Extension cretacica a gran escala en el Norte de Chile (Puquios Sierra Fraga, 27°S): significado para la evolucion tectonica de los Andes

Abstract

ABSTRACT. Large scale Cretaceous extensional tectonics in the Andes of northern Chile (Puquios-Sierra Fraga, 27°S). Extensional structures are commonly inferred, but seldom observed or described in the Andean Orogen. In the Andes of Northern Chile, the Sierra Fraga-Puquios region (27°S) exposes a complete record of superposed deformation which affected this part of the orogen during the Mesozoic and Cenozoico A major extensional event, which probably occurred between the Aptian and Cenomanian (?), generated a suite of structures remarkably similar to those observed in the metamorphic core complexes of western North America. Four allocthons, involving Paleozoic and Mesozoic strata are bounded by low-angle normal faults which place younger rocks over older, omitting as much as several kilometers of stratigraphic section. In one area, an extensional “chaos” (Puquios chaos) is composed of domino and boudined blocks of Neocomian limestone floating in a matrix of ductile, cataclastically deformed volcanics. Domino orientation, boudin asymmetry, sheath folds and minor shear sense indicators identify two senses of movement on the detachments. The first and less important was top to the northwest and the second event was top to the northeast. The upper Paleozoic basement core of the extensional terrane differs from the metamorphic core complexes in that it is not highly metamorphosed or ductily deformed. We attribute this difference to the lack of a prior crustal thickening event in the Andes and to the position of Sierra Fraga-Puquios at the margin of the central Chile aborted marginal basin. It is inferred that the extensional events occurred during a global episode of rapid spreading which was accompanied, along the Andean margin, by a substantial increase in magmatism.