Ernesto O. Cristallini

Cenozoic tectonics of the High Andes of west-central Argentina (30–36°S latitude)

Abstract The structure of the Central Andes shows three distinctive segments characterized by different geometries. These geometries are superimposed on the present large-scale plate tectonic setting characterized by distinct subduction segments. The northern La Ramada segment is a thick-skinned fold and thrust belt formed by tectonic inversion of a Late Triassic rift. The central Aconcagua segment consists of a thin-skinned fold and thrust belt while the southern Malargue segment like the first one is a thick-skinned fold and thrust belt developed by tectonic inversion of a Late Triassic-Early Jurassic rift system during late Cenozoic times. The amount of shortening gradually decreases from north to south, as indicated by the crustal roots of the Central Andes. The different geometries along the Principal Cordillera controlled the abrupt changes in the shortening among segments. The structure of Precordillera and Sierras Pampeanas has also been considered in order to account for the total shortening. In the La Ramada segment the main shortening occurred in the Precordillera; in the Aconcagua segment in the Principal Cordillera while in the Malargue segment the shortening is widely distributed in a broader Principal Cordillera, because south of the flat-slab subduction segment the Precordillera and Sierras Pampeanas are missing.

Deep structure of the Metan-Guachipas region: tectonic inversion in Northwestern Argentina

Abstract New deep seismic-reflection data obtained by reprocessing an industrial line from the eastern sector of the Salta rift system, Northwestern Argentina, are examined. An oil well located along the line provides stratigraphic control for the seismic interpretation. The corresponding synthetic seismogram extends this interpretation through the Metan trough. The data show the deep structure of the rift, consisting of a series of half-grabens, filled with Cretaceous red beds of the Pirgua Subgroup. The sag phase deposits consist of limestones, marls, shales and evaporites of the Balbuena and Santa Barbara Subgroups, deposited in a transitional marine to lacustrine environment. Several oblique east-dipping reflectors at 10 to 20–24 km depth, are interpreted as master shears that control the extensional mechanics of the rift system, partially reactivated during Andean compression. The structure of this eastern sector of the rift is integrated through a regional cross-section through the entire rift system. The section clearly shows that troughs and highs may be related to flats and ramps of a regional master shear dipping to the east.

Crustal wedging triggering recent deformation in the Andean thrust front between 31°S and 33°S: Sierras Pampeanas-Precordillera interaction

[1] We document a new model of crustal structure of the Andean front in Argentina where numerous historic earthquakes destroyed the cities of Mendoza in 1861 (Ms = � 7) and San Juan in 1944 (Mw = 7.0). The Cerro Salinas anticline is formed above the west directed Cerro Salinas thrust. An east facing monocline with an amplitude of about 2 km folds the Cerro Salinas thrust and overlying Neogene succession. This monocline is formed above a blind crustal thrust in the basement. Its dip of 14 west is inferred from fold geometry. This thick-skinned east directed blind thrust and the thin-skinned west directed Cerro Salinas thrust define a tectonic wedge; the wedge tip occurs at a depth of 5.4 km. Growth of the monocline after � 8.5 Ma is revealed on multichannel seismic (MSC) profile 31017 (Repsol-YPF). Rates of Cerro Salinas thrust displacement are of the order of 1 mm/yr, whereas vertical uplift of � 0.45 mm/yr results from the combined displacement on the Cerro Salinas thrust and growth of east facing monocline. The lateral extent of the east directed crustal blind ramp corresponds with the along-strike extent of the Eastern Precordillera. When combined with the low displacement rate, a long earthquake recurrence interval is implied. Smaller magnitude earthquakes, however, indicate that segments of the blind thrust ramps ruptured in historic events. If all the segments of the blind thrust ruptured together the fault area is � 7000 km 2 and could produce a Mw � 7.7 earthquake. The crustal wedge model provides new constraints on the origin and potential size of earthquakes that threaten the densely populated region.

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.

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.