Abstract The Andes, the world's largest non-collisional orogen, is considered the paradigm for geodynamic processes associated with the subduction of an oceanic plate below a continental plate margin. In the framework of UNESCO-sponsored IGCP 586-Y project, this Special Publication includes state-of-the-art reviews and original articles from a range of Earth Science disciplines that investigate the complex interactions of tectonics and surface processes in the subduction-related orogen of the Andes of central Chile and Argentina ( c. 27–39°S). This introduction provides the geological context of the transition from flat slab to normal subduction angles, where this volume is focused, along with a brief description of the individual contributions ranging from internal geodynamics and tectonics, Quaternary tectonics and related geohazards, to landscape evolution of this particular segment of the Andes.

Abstract In this classic segment, many tectonic processes, like flat-subduction, terrane accretion and steepening of the subduction, among others, provide a robust framework for their understanding. Five orogenic cycles, with variations in location and type of magmatism, tectonic regimes and development of different accretionary prisms, show a complex evolution. Accretion of a continental terrane in the Pampean cycle exhumed lower to middle crust in Early Cambrian. The Ordovician magmatic arc, associated metamorphism and foreland basin formation characterized the Famatinian cycle. In Late Devonian, the collision of Chilenia and associated high-pressure/low-temperature metamorphism contrasts with the late Palaeozoic accretionary prisms. Contractional deformation in Early to Middle Permian was followed by extension and rhyolitic (Choiyoi) magmatism. Triassic to earliest Jurassic rifting was followed by subduction and extension, dominated by Pacific marine ingressions, during Jurassic and Early Cretaceous. The Late Cretaceous was characterized by uplift and exhumation of the Andean Cordillera. An Atlantic ingression occurred in latest Cretaceous. Cenozoic contraction and uplift pulses alternate with Oligocene extension. Late Cenozoic subduction was characterized by the Pampean flat-subduction, the clockwise block tectonic rotations in the normal subduction segments and the magmatism in Payenia. These processes provide evidence that the Andean tectonic model is far from a straightforward geological evolution.

Abstract We propose an integrated kinematic model with mechanical constrains of the Maipo–Tunuyán transect (33°40′S) across the Andes. The model describes the relation between horizontal shortening, uplift, crustal thickening and activity of the magmatic arc, while accounting for the main deep processes that have shaped the Andes since Early Miocene time. We construct a conceptual model of the mechanical interplay between deep and shallow deformational processes, which considers a locked subduction interface cyclically released during megathrust earthquakes. During the coupling phase, long-term deformation is confined to the thermally and mechanically weakened Andean strip, where plastic deformation is achieved by movement along a main décollement located at the base of the upper brittle crust. The model proposes a passive surface uplift in the Coastal Range as the master décollement decreases its slip eastwards, transferring shortening to a broad area above a theoretical point S where the master detachment touches the Moho horizon. When the crustal root achieves its actual thickness of 50 km between 12 and 10 Ma, it resists further thickening and gravity-driven forces and thrusting shifts eastwards into the lowlands achieving a total Miocene–Holocene shortening of 71 km.

Abstract We use analogue modelling to investigate the response of compressional deformation superimposed on an extensional basin with along-strike changes in width. Parameters described include extension and shortening distribution and directions, orientation of structures and degree of basin inversion. Two types of model are presented: in the first (Type I), an extensional basin is constructed with variable width (applying differential extension) and subsequently inverted by homogeneous shortening; in the second (Type II), an extensional basin with constant width is subsequently inverted by inhomogeneous shortening (differential compression). From our observations, we compare both types of model to structural patterns observed in some natural cases from the Central Andes. Both models generate oblique structures, but in the Type II model a significant rotation is characteristic. Our results suggest that in the Central Andes region between 32° and 33°S, the Abanico Basin may correspond to a basin of smaller area compared to the larger basin south of 33°S. Our Type I model further explains some patterns observed there, from which we conclude that the control exercised by the width of a pre-existing basin should be considered when interpreting the geological evolution of that area of the Andes.

Abstract The magmatic history and tectonic evolution of the Valle del Cura region has received the attention of several studies in recent years, particularly as part of a larger area of interest named the Indio Belt. These studies have suggested an Eocene volcanic sequence known as the Valle del Cura Formation. The present study, based on extensive field work, robust geochronological and geochemical datasets, shows an Oligocene to early Miocene age for this unit, similar to the Doña Ana Group. The tectonic setting that controlled the volcanism of the Valle del Cura Formation was extensional and corresponds to a retro-arc position of the main arc volcanism of the Doña Ana Group. The field evidence combined with radiometric and geochemical data demonstrate the synextensional characteristic of the volcanic sequence of the Valle del Cura Formation and the Doña Ana Group at these latitudes. This characteristic was dominant at the central part of the Pampean Flat Slab (29°–30°S).

Abstract New U–Pb detrital zircon ages are presented for the Tordillo Formation. The ages indicate that the most important source region of sediment supply was the Jurassic Andean arc (peaks at c. 144, 153 and 178 Ma), although two secondary sources were defined at c. 218 and 275 Ma. Temporal variation in the provenance indicates that at the beginning of the sedimentation, Carboniferous to Lower Jurassic magmatic rocks and Lower Palaeozoic metamorphic rocks were the most important sources. Towards the top, the data suggest that the Andean arc becomes the main source region. The comparison between provenance patterns of the Tordillo Formation and of the Avilé Member (Agrio Formation) showed some differences. In the former, the arc region played a considerable role as a source region, but this is not identified in the latter. The results permit a statistically robust estimation of the maximum deposition age for the Tordillo Formation at c. 144 Ma. This younger age represents a discrepancy of at least 7 Ma from the absolute age of the Kimmeridgian and Tithonian boundary (from the chronostratigraphic timescale accepted by the International Commission of Stratigraphy, IUGS), and has strong implications for the absolute age of the Jurassic–Cretaceous boundary. Supplementary material: Sample coordinates, values of the sandstone compositional framework and U–Pb (LAM-MC-ICP-MS) age measurements of zircons grains are available at http://www.geolsoc.org.uk/SUP18718

Abstract Volcanic and pyroclastic rocks of intermediate composition, dating from Kimmeridgian time with peak ages during both the early–middle Tithonian and the late Hauterivian, characterize several localities of the north-western margin of the Neuquén basin along the main Andean Range at 33–34°S. The latter is thought responsible for the desiccation of the basin during Aptian–Albian time. The thick Tithonian submarine lavas at the Volcán valley (33°30′S), on the other hand, coincide spatially with the only pass-way recognized at that time in the Chilean Coast Range. This coincidence may be related to an isolated relic rift-like structure that coexisted with the regional intra-arc extensional setting during Tithonian time. Continental-scale rifting in the southern Andes has been recognized only for the Late Triassic–Early Jurassic period.

Abstract The Nazca Plate subducting beneath the South American Plate has strongly influenced Cenozoic mountain growth in western Argentina and Chile sectors (32–34°S; 70–66°W). At these latitudes, the Pampean flat slab has induced the development of prominent mountain systems such as the Frontal Cordillera, the Precordillera, and the associated Sierras Pampeanas in the eastwards foreland region. Through a gravity study from the Frontal Cordillera to the Sierras Pampeanas region between 32 and 34°S, we delimit a series of geological structures that are accommodating shortening in the upper crust and others of regional and subsurface development, without any clearly defined mechanics of deformation. Additionally, through an isostatic residual anomaly map based on the Airy-Heiskanen local compensation model, we obtain a decompensative gravity anomaly map that highlights anomalous gravity sources emplaced in the upper crust, related to known geological structures. In particular, by applying the Tilt method which enhances the gravity anomalies, the NW-trending Tunuyan Lineament is depicted south of 33.4°S following previous proposals. Using the decompensative gravity anomaly, two profiles were modelled through the northern sector of the study area using deep seismic refraction lines, borehole data and geological information as constraints. These density models of the upper crust of this structurally complex area accurately represent basin geometries and basement topography and constitute a framework for future geological analysis.

Abstract The subduction of the Nazca oceanic plate under the South American plate in the south-central Andes region is characterized by the oblique collision of the Juan Fernandez Ridge against the continental margin. The upper plate is characterized by a broken foreland, a thrust-and-fold belt and eastward migration of the volcanic arc promoted by the flattening of the slab. Topographic load, thermal state and plate rheology determine the isostatic state of the continental plate. We calculated the vertical gravity gradient from GOCE satellite data in order to delineate the main tectonic features related to density variations resulting from internal and external loads. Then, using the Bouguer anomaly, we calculated the crust–mantle discontinuity and the elastic thickness in the frame of the isostatic lithospheric flexure model applying the convolution method approach. The results obtained show substantial variations in the structure of the continental lithosphere related to variations in the subduction angle of the Nazca plate. These variations are reflected in the varying Moho depths and in the plate rigidity, presenting a distinct behaviour in the southern zone, where the oceanic plate subducts with an approximate ‘normal’ angle with respect to the northern zone of the study area where the flat slab occurs.

Abstract Isolated marine sedimentary Lower Cretaceous deposits crop out in the foreland of the Neuquén Basin, west-central Argentina. They are the result of an anomalous uplift of the Sierra de Chachahuén in the far foreland region. These outcrops are assigned to the Agrio Formation based on their rich fossil contents. In particular, the study reveals a unique outcrop of continental facies along the eastern proximal margin of the basin that were known only from core wells, and constitutes the first exposed evidence at the surface. These deformed deposits are 70 km from the Andean orogenic front and present 2 km of local uplift produced by high-angle basement reverse faults that reactivated a previous Early Mesozoic rift system. The increase in compression was related to the decrease in the subduction angle. This fact, together with the expansion of the magmatic arc, controlled the Chachahuén calc-alkaline Late Miocene volcanic centre and the uplift of the Mesozoic deposits in the foreland. This broken foreland was associated with localized heating of the Miocene volcanic centre that produced the rising of the brittle-ductile transitions. This fact weakens the foreland area, which was broken by compression during the development of the Payenia flat-slab.

Abstract Piggyback basins are one of the most important sediment storage systems for foredeep basins within foreland basin systems, so understanding the dynamics of sediment accumulation and allocyclic changes is essential. Three alluvial systems are proposed here to depict sediment movement along the piggyback basin: piedmont, axial and transference systems. We propose differentiation between open continental piggyback basins that include a transference system that is able to deliver sediment to the foredeep and closed piggyback basins that are isolated. Two idealized models of sedimentation in piggyback basins are proposed. For open piggyback basins we identify four stages: (a) the incision stage; (b) the confined low accommodation system tract; (c) the high accommodation system tract; and (d) the unconfined low accommodation system tract. Meanwhile two stages are proposed for closed ones: (a) the high accommodation system tract; and (b) the low accommodation system tract. To test these models, Quaternary deposits and a Miocene unit are analysed. The first one is controlled by climatic changes, and the second is related to tectonic activity in the Precordillera.

Abstract The NNW-trending Las Peñas Thrust System is one of the key structures along the Andean orogenic front between 32°15′ and 32°40′S in the Southern Precordillera of Argentina. This east-verging structure crops out over a distance of c. 40 km and provides one of the best opportunities for a detailed field survey of Quaternary thrusting in the Andean frontal deformation zone. We present a systematic description of the geometry and geomorphic signatures of the main thrust deformation zone, which emplaces Neogene rocks over Quaternary alluvium, and usually behaves as a blind propagating thrust into the youngest (Late Pleistocene–Holocene) alluvial deposits. The Las Peñas Thrust System is understood to represent the latest stage of the eastward migration of an imbricated fan structure, which has driven the neotectonic uplift of the Las Peñas–Las Higueras range. Excellent outcrops provided by well-incised creek outlets reveal that the thrust system is made up either by a single fault surface or by two or more frontal splays. Several sections along its length can be differentiated on the basis of thrust geometries and/or morphotectonic features. The northern sections are characterized by isolated outcrops of Neogene rocks in the hanging wall, surrounded by alluvial bajadas. Remnants of fold limbs scarps depict the geomorphic signature of the thrust propagation into the Quaternary layers, although the preserved topographic relief always underestimates the cumulated thrust slip during the Quaternary. The southern part of this thrust system is defined by a frontal range, cored by a transposed south-plunging anticline in bedrock. Our observations suggest a dynamic and unsteady interaction between thrust propagation and sedimentation/erosion processes along the thrust trace during deposition of the Quaternary alluvial layers.

Abstract The distribution of the Quaternary deformation in the outer retro-wedge of the Andes (31°30′–35°S) is controlled by the subduction geometry, the position of the structural front, and the location of oblique pre-Cenozoic mechanical anisotropies. In the Southern Precordillera, Quaternary structures tend to group along the Barreal–Las Peñas deformation zone with a NW to NNW trend. This Cenozoic belt (31°30′–32°40′S) developed on the northern segment of the Triassic Cuyo Basin and broadened laterally during the Quaternary. New radiocarbon ages on dam deposits confirm Holocene tectonic activity on the northwestern edge. The oldest ages of dam deposits are 5810±90 a BP (Cabeceras Creek) and 810±50 a BP (Dolores Creek). Palaeoseismological and seismic data suggest active tectonic growth on the NW and SE extremes. Quaternary tectonics has contributed to modifying the relief along this oblique belt. This contribution is evident from the tectonic uplift of blocks (minimum 90–120 m in the Barreal block), the initial development of intermontane basins (Pampa de los Burros Basin), the Quaternary rejuvenation of tectonic depressions (at least 32–37 m of tectonic subsidence in the Vizcacheras half-graben) and the incipient development of low-relief morphotectonic units by soft-linkage of Quaternary structures.

Abstract The piedmont of Cordillera Frontal between c. 33° and 34°S (Mendoza, Argentina) is a highly populated area deeply modified by human activities, known as Valle de Uco. It is situated within the borderland region of the geological provinces of Cordillera Frontal and Cuyo basin. The landscape is dominantly composed of both erosional and depositional landforms made of fluvio-aeolian deposits fractured and folded by tectonic processes together with some landforms of volcanic origin. Alluvial fans, related to several aggradational cycles of Quaternary age, are the most remarkable geomorphological units. Several tectonic features are present giving rise to conspicuous morphological features. Some of the streams are structurally controlled by faults while several drainage anomalies that indicate active tectonic processes have been identified. The Late Quaternary alluvial sequences, dominantly comprising sandy and silty deposits of volcaniclastic composition and secondarily metamorphic rocks, represent the fine-grained sedimentary facies of the fluvial systems accumulated in a distal fan environment. The alluvial deposits have been incised by several episodes of erosion since Pleistocene time.

Abstract The Andean retrowedge, located between 33°S and 34°S, lies in the transition region of the Pampean flat-slab subduction zone to the north and a normal subduction zone to the south. Neotectonic structures and shallow seismicity are very common north of this segment and become progressively less frequent southwards. The Frontal Cordillera and the Cerrilladas Pedemontanas are the main morphostructures involved in the Quaternary deformation of this region. The Frontal Cordillera is a thick-skinned fold-and-thrust belt uplifted since Late Miocene time. The Cerrilladas Pedemontanas are low-relief hills that represent the mild inversion of the Cuyo Triassic rift depocentre since Pliocene time. Middle Miocene–Holocene synorogenic strata cover the Cuyo basin and surrounding foreland areas. The Quaternary tectonic evolution of this area has been established through integration of new data from fieldwork in the Frontal Cordillera piedmont with subsurface information and previously published data. Mean Late Pleistocene uplift rates ranging between 0.21 and 0.92 mm a −1 and earthquake minimum moment magnitudes ( M w ) of c. 6.4–6.7 have been estimated for the morphostructural units analysed in this manuscript.

Abstract This review deals with an integration and update of the knowledge about large-volume landslides in the Central Andes at 32–34°S. An integrated landslide inventory for megalandslides in central Chilean and Argentinean Andean basins was developed, and dispersed chronological data on palaeolandslides were compiled, showing a dominance of Late Pleistocene and Holocene ages. Traditional hypotheses adopted for explaining landslide occurrence in the Central Andes are contrasted. Whereas seismic tremors have been widely suggested as the main triggering mechanism in Chilean collapses, palaeoclimatic conditions are considered as the main cause of Argentinean giant landslides. These different approaches denote the lack of multidisciplinary studies focused on the controversy about seismic or climate trigger mechanisms in the Central Andes. These studies are also essential to understand failure mechanisms and assessment of the related hazard and risk, which are essential to reduce social and economic impacts on vulnerable communities from future landslide events.

Abstract The morphology, sedimentology and mineralogy of deposits that previously had been associated with glacial advances (the Penitentes, Horcones and Almacenes drifts) were reinvestigated and dated using the terrestrial cosmogenic nuclide (TCN) 36 Cl. These results indicate that the deposits previously associated with the Horcones and Almacenes drifts are actually deposits of a rock slope failure from the southern face of Aconcagua mountain forming a debris–ice avalanche that were deposited 10 490±1120 years ago, while the deposits previously associated with the Penitentes drift is a rock avalanche from the Mario Ardito valley that deposited in the Las Cuevas valley 11 220±2020 years ago. Earlier in the Late Pleistocene a further rock–ice avalanche sourced from Aconcagua mountain and deposited in the Las Cuevas valley, predating related lake sediments with a calibrated 14 C age of 14 798–13 886 years and travertine deposits with a U-series age of 24 200±2000 years. In addition, three further rock-avalanche deposits were dated that sourced from Tolosa mountain, having 36 Cl mean ages of 14 740±1950 years, 12 090±1550 years and 9030±1410 years. No deposits of massive rock slope failures were found in those parts of the valleys that date younger, suggesting that climatic conditions at the transition from the Late Pleistocene to the Holocene, that were different from today's, caused the slopes to fail. Alternatively, the rock slope failures could have been seismically triggered. We suggest that the slope failures at the southern face of Aconcagua mountain have caused or contributed to a reorganization of glacial ice flow from Aconcagua mountain that might ultimately be the cause of the surging behaviour of the Horcones Inferior glacier today. Our results indicate that the glacial stratigraphy of this part of the Central Andes is still poorly understood and requires detailed mapping and dating. Supplementary material: Sample coordinates, sample porosity and density, Cl nuclide composition and geochemical composition are available at http://www.geolsoc.org.uk/SUP18753 .

Abstract Central western Argentina is identified as the most hazardous seismic zone in the country. Historical earthquakes with magnitudes greater than M s >6.0 frequently occur in this territory and are associated with the subduction of the Nazca plate. However, seismic hazards have not been fully assessed in this region. No secondary seismic effects of a potential earthquake with destructive consequences have been considered, nor has the existence of shallow Quaternary blind faults been identified by seismic surveys. Neotectonic studies performed up to the present describe only those Quaternary faults with some surficial expression. Lacking proper hazard assessment limits strategies to reduce the economic impact of drastic seismic events. This chapter is focused on the impact of major destructive earthquakes that have occurred in central western Argentina in order to understand the incidence of these phenomena in the past and to consider the vulnerability of the region in light of its increased urbanization and changing agricultural practices.

Abstract Apatite (U–Th)/He thermochronology from palaeosurface-bounded vertical transects collected in deeply incised river valleys with >2 km of relief, as well as geomorphic analysis, are used to examine the timing of uplift of the Frontal Cordillera and its relation to the evolution of the proximal portions of the Andean foreland between 32° and 34°S latitude. The results of apatite (U–Th)/He (AHe) analyses are complex. However, the data show positive age-elevation trends, with higher elevation samples yielding older AHe ages than samples at lower elevation. Slope breaks occur at c. 25 Ma in both profiles, separating very slow cooling and or residence within a partial retention zone (slope of c. 10 m/Myr) at the highest elevations from a slope of c. 60–100 m/Myr cooling rate at lower elevations. The older AHe ages suggest either (1) minimal burial of the Frontal Cordillera and/or (2) significant pre–middle Miocene local relief. Geomorphic analysis of the adjacent, east-draining Río Mendoza and Río Tunuyán catchments reveals a glacial imprint to the landscape at elevations above 3000 m, including greater channel steepness and lower profile concavities developed during glacial erosion. Detailed analysis of headwall heights provides evidence of ongoing rock uplift along the entire eastern flank of the Frontal Cordillera and in the eastern flank of the Principal Cordillera south of the slab dip transition.