Lucas Fennell

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.

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.