César Arriagada

Paleogene building of the Bolivian Orocline: Tectonic restoration of the central Andes in 2-D map view

Using available information on the magnitude and age of tectonic shortening, as well as paleomagnetically determined tectonic rotations, we have run a series of 2-D map view restorations of the central Andes. Neogene shortening in the foreland belt induced only slight orogenic curvature of the central Andes. The constraints on the ages of the large observed fore-arc rotations (average of 37° counterclockwise in southern Peru and 29° clockwise in northern Chile) indicate that the Bolivian Orocline formed during the Eocene-Oligocene as a consequence of differential shortening focused in the Eastern Cordillera. To minimize local block rotations along the fore arc, the restoration that best fits the central Andean rotation pattern requires about 400 km of total (Paleogene plus Neogene) shortening near the Arica bend. This value corresponds to the upper bound of estimates of maximum horizontal shortening for the central Andes. Along-strike variations in horizontal shortening in the back arc induced bending of the continental margin, block rotations with fore-arc along-strike extension, and/or orogen-parallel transport of upper crustal material toward the symmetry axis of the orocline.

Salar de Atacama basin: A record of compressional tectonics in the central Andes since the mid-Cretaceous

The Salar de Atacama basin lies in the inner fore arc of northern Chile. Topographically and structurally, it is a first-order feature of the central Andes. The sedimentary fill of the basin constrains the timing and extent of crustal deformation since the mid-Cretaceous. We have studied good exposures along the western edge of the basin and have correlated them with seismic reflection sections and data from an exploration well. Throughout most of its history, the basin developed in a foreland setting, during periods of thin-skinned and thick-skinned thrusting. Growth strata provide evidence for coeval sedimentation and thrust motions during mid-Cretaceous, Paleogene, and Neogene times. Pre-Neogene deformation was significant in the basin and in surounding areas of the early central Andes. Models that attempt to explain the current thickness of the central Andes should consider Late Cretaceous and Paleogene shortening, as well as the more obvious Neogene and Quaternary shortening.

Clockwise block rotations along the eastern border of the Cordillera de Domeyko, Northern Chile (22°45′–23°30′S)

Abstract We report new paleomagnetic results from a study of 38 sites in Cretaceous to Early Tertiary red beds and volcanic rocks belonging to the Purilactis Group, which outcrop along the western border of the Salar de Atacama Basin. After detailed thermal demagnetization, characteristic directions were determined for 32 units. In most cases, red bed sediments from the lower members of the Purilactis Group have a well-defined normal polarity magnetization probably carried by hematite forming an early diagenetic cement. A large decrease in the dispersion of the paleomagnetic directions upon tilt correction demonstrates that this magnetization is a pre-tectonic magnetization. We interpret the dominant occurrence of the normal polarity direction as evidence for acquisition of the magnetization during the Cretaceous normal polarity superchron. Large deviations of the paleomagnetic declinations from the expected ones for stable South America provides new evidence for clockwise tectonic rotations associated with Tertiary deformation in the Cordillera de Domeyko. These data confirm that clockwise tectonic rotations are one of the most significant structural characteristics of the north Chilean Andes. This study, however, indicates spatial variation in the magnitude of the rotation with rotations >60° in the Cerro Totola area. These rotations have occurred in conjunction with transpressional deformation that affected large tracts of the Cordillera de Domeyko during Eocene deformation. The systematic observation of clockwise rotations contemporaneous with sinistral displacements in the Cordillera de Domeyko can be explained by shear-traction at the base of the brittle crust.

Distribution, timing, and causes of Andean deformation across South America

Abstract The Andean Orogeny in South America has lasted over 100 Ma. It comprises the Peruvian, Incaic and Quechuan phases. The Nazca and South American plates have been converging at varying rates since the Palaeocene. The active tectonics of South America are relatively clear, from seismological and Global Positioning System (GPS) data. Horizontal shortening is responsible for a thick crust and high topography in the Andes, as well as in SE Brazil and Patagonia. We have integrated available data and have compiled four fault maps at the scale of South America, for the mid-Cretaceous, Late Cretaceous, Palaeogene and Neogene periods. Andean compression has been widespread since the Aptian. The continental margins have registered more deformation than the interior. For the Peruvian phase, not enough information is available to establish a tectonic context. During the Incaic phase, strike-slip faulting was common. During the Quechuan phase, crustal thickening has been the dominant mode of deformation. To investigate the mechanics of deformation, we have carried out 10 properly scaled experiments on physical models of the lithosphere, containing various plates. The dominant response to plate motion was subduction of oceanic lithosphere beneath continental South America. However, the model continent also deformed internally, especially at the margins and initial weaknesses.

The curved Magallanes fold and thrust belt: Tectonic insights from a paleomagnetic and anisotropy of magnetic susceptibility study

The Magallanes fold and thrust belt (FTB) presents a large-scale curvature from N-S oriented structures north of 52°S to nearly E-W in Tierra del Fuego Island. We present a paleomagnetic and anisotropy of magnetic susceptibility (AMS) study from 85 sites sampled in Cretaceous to Miocene marine sediments. Magnetic susceptibility is lower than 0.0005 SI for 76 sites and mainly controlled by paramagnetic minerals. AMS results indicate that the sedimentary fabric is preserved in the undeformed areas of Tierra del Fuego and the more external thrust sheets units, where an incipient lineation due to layer parallel shortening is recorded. Prolate AMS ellipsoids, indicating a significant tectonic imprint in the AMS fabric, are observed in the internal units of the belt. AMS results show a good correlation between the orientation of the magnetic lineation and the fold axes. However, in Peninsula Brunswick, the AMS lineations are at ~20° counterclockwise to the strike of the fold axes. Pretectonic stable characteristic remanent magnetizations (ChRM) were determined in seven sites. A counterclockwise rotation (21.2° ± 9.2°) is documented by ChRM data from four sites near the hinge of the belt in Peninsula Brunswick and near Canal Whiteside while there is no evidence of rotation near the nearly E-W oriented Vicuna thrust within Tierra del Fuego. The curved shape of the Cenozoic Magallanes FTB is not related to vertical axis rotation, and thus, the Magallanes FTB can be considered as a primary arc.

Mechanisms and Episodes of Deformation Along the Chilean–Pampean Flat-Slab Subduction Segment of the Central Andes in Northern Chile

The knowledge of the tectonic architecture, timing, and the mechanisms of deformation that affected the western slope of the Chilean–Pampean flat-slab subduction segment of the Central Andes in northern Chile are a key to understand the complete evolution of this Andean segment. In Chile, this segment is composed of two tectonic provinces: The Coastal and the Frontal Cordilleras. Traditionally, this broad intracontinental deformation zone that characterized this segment has been compared with the Rocky Mountains, in terms of structural styles and age of deformation, although the complex interaction between extensional Mesozoic structures and different styles of contractional structures suggest that this segment resulted from multiple episodes of deformation. We present the results of a study developed along the Coastal and Frontal Cordillera of northern Chile (27°–28° S), based on regional fieldwork, structural analysis, and geochronological dating of synorogenic deposits. Our results have revealed that the structure of this region consists of NNE-striking inverted and basement-involved contractional structures. The occurrence of these structural styles suggests that a hybrid tectonic mechanism dominated by tectonic inversion and basement-involving thrusting was responsible of its current configuration. On the other hand, the U–Pb ages determined in the synorogenic deposits exposed on the footwall of the main faults indicate that the Andean deformation could have initiated during the Late Cretaceous in the Coastal Cordillera associated with tectonic inversion and then this migrated to the east as basement-involved thrusting during the Paleocene–Miocene times.

Tectonic Rotations Along the Western Central Andes

Despite the evidence for protracted deformation, crustal shortening, and exhumation since at least 100 Ma along the Central Andes, the uplift of the Eastern Cordillera and Bolivian Orocline formation did not start until the Eocene–Oligocene. Moreover, the Central Andes also exhibit much younger recent surface uplift (e.g., 10 Ma) that would postdate significant shortening. Recent investigations were focused on the formation of the Bolivian Orocline by 2D Map-View Restoration of Non-plane Deformation experiments. Results from these 2D restorations support the hypothesis of the Paleogene formation of the Bolivian Orocline, due to differential shortening, concentrated in the Eastern Cordillera of Bolivia, Southern Peru, and northwestern Argentina. The “out of plane of cross-section” motion of material and the rotational components of deformation appear to be essential aspects for the formation of the Central Andes, although they are generally not included in models for orogenic systems evolution. One of the most remarkable results of the 2D restoration is the persistence of an “excess rotation” of 10–20° in northern Chile and 20–30° in Southern Peru which cannot be easily explained even if all the differential Paleogene shortening in the Eastern Cordillera and the Neogene shortening in the Sub-Andean zone were considered. Here, we discuss that a major proportion of rotation needs to be balanced in the forearc region by two major conjugate oblique shear zones (i.e., the Abancay Deflection and the Antofagasta–Calama Lineament). These shear zones are probably related to inherited lithospheric discontinuities and major changes in the magnitude of rotation likely occurring along these zones.

Geometric reconstruction and trishear model of folding: a case study in the western Principal Cordillera, Central Chile (34º15’S – 34º30’S)

The Subandean Basins of South America extending from Trinidad to Tierra del Fuego have been the object of intensive exploratory activities (Fig. 1). The largest amount of hydrocarbons discovered during the last 30 years in these basins was found in complex structural terrains. A total of 59 Billion Barrels of Oil Equivalent (BBOE) have been discovered in areas affected by compressional tectonics. Of these basins, the largest discoveries are in the Furrial Trend of Venezuela (24 BBOE), followed by the Chaco area in Bolivia and Argentina (13 BBOE), the Llanos Foothills of Colombia (4.4 BBOE), and the Madre de Dios Basin of Peru (4.2 BBOE).