Punta Negra Research Articles

Multiproxy provenance analyses in the Devonian Villavicencio Formation of the Mendoza Precordillera, Argentina: correlation and geotectonic implications for the SW Gondwana margin

Abstract This work focuses on the sedimentary provenance of the Villavicencio Formation of the Mendoza Precordillera and integrates the information obtained with previous work on other coeval units of the Precordillera Central of San Juan province (Gualilán Group: Talacasto and Punta Negra formations) in western Argentina. Multiproxy provenance analyses are carried out from different applied methodologies (petrography, geochemistry, morphological, and cathodoluminescence studies of detrital zircon grains, and analysis of U-Pb and Lu-Hf isotopes). The Villavicencio Formation is mostly composed of pelites and very fine-grained psammites. The major components are quartz, both monocrystalline and polycrystalline, and metamorphic lithics that associate this unit with a recycled orogen. Regarding geochemistry, the Chemical Index of Alteration (CIA) values are similar to the Post-Archean Australian Shales (PAAS), indicating a null to incipient degree of weathering. The ratios between different trace elements and rare earth elements (REEs) suggest the felsic composition of the source area. Th/U ratios differ, but a secondary uranium enrichment is inferred. The morphological analysis of the zircon grains reveals their mainly plutonic origin. The integration of U-Pb data with Lu-Hf data shows a juvenile-mantle origin in which the populations are dominantly Mesoproterozoic and ɛHf of positive values (up to 12), indicating poor differentiation. The Villavicencio Formation would be the product of deltaic deposits in which its components are dominantly from the Western Pampean Sierras associated with the Grenville orogen, assuming exhumation and erosion of the Mesoproterozoic basement. The data support the hypothesis of equivalence and correlation with the Punta Negra Formation in the Devonian depocenters of the south-central region of the San Juan Precordillera.

Emplacement and accumulation of magmatic products in contractional structures: How does it occur? Insights from Central Andes of northern Chile

Structural and stratigraphic data demonstrate that the magmatism in northern Chile was accompanied by shortening related to the development of fold and thrust belts. However, how magmatic products were emplaced and accumulated in the contractional structures is an unresolved question. We, therefore, integrated field and seismic data from the regions of the Pampa del Tamarugal, Salar de Punta Negra Basin, and the Hornitos Fold and Thrust Belt to determine the main structures that facilitated the magma accumulation. The syn-kinematic granitic intrusions in the Hornitos Fold and Thrust Belt were preferably emplaced along fold-related thrusts. In this region, magma migrated along thrust ramps and finally accumulated into the core of the anticlines and broken synclines. Subsidiary and vertical tensional fractures controlled the magma accumulation in the hinge zone and at the axis of the folds. The seismic data from the Pampa del Tamarugal and Salar de Punta Negra Basin present evidence of buried volcanoes on inversion anticlines. During the tectonic inversion of the pre-existing Late Paleozoic to Mesozoic half-graben structures in the regions, magma migrated along the master faults and finally accumulated into the core of the inversion anticlines as sills or was expulsed to the surface as volcanoes, as evidenced by the Punta Negra Monogenetic Volcano.

The temporal and spatial relationship between strike-slip and reverse faulting in subduction-related orogenic system: Insights from the Western slope of the Puna Plateau

The relationship between parallel and oblique to the orogen faults and the magmatic evolution is key to understanding the evolution of a hot orogen, such as the Central Andes. The Andean orogenesis along the southern Central Andes, during the Neogene is characterized by regional compression and magmatic processes associated with subduction. The outcome of this dynamic interaction between plate tectonics and magmatism has generated reverse, normal and strike-slip faults, both parallel and oblique to the trench. Despite the progress made on studying these fault systems, both their relationship with the stress field and their role in magma propagation into the shallow crust are still enigmatic. In this work, geomorphological observations are coupled with kinematic and dynamic analyses, as well as with kinematic forward modeling, to reconstruct the evolution of two main faults affecting the western slope of the Puna plateau, the Barrancas Blancas fault and the Tocomar fault, during the Neogene. The obtained data reveal that, between 17 and 10 Ma, the Barrancas Blancas fault had reverse activity, while the Tocomar fault had left-lateral strike-slip movement. At 10 Ma, the area was affected by the coeval reactivation of the Volcan de Punta Negra fault and the right-lateral activity of the Tocomar fault. During the last stage, strike-slip movement along the Tocomar fault favored the rise of magma, while the hydrothermal activity evolved along the Barrancas Blancas fault. The study results reveal that the oblique-to-the-orogen faults play a role in the segmentation of the reverse parallel-to-the-trench deformation and control the position of the volcanic centers, while the parallel-to-the-orogen faults control the relief development and the evolution of hydrothermal systems. The proposed model helps in understanding how magma rises to the surface associated with movement along reverse and strike-slip faults during the thickening of the crust.

New insights on the interpretation of the provenance and evolution of the Silurian units in the central Precordillera, Argentina

In the central region of the Precordillera, San Juan Province, Silurian silicoclastic sedimentites of the Los Espejos Formation crop out in the Jáchal River area. To the south of this region, an equivalent unit is recognized in the San Juan River area (Tambolar Formation). Both units present similar lithological characteristics, however, it has not yet been defined if they share source areas of detrital contributions. On the other hand, for the Jáchal River sector, it is proposed to establish if there were changes in the regions from where the sub-basin received sediment contributions during the Devonian, which can be seen reflected in the detrital zircon contribution patterns of the overlying unit (Talacasto Formation). The present work is part of a series of studies tending to determine the nature and provenance of the Silurian-Devonian sequences of the Central Precordillera. On this occasion, detrital zircon patterns of the detrital sources of the Los Espejos Formation are analyzed and compared with the information obtained for the Silurian Tambolar Formation (San Juan River area) and the overlying Devonian Talacasto and Punta Negra formations (Jáchal and San Juan rivers areas). To characterize and compare the studied units, different methodologies were applied, namely sedimentary petrography, heavy minerals studies, and morphological and isotopic analyses of detrital zircons. The analysis of thin sections allowed determining textural and compositional parameters. Through the predominance of detrital minerals, it was possible to establish that the studied units are composed of quartzite-type rocks coming from mature areas, with low percentages of lithic components and abundant opaque heavy minerals of the hematite group. The study of heavy minerals, especially morphological and typological parameters of detrital zircons, allowed to establish recycled and plutonic sources as main modes, as well as the changes that occurred during the basin filling dynamics for Silurian and Devonian times. On the other hand, U–Pb isotopic analysis in detrital zircons indicate that the Pampean-Brasiliano orogenic cycle composes the main source of detritus with ages between 511 and 816 Ma. In second place are the Mesoproterozoic ages, represented by the interval from 1000 to 1350 Ma. The youngest detrital ages show a maximum sedimentation age of 478.5 ± 4.4 Ma (Tremadocian), indicating that younger sources of contribution correspond to the Famatinian Orogen. The Kolmogorov-Smirnoff test revealed that the studied Silurian-Devonian units studied have similar patterns of sedimentary contributions, which suggests that the sources of provenance were common in both regions and remained active throughout the entire time interval, without significant changes.

Comparison of groundwater composition from the Monturaqui and Punta Negra Basins, northern Chile: implications for porphyry copper exploration

Groundwaters recovered from the Salar de Punta Negra and Monturaqui basins in the Atacama Desert of northern Chile exhibit distinctly different isotopic, major, trace, and porphyry copper elemental compositions related to contrasting morphostructural, geochemical, and hydrodynamic settings. Comparison of these distinct groundwater signatures with groundwaters from known porphyry copper deposits (Salar de Hamburgo and Spence Deposit), can be used to determine if either basin might be prospective for porphyry copper mineralization. Groundwaters within the Punta Negra Basin exhibit geochemical characteristics consistent with other closed basin settings throughout the arid Andes. Elemental and isotopic compositions within the Punta Negra Basin reflect closed basin evaporitic processes consistent with the hyperarid, volcanic setting of the central Andes. Pathfinder metals and isotopic compositions are not consistent with porphyry copper type mineralization as described for groundwaters within the Salar de Hamburgo Basin, and the Spence Deposit. Within the Monturaqui Basin the geochemical composition of groundwaters are characteristic of diffuse hydrothermal activity. Sulfur isotopic composition within the waters of the northern Monturaqui Basin exhibit δ 34 S CDT isotopic signatures that fall within the range for sulfide mineralization as reported for groundwaters in and around the Spence Deposit and the Salar de Hamburgo. However, porphyry copper related elements within the Monturaqui Basin are impoverished relative to groundwaters of the Spence and Escondida Deposit. Such impoverishment in porphyry related elements, taken in conjunction with enriched δ 13 C PDB compositions, elevated groundwater temperatures, groundwater compositions dominated by HCO 3 , SO 4 , and Si, and the proximity of the Monturaqui Basin to the current magmatic arc are consistent with a volcanic hydrothermal origin. In summary, the groundwater geochemistry of the Monturaqui and Punta Negra Basins are not indicative of porphyry copper-type mineralization. Thematic collection: This article is part of the Hydrochemistry related to exploration and environmental issues collection available at: https://www.lyellcollection.org/cc/hydrochemistry-related-to-exploration-and-environmental-issues

Provenance evolution of the San Juan Precordillera Silurian-Devonian basin (Argentina): Linking with other depocentres in Cuyania terrane

This paper explores the sedimentary provenance of the Silurian to Devonian siliciclastic successions in the Central region of the San Juan Precordillera, in central-western Argentina. The comprehensive provenance analysis of the Tambolar Formation (upper Silurian), Talacasto Formation (Lower Devonian) and Punta Negra Formation (Lower-Middle Devonian) from the San Juan River section is based on petrography, detrital zircon analyses and combined U–Pb and Lu–Hf isotopic data. Modal composition of the three units here studied shows an evolution trend from quartzose recycled, mixed, to transitional arc. Detrital zircon morphologies indicate derivation from dominantly plutonic and metamorphic sources with little evidence of transportation. The most representative detrital zircon U–Pb age populations are Famatinian (middle Cambrian - Devonian), Pampean–Brazilian (Tonian - lower Cambrian), and Grenvillian (Ectasian - Stenian) cycles derived rocks. Hf isotopes determined mainly in Mesoproterozoic detrital zircons reflect a juvenile mantle source during crystallization, with less significant inputs from those with a crustal and recycled crust origin. Results from this work are compared to data from outcrops of the same units developed in the North-Central region of the Precordillera, presented in previous works. Provenance proxies provide insights regarding the correlation of probably equivalent units deposited in other foreland basins within the Cuyania terrane, such as the Villavicencio Formation (Mendoza Precordillera), Río Seco de los Castaños and La Horqueta Formations (San Rafael Block). The K–S test suggests a low correlation between the units studied at the San Juan River and the equivalent outcrops exposed towards the central and northern parts of the San Juan Precordillera, as well as with the known records of the Río Seco de los Castaños and La Horqueta Formations of the San Rafael Block. In contrast, the samples from the Villavicencio Formation show a high correlation with the sample from the Punta Negra Formation, indicating a possible common sediment source. The broad variations of U–Pb detrital age pattern distributions observed between Silurian and Devonian successions throughout the Precordillera and the San Rafael Block point to provenance shifts linked to variability in east-to-west sediment dispersal patterns within an axial peripheral foreland basin. Fluctuations in the sedimentary composition are ascribed to the differential exhumation rate of source rocks and a progression from a convergent tectonic setting in the San Rafael block towards a coeval collisional environment in the Precordillera. The Silurian - Early Devonian foreland basin is tectonically linked to the Ocloyic Orogen that resulted from the collision of Cuyania terrane to the Western margin of Gondwana. Source rocks were most likely located within the Mesoproterozoic basement of Cuyania, exhumed during the Middle Devonian as a response to the approximation of Chilenia.

Deciphering the Late Paleozoic–Cenozoic Tectonic History of the Inner Central Andes Forearc: An Update From the Salar de Punta Negra Basin of Northern Chile

We integrated new and existing geological, geochronological, thermochronological, and two-dimensional (2D) seismic data from the Salar de Punta Negra Basin to define the Late Paleozoic–Cenozoic tectonic evolution of the inner Andean forearc of northern Chile more precisely. Our results indicate that this region experienced early Late Paleozoic–Mesozoic crustal extension, creating several basement half-graben structures bounded by east- and west-dipping master faults. These extensional basins were filled by Upper Permian to Jurassic volcanic and sedimentary (continental and marine) syn-rift deposits. The genesis of these structures is related to the early breakup of the western Gondwana continent and the development of the large Tarapacá Basin in northern Chile and southern Perú. Subsequently, Late Cretaceous to Paleocene contraction occurred, which led to the tectonic inversion of the pre-existing rift system and the uplift of the Paleozoic–Mesozoic syn-rift deposits. Seismic data show that Upper Cretaceous and Paleocene synorogenic deposits accumulated along and over inversion anticlines, recording the initial contraction and marking the change from an extensional to a contractional tectonic setting. During the final episodes of basin inversion, crustal shortening was accommodated by the Eocene to recent basement reverse faulting accompanied by the rapid exhumation of basement pre-rift blocks, which served as the principal sources for the sediments that filled the pre-Andean basins during the Late Cenozoic. Finally, the exhumed basement pre-rift blocks and the reverse faults compartmentalized the contractional intermontane basins, which constitute the main low topographic relief of the inner forearc of northern Chile.

Early Devonian paleogeographic evolution of SW Gondwana (Precordillera Argentina). How can the record of plants help us?

In this work we refer new fossil plant records and revise the systematics of previously described taxa. The finding of Sporogonites excellens and the analysis of its fossil record allows us to propose it as a Lower Devonian indicator. The finding of a leafy lycophyte remains allows us to identify Haskinsia sp. in Precordillera and confirms its presence since the Emsian. From the registry of the plants associations found and taking into account the good age control in the sections to the north of the San Juan river, we propose a Devonian age (pre-Emsian) for the southern region (Banchongo Farm and vicinity). In this way, the Precordillera basin would be separated by a positive structure in at least two depocentres at this time, developing the Talacasto Formation to the north and center of the basin while the deposits of the Punta Negra and Villavicencio Formations occurred simultaneously to the south. During the Emsian- Middle Devonian, the rise in sea level, higher rates of sediment inputs and a marked subsidence made possible the deposition of the Punta Negra formation throughout the basin. The above conclusions affirm the importance of biostratigraphy plant-based in successions that do not have the presence of other markers.

The spatial and trophic ecology of culpeo foxes (Lycalopex culpaeus) in the high Andes of northern Chile

ABSTRACT The spatial and trophic ecology of the culpeo fox (Lycalopex culpaeus) was studied in the Salar de Punta Negra basin and the Llullaillaco National Park, in the high Andes of the Region of Antofagasta in northern Chile. The objectives of this study were to understand the feeding ecology, habitat use, and home range and activity pattern of culpeo fox. The study was conducted in an area of approximately 2800 km2 inside the Punta Negra salt flat basin, Llullaillaco NP, the surrounding mountains, and within areas of mining operations. We used methodologies including camera trapping, live trapping and satellite monitoring of L. culpaeus individuals, and diet analysis. Fifteen captures and five recaptures of culpeos were made, and three of them were equipped with satellite tracking collars. The home ranges estimated using the Minimum Convex Polygon (MPC 95%) were: 1029 km2 (female), 809 km2 (female) and 215 km2 (male). Culpeo foxes were nocturnal animals. A larger sample of 282 fecal scats was collected, with the main trophic prey in culpeo fox diet by frequency of occurrence being rodents (74.3%), arthropods (48%), camelids carrion (43.6%), birds (16.3%), vegetation (13.8%) and reptiles (2.5%). Niche breadth was low (0.367).

On the origin of saline compounds in acidic salt flats (Central Andean Altiplano)

Volcanism, aridity, and endorheism converge in the central zone of the Andean Cordillera (Bolivia, Chile, and Argentina, between latitudes 19° S and 27° S). The Gorbea and Ignorado basins are pristine Andean sites in which the transfer of saline compounds from endogenous to exogenous environments occurs. In this area the regional volcanic rocks display strong argillic alteration, with Al and Fe (alunite, jarosite) and Ca (gypsum/anhydrite) sulfates. Native sulfur is also present in paleosolfataras. The Gorbea salt flat is covered by a discontinuous layer of selenitic gypsum of varying thickness (maximum 2 m). The discontinuity of the layer as well as its variable thickness is due to the fact that the original bed has been partially destroyed mainly by dissolution but also by deflation. Saline pond brines (Cl-SO4-Na [-Mg]) are strongly acidic reaching pH values lower than 2. The high temperature processes that caused the hydrothermal alteration in the Gorbea and Ignorado basins occurred in the Miocene (14 Ma) coinciding with a wet period that reached 9 Ma. Subsequently, the weather up to 120 ka was predominantly hyper-arid with a less arid interval between 6 and 3 Ma and the epithermal sulfates were recycled in saline lakes mainly in the Late Pleistocene wet period (120 to 11.7 ka). Evolution into the current salt flat occurred in the mid-early Holocene (11.7 to 4 ka), through a salt lake that first evolved into a ‘salina’ environment that gave rise to a selenitic gypsum layer (6.4 ka), and later to the final dryness. The highest values of δ34SVCDT and δ18OVSMOW found in the selenitic gypsum layer (+20‰ and + 28‰, respectively) show that the recycling was locally produced, mainly from isotopically heavier hypogene sulfates. The δ18OVSMOW (and less clearly, δ34SVCDT) values are higher in the basal part of the gypsum crust (about +27‰), which suggests an initial hypogene source that decreased towards the top due to mixing with supergene sulfate inputs. Bacterial activity, although catalyzing the supergene reactions, does not appear to have had a significant influence on the isotopic composition of sulfates. The crystallization water of the gypsum is isotopically lighter in the basal part of the selenitic layer (δ18OVSMOW ≈ +7‰), which indicates that the brines of the saline lake were still poorly evolved. These brines evolved to heavier (approximately +13‰) towards the top of the layer and towards the center of the salt flat, before the final drying. The partial destruction of the selenitic gypsum layer occurred during more recent wet periods over the last 4 ka that have been identified in wetlands and lakes in the Central Andean area. Isotopic data (δ34SVCDT and δ18OVSMOW) clearly suggest that sulfates in the saline compounds and evaporites from the entire Central Andean arid area have mostly a thermal origin with contributions from atmospheric deposition and locally, near the Pacific coast, marine aerosols. In addition, the great difference in altitude (approximately 3000 m) between the Altiplano and the lands located to the west, up to the Pacific coast, generated a constant flow of groundwater containing saline compounds that gave rise, to the salt flats of the basins located in lower topographic areas (Atacama, Punta Negra, Hilaricos, Soledad, Tamarugal, Salar de Pintados, Salar Grande) throughout the Cenozoic. Such salt flats have lower δ34SVCDT and δ18OVSMOW values mostly for two reasons: the secular mixing with atmospheric sulfate, and the isotope fractionation related to repetitive dissolution (or leaking)—migration—precipitation along the hydraulic gradient, a process that occurred throughout a large part of the Cenozoic. The last process also explains that the evaporites of some of these salt flats (e.g., Salar de Atacama, Salar Grande) display very high Cl/SO4 ratios.

Late Pleistocene–Early Holocene human settlement and environmental dynamics in the southern Atacama Desert highlands (24.0°S–24.5°S, Northern Chile)

AbstractThe earliest known population of the Atacama Desert coincided with the Central Andean Pluvial Event II (CAPE II), an extensive pluvial event recorded during the Late Pleistocene–Early Holocene (13,800–8,500 cal years BP). A large number of archaeological sites from this period have been found along the borders of the Imilac and Punta Negra (24.5°S) high‐altitude basins (ca. 3,000 masl). By combining the results of proxy data obtained from geomorphological, sedimentological, and paleoecological (rodent middens) archives and the spatial location of archaeological sites and radiocarbon series analyses, this paper examines the links between human activities and paleoenvironmental dynamics at different times during CAPE II in the southern Atacama Desert. This is done at a finer spatial and temporal scale than previous studies. The first archaeological sites (ca. 12,000–11,200 cal years BP) are concomitant with increased paleowetland occurrence frequency. An occupational hiatus is recorded between 11,200 and 10,800 cal years BP, synchronous with a drop in precipitation and wetland development. This is followed by a displacement of the archaeological sites toward the south‐east of the Punta Negra Basin and concomitant again with a second peak of wetland development in the area (10,800–10,400 cal years BP) before the final abandonment of the salt flat margins.

Complex Basement‐Involved Contractional Structures in the Pre‐Andean Basins of Northern Chile: A Review From Seismic Data

AbstractFor many years, the geometry, kinematic, and the age of the basement‐involved structures of the Pre‐Andean basins of northern Chile have been debated. Even, many tectonic models supported by surface geological data have ignored how is the continuity of these structures in the subsurface, and also their possible relation with ancient preorogenic structures, thus difficulting the understanding of the main tectonic mechanisms that acted during the Andean uplift. To solve this problem, we discussed in this study the geometry and timing of the basement‐involved contractional structures present in the Pre‐Andean basins of northern Chile. We present field and seismic evidences of different basement‐involved structural styles, including reverse faults, inverted normal faults, basement thrust ramps, and rotated and reworked basin margins, and use it them to produce three large structural cross‐sections showing the geometries and distribution of structures along the inner forearc region. The structures are interpreted to have resulted from basin inversion, which was followed by large reverse faulting accumulating 43 km in the Salar de Atacama, 10 km in the Salar de Punta Negra, and 27 km in the Salar de Pedernales. Major reverse faults are predominantly located along the western and eastern edges of the basins, whereas inverted normal faults, basement thrust ramps, and other structures are confined to the central sections. In this context, large basement thrust ramps and reverse faults are the most effective structures for generating crustal thickening. Previous analyses (U‐Pb dating) of synorogenic deposits over inverted structures and apatite fission track data from Paleozoic basement prerift rocks suggest that contraction in the region began in the Late Cretaceous‐Paleocene and continued throughout the Cenozoic; however, basement rocks experienced rapid cooling due to tectonic uplift during the Eocene. Further, the results indicate that basement‐involved contractional structures are not only generated by flat‐slab subduction processes, but also observed in other regions of northern Chile (e.g., Frontal Cordillera). Finally, we conclude that the observed structural complexity predominantly results from the initial distribution of preorogenic extensional structures.

Provenance analysis of Devonian peripheral foreland basins in SW Gondwana, case of the Gualilán Group, Precordillera Argentina

We carried out a sedimentary provenance analysis of the Devonian Gualilan Group, Central Precordillera of western Argentina, focusing on the siliciclastic record of the Talacasto (Lower Devonian) and the Punta Negra (Lower-Middle Devonian) formations. Provenance is determined based on petrography, heavy minerals, whole-rock geochemistry, Sm–Nd isotopes, and U–Pb detrital zircon dating. Sandstones are composed of quartz, feldspar, and metamorphic and plutonic lithoclasts. The Gualilan Group underwent moderate weathering indicated by CIA values and Th/U ratios below 76 and 4.9, respectively. Th/Sc, Zr/Sc, Cr/V and La/Th ratios, negative Eu-anomalies and REE patterns point to felsic source compositions. The heavy minerals and zircon morphologies indicate that high-grade metamorphic and igneous rocks provided the bulk of detritus. TDM ages between 1.40 and 1.46 Ga and eNd(t) values ranging from − 9.21 to − 11.13 constrain the signature of the sources. U–Pb detrital zircon ages for the Talacasto Formation are equally distributed between the Famatinian (Late Cambrian–Late Devonian), Pampean–Brazilian (Neoproterozoic–Early Cambrian) and Grenvillian-Sunsas (Mesoproterozoic) orogenic cycles. Detrital zircon ages of the Punta Negra Formation were dominantly supplied from Mesoproterozoic rocks, linked to the Grenvillian-Sunsas orogeny. The data indicate a provenance from rocks located eastwards of the basin represented by the basement of the Pampean Ranges. The peripheral foreland basin developed during the post-collisional regime that followed the accretion of Cuyania against Gondwana. Whereas eastern and western Pampean Ranges provided detritus to the Talacasto Formation, the Punta Negra Formation was sourced by the western Pampean Ranges, implying exhumation and erosion of such basement during the Devonian.

Hydropower Resource Development of the San Juan River in Argentina

This article discusses the design, organization of construction, installation and commissioning processes of the Los Caracoles and Punta Negra Hydro Power Plants (HPPs), as well as a plan for the development of the existing cascade of HPPs on the San Juan River in Argentina through the construction of the Tambolar HPP.

Effects of pre‐orogenic tectonic structures on the Cenozoic evolution of Andean deformed belts: Evidence from the Salar de Punta Negra Basin in the Central Andes of Northern Chile

AbstractWe integrated new field observations, two‐dimensional (2‐D) seismic profiles and new and previously reported chronological data to understand the effects of pre‐orogenic structures on the tectonic evolution of the Salar de Punta Negra in the Central Andes. For first time a series of restored geological cross‐sections are presented, thus showing the pre‐orogenic tectonic architecture of the region and new ideas about the tectonic evolution of the inner forearc of the Central Andes. Our results show a series of east‐dipping normal faults as the main pre‐orogenic structures in the region, which resulted from lithospheric stretching of the western continental margin during the Paleozoic to Mesozoic (Triassic–Jurassic). These were later incorporated into the Andean orogen by tectonic inversion, forming west‐verging inversion anticlines. The beginning of the tectonic inversion is constrained by the first on‐lap of the Upper Cretaceous‐Palaeocene syn‐kinematic deposits on the top of the Mesozoic syn‐rift successions, highlighting that inversion occurred during this period. These syn‐kinematic deposits display zircons with older age peaks between ca. 200 and 300 Ma, thus indicating that some Carboniferous to Triassic sources of sediments were eroded during the uplift of the orogen. Other basement reverse faults affect the footwalls of normal inverted faults and the shoulders of ancient half‐graben structures. These truncate and decapitate previous inverted faults and completely cut the infill of the basin, leading to exhumation of the pre‐rift basement rocks. We propose that the propagation of these structures was favoured by the modified thermal‐tectonic state of the lithosphere from the eastward migration of the volcanic arc, and not by the previous pre‐orogenic structures. The structural and stratigraphic relationships recognized both in the field and 2‐D seismic profiles indicate that many reverse faults originated after the initial tectonic inversion and continued to be active from the Eocene until the Pleistocene period.

Geometry and development of a hybrid thrust belt in an inner forearc setting: Insights from the Potrerillos Belt in the Central Andes, northern Chile

Thick- and thin-skinned “hybrid” thrust belts are typically developed in areas where crustal shortening was preceded by extension and normal faulting. They have been recognized in many places within the Central Andean backarc regions of Perú, Argentina, and Bolivia. In contrast, these structures have not been broadly studied in forearc regions. To understand the geometry and kinematics of this style of tectonism in a forearc setting related to Andean-type subduction zones, we used field and industrial 2D seismic data to study the Potrerillos thrust and fold belt and the Salar de Pedernales Basin in northern Chile (26°S). Our results indicate that the structure in this region consists of a dominantly east-verging contracting system composed of large basement thrusts, Mesozoic inverted normal faults, and shallow thrust-related folds. Large basement thrust ramps and imbricated wedges represent the most prominent basement structures, which accommodated major crustal shortening. Reactivated and tectonically inverted Mesozoic extensional structures also constitute an important structural component, especially beneath the Salar de Pedernales Basin. These exhibit asymmetrical inverted anticlines with arrowhead shapes on the hanging walls of the inverted normal faults. The subsidiary shallow thrust-related folds commonly display fault bend and propagation folds that primarily accommodated the shortening of the Mesozoic and Cenozoic stratified deposits overlying the basement thrusts and cored anticlines. The age of the contraction in this region is poorly constrained; however, many of the oldest sequences accumulated in the top of the syn-rift sequences and related to synorogenic deposits, can be correlated with those identified in neighboring regions (e.g., Salar de Atacama, Salar de Punta Negra basins and Frontal Cordillera, among others), which have reported Upper Cretaceous ages. Based on this stratigraphic correlation we suggest that orogenesis may have begun in the Late Cretaceous with basin inversion, and then continued during the Cenozoic with basement thrusting.

Testing the occurrence of thick-skinned triangle zones in the Central Andes forearc: Example from the Salar de Punta Negra Basin in northern Chile

The Salar de Punta Negra Basin in the forearc of the Central Andes serves as a natural laboratory to understand the evolution of thick-skinned triangle zones dominated by opposite-verging, contractional systems. We combined field data and W-E 2D seismic profiles along parts of the basin where stratigraphic units are well constrained. The seismic data show the occurrence of thick-skinned triangle zones under the basin with along-strike variation. These are composed of west and east-verging, basement-involved faults and thrust ramps, or partially inverted normal faults. The plan-view structural configuration of the basin usually coincides with the sub-surface position of the triangle zones. The interplay between purely reverse and inverted faults in the region indicates that the thick-skinned triangle zones could be kinematically related to the superposition of two contractional episodes, consisting of: i) partial tectonic inversion of half-graben structures; and ii) basement deformation dominated by reverse faults and thrust and anticline ramps. The post-extensional tectono-sequences interpreted on the seismic profiles generally correlate with the Upper Cretaceous to Paleocene synorogenic successions exposed on surface, which allow us propose an age of deformation correlated with the “Peruvian” and “K-T” Andean Orogeny. The triangle zone could still be an active structure.

Tectonic interaction between Mesozoic to Cenozoic extensional and contractional structures in the Preandean Depression (23°–25°S): Geologic implications for the Central Andes

A multidisciplinary study supported by field data and 2-D seismic information was conducted to understand the tectonic interactions between Mesozoic to Cenozoic extensional and contractional structures along the Preandean Depression (23°–25°S) in northern Chile, which have been largely debated. This work gives some new ideas about the control that ancient basement extensional faults exert on the crustal shortening and growth of those orogenic belts located over normal subduction zones similar to the Central Andes. In this work, two sedimentary basins were analyzed: the Salar de Atacama and the Salar de Punta Negra, both of which correspond to intra-montane basins bounded by the Domeyko Cordillera and the current volcanic arc of the Central Andes. The interpretation of a series of west to east-oriented 2-D seismic profiles revealed three types of structural interaction: i) pure positive reactivation; ii) decapitation of previous normal faults by reverse faults; and iii) transport of previous normal faults by thrust ramps. Inversion anticlines and basement-involved contractional structures were the most common observed structures, indicating that the geometry and distribution of inherited normal faults of the Paleozoic and Mesozoic basement played an important role in basin contraction. The different modes of tectonic interaction frequently created variations on the geometry and kinematic of the final structural styles present in the region. Commonly the Upper Cretaceous to Cenozoic synorogenic tectonosequences recorded the ages of contractional deformation that affected the region. Similar to other regions of northern Chile, basin inversion occurred during the Late Cretaceous and subsequent propagation of reverse faults occurred from the upper Late Cretaceous to the present. The basement-involved reverse fault structures usually create the most important positive topographic relief in this part of the western Central Andes. Using as example our study area, we finally conclude that the original position and distribution of ancient basement pre-orogenic normal faults in the orogenic belts condition the final vergence and kinematic of the subsequent thrust systems related to shortening episodes.

Palynomorphs from the Devonian Talacasto and Punta Negra Formations, Argentinean Precordillera: New biostratigraphic approach

Marine and terrestrial palynomorphs from the Loma de los Piojos section of the Precordillera Basin, western Argentina, were analyzed. The studied section embraces the upper part of the Lower Devonian Talacasto Formation and the whole Lower-Middle? Devonian Punta Negra Formation, which constitute the Gualilán Group. Terrestrial palynomorphs dominate the Talacasto Formation whereas marine palynomorphs dominate the Punta Negra Formation. Even though palynomorphs are scarce, poorly preserved, and marked by low diversity, they enable, together with previous palynological data, a new biostratigraphic interpretation for the Devonian units of the Gualilán Group. In addition, an age reassessment of the guide horizon of Keidel (1921), and a correlation of sections from different parts of the basin are provided. Based on palynological data, the age of the Talacasto Formation is constrained to the Lochkovian-late Pragian and the age of the Punta Negra Formation is now considered to be restricted to the late Pragian-Emsian, although a younger age for the uppermost part of Punta Negra is possible.

Early Devonian organic-walled phytoplankton and miospores from the Precordillera Basin of San Juan, Argentina: biostratigraphic and paleobiogeographic implications

ABSTRACTThe palynological assemblages of the Talacasto and Punta Negra formations, from the Argentinean Precordillera, were analysed. Marine phytoplankton is dominant in both stratigraphic units. ?Cymatiosphaera florida, Evittia sommeri, Palacanthus ledanoisii, Riculasphaera fissa, Schizocystia pilosa, Tysanoprobolus polykion and Winwaloeusia distracta are among the most age-significant phytoplankton species of the Talacasto Formation, while Amicosporites sp. cf. A. streelii, Dictyotriletes emsiensis Morphon, Dictyotriletes favosus and ?Knoxisporites riondae prove the most biostratigraphically significant miospore species. A Lochkovian to probably late Pragian age is interpreted based on the marine and terrestrial palynomorphs. The lower part of the Punta Negra Formation contains phytoplankton species such as Cordobesia oriental, Cordobesia uruguayensis, Duvernaysphaera stellata, Navifusa bacilla, Polyedryxium fragosulum and Pterospermella sp. cf. P. pernambucensis as well as the miospores Acinosporites sp. cf. A. lindlarensis and Apiculatasporites microconus, which suggests an early Emsian age. Taxa such as Cordobesia, Schizocystia and Winwaloeusia support affinities of the Argentinean assemblages with Gondwana.