Opening and Closure of the Neuquén Basin in the Southern Andes

The Neuquén Basin is constituted by a system of NW-SE rift clusters formed during the Late Triassic–Early Jurassic at the western margin of Gondwana. The rifting is the result of an extensional tectonic regime triggered by the Gondwana break-up that attained a NE-trending direction (λ1) and a magnitude of extension of around 10%. The syn-rift climax occurred synchronously with an important volcanism, shortly after the beginning of crustal extension. The tectono-stratigraphy of the syn-rift indicates that after a short lapse of crustal extension related to mainly epiclastic sedimentation, the rifting was accompanied by widespread volcanism, characterized by sub-alkaline series, with orogenic signature (i.e. Precuyano Cycle). Recent isotope data revealed a purely crustal origin and mantle-derived rocks with high proportion of crustal participation for the magmatic source. The initiation and cessation of the magmatism, as well as the mechanical extension were diachronic through the basin, showing younger ages from North to South. This would have driven the diachronic post-rift initiations that tracked the rift evolution. The combined effect of the ascending global sea level and the diachronic onset of the sag phase determined that the lower Jurassic marine sedimentation (i.e. Lower Cuyano Cycle) had been set either in a syn-rift or post-rift scenarios, depending on its position in the basin. The understanding (in time and space) of the tectono-magmatic-eustacy factors allowed defining a conceptual scheme with the following endmembers recorded in the syn-rift megasequence: (i) continental non-volcano-dominated syn-rift, (ii) continental volcano-dominated syn-rift; (iii) marine volcano-dominated syn-rift, and (iv) marine non-volcano-dominated syn-rift depositional scenarios. The principal controls on the syn-rift stratigraphy were the mechanical subsidence, driven by normal faulting, and the volcanism, which depending on the type and magnitude determined stacking patterns of the infill. In extreme situation volcanic processes captured the tectonic structures triggering volcano-tectonic subsidence events and graben-calderas. The relative sea-level change (eustacy + tectonics) and the climate conditions acted as secondary factors, mainly during the final rifting stage. Structural, volcanic and tectono-stratigraphical features suggest that the Neuquén Basin at its initial stages corresponded to a wide rift, triggered by lithospheric extension driven by far-field stresses, controlling magmatism in a context of a “passive” rift.

The Neuquén Basin presents an almost continuous record from the Late Triassic until the Paleocene, making it an excellent case study of the most relevant tectonic stages of southern South America during the Mesozoic. It was initiated in Late Triassic to Early Jurassic times as a continental rift basin in the context of a widespread extensional stage that affected western Gondwana and culminated with the break-up of the supercontinent. The Atuel depocenter is located in the northern sector of the Neuquén Basin. Its synrift and sag units are represented by Upper Triassic to Lower Jurassic siliciclastic marine and continental sedimentary rocks including the oldest marine deposits of the basin, of Late Triassic age. The depocenter infill has been deformed and exhumed during the Andean orogeny, being presently exposed in the northern sector of the Malargüe fold and thrust belt. In this review, we have integrated a large set of stratigraphic, sedimentologic, geochronologic, and structural data in order to unravel the tectono-sedimentary evolution of the Atuel depocenter and to evaluate the main controlling factors of the synrift stage. We analyzed data from the synrift units, such as facies and thickness distribution, sandstone provenance, detrital zircon geochronology data, kinematic data from outcrop-scale normal faults, angular and progressive unconformities, and subsurface information. Reactivation of preexisting NNW-striking anisotropies under a regional NNE extension resulted in an oblique rift setting, which generated a bimodal distribution of NNW- and WNW-striking major normal faults. Reduced strain and stress tensors obtained from the kinematic and dynamic analysis of structural data show a complex heterogeneity that we interpreted as a result of local stress permutations due to the activity of the larger faults and to strain partitioning inside the Atuel depocenter. Sedimentologic and petrographic data revealed a complex evolution with strong lateral variations of the depositional environments during the synrift phase, which lasted from Rhaetian to Pliensbachian times. We identified several stages that were controlled by processes of initiation, propagation, growth, linkage, and deactivation of new and reactivated faults along the depocenter evolution, in combination with sea-level changes related to global eustatic variations. Sandstone provenance data suggest an important basin reorganization by the Toarcian, probably related to the initiation of the sag stage in this depocenter.

La Manga Formation is a vast carbonate system developed in the Neuquén Basin. The age is based in ammonite faunas, ranging from Early Callovian (Bodenbenderi-Proximum Zone) to Middle Oxfordian (Cordatum Standard Zone to Transversarium Standard Zone, and probably to the lower part of the Bifurcatus Standard Zone). A stratigraphical and sedimentological analysis, in the outcrops exposed in the south of Mendoza province, enabled the recognition of five facies associations of a carbonate ramp corresponding to (1) distal outer ramp, (2) proximal outer to distal middle ramp, (3) proximal middle ramp, (4) inner ramp deposits (shoreface, shoal, patch reef, shallow subtidal lagoon and tidal flat) and (5) paleokarstic facies. These facies correspond to homoclinal to distally steepened carbonate ramp. The facies associations are included into three third-order depositional sequences (DS-1, DS-2, DS-3) represented by transgressive and highstand systems tracts with sequence boundaries of regional character. Different controlling factors can be recognised in the deposition of this unit. The abrupt changes of facies, as well as paleokarst and epikarst discontinuity surfaces in the successions provide important evidence in terms of depositional environment and vertical evolution of the carbonate ramp. Facies patterns are variable across the outcrop area and vertically through time because of a combination of ramp morphology, siliciclastic supply, sea level changes and tectonic effects. In the southern sections, siliciclastic influx influenced the deposition of proximal middle ramp facies later overlain by scleractinian patch reefs which grew up throughout progressive stages from aggradational to progradational facies in response to climate controls and nutrient levels influence. In northern outcrops, tectonic controls affected the ramp topography and influenced the development of distal deep marine facies. Shallow subtidal and peritidal cycles indicate a combination of allocyclic and autocyclic processes controlling accommodation space and sediment accumulation.

The early Andean arc in Southern Central Chile and Argentina is a main feature of the youngest Andean margin paleogeography, acting as the western edge of the Neuquén Basin sedimentation and sometimes as a barrier to the connection between the open sea and the back-arc basin. Earlier stages of evolution of the magmatic arc are probably related to three pulses of magmatism. The first is related to S-type granites with slightly subduction features and apparently contemporaneous intermediate to highly differentiated volcanic rocks during Late Triassic (~225–220 Ma), that probably represent the first event of magmatism in the area since the Carboniferous. A second is more diverse, with bimodal features, presence of A-type granites and more developed calc-alkaline affinities that took place during the Triassic–Jurassic boundary (210–197 Ma). This event is developed as a paired belt of intrusive outcrops in the present Coastal Cordillera and volcanic deposits in the proximities of the Chile–Argentina border, suggesting an arc—back-arc configuration which is most likely a submerged or slightly emerged immature arc. Finally, the third and definitive instauration and development of the Andean Arc seems to have taken place since late Lower Jurassic, with the development of larger granitic bodies and thick volcanic sequences well exposed in the Coastal Cordillera between 33° and 35° S and close to the Chile–Argentina border between 38° and 40° S. Geochemical and isotopically available data suggest that this early arc was emplaced and evolved through a thin crust with none to little contribution of it during the magma genesis.

The Tordillo Formation is a continental succession of redbeds deposited in the Neuquén Basin during the Kimmeridgian. Based on sedimentological and structural evidence we document an extensional setting between 34° and 36° S, in the northern part of the basin, which contrasts with a transpressional environment in the Huincul arch region (39° S). The tectonic setting of the Neuquén Basin, during the deposition of the Tordillo Formation, was controlled by the combined stresses generated by the subduction system to the west and the break-up of Pangea to the east. We propose that trench roll-back in the Late Jurassic caused regional extension in the back-arc zone. This stress field was locally modified by the clockwise rotation of South America due to the initial break-up of Pangea that ended with the opening of the South Atlantic Ocean. This rotation produced a transpressive stress field in the southern Neuquén Basin in which the structures generated during the Permian collision of the Patagonia terrane were reactivated as strike-slip and reverse faults leading to variable basin geometries and subsidence patterns recorded by the Tordillo Formation in the north and south.

At the northwestern Mendoza province, the Mesozoic infill of the Neuquén Basin is tectonically repeated in the Aconcagua fold-and-thrust belt. Particularly, the Tordillo Formation (commonly associated with the Kimmeridgian) represents a local low stand period of sea level, with mainly alluvial and fluvial sediments. Toward the western sector of the belt, it interfingers with volcanic and volcaniclastic materials and presents a marked increase in the thickness. This unit was studied in two localities at the Blanco River valley, at the undeformed sector and over the second thrust that produces a second repetition of the Upper Jurassic and Lower Cretaceous sequences in the Aconcagua fold-and-thrust belt. This transect exposes facies variations and a significant increase in thickness to the west. Additionally, provenance analysis and paleocurrent directions indicate that the sediment supply was located to the E-SE, and that the underlying units were exhumed at the time of deposition of the Late Jurassic red beds. A consistent thickness increment of the Upper Jurassic deposits to the west through the Aconcagua fold-and-thrust belt suggests that sedimentation was controlled by NNW-directed structures. This is also supported by facies analyses that demonstrate high topographic breaks affecting a smooth west-dipping fluvial ramp toward the volcanic arc. These features support an extensional setting for the deposition of the Tordillo Formation at the latitudes of the Aconcagua fold-and-thrust belt, as other authors have proposed for the Malargüe fold-and-thrust belt to the south. Plate tectonic reconstructions suggest trench rollback during this time previous to the westward migration of the South American plate, which is consistent with the back-arc extension proposed in the previous works.

The first magnetostratigraphic scales for the Jurassic through Early Cretaceous from the Southern Hemisphere have been constructed over the last decades from marine sections in the Neuquén Basin. Paleomagnetic sites were tied to ammonite zones in order to achieve well-refined ages of studied sections. Diverse field tests for the paleomagnetic stability proved the primary origin of isolated magnetizations. In the case of Upper Jurassic–Lower Cretaceous studies, magnetostratigraphic and biostratigraphic data were combined with cyclostratigraphy. Finally, polarities were tied to Andean ammonite zones and from their correlation with the standard zones, calibrated to the GTS2016 (Geomagnetic Polarity Time Scale 2016). For the Early Jurassic, a composite magnetostratigraphic scale was derived out of five sections spanning the Hettangian–Toarcian. The magnetostratigraphic scale portrays 16 reverse (Jr1–Jr16) and 16 normal (Jn1–Jn16) polarity zones that encompass at least 19 ammonite zones. A major difference between both scales rises in the Hettangian involving the Jr1–Jr3 polarity zones. For the Middle Jurassic, the resultant magnetostratigraphy obtained in the Lajas Formation outlines a dominantly reverse polarity pattern. According to the correlation with the GTS2016, the studied section is assigned to the Lower-uppermost Middle Bathonian (Chrons M41 through M39). For the Late Jurassic–Early Cretaceous, the magnetostratigraphic scale obtained in the Vaca Muerta Formation comprises Subchrons M22r.2r through M15r, spanning the V. andesensis (Lower Tithonian)–S. damesi Zones (Upper Berriasian). The use of diverse chronostratigraphic tools such as biostratigraphy, magnetostratigraphy and cyclostratigraphy, enabled to determine with unprecedented precision the position of the Jurassic–Cretaceous boundary, as well as to assess durations of ammonite zones

Detailed cyclostratigraphic analyses have been made from seven Tithonian–Hauterivian sections of the Vaca Muerta and Agrio Formations, exposed in southern Mendoza area of the Neuquén Basin. Both lithostratigraphic units are characterized by decimeter-scale rhythmic alternations of marlstones and limestones, showing a well-ordered hierarchy of cycles, including elementary cycles, bundles, and superbundles. According to biostratigraphic data, elementary cycles have a periodicity of ~18–21 ky, which correlates with the precessional cycle of the Earth’s axis. Spectral analysis based on time series of elementary cycle thicknesses allowed us to identify frequencies of ~400 ky, and ~90–120 ky, which we interpret as the modulation of the precessional cycle by the Earth’s orbital eccentricity. A third band frequency of ~40 ky was also identified that can be assigned to the obliquity cycle. Cyclostratigraphy enabled the construction of almost continuous floating astronomical time scale for the Tithonian–Hauterivian, for which a minimum duration of 5.67 myr for the Tithonian, 5.27 myr for the Berriasian, >3.45 myr for the Valanginian, and 5.96 myr for the Hauterivian have been assessed. Additionally, the likely transference mechanisms of the orbital signal to the sedimentary record are analyzed, proposing the coexistence of carbonate exportation and dilution as the dominant mechanisms.

The Agrio Formation (late Early Valanginian–earliest Barremian) is an environmentally complex marine and continental succession that involves unconformities and flooding surfaces that respond to tectonic thermal subsidence. Internally, the Pilmatué Member contains five third-order sequences eustatically controlled. The Avilé Member starts over a regional unconformity that is the result of erosion and bypass by tectonic quiescence and only punctuated subsidence permits to explain the abnormal thickness in some areas. The Agua de la Mula Member starts with an isochronous and geologically instantaneous inundation which is better explained by tectonic subsidence rather than global eustatism. It contains four sequences but of fourth order. It also shows a sedimentary input from the east in some areas, largely neglected in the literature. The Chorreado Member, from a sequence stratigraphy point of view, is part of the Mendoza Group as it does not represent a basin expansion after a minor unconformity during the Barremian. Contrarily, the unconformity that marks the base of the Troncoso Member of the Huitrín Formation is an evidence of intense regional basin reorganization. The depocentres in the discussed intervals and units shift to the northwest while the thickness in proximal areas in the two marine members of the Agrio Formation point out to accommodation space created by tectonism. This is the first sequence stratigraphic model for the interval in two decades after the first absolute ages and latest biozone calibration provided for the Agrio Formation.

U–Pb dating of detrital and igneous zircons from the retroarc deposits of the Neuquén Basin has shed light over the Mesozoic evolution of the western border of South America, yet the coeval arc and forearc regions remain mostly indirectly characterized. Furthermore, recent paleogeographic reconstructions consider the arc and forearc regions as a tectonically stable and static region at least until Late Cretaceous. In this chapter, we aim to contribute to the Middle Jurassic-Late Cretaceous paleogeographic reconstructions of the western margin of South America from a western point of view integrating the coeval arc and forearc evolution, between 34° 30′ and 36° S. We focus here in the deposits exposed along the Chilean slope of the Principal Cordillera and use four new detrital zircon age data to determine their ages and main source areas. These ages are compared with 38 published U–Pb detrital zircon ages and integrated into a series of paleogeographic cross sections which illustrate the Mesozoic evolution along the Southern Central Andes encompassing the forearc, arc, and retroarc regions. Our data show that the arc and forearc regions were active at least since the Middle Jurassic. Evidence for this tectonic activity corresponds to the development of forearc basins in the Middle Jurassic and Early Cretaceous times. New ages along the Chilean slope of the Andes allow suggesting an early beginning for the compressive period during the latest Early Cretaceous. The formation of a geographic barrier, as a consequence of the compressive regime, would explain the differences in the sediments provenance between western and eastern deposits during the latest Late Cretaceous. Finally, the almost complete record of Mesozoic ages in the detrital and volcanic deposits of the western slope of the Southern Central Andes constitutes a counter-argument about the null or waning activity proposed for the Middle Jurassic or Late Cretaceous from U–Pb detrital zircon analysis of the eastern Mesozoic deposits. Conversely, our data indicate a continued activity of the arc-related volcanism and magmatism throughout all the Mesozoic time.

The first regional tectonic uplift was registered in the Neuquén Basin at ca. 100 Ma, resulting in the inception of the first foreland basin of the Andes at these latitudes. The infill of this foreland basin is represented by the nonmarine deposits of the Neuquén Group, characterized by the presence of growth strata and fold-thrust belt detrital derivation, typical of wedge-top depozones. The presence of a long-lasting hiatus and angular unconformities in structures west of the wedge-top area indicate that these constituted the main detrital source areas of the Late Cretaceous foreland basin. These structures were uplifted by tectonic inversion of pre-existing normal faults, a mechanism that was also responsible for isolated basement block uplifts detected in the foredeep depozone. The regional unconformity between the synorogenic deposits of the Neuquén Group and the back-bulge deposits of the Bajada del Agrio Group represents the migration of the forebulge, which constituted another important detrital source of the foreland basin during its mature phase. An active fold-thrust belt, disconnected basement block uplifts and the identification of the typical depozones of present foreland basins provide a complete picture of the Late Cretaceous orogenic system responsible for the early inversion of the Neuquén Basin.

This chapter reviews the evolution of the Chos Malal fold and thrust belt based on structural and thermochronological data. This fold and thrust belt can be divided into an inner-western zone, characterized by exposed basement-involved structures that are inserted in the sedimentary cover and generated a wide region with thin-skinned deformation, and an outer-eastern zone, where blind thrusts, involving basement rocks, produce deformation in the cover restricted to the deformation front where the main hydrocarbon deposits of the region are located. Zircon (U–Th)/He and apatite fission-track cooling ages at the Cordillera del Viento, ranging from 72 to 51 Ma, evidence a period of uplift and exhumation in the inner zone during the Late Cretaceous–Paleocene. These data are in agreement with the notable unconformity and hiatus observed between Paleogene volcanics and Paleozoic basement rocks. The contraction in this zone continued during the Miocene giving rise to most of the thick and thin-skinned structures of the Chos Malal fold and thrust belt, as revealed by apatite fission-track ages between ~15 and 10 Ma. Furthermore, folded volcanic sequences with ⁴⁰Ar/³⁹Ar ages of ~15 Ma and intrusive rocks with U–Pb ages of 11.5 Ma evidence this compressive episode. The deformation advanced toward the foreland during the Late Miocene, where samples from the basement-involved Las Yeseras–Pampa Tril anticlines show apatite fission-tracks ages of ~9–7 Ma, which led to the uplift and exhumation of the outer zone of the Chos Malal fold and thrust belt.

The Neuquén Basin of west-central Argentina preserves a complex structural and stratigraphic record of the Andean orogeny marked by intermittent periods of foreland sediment accumulation, bypass, and regional exhumation and erosion. Nonmarine deposits of the Cenomanian–Campanian Neuquén Group are the earliest of foreland basin affinity, with clear input from western sediment sources and relatively rapid subsidence rates. However, available provenance and structural data fail to demonstrate a Late Cretaceous regional fold-thrust belt capable of creating rapid accommodation and voluminous sediment supply, suggesting that the magmatic arc may have played an important role as a flexural load and sediment source. The Upper Cretaceous basin architecture defined by the Neuquén Group seems to mimic that of underlying Jurassic–Lower Cretaceous units, indicating either a continued thermal contribution to basin subsidence and/or flexure concentrated along previously thinned lithosphere. Fine-grained marine and nonmarine facies of the overlying Campanian–Eocene Malargüe Group were deposited under slowing rates of subsidence and unclear tectonic conditions. These deposits are capped by a condensed section or unconformity of late Eocene to earliest Miocene age. This foreland hiatus is contemporaneous with the development of extensional basins in the orogen interior, possibly resulting from slab rollback and decoupling along the subduction plate margin, with sediment bypass and minor erosion in the foreland basin aided by a global sea-level lowstand. At ca. 20 Ma, proximal areas adjacent to the advancing fold-thrust belt started accumulating coarse clastic fluvial and alluvial fan deposits in an unambiguous flexural foreland basin. However, Neogene deposition farther east remained irregular, with only relatively thin accumulations in localized zones. In the distal foreland, erosional exhumation rather than sediment accumulation was the dominant process since late Eocene to Oligocene time.

The structural evolution of the southern Central Andes has been widely studied with a general consensus on its geometry and regional architecture. However, the timing of uplift is poorly constrained, with estimates ranging from Late Cretaceous to Pliocene. New thermochronometric results from the Malargüe fold-thrust belt (MFTB) reveal diachronous Neogene cooling and exhumation associated with contractional deformation in this sector of the Central Andes. The Malargüe fold-thrust belt is an excellent example of inversion structures superimposed on a backarc extensional basin (the Late Triassic–Cretaceous Neuquén Basin). As in many other inverted structural systems, preexisting discontinuities play a substantial role in the distribution of shortening and erosional exhumation. Apatite fission track and (U–Th)/He data together with thermal modeling help to define early Miocene (~20 Ma) rapid cooling in hinterland sectors of the MFTB coupled with thin-skinned shortening and retroarc foreland basin development to the east. By 10–7 Ma, continued shortening reactivated the easternmost frontal structures (e.g., Río Atuel fault) and partially inverted the Neuquén Basin. Subsequent deformation exhumed hinterland structures (e.g., Dedos-Silla block) by 5–2 Ma. These temporal constraints reveal that shortening-induced exhumation operated in a disparate manner, rather than following a systematic progression from hinterland to foreland. Additionally, thermal models of the fission track and (U–Th)/He data tightly constrain a long period (~40 Myr) of very slow cooling during an early–middle Cenozoic phase of neutral stress conditions.

Geochemical variations in arc- and within-plate magmatic associations since Late Cretaceous times are analyzed and correlated with the main tectonic changes that influenced the Neuquén Basin evolution. The collision and southward migration of the Farallon-Aluk mid-ocean ridge along the Chilean trench since 80 Ma have played an important role in controlling the Late Cretaceous to Oligocene magmatic evolution of the arc and retroarc zones. The passage of this spreading center through the Chilean trench induced the development of geochemically distinct magmatic associations since Late Cretaceous to Eocene times associated with the extensional reactivation of the Cretaceous fold and thrust belt. Then, by Late Oligocene times, a major plate tectonic reorganization occurred when the Farallon plate broke apart and the resulting Nazca plate started an orthogonal subduction regime beneath the South American plate with higher convergence rates. Then extensional basins and associated magmatism developed at this time destabilizing the Paleogene fold and thrust belt and establishing a more homogeneous tholeiitic signature along the Andean axis.

Geological observations in the Neuquén Basin indicate a Late Oligocene to Early Miocene episode of extension followed by an abrupt shift towards regional compression. However, the reasons behind this brief extensional episode and the Oligo-Miocene tectonic mode switch are not fully understood. Through the aid of numerical modelling, it has been shown that after a period of limited subduction in Early Palaeogene times, the penetration of Nazca’s slab tip into the mantle transition zone in Late Oligocene times resulted in the renewal of effective subduction due to the effect of the slab pull force. This renewed subduction consists of an initial stage of higher trench hinge retreat and steep slab dips, leading to extension and mantelic upwelling processes east of the trench. Then, the natural evolution of the slab produces a deceleration of roll-back and shallowing of the subduction angle once it reaches the lower mantle, resulting in horizontal shortening. These results indicate that the effect of the slab pull force is a potential responsible for the Oligo-Miocene tectonic mode switch, causing the opening of a series of intra-arc basins and widespread magmatism partially devoid of arc-like components followed by an increasing influence of the slab in the magmatic arc and the final closure of the Neuquén Basin.

The tectonic regime associated with the Oligo-Miocene Cura-Mallín Formation and equivalents in the Main Andean Cordillera between 35° and 40° S, the Ventana and Abanico formations to the south and north, respectively, is still matter of debate. While most authors have agreed in relating them to an extensional regime that could have interrupted Andean orogenesis, others have provided evidence of coexistence with an ongoing compressional regime at least for the upper part of their record. Available geochronological, structural, geochemical, thermochronological, and basin subsidence data between 33° and 43° S are compiled and analyzed in this chapter in order to provide a tectonic framework for these rocks. Based on this analysis, two different mechanisms seem to have succeeded through the accumulation of these sequences, a late Oligocene to earliest Miocene period characterized by syn-extensional deposits in an attenuated crust evidenced by low La/Yb ratios in contemporaneous magmatism, and a younger time period in the mid- to late Miocene that coincides with regional exhumation and compression revealed by thermochronological data and syncontractional sedimentation.

Late Oligocene to Pliocene magmatism at the latitudes of Malargüe and Chos Malal fold and thrust belts, in the Neuquén Basin, is distributed from the main Andean axis to the retroarc zone. While arc magmatism maintained relatively similar compositional and geochemical features during late Oligocene to Pliocene times, major variations are seen in volcanic sequences developed in the retroarc zone, due to the development of a shallow subduction regime by mid-late Miocene times. Thus, late Oligocene-early Miocene period is characterized by an extensional regime that conditioned mainly tholeiitic magmas in the main Andean axis and alkaline intraplate magmas in the retroarc zone. Early-late Miocene marks a change to compressional tectonics in the Andean margin. Main arc magmatism showed a change to clearer arc, calc-alkaline signature, while retroarc magmatism showed the progressing input of slab-derived products as the shallow subduction regime triggered the eastward migration of the asthenospheric wedge. Thus, arc-derived lavas expanded into the mid to far retroarc zone (~69° 30″ to 68° 30″) with arc-like andesitic to dacitic compositions. With the progressive influence of the shallow subduction regime, arc-derived products reached almost 500 km away from the Chilean trench at ∽36° S by latest Miocene-Pliocene. By the middle Pliocene, re-steepening of the slabconditioned extensional tectonics that favored a widespread alkaline volcanism in the present-day Payenia retroarc and rhyolitic calderas in the main arc zone. After ∽3.5 Ma, retroarc magmatism at the latitudes of the Malargüe and Chos Malal fold and thrust belts lack of clear arc-related geochemical features.

The occurrence of a Neogene shallow subduction stage, as well as, a Pliocene slab-tearing, and steepening of the Nazca plate in the southern Central Andes are well established. However, a satisfactory explanation for the origin and connection between these complex processes is still elusive. In this contribution, we revise the late Cenozoic tectonic and magmatic evolution of the southern Central Andes between 35° and 38° S and discuss different proposals for the Miocene slab shallowing and its Pliocene destabilization. Recent plate kinematic reconstructions show that Neogene arc-front expansion linked to slab shallowing, fold belt reactivation in the main cordillera and intraplate contraction in the San Rafael Block correlates with the subduction of the ancient Payenia plume, a deep mantle anomaly potentially rooted in the lower mantle. Also, the Nazca slab tear determined from tomographic analyses and subsequent slab steepening may also be a direct consequence of this plume subduction process. Considering the westward drift of South America and the presence of several neighbor hotspots over the Nazca plate, the Payenia plume overriding could be the first of future episodes of plume–trench interaction in the Andes.

Quaternary deformations described in the retro-arc region at the latitudes of the Neuquén Basin can be divided into two main groups: a northern group characterized by Quaternary deformation zones concentrated near the main topographic breaks of different morphostructural systems, and a second group located in the southern Neuquén Basin distinguished by disconnected, sparced and noncontinuous Quaternary deformational zones. In the northern Neuquén Basin, evidence of active deformation is associated with the Frontal Cordillera (33°–34° S); while in the foreland area, young deformations concentrate in the San Rafael Block. In the southern Neuquén Basin, a western deformational belt constitutes the continuation to the north of an intra-arc fault system associated with dip- and strike-slip displacements in a strain-partitioned regime (Liquiñe–Ofqui fault system). At these latitudes, to the east, isolated evidence of Quaternary deformation, regional uplift and development of non-equilibrated fluvial profiles are recognized in the Tromen and Auca Mahuida volcanic plateaux and sierra de Cara Cura-sierra de Reyes area. These systems are short and unconnected and have been explained through an intricate pattern of asthenospheric anomalies evidenced from magnetotelluric data. These mantle anomalies could be related to the tearing of the subducted Nazca plate at depth evidenced by seismic-tomographic data. We therefore suggest that the thermally weakened crust at the southern Neuquén Basin latitudes could be the main control responsible for focalizing contractional, extensional and transpressional deformations in isolated mountain systems.

The Loncopué Trough is an extensional basin produced by the extensional reactivation of the hinterland area of the Southern Central Andes. Neotectonic extensional structures in this basin bound a broad topographic low filled with volcanic and volcaniclastic rocks. The studies carried out in the area of the Loncopué Trough have concentrated on the study of its neotectonic activity, volcano-sedimentary infill and the surface structure. Less effort has been paid to characterize the magnetic properties of the crust and to unravel the deep geometry of this Pliocene to Quaternary extensional setting. Therefore, magnetic and gravimetric data were used to highlight the boundaries of the magnetic sources and to obtain a crustal-scale 2D density model at 38°S. To complement this work, an effective susceptibility model using the Magnetization Vector Inversion method was estimated, which takes into account the combined effects of remanence and induced magnetization. Additionally, the Curie depth points were calculated through the spectral analysis technique in order to determine the thermal structure of the retroarc area. From this analysis, we were able to characterize the main structures associated with this extensional trough. Based on this analysis, only the Loncopué eastern fault system is considered as having a crustal-scale hierarchy. Additionally, the susceptibility model revealed possible fluid (magmatic and or hydrothermal) reservoirs in the area of the Copahue volcano and the Codihue and Cajón de Almanza depocenters/volcanic fields. These spots coincide with shallower values of the calculated Curie depth point, implying higher heat flows. Finally, the 2D density model shows an area of lower crustal attenuation that is coincident with one of the described potential magmatic/hydrothermal reservoirs and is decoupled from the upper crust extensional structures immediately to the west in the Loncopué Trough. This crustal configuration could be explained by a simple shear deformation model with a crustal-scale master fault dipping to the east.