CONICET | Buscador de Institutos y Recursos Humanos

During the last years increasing evidence and discussion about the role of shallow and flat subduction settings in the construction and destruction of the Andes and North American cordillera have emerged. These models have at a certain extent explained the pattern of magmatic variability through time as a product of changes in the angle of subduction. These subduction geometry variations are thought to have triggered heat flux changes, associated with injection and extrusion of important volumes of asthenospheric material, potentially accompanied by thermal and mechanical removal of the lower lithosphere. Deformational stages would have also been linked to these processes and associated with the upraise of fragile/ductile transitions at the middle crust in the retroarc zone as arc expanded landwards, as well as with lower crust delamination. The Northern Patagonian Andes have been built through these processes. A first important shortening took place in Late Cretaceous at the time of eastward arc expansion potentially linked to three areas of subducted slab shallowings of 200 and 800 km wide respectively. One of these shallowings has evolved to one smaller flat slab in Eocene times, while the others have steepened prematurely in Paleocene times. During Late Oligocene times a broad area of steepening developed with the emplacement of voluminous basaltic and rhyolitic plateaux in central Patagonia and extensional basins in the hinterland zone. Lately three Miocene shallow subduction settings more than 400 km long each, evidenced by arc expansions and associated with Andean uplift occurred partially superimposed to the previous shallow configurations. The Northern Patagonian Andes have been constructed through multiple mechanisms that range from i) primary contractional structures detached from fragile/ductile transitions when the arc expanded towards the foreland and consequently the crustal thermal structure changed to ii) tectonic inversion of peri-Gondwanic Mesozoic and Oligocene extensional structures when no arc migration existed and the crust yielded through previous anisotropies. These two mechanisms have acted during two distinctive orogenic stages, first at the base of Late Cretaceous and lately in Miocene times. Late Oligocene extension separates these two constructional periods and is recorded by half-grabens developed from the arc to the retroarc region. The last constructional stage (late Early to Late Miocene) was maximum where arc expanded, particularly at 34.5º-38ºS and 41º-43.5ºS. As a consequence of this, two foreland basins have developed since 18 Ma that have been subsequently cannibalized exposing their remnants in the hinterland zone. Blind and emergent structures formed a broken foreland at the frontal zone inferred from growth strata geometries. These Neogene shallow subduction zones determined most of the Present morphological characteristics of the Southern Andes, as well as controlled magmatic mechanisms associated with their final steepening. Then these areas were extensionally/transtensionally reactivated in the last 5 My at the time of retraction and steepening of formerly shallow subduction zones, being associated with voluminous mantle derived materials and shallow asthenospheric injection, evidenced by S wave tomographies, resistivity surveys, receiver function analyses, geoid anomalies, anomalous heat flux determined from magnetic data and important volumes of within-plate foreland eruptions and minor synextensional silicic collapse calderas in the hinterland zone. Even though Pliocene to Quaternary times at the retroarc area are dominated by extensional processes associated with within-plate volcanic activity, more locally, seismic refraction data and gravimetric models revealed a shallower sector of the Nazca plate at 37ºS in the order of 8º respect to neighbor sectors. This shallower setting coincides with the collision of the Mocha oceanic plateau, formed by highly serpentinized oceanic rocks sheared at the Mocha fracture zone. This plateau has collided against the Chilean trench between 36º and 39ºS in the last 4 Ma producing mountain building processes that acted initially at the Chilean coast, revealed by field and fission track data and propagated into the retroarc area reactivating previous Late Cretaceous structures and creating a new wave of contractional deformation. Moreover, the arc front has eastwardly migrated in this segment some 40 km indicating that slab shallowing is the cause of the renewed wave of contractional deformation at these latitudes in Late Pliocene to Quaternary times.