Atacama Fault Research Articles

The Andean subduction zone between 22 and 25°S (northern Chile): precise geometry and state of stress

One year of seismicity recorded by a local network is used to obtain more precision about the geometry and the stress regime of the Andean subduction between 22 and 25°S in the northern Chile seismic gap. A sharp image of the Wadati-Benioff Zone (WBZ) is obtained down to 270 km in depth. A seismically quasi-quiescent zone is observed in the WBZ below the volcanic arc, between 150 and 210 km in depth. Hypocentres of distant intermediate depth earthquakes located with the local network are compared with worldwide seismic network hypocentres in order to evaluate the accuracy of the WBZ image at depth greater than 100–150 km in depth. No shallow microearthquakes have been observed in the continental crust but some seismic activity is likely to occur locally at the deep root of the Atacama Fault. The stress field and the characteristics of faulting along the subducted slab are investigated. Underthrusting and localized reverse faulting earthquakes define the seismically coupled plate interface from 20 to 50 km in depth (Locked zone). Downdip, intra-slab normal faulting prevails (Tensile zone), but some strike-slip faulting is observed. A transition between normal faulting with variable fault azimuth and normal faulting with nearly homogeneous NNW- to NW-oriented fault plane is found at about 80 km in depth. It is found that the stress axes σ 1 and σ 3 in the Locked zone are oriented in the convergence direction (75–80°E). Downdip, in the tensile zone, σ 3 has a mean azimuth 60–65°E. There, the slab is hence submitted to a tensional force (slab pull) oblique relatively to the convergence. The transition between seismic underthrusting and intraplate normal faulting downdip occurs at the depth where the continental Moho encounters the Wadati-Benioff Zone, suggesting that a relationship exists between seismic coupling and the presence of continental crust at the plate interface. The pre-seismic state of this segment of the Andean subduction zone is confirmed by the occurrence of strong earthquakes located by the global network around the presumed rupture area and by the stress regime found along the Wadati-Benioff Zone.

Variations in F and Cl contents in apatites from magnetite—apatite ores in northern chile, and their ore-genetic implications

AbstractMagnetite—apatite deposits associated with the Atacama Fault Zone of northern Chile are interpreted here, on field criteria, as being the products either of hydrothermal fluids with a strong magmatic signature, or of late-stage Fe-rich magmas mixed with an aqueous fluid. Even in the Chilean iron belt, apatite-rich magnetite deposits are a rarity. Variations in F- and Cl- contents in apatites, strongly zoned with respect to halogens, are indicative of primary variations in fHF and fHCI in the hydrothermal fluids. Small variations in halogen fugacities in the aqueous fluid are capable of buffering large variations in halogen content within apatite crystals in equilibrium with that fluid. The recorded halogen zonation profiles are inconsistent with crystallization of the apatites simply from a volatile-rich, late-stage fractionation Fe-rich magma, or its derived magmatic vapour. It is more likely that they are the result of mineral—fluid buffering with a fluid that represents the mixing of a magmatically-derived aqueous fluid with a meteoric fluid that has variably scavenged Ca and Cl from within the country rocks. The source magma of the former is probably an Fe-P enriched acidic magma, derived by fractionation of primary calc-alkaline basic magmas.

Mesozoic Magmatic and Tectonic Events within the Andean Plate Boundary Zone, 26°-27°30'S, North Chile: Constraints from40Ar/39Ar Mineral Ages

In North Chile, Jurassic-Cretaceous arc magmatism is represented by several narrow belts of plutons and two separate volcanic sequences. New $^{40}Ar/^{39}Ar$ mineral ages confirm five distinct episodes of magma emplacement as plutonic complexes (c. 202-188 Ma, c. 160-153 Ma, c. 142-138 Ma, c. 130-127 Ma, and c. 106-103 Ma). Conjugate dike sets were emplaced immediately before periods of plutonism, and each distinct episode of magmatism prior to c. 127 Ma was located successively eastward, inboard from the subduction boundary, features interpreted to result from regional extension within the magmatic arc. Steeply dipping, ductile mylonitic shear zones were developed in wall-rocks along western contacts of Late Jurassic and Early Cretaceous plutons. Steeply-plunging mineral stretching lineations and a variety of kinematic indicators record an east-side-down extension. Along eastern faults of the Atacama Fault System steeply dipping, ductile mylonitic shear zones display subhorizontal mineral stretching lin...

40Ar/ 39Ar and RbSr analyses from ductile shear zones from the Atacama Fault Zone, northern Chile: the age of deformation

The influence of deformation on the K-Ar and the Rb-Sr isotope system is investigated. It is assumed that, due to the diffusion processes involved, deformation has a similar effect on isotopic equilibrium as has temperature. In order to examine the influence of deformation on the K-Ar and the Rb-Sr isotope systems two shear zones from the Atacama Fault Zone (AFZ), situated in the north Chilean Coastal Cordillera, have been investigated. The AFZ, which was active as a sinistral strike-slip fault during the Mesozoic, has two sets of shear zones, one formed under amphibolite (SZ1), one under greenschist facies conditions (SZ2), Rb-Sr and 40Ar/ 39Ar age determinations were conducted on samples from cross sections of each set. In SZ1 the hornblendes and bioties from a weakly deformed sample reveal cooling ages of 153-152 and 150 ± 1 Ma, respectively. Biotite from the center of the shear zone of SZ1 gave an isochron of 143.9 ± 0.3 Ma (MSWD = 0.04) which is interpreted as the age of deformation which produced resetting of the mineral system. In SZ2 hornblendes yielded 40Ar/ 39Ar plateau (cooling) ages of ∼ 138 Ma. Biotites from undeformed samples gave Rb-Sr and 40Ar/ 39Ar total degassing ages of 130 ± 1 Ma, whereas biotite from the mylonitic rocks yielded 126-125 Ma which dates the time of deformation. Sr isotope homogenization occurred in the mylonitic rocks, and is most likely a result of deformation. The formation of SZ1 can be correlated to the Araucanian (= Nevadan) phase. The deformation in SZ2 is related to the onset of uplift and cooling of the Coastal Cordilleran magmatic arc.

Paleomagnetic and structural constraints on rotations in the North Chilean coast ranges

Paleomagnetic analysis of seventy-six samples from six sites in the Jurassic to Cretaceous plutons (radiometric ages from 157 to 128 Ma) of the Coastal Cordillera of northern Chile yielded stable remanent magnetization directions. The plutons record both a normal and a reversed characteristics direction. The mean normal direction has a declination of 3.4° an inclination of −38.5°, and an alpha 95 of 10.5°. The mean reversed direction has a declination of 207.8°, an inclination of 33.8°, and an alpha 95 of 10.8°. Paleopoles calculated from these mean and latitude=63.5° S, longitude=191.9° E respectively. In comparison to an APW path, the reversed pole indicates a clockwise rotation of approximately 28°, similar to other regional results, whereas the normal pole is concordant with the present day magnetic field. These results, in and of themselves, could support oroclinal bending or local block rotations. However arrays of secondary faults to the Atacama fault system in the sampling area show characteristic spacings, orientations, and displacements. Restoration of displacements results in a 31° clockwise rotation of fault blocks; essentially removing the observed paleomagnetic discordance. Thus, structural data confirms the local block rotation model. The existence of clockwise discordant paleomagnetic data within the sinistral (counterclockwise) Atacama Fault zone (AFZ) underscores the ambiguity of using the sense of rotation to infer the sense of motion on the larger shear zone. Age constraints for motions along the AFZ suggest that these rotations were completed in the Early Cretaceous; essentially coeval with magmatic activity within the Coast Ranges, and underscores the magmatic arc, rather than forearc, setting for this tectonic event.

Determination of seismogenic interplate contact zone and crustal seismicity around Antofagasta, northern Chile using local data

SUMMARY During September and October 1988 a microseismic field experiment was carried out around the city of Antofagasta, northern Chile, with 29 portable analogue and digital stations. A total of 197 reliable microearthquake locations and 19 focal mechanisms were determined. It is proposed that it is possible to estimate the maximum depth of the coupled-uncoupled transition of the subducting lithosphere using local data, defined by the depth of the expected change of the stress field from compressional to tensional along the slab. This change is observed at about 70 km depth in the Antofagasta field work. Two estimations of the width of the seismogenic interplate contact are discussed: (1) the maximum depth of the coupled zone defined by the observed maximum depth of the shallow-dipping thrust events recorded during the experiment of 47 km, corresponding to a width of the seismogenic contact zone of about 90 km; and (2) the maximum depth of the coupled zone defined by the depth of the observed change from a compressional to tensional stress field, which is 70 km and corresponds to a width of the seismogenic contact zone of about 130 km. With both values, the maximum magnitude Ms estimated for the region varies between 8.6 and 8.7. No shallow event associated with the Atacama fault system was observed during the experiment.

Displacement history of the Atacama fault system 25°00′S-27° 00′S, northern Chile

In the Cordillera de la Costa of the central Andes in northern Chile, Mesozoic arc complexes are cut by a trench-parallel strike-slip fault system: the Atacama fault system. Brittle faulting in the Atacama fault system is superposed on steeply dipping foliations in ductile shear belts. Between 25°S and 27°S, the western part of the fault system was active in Early Cretaceous time as an upper amphibolite facies, down-to-the-east, dip-slip ductile shear zone. In the eastern part of the fault system, ductile deformation is of similar Early Cretaceous age but occurred under lower-grade metamorphic conditions at the greenschist/lower amphibolite facies transition. The mylonites in the eastern part of the fault system were formed by sinistral strike-slip displacement. The dip-slip and sinistral strike-slip displacements are contemporary with the development of a magmatic arc, and they imply that the tectonic environment in this part of the arc was transtensional. The ductile deformation was partitioned spatially into a dip-slip component associated with the emplacement of magmas and a sinistral strike-slip component. Brittle fault zones in the El Salado segment of the Atacama fault system define large-scale sidewall ripout structures. Subhorizontai slickenlines, ripout asymmetry, and S-C-type fabrics in fault gouge indicate that brittle deformation involved sinistral strike-slip displacements. The transition from ductile to brittle sinistral strike-slip displacements may have occurred due to cooling, in mid-Cretaceous time, when the magmatic arc was abandoned.

North Chilean forearc tectonics and cenozoic plate kinematics

The continental forearc of northern Chile has been subjected to contemporaneous extension and compression. Here, cross-sections constructed across the forearc are presented which show that since initial shortening, deformation of the forearc has occurred in two tectonically distinct areas. These inner and outer forearc areas are separated by the strain discontinuity of the Atacama fault system and the tectonically neutral Central Depression. The outer forearc, the Coastal Cordillera, exhibits extensional tectonics, with large (up to 300 m) normal fault scarps preserved. These faults cut the earlier thrusts responsible for the elevation of Jurassic rocks at the coast above their regional elevation. The normal faults have been re-activated, displacing Quaternary salt deposits in the Salar Grande. This re-activation of the basement faults is probably due to the subduction of anomalously thick oceanic crust, producing an isostatic imbalance in the outer forearc. In the inner forearc, cross-sections through the Sierra del Medio and Cordillera de Domeyko show that structures of the Pre-Cordillera are best explained by a thick-skinned thrust system, with localized thin-skinned tectonics controlled by evaporite detachment horizons. Current forearc deformation features indicate a strong degree of correlation between subduction zone geometry and forearc tectonics. The timing of Cenozoic tectonism also fits well with established plate motion parameters, and the spatial and temporal variation in the state of stress of the forearc shows a close relationship throughout the Cenozoic to the plate kinematics and morphology of the subducting Nazca plate.

Active tectonics of the Andes

Nearly 90 mm a −1 of relative plate convergence is absorbed in the Andean plate-boundary zone. The pattern of active tectonics shows remarkable variations in the way in which the plate slip vector is partitioned into displacement and strain and the ways in which compatibility between different segments is solved. Along any traverse across the plate-boundary zone, the sum of relative velocities between points must equal the relative plate motion. We have developed a kinematic synthesis of displacement and strain partitioning in the Andes from 47°S to 5°N relevant for the last 5 Ma based upon: (1) relative plate motion deduced from oceanic circuits giving a roughly constant azimuth between 075 and 080; (2) moment tensor solutions for over 120 crustal earthquakes since 1960; (3) structural studies of deformed Plio-Pleistocene rocks; (4) topographic/geomorphic studies; (5) palaeomagnetic data; and (6) geodetic data. We recognize four neotectonic zones, with subzones and boundary transfer zones, that are partitioned in different ways. These zones are not coincident with the ‘classic’ zones defined by the presence or absence of a volcanic chain or differences in finite displacements and strains and tectonic form; the long-term segmentation and finite evolution of the Andes may not occur in constantly defined segments in space and time. In Segment 1 (47°–39°S), the slip vector is partitioned into roughly orthogonal Benioff Zone slip with large magnitude/large slip-surface earthquakes and both distributed dextral shear giving clockwise rotations of up to 50° and dextral slip in the curved Liquine-Ofqui Fault System giving 5°–10° of anticlockwise fore-arc rotation. In Segment 2 (39°–20°S), the slip vector is partitioned into Benioff Zone slip roughly parallel with the slip vector, Andean crustal shortening and a very small component of dextral slip, including that on the Atacama Fault System. Between 39° and 34°S, a cross-strike dextral transfer, which deflects the Chile trench and the volcanic arc, absorbs the shortening contrast between Segments 1 and 2. In Segment 3 (20°–6°S), the slip vector is partitioned into roughly orthogonal Benioff Zone slip, crustal shortening, sinistral trench-parallel faulting and northeast-southwest extension. Compatibility between Segments 2 and 3 is maintained by the sinistral east-southeast-trending Cochabamba shear zone and north-trending dextral faults. In Segment 4 (6°S to 5°N), the slip vector is partitioned into roughly orthogonal Benioff Zone slip and dextral strike-slip faulting in the fore-arc and volcanic chain.

Crustal seismicity and subduction morphology around Antofagasta, Chile: preliminary results from a microearthquake survey

During September-October 1988, 13 analog and 16 digital seismographs were installed in northern Chile within 100 km around the city of Antofagasta (22.5–24.5°S; 68.5–70.5°W). The purposes of this study were to observe the microseismicity, to describe the morphology of the subducting slab near the southern edge of the rupture of the last great 1877 earthquake ( M w = 8.8) in the northern Chile seismic gap, and to monitor the seismic activity probably associated with the Atacama fault system that is roughly parallel to the coast. The analysis of the analog records provides a total of 552 reliable events (2.0 < M < 5.0), whose hypocentres delineate the morphology of the subducting plate in the region. The Nazca plate subducts to the east with a dip of 10° along the trench from 22°S to 25°S down to 30 km depth. At 30–60 km depth a slight variation in the dip angle is observed from 17° (22–23.5°S) to 14° (24–25°S). Downplate, from 60 to 100 km in depth, the dip angle increases more rapidly to the north of 23.5°S than to the south of this latitude, where an almost constant dip (14–16°) is observed and the subducting plate becomes more subhorizontal. For greater depths (100–150 km), the dip of the subducting Nazca plate gradually varies from 36° to 18° between 22°S and 24.5°S. South of 24°S and below 100 km depth, an absence of seismicity is observed. However, a cluster of intermediate depth activity is located near the hypocentre of the December 9, 1950 ( M w = 8.2) intraplate normal fault earthquake, around 500 km inland from the trench. Shallow seismicity (depth ⩽ 30 km) is located near the Atacama fault system. Focal mechanisms show normal faulting with slight left-lateral motion along an average strike in the north-northeast-south-southwest direction, which is in agreement with the observed superficial orientation of the fault. Shallow seismicity is also observed on the Mejillones Peninsula, the main irregularity along the coastline. Focal mechanisms of microearthquakes located near the Cerro Moreno fault, which is in the Mejillones Peninsula show left-lateral motion along a north-south fault plane, similar to the fault orientation observed in the field. South of Antofagasta, the Coastal Scarp presents shallow seismicity. Focal mechanisms were possible to obtain only for events with depths between 20 and 30 km which are characterized by thrust faults and are probably associated with the interplate subducting zone.

Active faulting in northern Chile: ramp stacking and lateral decoupling along a subduction plate boundary?

Two large features parallel to the coastline of northern Chile have long been suspected to be the sites of young or active deformation: (1) The 700-km long Coastal Scarp, with average height (above sea level) of about 1000 m; (2) The Atacama Fault zone, that stretches linearly for about 1100 km at an average distance of 30–50 km from the coastline. New field observations combined with extensive analysis of aerial photographs demonstrate that both the Coastal Scarp and the Atacama Fault are zones of Quaternary and current fault activity. Little-degraded surface breaks observed in the field indicate that these fault zones have recently generated large earthquakes ( M = 7–8). Normal fault offsets observed in marine terraces in the Coastal Scarp (at Mejillones Peninsula) require tectonic extension roughly orthogonal to the compressional plate boundary. Strike-slip offsets of drainage observed along the Salar del Carmen and Cerro Moreno faults (Atacama Fault system) imply left-lateral displacements nearly parallel to the plate boundary. The left-lateral movement observed along the Atacama Fault zone may be a local consequence of E-W extension along the Coastal Scarp. But if also found everywhere along strike, left-lateral decoupling along the Atacama Fault zone would be in contradiction with the right lateral component of Nazca-South America motion predicted by models of present plate kinematics. Clockwise rotation with left-lateral slicing of the Andean orogen south of the Arica bend is one way to resolve this contradiction. The Coastal Scarp and the Atacama Fault zone are the most prominent features with clear traces of activity within the leading edge of continental South America. The great length and parallelism of these features with the subduction zone suggest that they may interact with the subduction interface at depth. We interpret the Coastal Scarp to be a west-dipping normal fault or flexure and propose that it is located over an east-dipping ramp stack at the subduction interface. The similar flexure at the western edge of the Altiplano may have the same origin. Ramp stacking along the subduction zone, a mechanism disregarded so far, may be an important thickening process in the Andes and perhaps the basic cause of the uplift of the Altiplano.

The kinematic and geodynamic significance of the Atacama fault zone, northern Chile

The Atacama fault zone (AFZ) is the dominant feature in the structure of the North Chilean Coastal Cordillera. Left lateral displacement took place along its system of longitudinal faults during the Jurassic and early Cretaceous. This development was contemporaneous with arc magmatism and was later reactivated, resulting in a steep normal fault. Strike-slip movements along the AFZ consist of two sets of ductile shear zones of different ages: one Jurassic, formed under amphibolite-facies conditions; the other early Cretaceous, with greenschist-facies mylonites. Structural asymmetries point to a sinistral sense of shear in both sets. The AFZ can be interpreted as a magmatic arc structure which accommodated the oblique subduction of an oceanic plate (trench-linked strike-slip fault). The sinistral sense of shear is consistent with reconstructions of late Jurassic to early Cretaceous plate configurations in the SE Pacific.

Palaeomagnetism of the Cordillera de la Costa, northern Chile: evidence for local forearc rotation

Palaeomagnetic studies of Lower Jurassic to Lower Cretaceous volcanic, plutonic and sedimentary rocks from the Cordillera de la Costa of northern Chile, indicate that there has been no significant latitudinal movement of northern Chile since early Cretaceous times. A palaeomagnetic pole for the Cordillera de la Costa (63°S, 187°E; α 95 = 7.7° ) was derived from combining the results of three studied units, the La Negra Formation (Jurassic volcanics); Jurassic/Lower Cretaceous granodioritic plutons; and the E1 Way Formation (Hauterivian marine carbonates) together with the Coloso Formation (Lower Cretaceous red beds). Comparison with a Cretaceous reference pole for the stable shield area of South America (87°S, 235°E; α 95 = 2.4° ) indicates that 24 ± 7° of clockwise rotation has taken place. Possible mechanisms for the rotation of the Andean forearc in this area have been examined in relation to the kinetics of the subducting Nazca and over-riding South American plates. The Cordillera de la Costa has been subjected to both compressional and extensional tectonics since the Lower Cretaceous. Major eastwardly directed thrusting in mid-Cretaceous and early Tertiary times, possibly associated with a period of oblique subduction of the Nazca plate, contributed to the accretion of the Cordillera to the main forearc; rotation may have occurred during this process. In Oligo-Miocene and later Tertiary times, extensional faulting (possibly associated with roll-back of the Nazca plate) led to the reactivation of the Atacama fault zone and the formation of oblique extension faults in the Cordillera de la Costa. Structural rotation associated with this phase of extension could equally explain the observed palaeomagnetic results.

北部チリのアタカマ断層系および中南部チリ（プエルトモント周辺）の氷河地形調査の問題点

北部チリのアタカマ断層系および中南部チリ（プエルトモント周辺）の氷河地形調査の問題点

Geologic structure of the Taltal Area, Northern Chile, in relation to the earthquake of December 28, 1966

abstractThe Taltal area, which lies within the coastal cordillera of northern Chile, is dominated by a group of major active faults that cut a eugeosynclinal section of predominantly Jurassic andesites overlying Paleozoic metamorphic and plutonic rocks and intruded by Late Mesozoic plutons. The Atacama fault, a suggested regional strike-slip fracture parallel to the coastline, has been obliquely cut and left-laterally offset 10 km by the Taltal fault, which passes through the town of Taltal. Three distinctive features were found to be consistently offset 10 km by the Taltal fault: the easternmost strand of the Atacama fault, an intrusive contact, and a unique volcanic unit. Former continuity of the Atacama fault through the Taltal region is proposed, and subsequent disruption by the Taltal fault appears to have caused major structural readjustments in the still-active Atacama fault zone.The tentative offshore epicenter and aftershock distribution of the December 28 earthquake are not directly correlative with faults that have been mapped in the nearby on-shore areas; this lack of correlation is not surprising in view of the suggested depths of hypocenters in the lower crust or upper mantle.