Cordillera De Domeyko Research Articles

Conditions of groundwater recharge in the hyperarid southern Atacama Desert

Weather systems that produced recharge to the aquifers in the Atacama Desert between 24.5 and 25.5°S were investigated using δ 18 O and δ 2 H data in groundwater and precipitation combined with remote sensing methods. Atmospheric systems that produced rain and snow from 1984 to 2017 were analysed using isotopic data for precipitation from the literature and this investigation. δ 18 O-lapse rates on a west to east transect for a series of precipitation events ranged from −5.0 to −1.1 ‰/km, which is similar to the global average. Groundwater stable isotopes plotted against the average elevation of the catchment basin for each spring or well sampled produce a lapse rate identical to precipitation isotopes but are enriched in δ 18 O relative to δ 2 H due to the effects of evaporation before water enters the groundwater system. Data show a seasonal cycle in the isotopic composition of precipitation, as does δ 2 H data obtained from Tropospheric Emission Spectrometer (TES) on-board the Aura satellite. These analyses demonstrate that an important source of moisture is the Pacific Ocean. Although usually discounted in climate and paleoclimate analyses of northern Chile, this source of moisture is consistent with two features of the regional land-atmosphere system. First, the hyper-arid core of the Atacama Desert at these latitudes lies in a secondary rain shadow created by the Cordillera de Domeyko which exceeds 4000 m a.s.l., which blocks further westward passage of moisture that might have crossed from the east over the Andes. Second, the weather events responsible for precipitation between 2000 and 2018 are distributed between winter systems which initiate from the south, and exceptional events at other times of the year in which moisture originates from the tropics. Particle tracking indicates that air masses resided between 10 and 30°S over the Pacific at least 4 days prior to weather events which produced rain, or snow that persisted on the ground for more than one month between 2000 and 2018, even though the triggers for these events originated in higher latitudes. Pacific moisture sources need to be better accounted for in paleoclimate studies and groundwater resources evaluation. • Precipitation collected in the southern part of the Atacama Desert is isotopically characterized. • Seasonal patterns are found in the isotope composition of precipitation and atmospheric water vapor. • Isotope data indicate moisture is derived from the Pacific Ocean, and is confirmed from particle tracking. • Groundwater isotope data indicates moisture has been derived from the Pacific Ocean on millennial timescales.

Kinematic evolution of the central Andean retroarc thrust belt in northwestern Argentina and implications for coupling between shortening and crustal thickening

Abstract The Andes are the culmination of shortening and crustal thickening that commenced during Late Cretaceous time. First-order questions regarding the tectonic evolution of the central Andes include the magnitude and timing of shortening, and controls on the along-strike variability in observed styles of shortening and deformation. Along-strike differences in the time of surface uplift have spawned two contrasting hypotheses: (1) uplift is related to dynamic and isostatic processes accompanying lithospheric removal and is decoupled in space and time from crustal thickening and shortening, and (2) uplift is directly coupled with shortening and crustal thickening. Although considerable work has been done in Bolivia to address these hypotheses, work in northern Argentina has not yet produced a transorogenic balanced structural cross section from which the total amount and kinematic history of shortening can be evaluated. To help understand the evolution of the thrust belt in northernmost Argentina, we present a regional, retrodeformable cross section at 23°–24°S across the Puna and Eastern Cordillera. New apatite fission-track thermochronological data integrated with other geochronological, sedimentological, and structural data constrain incremental retrodeformation of the cross section between ca. 45 and 6.5 Ma. Regional shortening was facilitated by at least 12 major thrust systems, linked to a regional mid-crustal décollement. Deformation generally propagated eastward through time and involved two major episodes of eastward advance of the orogenic front, separated by periods of internal out-of-sequence shortening and kinematic stagnation of the orogenic front. A new minimum estimate of ∼271 km of total shortening from the Cordillera de Domeyko to the eastern orogenic front explains crustal thickening at northern Puna latitudes. Together with previously published paleoaltimetry data, our new structural and thermochronologic data indicate that regional uplift in the northern Argentine Puna and Eastern Cordillera was synchronous with, and thus directly linked to, crustal shortening and thickening.

Multi-proxy insights into the structure and geometry of the tectonic boundary at the Cordillera de Domeyko-Salar de Atacama border: An example of the interplay between basement and foreland basins

The study of the Cordillera de Domeyko and the Salar de Atacama Basin in northern Chile has been critical to understand the generation of the Andean Cordillera and the migration of the deformation front eastwards. However, the relation between the uplift of the range, its temporal pattern, and the formation of the Preandean Depression have not been fully understood. In this article, we forward modeled two gravimetry surveys, consisting in 185 stations crossing the boundary between the Salar de Atacama Basin and Cordillera de Domeyko, with the goal of understanding the physical traits of each domain and the variations in basin geometry. We interpreted two seismic reflection profiles, along with log data from one well in the basin center, and additionally, we analyzed an inversion of the magnetic anomaly for both lines. The results of the analysis show the segmentation of physical properties into two domains. The first is interpreted as basement (Triassic-older) units, while the other shows the presence of gravity lows correlated with late Mesozoic-Cenozoic units cropping out along the western edge of the basin. The magnetic anomalies identified were correlated with variations within the basement and outcrops of crystalline intrusives. Within the sedimentary units, the Late Cretaceous-Paleogene Purilactis Group and Paleogene-Eocene formations were distinguished from the evaporite-rich Oligocene-Miocene Paciencia Group. The latter was deposited after a compressive event (Incaic Event) which thrusted basement units eastwards, inverted Mesozoic extensional faults, and generated the accommodation space required for its deposition. The basin then experienced renewed compression during the Miocene. This interplay between basement uplift and thrusting, along with fault inversion and basin generation, is a characteristic found in other parts of the Salar de Atacama Basin and the orogen, suggesting that this tectonic style might be prevalent in other basins comprising the Preandean Depression and along the Central Andes.

Using spatial patterns of fluvial incision to constrain continental-scale uplift in the Andes

Abstract Geomorphic archives, particularly longitudinal river profiles, are increasingly used as a proxy to reconstruct uplift rates in mountainous regions. Within the Atacama Desert, Northern Chile, slow, long-term erosion creates exceptional preservation of fluvial and alluvial surfaces. This enables river incision patterns to be used on a continental-scale (>250 km) along the western margin of the Andes (18°00'S to 20°15′S) and over a time frame from Miocene to Present day. The data show marked compartmentalisation of fluvial system behaviour with changes in incision rates from south to north creating 3 distinctly different regions. Within these different sectors, incision rates are broadly consistent between rivers suggesting a regional rather than a river specific control on rates. In Sector 1 (18°05′S to 19°20'S) the fluvial systems are exorheic with a terminal base level (the lowest base level to which the river system can erode) in the Pacific Ocean and span the Coastal Cordillera, Longitudinal Valley, Precordillera and western edge of the Western Cordillera. This constrains the total uplift over these regions to a minimum of 1200 m in 11 Myr with incision rates of ~200–120 m/Myr consistent with rapid but sustained uplift of the Andes in the Late Miocene. In Sector 2 (19°20'S to 19°50'S), to the immediate south, the rivers are shorter and terminate in the Longitudinal Valley, spanning only the Longitudinal Valley, Precordillera and the western edge of the Western Cordillera with lower incision rates of 100–50 m/Myr. Comparison of incision rates between Sector 1 and 2 can constrain the uplift of the Coastal Cordillera to 60 m/Myr which is in keeping with previous studies from the region. In southernmost Sector 3 (19°50'S to 20°10'S), the fluvial systems terminate in the Longitudinal Valley and span only the Longitudinal Valley and eastern part of the Precordillera with low incision rates of 50 to 25 m/Myr. Differences between Sectors 2 and 3 are attributable to drainage loss by tectonic beheading of catchments through uplift of the Cordillera de Domeyko fault system, placing a minimum constrain on uplift in this region of 50 to 25 m/Myr. This study demonstrates the applicability of large-scale fluvial archives to access not just the timing of uplift on a continental scale, but also the relative uplift of individual tectonic provinces.

The origin of solutes in groundwater in a hyper-arid environment: A chemical and multi-isotope approach in the Atacama Desert, Chile

The major ion and the multi-isotopic composition (87Sr/86Sr, δ11B, δ34S(SO4) and δ18O(SO4)) of groundwater from the Central Depression in northern Chile is investigated to identify the origin of groundwater solutes in the hyper-arid core of the Atacama Desert. The study area is between the Cordillera de Domeyko and the Central Depression, at latitudes 24–25°S, and is characterized by near-zero air moisture conditions, rare precipitation and very limited runoff. Groundwater composition varies from Ca-HCO3 to Ca, Na-SO4 type below elevations of 3400 m a.s.l. The rCl/rBr ratio of meteoric waters and groundwater overlap, but significantly increase in the aquifer as salinity goes up due to evapoconcentration far from the Domeyko Cordillera. The wind-displaced dust originating in the Central Depression (87Sr/86Sr: 0.706558–0.710645; δ34S(SO4): 0 to +4‰) affects the precipitation composition in the highest parts of the Domeyko Cordillera (87Sr/86Sr: 0.706746–0.709511; δ34S(SO4): +1 to +6‰), whose δ34S(SO4) and δ11B values are greatly different from marine aerosols, discarding its contribution to dust at this distance inland. Sr and S isotopic values in groundwater indicate a strong relation with three main geological units: i) Paleozoic rocks contribute high radiogenic strontium isotope ratios to groundwater (0.707011–0.714862), while sulphate isotopic composition is probably acquired from atmospheric dust (>− 1.4‰), ii) Jurassic marine limestones contribute low-radiogenic strontium isotopic ratios to groundwater (<0.70784), while sulphate can be related to oxidized sulphides that change the isotopic signatures of sulphur (<−1.2‰), and iii) mixed salts in the Atacama Gravels contribute lower radiogenic strontium isotopic ratios and sulphate to groundwater (87Sr/86Sr: <0.707324; δ34S(SO4): +0.1 to +7.7).These three processes reflect water-rock interactions. The δ11B of groundwater generally up to +13‰, does not increase along the regional groundwater flow path, discarding fractionation by interaction with clays. These results improve the understanding of the groundwater evolution in hyper-arid systems through a new conceptual model.

Cretaceous to Middle Cenozoic Exhumation History of the Cordillera de Domeyko and Salar de Atacama Basin, Northern Chile

AbstractSpatiotemporal patterns of deformation and exhumation in the central Andes are key parameters for reconstructing the kinematic history of the orogenic belt. Previous studies of the retroarc thrust belt document overall eastward propagation of deformation since the late Eocene, but the amount and timing of exhumation during the early phase of Andean orogeny remains largely unconstrained, particularly in the modern forearc region. In order to determine the timing and amount of exhumation prior to the late Eocene, we employed a multidating approach combining zircon U‐Pb geochronology with apatite fission track and apatite (U‐Th)/He thermochronology. We focus on the low‐temperature cooling history of the Cordillera de Domeyko thrust belt and synorogenic deposits in the Salar de Atacama basin. Our results show Late Cretaceous to Oligocene cooling and exhumation in the Cordillera de Domeyko. The distribution of cooling ages in the forearc indicates three periods of exhumation: ~86–65, ~65–50, and 50–28 Ma. The amount of cooling was variable in space and time but requires total exhumation of ~2.5–3.3 km of rocks above major structures in the thrust belt. Regional unconformities in the Salar de Atacama basin correlate with periods of eastward migration of the orogenic front at ~65 Ma and ~50–40 Ma. Pulses of deformation at the front of the thrust belt alternated with periods of out‐of‐sequence hinterland deformation and exhumation. Overall, our data show that shortening in the central Andes commenced during the Late Cretaceous (as early as ~86 Ma) and that deformation (shortening) and exhumation were coupled in space and time.

Callovian and Oxfordian (Jurassic) teuthids (Coleoidea, Cephalopoda) from Chile

AbstractColeoid specimens from the Jurassic of northern Chile are included in two different species ofTrachyteuthis, i.e.,T. covacevichiFuchs and Schultze, 2008 andT. chilensisn. sp., and in a new genus and a new species,Pseudoteudopsis perezin. gen. n. sp. The specimens described and figured are from two different areas in northern Chile. Those referred toPseudoteudopsis perezin. gen. n. sp. came from a locality north of Calama and are associated with ammonites indicating the lower Callovian uppermostbodenbenderito lowermostproximumzones (≈gracilisStandard Zone) of the Andean ammonite zonation. Those described asT. covacevichiandT. chilensisn. sp. came from the Cordillera de Domeyko, northeast of Taltal, and are associated with ammonites indicating the middle OxfordiantransversariumZone.

Unraveling the Peruvian Phase of the Central Andes: stratigraphy, sedimentology and geochronology of the Salar de Atacama Basin (22°30–23°S), northern Chile

AbstractThe Salar de Atacama Basin holds important information regarding the tectonic activity, sedimentary environments and their variations in northern Chile during Cretaceous times. About 4000 m of high‐resolution stratigraphic columns of the Tonel, Purilactis and Barros Arana Formations reveal braided fluvial and alluvial facies, typical of arid to semi‐arid environments, interrupted by scarce intervals with evaporitic, aeolian and lacustrine sedimentation, displaying an overall coarsening‐upward trend. Clast‐count and point‐count data evidence the progressive erosion from Mesozoic volcanic rocks to Palaeozoic basement granitoids and deposits located around the Cordillera de Domeyko area, which is indicative of an unroofing process. The palaeocurrent data show that the source area was located to the west. The U/Pb detrital zircon geochronological data give maximum depositional ages of 149 Ma for the base of the Tonel Formation (Agua Salada Member), and 107 Ma for its middle member (La Escalera Member); 79 Ma for the lower Purilactis Formation (Limón Verde Member), and 73 Ma for the Barros Arana Formation. The sources of these zircons were located mainly to the west, and comprised from the Coastal Cordillera to the Precordillera. The ages and pulses record the tectonic activity during the Peruvian Phase, which can be split into two large events; an early phase, around 107 Ma, showing uplift of the Coastal Cordillera area, and a late phase around 79 Ma indicating an eastward jump of the deformation front to the Cordillera de Domeyko area. The lack of internal deformation and the thicknesses measured suggest that deposition of the units occurred in the foredeep zone of an eastward‐verging basin. This sedimentation would have ended with the K‐T phase, recognized in most of northern Chile.

Transcurrencia a lo largo de la Falla Sierra de Varas (Sistema de fallas de la Cordillera de Domeyko), norte de Chile

Strike-slip along the Sierra de Varas Fault (Cordillera de Domeyko Fault-System), northern Chile. The north-south trending Cordillera de Domeyko Fault System in northern Chile considered herein is parallel to the Peru-Chile trench. The displacement history of the Sierra de Varas Fault, a master fault of the mentioned system, was examined in detail in the Aguada del Hornito-Aguada del Cerro Alto de Varas segment. Upper Paleozoic granitoids of the same composition, internal structure and age were cut and displaced by the fault. A sinistral horizontal separation of 15.6±1 km with a vertical component of 4.9±0.1 km, suggests a sinistral-reverse net displacement of 16.4±1 km. This is consistent with the local stratigraphic section that was eroded from the eastern block. A kinematic and dynamic analysis of mesofaults spatially related to the SVF displacements was conducted to identify the different fault populations and to obtain the stress tensor. Two structural systems were identifi ed: an early reverse-strike-slip system and a late dextral superposed system. The fi rst one ocurred during late middle Eocene, and the second is post-Miocene with an horizontal displacement of 0.6 km. The presence of coeval strike-slip displacements along the Sierra de Varas Fault and reverse displacements in a 'reverse fl ower' in the studied segment show that the structural evolution of the Sierra de Varas was dominated by a bulk transpression during the late middle Eocene. The left-lateral displacement here demonstrated for the Sierra de Varas Fault and its infl ection to the SE, south of the Aguada del Cerro Alto de Varas are compatible with the westward vergence of the folds and reverse faults in the El Profeta fault-and-thrust belt, which should be also the result of the transpression.

The role of inherited tectono-sedimentary architecture in the development of the central Andean mountain belt: Insights from the Cordillera de Domeyko

The structure of the Cordillera de Domeyko is dominated by a number of elongated N–S-trending basement ridges. These ridges were uplifted by steep reverse N–S faults that deformed the Mesozoic–Cenozoic cover. The vergence of the fault system varies along strike, conferring an apparent doubly vergent “pop-up” geometry to the axial zone. Two Mesozoic pre-compressional extensional events were recorded in the area. New structural data presented in this paper indicate that most of the generated N–S-trending thrusts and related folds were controlled by the inversion of the pre-existing Mesozoic extensional faults. Thin-skin structures in the Mesozoic–Cenozoic cover are genetically linked to major basement upthrusts, which could be interpreted as basement short-cuts formed during inversion rather than as uplifted blocks associated with major Cenozoic strike-slip faults. Growth-strata geometries date the beginning of the Andean compressional event, which generates the Chilean Precordillera, as far back as 90 Ma ago; the resulting structural architecture is strongly controlled by inherited pre-Andean extensional structures. The association of porphyry intrusives with major reverse faults suggests that the emplacement of the Eocene–Oligocene porphyry Cu–Mo deposits in Northern Chile can be explained by an oblique-inversion Tectonics Model. The upper Eocene–lower Oligocene giant porphyry copper bodies (Chuquicamata, La Escondida, El Salvador) located in the Cordillera de Domeyko show an adakitic affinity. This magma affinity, together with structural evidence presented in this work, indicates that porphyry emplacement occurred at the end of the basement-involved contractional stage that generates the anomalous thickened crust needed to generate these magmas. This tectonic evolution is coherent with the existence of a flat-slab subducting beneath the Central Andes (22°–26° S) during early Cenozoic, that will also produce the eastward migrating of the compressional regime in the upper plate since Late Cretaceous.

Structural control of the emplacement of the Portrerillos porphyry copper, central Andes of Chile

The Potrerillos porphyry is located in the Late Eocene–Oligocene metallogenic belt of porphyry copper deposits of Chile, Cordillera de Domeyko, central Andes. It is formed of a porphyritic biotite–hornblende monzonite that was emplaced at less than 1000 m depth. The intrusion has a cylindrical vertical form at depth, whereas its uppermost section resembles a mushroom whose stem is inclined 20–25° to the northwest. The location of the Potrerillos porphyry coincides with a major facies and east–west thickness change in Jurassic to Late Cretaceous marine sedimentary and Tertiary volcanic host rocks. Structurally, the porphyry was emplaced at the intersection between the northeast-oriented Potrerillos fold-and-thrust belt, and a system of northwest-trending strike-slip faults of the Ciénaga fault system. Both structural elements are related to a sinistral regional transpression that was active during the Middle to Late Eocene. The north–northeast trending Sierra Castillo-Agua Amarga fault system was trench-linked with an oblique subduction along the Peru–Chile trench in the southwestern part of South America at this time. One of the main structural elements of the Potrerillos fold-and-thrust belt is the eastward verging Potrerillos Mine reverse fault. We demonstrate a syntectonic relationship between the Potrerillos porphyry emplacement and the onset of the Potrerillos Mine fault. The control at the shallowest levels of the Potrerillos porphyry emplacement was studied by means of a structural analysis of folds, cleavage and deformed oolites in the country rock. This analysis suggests that the porphyry occupied the core of an anticline. The form of this anticline was modeled as a fault-propagation-fold taking into account a 200 m displacement along the basal fault and a local shortening of 45.5% occurred. This model is compatible with a limited displacement along the Potrerillos Mine fault under hot, ductile conditions. The remaining displacement of 1600 m would have taken place afterwards under cold, brittle conditions. The Potrerillos porphyry was emplaced during a transpressional regime within the Potrerillos fold-and-thrust belt. The fact that the cleavage does not show a concentric distribution around the Potrerillos porphyry, discounts the possibility of a forced emplacement of the porphyry, pushing aside its wall rocks. This is reinforced by the deformed oolites which do not show a strain gradient from outside towards the edge of the body. The clockwise transection of the fold axes by the cleavage and the southeast direction of displacement along the Potrerillos Mine fault are interpreted as related to the regional sinistral transpression related to the Sierra Castillo Fault movement.

Eocene exhumation and basin development in the Puna of northwestern Argentina

The Puna is part of the larger Puna‐Altiplano Plateau (also known as the Central Andean Plateau), characterized by high elevation, low relief, and aridity, located in the central Andes of Bolivia and Argentina. Tertiary sedimentary rocks preserved within the Puna contain a unique archive of information regarding the paleogeography, depositional environments, and timing of sediment source exhumation during the early stages of Andean mountain building. The Eocene Geste Formation in the Salar de Pastos Grandes area (within the central Puna of northwestern Argentina) consists of deposits that are the result of confined to unconfined flows in a sandy to gravelly, braided fluvial system and alluvial fans proximal to the source terrane. Paleocurrent data document an overall eastward flow direction. Up‐section coarsening of the Geste Formation suggests that topographic relief in the source area increased through time, possibly owing to enhanced tectonic activity and source terrane unroofing. Sandstone petrography and conglomerate clast‐count data document quartzose and phyllitic compositions typical of Ordovician rocks preserved just west of the Salar de Pastos Grandes area. Paleocene‐Eocene detrital apatite fission track age populations (P1: ∼35–52 Ma; P2: ∼52–65 Ma) of the Geste Formation and their consistent trends up‐section suggest moderate to rapid (∼0.4 mm/a to &gt;1 mm/a) exhumation of western sediment sources during the early to mid‐Tertiary stages of Andean mountain building. Sedimentation rates increase up‐section from ∼0.1 mm/a to 1 mm/a. Our data, when combined with other structural, stratigraphic and seismic evidence from surrounding regions, suggest that the Geste Formation was deposited in response to crustal shortening and resulting erosion and sedimentation, which started as early as Cretaceous in the Chilean Cordillera de Domeyko and in the Salar de Pastos Grandes area by Eocene time. The Geste Formation could be interpreted either as a local wedge‐top accumulation on the eastward propagating central Andean orogenic wedge, or as a local intermontane basin. The similarities between wedge‐top deposits preserved in Bolivia and Eocene deposits in northwestern Argentina, south of ∼25°S, lead us to favor the wedge‐top scenario for the Geste Formation. If correct, this implies that the deformation front of the Andean orogenic wedge incorporated both thin‐ and thick‐skinned structures as it migrated, possibly unsteadily, from the Cordillera de Domeyko during the Cretaceous‐Paleocene to areas within the Puna and Eastern Cordillera by mid‐late Eocene time. Contemporaneously, a regional‐scale foreland basin system developed over an along‐strike distance of at least 650 km.

Cenozoic subsurface stratigraphy and structure of the Salar de Atacama Basin, northern Chile

Sequence mapping of industry seismic lines and their correlation to exposed stratigraphic formations enable a description of the evolution of the nonmarine Salar de Atacama Basin. This major tectonic basin, located in the present-day forearc of the northern Chilean Andes, was first defined topographically by late Cretaceous inversion of the Jurassic–early Cretaceous extensional Tarapacá backarc Basin. Inversion led to both the uplift of the Cordillera de Domeyko and subsidence of the Salar de Atacama Basin along its eastern flank. The basin evolved from a continental backarc in the Cretaceous and Paleogene to a forearc tectonic setting during the Neogene. The principal causes of basin-scale tectonic subsidence include late Cretaceous and earliest Paleocene shortening and Oligocene–early Miocene localized extension. The basin was not completely filled by late Cretaceous (Purilactis Group, sequence G) and Paleocene (sequence H) strata, and its empty space persisted through the Cenozoic. Eocene deformation caused long-wavelength rotation of a deeply weathered surface, generating an erosional unconformity across which coarse clastic strata accumulated (sequence J). Oligocene–early Miocene normal faulting, perhaps in a transtensional environment, repositioned the western basin margin and localized hangingwall subsidence, leading to the accumulation of thousands of meters of evaporitic strata (sequence K, Paciencia Group). By the close of the early Miocene, shortening resumed, first uplifting the intrabasinal Cordillera de la Sal and later generating Pliocene blind reverse faults within the topographically lowest part of the basin. Unequal deposition and tilting across the nascent Cordillera de la Sal induced diapirism of the Paciencia Group halite. In combination, inherited accommodation space and new tectonic subsidence, plus local salt-withdrawal subsidence, shaped the distribution of Upper Miocene–Recent ignimbrites, evaporites, and clastics (sequence M and Vilama Formation).

Late Mesozoic to Paleogene stratigraphy of the Salar de Atacama Basin, Antofagasta, Northern Chile: Implications for the tectonic evolution of the Central Andes

The Salar de Atacama basin, the largest “pre-Andean” basin in Northern Chile, was formed in the early Late Cretaceous as a consequence of the tectonic closure and inversion of the Jurassic–Early Cretaceous Tarapacá back arc basin. Inversion led to uplift of the Cordillera de Domeyko (CD), a thick-skinned basement range bounded by a system of reverse faults and blind thrusts with alternating vergence along strike. The almost 6000-m-thick, upper Cretaceous to lower Paleocene sequences (Purilactis Group) infilling the Salar de Atacama basin reflects rapid local subsidence to the east of the CD. Its oldest outcropping unit (Tonel Formation) comprises more than 1000 m of continental red sandstones and evaporites, which began to accumulate as syntectonic growth strata during the initial stages of CD uplift. Tonel strata are capped by almost 3000 m of sandstones and conglomerates of western provenance, representing the sedimentary response to renewed pulses of tectonic shortening, which were deposited in alluvial fan, fluvial and eolian settings together with minor lacustrine mudstone (Purilactis Formation). These are covered by 500 m of coarse, proximal alluvial fan conglomerates (Barros Arana Formation). The top of the Purilactis Group consists of Maastrichtian-Danian alkaline lava and minor welded tuffs and red beds (Cerro Totola Formation: 70–64 Ma K/Ar) deposited during an interval of tectonic quiescence when the El Molino–Yacoraite Late Cretaceous sea covered large tracts of the nearby Altiplano-Puna domain. Limestones interbedded with the Totola volcanics indicate that this marine incursion advanced westwards to reach the eastern CD slope. CD shortening in the Late Cretaceous was accompanied by volcanism and continental sedimentation in fault bounded basins associated to strike slip along the north Chilean magmatic arc to the west of the CD domain, indicating that oblique plate convergence prevailed during the Late Cretaceous. Oblique convergence seems to have been resolved into a highly partitioned strain system where margin-parallel displacements along the thermally weakened arc coexisted with margin-orthogonal shortening associated with syntectonic sedimentation in the Salar de Atacama basin. A regionally important Early Paleocene compressional event is echoed, in the Salar de Atacama basin by a, distinctive, angular unconformity which separates Paleocene continental sediments from Purilactis Group strata. The basin also records the Eocene–Early Oligocene Incaic transpressional episode, which produced, renewed uplift in the Cordillera de Domeyko and triggered the accumulation of a thick blanket of syntectonic gravels (Loma Amarilla Formation).

Geologic Evolution of the Escondida Area, Northern Chile: A Model for Spatial and Temporal Localization of Porphyry Cu Mineralization

A program of geologic mapping and lithogeochemical and geochronological sampling has been carried out over a 745-km2 area of the Atacama Desert surrounding the porphyry Cu deposits at Escondida, Zaldivar, and Chimborazo (Cordillera de Domeyko, northern Chile). The purpose of this study was to examine the regional tectonic and magmatic setting of this preeminent porphyry Cu district for evidence of features or processes that might explain the giant scale of mineralization at Escondida and provide predictive tools for exploration in other areas. The geologic history of this area as recorded by exposed rocks begins with voluminous, intermediate to felsic Permo-Carboniferous volcanism (La Tabla Formation), and these rocks appear to constitute the crystalline basement throughout much of the porphyry belt of northern Chile. Geochemically, they are I-type in character, but the parental magmas were relatively dry, and thus did not generate effective magmatic-hydrothermal systems (few significant ore deposits are known to be associated with them). Andean cycle arc magmatism began in the Triassic, centered on the La Negra magmatic arc (now located near the Chilean coast). Farther inland, near Escondida, back-arc processes led to the eruption of intermediate to felsic lavas and tuffs and the deposition of marine sediments in rift basins. Closure of these basins in the Late Cretaceous resulted in deformation of the volcano-sedimentary sequences and was followed by emplacement of small alkali gabbro stocks and dikes. The axis of arc magmatism moved eastward in the Paleocene (Central Valley arc) and produced widespread calc-alkaline intermediate to felsic volcanism through to the Eocene. East- to northeast-directed convergence maintained a dextral transpressive regime during this period, and early movements in the West Fissure zone, a corridor of orogen-parallel faults that runs the length of the Cordillera de Domeyko (over 1,000 km), reflect this couple. At the end of the Eocene, however, stresses in the arc appear to have relaxed, and by the late Oligocene, strike-slip movement along the West Fissure zone had reversed to sinistral. This period of stress relaxation at the end of the Eocene period coincided with the voluminous emplacement of dioritic magmas at shallow crustal levels and also with porphyry development. Six samples of hornblende from these diorites yield 40Ar/39Ar dates between 38.28 ± 0.32 and 36.94 ± 0.46 Ma (2 σ ). Porphyry emplacement at Escondida, Zaldivar, and Chimborazo was coeval with this dioritic magmatism at ~38 Ma. Where plutonism was intense, the dioritic magma is interpreted to have evolved by processes of assimilation and fractional crystallization to more felsic compositions characteristic of the ore-forming porphyry intrusions. Whole-rock trace element data indicate that hornblende fractionation was an important control on chemical evolution of the diorites and attests to high-magmatic water contents (>4 wt % H2O). Volatile saturation would have occurred during further differentiation of these magmas, evidence for which is provided by the porphyry ore deposits. Porphyry emplacement was localized within a broad zone of intersection between the West Fissure zone and a regionally extensive northwest-trending structural corridor (the Archibarca lineament). It is proposed that the geometry of this junction was conducive to the formation of transtensional pull-apart structures during relaxation or reversal of dextral shear on the West Fissure zone. Such dilational structures would have focused the ascent and pooling of magma in the upper crust and maximized the potential for formation of magmatic-hydrothermal ore deposits. The formation of giant porphyry systems such as Escondida is, therefore, considered to be the result of a fortuitous coincidence of processes, including generation of suitable volumes and compositions of magma, appropriate lithospheric stress conditions, and structural focusing of emplacement; in addition, the development of thick supergene enrichment blankets has been critical to the economic value of these deposits. None of these contributory processes are in themselves unusual or rare, but because they are largely independent of one another, their constructive cooperation in ore formation is not necessarily repeatable at different places and at different times, thus explaining the relative rarity of giant porphyry deposits.

Clockwise block rotations along the eastern border of the Cordillera de Domeyko, Northern Chile (22°45′–23°30′S)

We report new paleomagnetic results from a study of 38 sites in Cretaceous to Early Tertiary red beds and volcanic rocks belonging to the Purilactis Group, which outcrop along the western border of the Salar de Atacama Basin. After detailed thermal demagnetization, characteristic directions were determined for 32 units. In most cases, red bed sediments from the lower members of the Purilactis Group have a well-defined normal polarity magnetization probably carried by hematite forming an early diagenetic cement. A large decrease in the dispersion of the paleomagnetic directions upon tilt correction demonstrates that this magnetization is a pre-tectonic magnetization. We interpret the dominant occurrence of the normal polarity direction as evidence for acquisition of the magnetization during the Cretaceous normal polarity superchron. Large deviations of the paleomagnetic declinations from the expected ones for stable South America provides new evidence for clockwise tectonic rotations associated with Tertiary deformation in the Cordillera de Domeyko. These data confirm that clockwise tectonic rotations are one of the most significant structural characteristics of the north Chilean Andes. This study, however, indicates spatial variation in the magnitude of the rotation with rotations >60° in the Cerro Totola area. These rotations have occurred in conjunction with transpressional deformation that affected large tracts of the Cordillera de Domeyko during Eocene deformation. The systematic observation of clockwise rotations contemporaneous with sinistral displacements in the Cordillera de Domeyko can be explained by shear-traction at the base of the brittle crust.

Strike-slip faulting and nested block rotations: structural evidence from the Cordillera de Domeyko, northern Chile

Based on a structural analysis, tectonic rotations around vertical axes have been found affecting the sedimentary sequence of the Purilactis Group from the Cordillera de Domeyko of northern Chile. The affected rocks form the northern part of a pre-existing syncline that was offset by two sets of strike-slip faults during the Upper Eocene Incaic Orogeny. Uplift and shortening in the hangingwall of two main bounding thrusts initiated the strike-slip faults, which divided the Purilactis Group into several fault blocks. Further contraction of the enclosed block-faulted area led to domino-style anticlockwise rotation of the central main blocks and the bounding block faults with rotations up to 40°. Additionally, eastward displacement of these blocks was accompanied by local indentation of the adjacent smaller blocks to the east giving rise to opposite rotation directions. The block rotations may have been accommodated by a décollement located in the mudstone and evaporite of the basal part of the sequence (Tonel Formation). Based on a reconstruction of the initial rock distribution, different fault models are discussed and a model resembling pure shear deformation is proposed to explain the initiation of strike-slip faults, the occurrence of block rotations and the distortion of the block-faulted domain.

The upper Bajocian and Bathonian in the Cordillera de Domeyko, North-Chilean precordillera: Sedimentological and biostratigraphical results

The Jurassic ammonites of the Cordillera de Domeyko have been studied in several monographies. The classicmemoirs of Steinmann (1881), Tornquist (1898), Burckhardt (1903) and Stehn (1923) are particularly relevant. However, the biostratigraphic problems concerning stage boundaries arisen only in recent dates. New sedimentological and biostratigraphical data have been obtained in relation to the Bajocian/Bathonian of this region. Several sections have been studied in two separate areas: Caracoles and Sierra de Varas. More than one thousand ammonites were collected in situ from these localites. Facies analysis shows that sedimentation during Late Bajocian and Bathonian occurs in a marine environment on a carbonate platform with abundant siliciclastic supply. Facies distribution was controlled by structural segmentation of the platform and processes of differential subsidence. The stratigraphical-sedimentological results together with taphonomical-palaeoecological observations are a diagnostic tool for sequence analysis. The successive ammonite assemblages which were found have allowed the establishment of four biostratigraphic units, spanning the interval from Upper Bajocian to Upper Bathonian. The sedimentological and biostratigraphical results indicate the existence of a regional discontinuity at the base of the Bathonian.

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.

Paleomagnetic evidence for rotation in the Precordillera of northern Chile: structural constraints and implications for the evolution of the Andean forearc

Palaeomagnetic results are reported for two late Mesozoic to early Cenozoic continental sedimentary sequences from the Precordillera (Cordillera de Domeyko) of northern Chile. Comparison of isolated components of magnetisation from the Late Cretaceous-Palaeocene Purilactis Formation and the unconformably overlying Oligo-Miocene Paciencia Group with reference poles for South America indicate that significant post-Palaeocene and post-Lower Miocene clockwise rotation has taken place. A structural study of the area has revealed the presence of a number of thrust sheets that were periodically active throughout the Cenozoic. Stratigraphie, structural and palaeomagnetic data indicate that thrust sheet movement took place following deposition of both the Purilactis Formation (?lower Eocene Incaic Orogeny) and the Paciencia Group (?lowest Miocene Peheunche Orogeny) and resulted in the folding, uplift and clockwise rotation of the two sequences. Differences in the amount of rotation between the two units reflect incremental movement of the thrust sheets, 12° of rotation took place prior to deposition of the Paciencia Group followed by 17° of post-Lower Miocene clockwise rotation. This study demonstrates that palaeomagnetically detected rotation in the Precordillera of northern Chile, 250 km inland of the Peru-Chile trench, is related to the periodic movement of an easterly propagating thin-skinned thrust front. Between 22°30′ and 23°30′S the north Chilean forearc contains areas of non-rotation, thin-skinned compressional related rotations and large, block fault rotations indicating that the simplistic models previously presented to account for forearc rotation in the Central Andes should be modified. In particular, evidence implies that rotation could be related to a number of tectonic events and may not simply be due to one deformation event.