Uplift of the Central Andean Plateau and bending of the Bolivian Orocline

Abstract

The topography of the central Andes can be considered the primary tectonic “signal” of late Cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation. The spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data. A color‐coded image of digitized topography between 12°S and 37°S highlights the Altiplano‐Puna, one of the world's most remarkable plateaus, and reveals important physiographic clues about the formation of that major feature. The topographic data combined with information on structure, magmatism, seismicity, and paleomagnetism support a simple kinematical model for the late Cenozoic evolution of the central Andes. The model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning. The model interrelates Andean uplift, a changing geometry of the subducted Nazca plate, and a changing outline (in map view) of the leading edge of the South American plate. Crustal shortening has accommodated convergence between the Chilean‐Peruvian forearc and the South American foreland. The Altiplano‐Puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping (20°–30°) subducted plate. The seawardly concave bend of western South America, the “Bolivian orocline,” was enhanced but not completely produced by an along‐strike variation in the amount of late Cenozoic shortening. Maximum shortening in Bolivia both produced the widest part of the plateau and increased the seaward concavity of the Bolivian orocline. The along‐strike variations of shortening are hypothesized to result from corresponding along‐strike variations in the width of a weakened zone in the overriding plate. Weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates; hence the width of the zone of weakening depends upon the dip of the subducted plate. Two types of shortening are recognized: (1) a widespread, basin‐and‐range, Laramide‐like shortening that characterizes modem activity in the Sierras Pampeanas and late Miocene deformation of the Altiplano‐Puna and (2) on the eastern side of the Cordilleras and plateau, an east verging foreland fold‐thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust. The second type of shortening can be applied to Plio‐Quaternary deformations throughout the central Andes but with a substantial narrowing of the region of plateau uplift in Peru and south of 28°S. A proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central Andes. The monocline is located above the tip of the asthenospheric wedge between the converging plates; it is postulated to occur above the western limit of lower crustal thickening. In the regions of horizontal subduction the monocline can be associated with a late Miocene asthenospheric wedge tip.