Guido Gianni

Late Oligocene–early Miocene submarine volcanism and deep-marine sedimentation in an extensional basin of southern Chile: Implications for the tectonic development of the North Patagonian Andes

The Chilean margin has been used as the model of an ocean-continent convergent system dominated by compression and active mountain building as a consequence of the strong mechanical coupling between the upper and the lower plates. The Andean Cordillera, however, shows evidence of alternating phases of compressional and extensional deformation. Volcano-sedimentary marine strata in the Aysen region of southern Chile contribute to an understanding of the causes of extensional tectonics and crustal thinning that occurred in the Andean orogeny because these deposits constitute the only reliable record of submarine suprasubduction volcanism during the Cenozoic in southern South America. In order to discern the age and tectono-sedimentary setting of these strata, referred to as the Traiguen Formation, we integrated sedimentology, ichnology, petrography, geochemistry, structural geology, foraminiferal micropaleontology, and U-Pb geochronology. Our results indicate that the Traiguen Formation was deposited in a deep-marine extensional basin during the late Oligocene–earliest Miocene. The geochemistry and petrography of the pillow basalts suggest that they formed in a convergent margin on a thinned crust rather than at an oceanic spreading center. We attribute the origin of the Traiguen Basin to a transient period of slab rollback and vigorous asthenospheric wedge circulation that was caused by an increase in trench-normal convergence rate at ca. 26–28 Ma and that resulted in a regional event of extension and widespread volcanism.

Neogene Growth of the Patagonian Andes

After a Late Cretaceous to Paleocene stage of mountain building, the North Patagonian Andes were extensionally reactivated leading to a period of crustal attenuation. The result was the marine Traiguen Basin characterized by submarine volcanism and deep-marine sedimentation over a quasi-oceanic basement floor that spread between 27 and 22 Ma and closed by 20 Ma, age of syndeformational granitoids that cut the basin infill. As a result of basin closure, accretion of the Upper Triassic metamorphic Chonos Archipelago took place against the Chilean margin, overthrusting a stripe of high-density (mafic) rocks on the upper crust, traced by gravity data through the Chonos Archipielago. After this, contractional deformation had a rapid propagation between 19 and 14.8 Ma rebuilding the Patagonian Andes and producing a wide broken foreland zone. This rapid advance of the deformational front, registered in synorogenic sedimentation, was accompanied at the latitudes of the North Patagonian Andes by an expansion of the arc magmatism between 19 and 14 Ma, suggesting a change in the subduction geometry at that time. Then a sudden retraction of the contractional activity took place around 13.5–11.3 Ma, accompanied by a retraction of magmatism and an extensional reactivation of the Andean zone that controlled retroarc volcanism up to 7.3–(4.6?) Ma. This particular evolution is explained by a shallow subduction regime in the northernmost Patagonian Andes, probably facilitated by the presence of the North Patagonian massif lithospheric anchor that would have blocked drag basal forces creating low-pressure conditions for slab shallowing. Contrastingly, to the south, the accretion of the Chonos Archipelago explains rapid propagation of the deformation across the retroarc zone. These processes occurred at the time of rather orthogonal to the margin convergence between Nazca and South American plates after a long period of high oblique convergence. Finally, convergence deceleration in the last 10 My could have led to extensional relaxation of the orogen.

The North Patagonian Orogen: Meso-Cenozoic Evolution from the Andes to the Foreland Area

In the last decades, an important amount of studies have dealt with the Patagonian orogen evolution. However, a holistic approach on the evolution of this sector has not been addressed yet. A review of recent advances in different aspects of the Patagonian orogen and its related broken foreland system reveals a close relation between the evolution of both sectors. This enabled us to integrate them in an evolutionary model connecting tectonic events from the North Patagonian Andes to the broken foreland area throughout the Mesozoic and Cenozoic. During the breakup of Western Gondwana, beginning in Jurassic times, several extensional basins developed in the Patagonian region. In late Early Cretaceous to Paleocene, a switch in the tectonic regime caused the initial uplift of the North Patagonian Andes and the fragmentation of the foreland area. Synchronously, an eastward magmatic arc expansion is documented at the retroarc zone. At this moment, a series of mid-ocean ridges collided one after another against the Patagonian margin. A causative relation between young lithosphere subduction , slab shallowing , orogenesis and eruption of mafic magmatism at the arc and retroarc region has been proposed. In concert to regional compression , synorogenic foreland rifting occurred transversally to the main Andean trend in the San Jorge Gulf Basin , describing an exceptional setting for this type of rifting mechanism. From the Eocene to early Miocene, a westward retraction of the magmatic arc, possibly related to roll-back , was synchronous to the Traiguen Basin, formed over highly attenuated crust that splitted the arc and forearc areas. To the east, extensive intraplate magmatism began in the Patagonian foreland covering partially the broken foreland orogen. During the Neogene, an acceleration of the convergence rate between Nazca and South American plate s caused the renewal of Patagonian Andes uplift and reactivation of the broken foreland system . Patagonian orogenesis along with the Late Cenozoic global cooling event triggered aridization of the foreland zone, having dramatic consequences for the Patagonian fauna and flora.

Cretaceous Orogeny and Marine Transgression in the Southern Central and Northern Patagonian Andes: Aftermath of a Large-Scale Flat-Subduction Event?

This review synthesizes the tectonomagmatic evolution of the southern Central and Northern Patagonian Andes between 35°30′S and 48° S with the aim to spotlight early contractional phases on Andean orogenic building and to analyze their potential driving processes. We examine early tectonic stages of the different fold and thrust belts that compose this Andean segment. Additionally, we study the magmatic arc behavior from a regional perspective as an indicator of potential past subduction configurations during critical tectonic stages of orogenic construction. This revision proposes the existence of a continuous large-scale flat-subduction with a similar size to the present-largest flat-slab setting on earth. This particular process would have initiated diachronically in late Early Cretaceous times and achieved full development in Late Cretaceous to earliest Paleocene, constructing a series of fold-thrust belts on the retroarc zone from 35°30′S to 48° S. Furthermore, dynamic subsidence focused at the edges of the slab flattening before re-steepening beneath the foreland zone may explain sudden paleogeographic changes in Maastrichtian–Danian times previously linked to continental tilting and orogenic loading during a high sea level global stage.

Lower Jurassic to Early Paleogene Intraplate Contraction in Central Patagonia

Breakup and dispersion stages of Gondwana were ruled by crustal extension. In Patagonia, this regime was associated with the opening of extensional basins from the Jurassic onward, a process that was interrupted by the Andean Orogeny. New data generated from the hydrocarbon exploration allowed identifying Jurassic to Eocene contractional deformations, previously not registered in Central Patagonia. We summarize in this chapter evidence of five compressional events intercalated with the extensional regime that affected Central Patagonia from the Early Jurassic to the Paleogene. These events, denominated “C1,” “C2,” “C3,” “C4,” and “C5,” acted diachronicronously producing tectonic inversion of the Jurassic–Cretaceous depocenters. The first three contractional pulses occurred during the Jurassic, while the two remaining were Late Lower Cretaceous and Early Paleogene. The origin of this compressive activity would be linked to different processes that comprehended from thermal weakening of the crust produced by expansion of the Karoo thermal anomaly in Mid- to Late Jurassic times; the southward continental drift since the Early Jurassic; the ridge push generated by the opening of Weddell Sea since Mid-Jurassic times; and two mid-ocean ridge collisions during the Cretaceous.

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.