Lithospheric structure and along-strike segmentation of the Central Andean Plateau: seismic Q, magmatism, flexure, topography and tectonics

Abstract

The Central Andean plateau is a 300-km-wide, nearly 4-km-high plateau which is situated above a 30°E-dipping segment of the subducted Nazca plate. Systematic changes in the topography, the upper mantle seismic attenuation ( Q) structure, the lithospheric flexural rigidity and the distribution and chemistry of back-arc lavas indicate that the upper mantle structure changes along strike beneath the plateau. South of 23°S, the upper mantle beneath the plateau becomes hotter, and the lithosphere becomes thinner and weaker. This lateral variation in upper mantle structure coincides with variations in the tectonic style and timing of deformation in two physiographically distinct segments of the plateau and its adjacent foreland thrust belt to the east: the Bolivian Altiplano and Subandean ranges in the north and the Argentine Puna and Santa Barbara system in the south. The segments' different mantle lid thicknesses result in an Altiplano supported mainly by crustal thickening and a Puna supported by a combination of a crustal root and a thermal mantle root. A thicker, stronger foreland lithosphere in the north favors thin-skinned underthrusting of the foreland beneath the plateau, while to the south, the thinner, weaker lithosphere allows more basement-involved shortening within the foreland and less underthrusting beneath the plateau. A weaker lithosphere results in a longer period of compressional deformation and greater local and structural relief in the Puna than in the relatively stable Altiplano to the north. Ultimately, this observed NS-directed segmentation on the plateau and in the foreland may reflect the different responses of strong and weak continental lithosphere to an “Andean” type subduction margin.