Jonathan Tobal

A Review of the Geology, Structural Controls, and Tectonic Setting of Copahue Volcano, Southern Volcanic Zone, Andes, Argentina

Copahue Volcano lies in the Southern Volcanic Zone of the Andes Mountains, although its geology and local structural controls differ from nearby active volcanic centers. Most of its geology is substantially older than active volcanoes at these latitudes, as the postglacial component is relatively minor. The basement of Copahue Volcano, represented by the Agrio Caldera products and its basal sections, accumulated in extensional depocenters when the arc narrowed from a broad geometry on both sides of the Andes to its present configuration. Initial stages comprise early Pliocene basaltic-andesitic eruptions associated with extensional (transtensional?) processes that ended with the formation of a series of rhombohedral calderas that emitted important amounts of ignimbrites in latest Pliocene-early Pleistocene time. Copahue Volcano concentrates the Pleistocene activity of one of these calderas, the Agrio Caldera, before the emplacement and development of the Present arc front to the west. Volcano morphology reflects this particular evolution, looking more degraded than Antuco, Callaqui and Lonquimay volcanoes located immediately to the west in the arc front. Most of Copahue’s volume is early Pleistocene in age, showing a thin resurfacing cover in synglacial (>27 ka) and postglacial times. A synglacial stage occurred mainly to the east of Copahue Volcano toward the caldera interior in a series of independent, mostly monogenetic centers. Postglacial eruptions occurred as both central and fissural emissions reactivating the old Pleistocene conduits. Its particular geological record and eastern longitudinal position indicate that Copahue was probably part of the late Pliocene-Pleistocene arc mostly developed in the axial and eastern Andes. Narrowing and westward retraction of the arc front, proposed in previous works for the last 5 Ma at 38°S, could have been the result of the eastward migration of the asthenospheric wedge during slab steepening. Reasons for this long-lived eruptive history at Copahue volcano could be related to the particular geometry of the active Liquine-Ofqui dextral strike-slip fault system that runs through the arc front from south to north when penetrates the retroarc area at the latitude of Copahue volcano. This behavior could be due to the collision of the oceanic Mocha plateau at these latitudes, as recently proposed. This jump and related deflection would have produced local transtensional deformation associated with abundant emissions of syn- and post-glacial products that could have partially resurfaced this long-lived center.

The Relation Between Neogene Denudation of the Southernmost Andes and Sedimentation in the Offshore Argentine and Malvinas Basins During the Opening of the Drake Passage

The Neogene orogenic growth of the Southern Patagonian Andes has been related to the approximation and collision of a series of segments of the Chile seismic ridge, which separates the Antarctic and Nazca plates, against South America. The compiled thermochronological data consistently indicates an eastward moving trend of exhumation, were uplift of the western basement domain occurred from ~34 to 15 Ma, and was followed by denudation of the basement front and the fold and thrust belt between ~20 and 5 Ma. There has been an assumption that tectonic growth in southern Patagonia ended in late Miocene times, largely based on the top age of the molasse deposits of the Santa Cruz Formation, spanning from ~22–19 to 14 Ma. There is, however, multiple thermochronological evidence that exhumation in the hinterland continued profusely, with large volumes of rock denudated rapidly between ~15 and 5 Ma, and steadily since ~7 Ma. However, continental sedimentation rate was very low in the Magallanes–Austral Basin of the Southernmost Andes after 14 Ma, an effect produced by the dynamic uplift of Patagonia. Contrastingly, the upper Miocene–lower Pliocene constitutes an aggradational period very well developed in the offshore Argentine continental margin. We propose that the great volumes of sediments produced by Miocene–Pliocene denudation of the Southernmost Andes bypassed Patagonia and reached the Argentine and Malvinas basins, where they were accommodated in thick sequences with high sedimentation rates. Those sediments were distributed along the Southern Atlantic margin by sub-Antarctic currents, which propagated into the Argentine continental margin during the deepening of the Drake Passage. The sediments were probably funneled through gargantuan fluvial and glacifluvial W–E systems, similar to those preserved in Patagonia from the last glaciation, and axially through the Fuegian Andes foothills toward the offshore basins.

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.