Lucía Sagripanti

Tectono-stratigraphic evolution of the Andean Orogen between 31 and 37°S (Chile and Western Argentina)

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.

The Neocomian of Chachahuén (Mendoza, Argentina): evidence of a broken foreland associated with the Payenia flat-slab

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.

Growth of the Southern Andes

GOCE satellite data and EGM2008 model are used to calculate the gravity anomaly and the vertical gravity gradient, both corrected by the topographic effect, in order to delineate main tectonic features related to density variations. In particular, using the Bouguer anomaly from GOCE, we calculated the crust–mantle discontinuity obtaining elastic thicknesses in the frame of the isostatic lithospheric flexure model applying the convolution method approach. Results show substantial variations in the density, compositional and thermal structure, and isostatic and flexural behavior of the continental lithosphere along the Southern Andes and adjacent foreland region.

Active Deformation, Uplift and Subsidence in Southern South America Throughout the Quaternary: A General Review About Their Development and Mechanisms

A broad range of processes act today and have acted simultaneously during the Quaternary, producing relief from the Chilean coast, where the Pacific Ocean floor is sinking underneath the South American margin, to the Brazilian and Argentine Atlantic Ocean platform area. This picture shows to be complex and responds to a variety of processes which are just started to be considered. These processes involve mountains created in a passive margin setting along vast sections of the Brazilian Atlantic Ocean coast and regions located inland, to “current” orogenic processes along the Andean zone. On one hand, mountains in the passive margin seem to be created in the area where the forearc region eastwardly shifts at a similar rate than the westward advancing continent and, therefore, it can be considered as relatively stationary and dynamically sustained by a perpendicular-to-the-margin asthenospheric flow. On the other hand, the orogenic processes associated with the eastern Andes show to be highly active at two particular areas: the Subandean region, where the trench is stationary and the Pampean flat subduction zone to the south, where a shallower geometry of the Nazca plate creates particular conditions for deformation and rapid propagation of the orogenic front generating a high-amplitude orogen. In the Southern Central and Patagonian Andes, mountain (orogenic) building processes are attenuated, and other mechanisms of regional uplift become dominant, such as the (i) crustal weakening and deformation linked to the impact of mantle plumes originated in the 660 km mantle transition, (ii) the retirement of ice masses from the Andes after the Pleistocene producing an isostatic rebound, (iii) the dynamic topography associated with the opening of asthenospheric windows during the subduction of the Chile ridge and slab tearing processes, (iv) the subduction of oceanic plateaux linked to transform zones and (v) the accretion of oceanic materials beneath the forearc region. Additionally and after recent geodetic studies, (vi) forearc coastal uplift due to co-seismic and post-seismic lithospheric stretching associated with large earthquakes along the subduction zone, also shows to be a factor associated with regional uplift that needs to be further considered as an additional mechanism from the Chilean coast to presumably the arc zone.