Extensional tectonics, Cretaceous Andes, northern Chile (27°S)

Abstract

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