustal levels a sequence of structures is described, starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions (Bolfin Complex). The high‐grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike‐slip and thrust kinematics with a NW–SE directed maximum horizontal shortening, i.e., parallel to the probable Late Jurassic vector of plate convergence. A kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds, thrusts, and high‐angle normal faults which formed during the same time interval as the discrete shear zones. During stage II (160–150 Ma), strong arc‐normal extension is revealed by brittle low‐angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels. During stage III (155–147 Ma), two reversals in the stress regime took place indicated by two generations of dikes, an older one trending NE–SW and a younger one trending NW–SE. Sinistral strike‐slip movements also prevailed during stage IV (until ∼125 Ma) when the Atacama Fault Zone originated as a sinistral trench‐linked strike‐slip fault. The tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates. The structures of stages I and IV suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations. However, decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages II and III.

Mesozoic fault systems, deformation and fault block rotation in the Andean forearc: a crustal scale strike-slip duplex in the Coastal Cordillera of northern Chile

In this paper we discuss the evolution and tectonic significance of the Mesozoic trench-parallel fault systems which affected the Coastal Cordillera and their relation to magmatism and crustal rotation. The oldest, extensional, fault system separates basement from rift-related Late Triassic and younger sedimentary units. This system [I] subsequently developed into a wider extensional fault system which acted as the locus of magma ascent and emplacement of the Coastal Batholith during much of the Jurassic to earliest Cretaceous period. This extensional fault system defined the forearc sliver during this period and was the consequence of a retreating subduction boundary. During the Early Cretaceous (c. 132–125 Ma) the kinematics of this fault system changed to transtension [II] and accommodated a major component of left-lateral strike-slip motion, the principal fault being the Atacama Fault Zone along which plutons continued to be emplaced. The final phase of pluton emplacement within the Coastal Cordillera appears to be c. 106 Ma, after which this magmatic arc and fault system was abandoned. An Late Cretaceous arc and fault system [III] developed some 20 Ma later and located some 50 km to the east in what is now the Central Valley of northern Chile. This paper seeks to show that the Coastal Cordillera was deformed as a whole by this Late Cretaceous fault system [III] which formed a crustal-scale left-lateral transpressional duplex. During this deformation the thermally weakened crust was dissected into a series of large-scale blocks bounded by NW-trending left-lateral strike-slip faults which merge into a NNE–SSW fault zone which forms the eastern boundary to the duplex. We term this eastern boundary zone the Central Valley Fault Zone (CVFZ) and this together with the NW-trending faults defines the duplex system which we refer to as a whole as the Coastal Cordillera Fault System (CCFS) [III]. We have traced the CCFS duplex between 25°S and 29°S and suspect that it continues northward. The timing of the deformation is constrained to be post 106 Ma, the age of Coastal Cordillera arc abandonment, and pre-Tertiary based on the deformation and pluton emplacement in and along the Central Valley Fault Zone. Palaeomagnetic data from the fault bound blocks within the CCFS duplex indicate 35°–45° of post-Early Cretaceous clockwise rotation with no substantial latitudinal motion. We suggest that the observed fault kinematics of the CCFS are consistent with this crustal-scale duplex model where rotations would have occurred in response to left-lateral transpression.

Recent crustal deformation in the Antofagasta region(northern Chile) and the subduction process

New neotectonic observations, along with a detailed aerial photograph analysis, allow a new interpretation of the recent tectonic behaviour of the outer forearc in northern Chile between 22.5°S and 24.5°S (Antofagasta region). Both the Coastal Cordillera and the Mejillones Peninsula are under E–W extension. Normal faults dipping east with an almost N–S orientation are predominant. Large-scale Neogene to Recent deformation is characterized by vertical uplift and subsidence related to normal faulting. Recent kinematics along the Atacama Fault System, including the Atacama Fault itself, are controlled by the regional extensional stress regime, and the predominant component of recent displacement along the Atacama Fault is vertical (normal). However, strike-slip components are also observed, but with moderate offsets, no larger than one to two tens of metres. The sense of shear, and the ratio between vertical and horizontal offsets, vary coherently with the fault azimuth, compatible with E–W extension. Oblique-slip faulting, with normal and left-lateral offsets of similar amplitude, is observed on the most northeasterly oriented segment of the Atacama Fault. This small left-lateral component of displacement and the strike-slip motion predicted by models of strain partitioning of the convergence along the Atacama Fault are in opposite senses. This occurs because the stress regime in the coastal region is not compressional but extensional. Very fresh recent ruptures are numerous, especially between 23°S and 24°S, and provide clear indications of the occurrence of moderate to large continental earthquakes during the Late Quaternary. As pointed out by several authors, indirect lines of evidence suggest active subduction erosion and underplating along the margin of northern Chile. We propose that part of the long-term extension is due to the broad-scale flexure of the outer forearc. This flexure may be controlled by offshore subsidence caused by subduction erosion near the trench, and by onshore uplift related to the underplating of eroded low-density material beneath the Coastal Cordillera. Neotectonic observations show that the state of stress in the outer forearc remains extensional (in a deviatoric sense) in the long term. However, it is proposed that the state of stress has a strong time-dependent component, which is intimately linked to the subduction seismic cycle. The Mw=8.0 Antofagasta earthquake of 1995 July 30 showed that large subduction earthquakes produce E–W extension in the coastal region. The overall E–W deviatoric extension in the outer forearc should be reduced by interseismic contraction in the period separating two subduction earthquakes. This reduction means that continental faults will remain locked and aseismic in the interseismic period of the subduction cycle, in agreement with microseismic observations, which indicate an absence of shallow crustal seismicity. We assume that the amount of extension produced by subduction earthquakes in the coastal area is larger than the cumulative interseismic contraction. This occurs because the displacement field generated by subduction earthquakes is concentrated in the outer forearc. The E–W extension should grow over prepeated subduction seismic cycles and lead to large continental earthquakes occurring within the overriding block, simultaneous with large subduction events. We show new surface ruptures associated with normal faulting along the Atacama Fault, which were most probably triggered by the 1995 Antofagasta subduction earthquake.

Salar Grande pull-apart basin, Atacama Fault System, northern Chile

Abstract The Salar Grande basin is a ‘poly-history’ strike-slip basin along the north-trending Atacama Fault System in northern Chile (21°S, 70°W). It exhibits a range of structures that indicate a complex evolution with multiple deformation styles. This paper presents a new interpretation of Salar Grande as a strike-slip pull-apart basin with three stages of evolution. In the first stage (Oligocene to Early Miocene), the basin opened as a sinistral strike-slip pull-apart structure at a left-stepping releasing jog of the Atacama Fault System. At the end of this stage, the basin had a rhomboidal geometry with dimensions of about 35 km length and 10 km width, bounded by a series of linked oblique-extensional basin-sidewall faults. The western sidewall faults linked to the northern fault strand of the Atacama Fault System whereas the eastern sidewall faults linked to the southern strand. The basin interior was a series of tilted fault blocks forming half-grabens. During the second stage (Oligocene to Early Miocene) a NNW-SSE basin short-cut fault developed, dissecting the basin with 6 km of sinistral offset. In the third stage (Pleistocene-Recent), NNW-SSE strike-slip faults were dextrally reactivated with displacements of up to 1 km. The understanding of field examples such as Salar Grande provides new insights into the geometries and evolution of complex multi-phase strike-slip terranes. The geometries recognized in Salar Grande compare well with analogue modelling studies of strike-slip fault systems.

TheMw= 8.0 Antofagasta (northern Chile) earthquake of 30 July 1995: A precursor to the end of the large 1877 gap

AbstractAn Mw = 8.0 earthquake occurred on 30 July 1995 in the Antofagasta region (northern Chile). The main rupture, corresponding to thrust faulting, developed from 10 to 50 km in depth along the subduction interface between the Nazca and the South American plates. The 1995 earthquake took place just south of the large seismic gap where a great earthquake (M = 9) had occurred in 1877. Most of the 1995 rupture was located within a local network consisting of nine short-period stations that had been previously installed at the southern end of the 1877 gap, and the aftershock sequence could be accurately monitored. Little destruction resulted from the 1995 earthquake in spite of its large size. Ground acceleration in Antofagasta reached 29% of gravity. A tsunami wave, 2 to 2.5 m high, was observed along the coast from Mejillones to Taltal. One strong foreshock (Mw = 6.2) occurred in the 1995 hypocentral region 6 months before the main event.Body-wave modeling of broadband seismograms from the global network, along with the analysis of the aftershock distribution, allows us to propose a well-constrained model for the whole rupture process. Some additional details of the rupture were obtained from an accelerometer record at Antofagasta. The main rupture started as a double even with thrust mechanism below the southern part of the Mejillones peninsula, and it propagated southward in a N200°E direction with an average velocity of 2.8 km/sec. It ended near the trench in normal faulting. The total rupture area and seismic moment are 185 × 90 km2 and 1.2 × 1028 dyne-cm, respectively. The aftershock distribution delineates a well-defined rupture surface along the subduction interface. The distribution of epicenters during the first 20 h of aftershock activity shows a sharp northern boundary beneath the Mejillones peninsula. Hence, the 1995 main rupture did not propagate north of the Mejillones peninsula into the 1877 gap. Aftershocks during the following 2 weeks indicate a growth of the initial rupture zone toward the north. The mechanisms of the strongest aftershocks are similar to that of the mainshock. The down-dip termination of the main rupture corresponds to the maximum depth (50 km) of the region that had been identified as the locked part of the subduction interface from the analysis of the microseismicity recorded by the local network prior to the 1995 event.A well-constrained dislocation model is proposed for the 1995 main rupture, which produces surface displacements in good agreement with available observations of coastal uplift and GPS measurements. The dislocation model, as well as Global Positioning System (GPS) measurements, indicate that the 1995 earthquake generated E-W extension in the coastal region of Antofagasta. The Atacama fault, located 40 to 50 km above the 1995 main rupture, showed small fresh surface ruptures near Sierra Remiendos (70 km to the SSE of Antofagasta) with a maximum vertical offset of 20 cm. This offset corresponds to normal faulting, which is in agreement with the E-W co-seismic extension.The Mejillones peninsula appears to be the surface expression of a barrier that interrupted the propagation of the 1995 rupture to the north into the region of the 1877 gap. Modeling of static stress changes induced by the Antofagasta earthquake indicates an increase in compressive stresses along a direction transverse to the trench immediately to the north of the 1995 rupture surface. Thus, the chances for the reactivation of the 1877 gap after this event are greater now.