Matías C. Ghiglione

Have the southernmost Andes been curved since Late Cretaceous time? An analog test for the Patagonian Orocline

The kinematic evolution of the enigmatic arc-shaped southernmost Andes of Patagonia and Tierra del Fuego has been a subject of debate for most of the past century. We compared the results from analog sandbox experiments with the tectonic evolution and actual configuration of the mountain chain in order to elucidate whether oroclinal bending took place during the Tertiary, or if the southernmost Andes have been a curved orogen since at least Late Cretaceous time. Experiments simulating oroclinal rotation produced strong along-strike variations in shortening and failed to account for structural data compiled from the Fuegian Andes. Results from experiments simulating an L-shaped, concave-to-foreland indenter were in agreement with the known Tertiary structural evolution of the southernmost Andes. The diachronicity of principal shortening events previously recognized in Patagonia and Tierra del Fuego could only be reproduced by moving the indenter in two successive orthogonal directions: first approximately northward to form the Fuegian fold-and-thrust belt, and then approximately eastward to propagate thrusting in the Patagonian Andes. This two-phase evolution is consistent with a recorded change in the convergence direction of the Farallon-Nazca plate that occurred at ca. 27 Ma.

Continental stretching preceding the opening of the Drake Passage: Evidence from Tierra del Fuego

Age estimates for the onset of a seaway through the Drake Passage range from middle Eocene to early Miocene, complicating interpretations of the relation between ocean circulation and Cenozoic global cooling. Here we present evidence for the presence of a latest Paleocene–early Eocene extensional basin (i.e., lateral rift) in Tierra del Fuego. An accurately dated postrift unconformity indicates that extensional faulting ended in the studied area ca. 49 Ma, in concurrence with a previously reported eightfold increase in South America–Antarctica separation rate, and the proposed onset of oceanic basins in the incipient Drake Passage. The coincidence of these facts indicates progressive strain concentration on the zone of future crustal separation (i.e., Drake Passage) after abandonment of lateral rifts ca. 49 Ma. Although the described extensional depocenters did not allow the exchange of water between the Pacifi c and Atlantic Oceans, they represent a direct indication of continental lithospheric stretching preceding the recently proposed Eocene opening of embryonic basins in the West Scotia Sea.

Tectonic Evolution of the Patagonian Andes

Publisher Summary Throughout the Cenozoic, the southern Andes have had a distinctive feature when compared to the central Andes. The presence of a continuous batholith belt that starts at nearly 39o S and ends in Cape Horn Islands in the southernmost tip of the Andes at about 52o S is evident. This chapter focuses on the importance of the different geological and tectonic processes that have uplifted the Andes at these latitudes and the ways in which the climate has had an important role not only in carving the landscape but also in controlling the uplift mechanisms through time. The Patagonian Cordillera records a complex Pre-Cenozoic history that controls the Andean structures. During the Cenozoic, the interaction of spreading ridges and subduction at the trench controlls the main deformation episodes, the volcanic arc gaps and the eruption of foreland plateau basalts. As a result, most of the Patagonian Cordillera south of the Aysen triple junction had an abnormal heat flux through time, which substantially decreases the viscosity of the underlying mantle. This fact enhanced the structural response to different climatic changes that affected the area since the Late Miocene.

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.

Growth of the Southern Patagonian Andes (46–53°S) and Their Relation to Subduction Processes

The Cretaceous-Cenozoic evolution of the Southern Patagonian Andes is one of the most prominent examples of coupling between subduction processes and climatic, magmatic, deformational, and sedimentary events. Three orogenic and magmatic cycles can be particularly related to processes in the subduction zone (1) Late Cretaceous closure of the Rocas Verdes marginal basin, (2) Paleogene collision of the Farallon-Aluk seismic ocean ridge, and (3) Miocene subduction of the Chile seismic ridge beneath South America. Andean orogenic growth started during Late Cretaceous times, in a tectonic scenario that included the Rocas Verdes back-arc oceanic basin, widening from 49°S toward the south. The Andean segment south of 49°S experienced a strong Cenomanian–Santonian deformational event during the closure of the back-arc basin and progressive subduction of its ocean floor. The final closure produced the Coniacian–Santonian exhumation of the Sarmiento Ophiolitic Complex and propagation of the orogenic front toward the foreland. The second cycle, during Paleogene deformation, coincided with an Eocene volcanic arc gap, and seems to be related to the Fallaron-Phoenix seismic ridge collision. The resulting slab window produced OIB volcanic plateaux represented by the Chile Chico and Posadas Basalts, erupted in the foothills and retroarc. The third cycle of accelerated Andean uplift started during the Oligocene, as a consequence of orthogonal and fast subduction of young lithosphere, while the Chile seismic ridge between Antarctica and Nazca was approaching the trench. Kinematic plate reconstructions show that at approximately 14–18 Ma the Chile oceanic ridge entered the South America trench and migrated northward from 53°S to its present-day position at 46°S. The early Miocene ridge collision and resulting slab window produced an extensive OIB magmatism between 10 and 3 Ma in extra-Andean Patagonia. The space-time unraveling of tectonic uplift is well known from geochronometers, and shows a migration from the basement domain to the external fold and thrust belt accompanied by lower Miocene synorogenic sedimentation. Orogenic growth led to middle Miocene rain shadow in the foothills, followed by the late Miocene—Pliocene desertification of Patagonia.

Wrenching Tectonism in the Southernmost Andes and the Scotia Sea Constrained from Fault Kinematic and Seismotectonic Overviews

The geodynamics of the study area includes subduction, orogen dynamics, and major transcurrent tectonics at a complex transform margin in the southern ocean. We present a synthesis of the fault system associated to major fault zones in the Fuegian Andes and the Scotia Sea and review the available kinematic databases from microtectonic measurements. Our synthesis of six independent fault kinematic studies is coherent with a very consistent shortening direction-oriented NE–SW in the Southernmost Andes. The stability of the stress pattern and orientation of the shortening axes on a more regional scale involving the Scotia Sea indicate a steady E–W to NNE–SSW σ1/shortening direction since middle Eocene times. This observations reflect that the global left-lateral motion between Antarctica–Scotia–South America plates circuit is the main driving force for the entire area and in particular for the Southernmost Andes, over the last ~45 Ma. Both the observed short-term geodetic and the long-term geological slip rates of the Magallanes–Fagnano fault system from Tierra del Fuego are moderate (~5 mm/year), and the expected time span between major M8 earthquakes would be around 10 kyr. Yet the time between the two most recent large earthquakes was about 70 years. Such a great discrepancy suggests a complex mechanics on the Magallanes–Fagnano fault system, leading to complex recurrence time history for the characteristic earthquake (M ≥ 7).

Role of basin width variation in tectonic inversion: insight from analogue modelling and implications for the tectonic inversion of the Abanico Basin, 32°–34°S, Central Andes

Abstract We use analogue modelling to investigate the response of compressional deformation superimposed on an extensional basin with along-strike changes in width. Parameters described include extension and shortening distribution and directions, orientation of structures and degree of basin inversion. Two types of model are presented: in the first (Type I), an extensional basin is constructed with variable width (applying differential extension) and subsequently inverted by homogeneous shortening; in the second (Type II), an extensional basin with constant width is subsequently inverted by inhomogeneous shortening (differential compression). From our observations, we compare both types of model to structural patterns observed in some natural cases from the Central Andes. Both models generate oblique structures, but in the Type II model a significant rotation is characteristic. Our results suggest that in the Central Andes region between 32° and 33°S, the Abanico Basin may correspond to a basin of smaller area compared to the larger basin south of 33°S. Our Type I model further explains some patterns observed there, from which we conclude that the control exercised by the width of a pre-existing basin should be considered when interpreting the geological evolution of that area of the Andes.

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.

An Introduction to the Southern Andes (33–50°S): Book Structure

This book intends to constitute a useful tool to access to data and a general discussion about the mechanisms that have been associated with the development of the Southern Andes. It is mainly conceived for Earth Science professionals working in academia and industry, as well as Ph.D and Ms students and interested readers in general.