Constantino Mpodozis

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.

Paleogene clockwise tectonic rotations in the forearc of central Andes, Antofagasta region, northern Chile

[1]xa0For the Central Valley of northern Chile (Antofagasta region), a paleomagnetic analysis of data from 108 sites, mainly in Mesozoic and Paleogene volcanic rocks, has yielded stable remanent magnetization directions for 86 sites. From these data, we infer clockwise tectonic rotations of up to 65° within the forearc domain of the central Andes. The apparent relationship between tectonic rotations and structural trends suggests that rotations occurred mainly during the Incaic orogenic event of Eocene–early Oligocene age. A few paleomagnetic results obtained in Neogene rocks do not show evidence of clockwise rotations. Hence the development of the Bolivian orocline during late Neogene time cannot be explained by simple bending of the whole margin. These results demonstrate that tectonic rotations within the forearc and pre-Cordillera are key elements of early Andean deformation, which should be taken into account by kinematic models of mountain building in the central Andes.

A genetic model based on evapoconcentration for sediment-hosted exotic-Cu mineralization in arid environments: the case of the El Tesoro Central copper deposit, Atacama Desert, Chile

Although the formation of exotic-Cu deposits is controlled by multiple factors, the role of the sedimentary environment has not been well defined. We present a case study of the El Tesoro Central exotic-Cu deposit located in the Atacama Desert of northern Chile. This deposit consists of two mineralized bodies hosted within Late Cenozoic gravels deposited in an arid continental environment dominated by alluvial fans with sub-surficial ponded water bodies formed at the foot of these fans or within the interfan areas. Both exotic-Cu orebodies mostly consist of chrysocolla, copper wad, atacamite, paratacamite, quartz, opal, and calcite. The most commonly observed paragenesis comprises chrysocolla, silica minerals, and calcite and records a progressive increase in pH, which is notably influenced by evaporation. The results of stable isotope analyses (δ13C and δ18O) and hydrogeochemical simulations confirm that evapoconcentration is the main controlling factor in the exotic-Cu mineralization at El Tesoro Central. This conclusion complements the traditional genetic model based on the gradual neutralization of highly oversaturated Cu-bearing solutions that progressively cement the gravels and underlying bedrock regardless of the depositional environment. This study concludes that in exotic-Cu deposits formed relatively far from the source, a favorable sedimentary environment and particular hydrologic and climatic conditions are essential to trap, accumulate, evapoconcentrate, neutralize and saturate Cu-bearing solutions to trigger mineralization. Thus, detailed sedimentological studies should be incorporated when devising exploration strategies in order to discover new exotic-Cu resources, particularly if they are expected to have formed relatively far from the metal sources.