Tectonics and magmatism in the Peruvian Andes from late Oligocene time to the present

An overview of the tectonic and magmatic evolution of the Peruvian Andes since late Oligocene time is presented. From 30 to 26 Ma, the weak deformation and the nearly total quiescence of magmatic activity correlates with a very low convergence rate between the Farallon and the South American plates. From 26 Ma to the Present, the tectonic regime outside the sub-Andean retroarc foreland has been unstable, characterized by long periods of tectonic quiescence separated by five short-lived generalized compressional events dated approximately at 26, 17, 10, 7, and 2 Ma, respectively. During the quiescent tectonic periods, Andean shortening was minor and occurred mainly in the sub-Andean foreland, whereas extensional tectonics prevailed within the High Andes and the fore-arc. Conversely, most of the late Cenozoic Andean shortening, even in the sub-Andean foreland, occurred during the generalized compressional events. The following model can be invoked to interpret this unstable tectonic regime: during the tectonically "quiet" periods, most of the westward drift of the South American plate is accommodated by an absolute westward overriding of the continental plate over a retreating Nazca slab. In this steady-state regime, the compressional events may be considered as instabilities of the dynamic equilibrium between lithospheric motion and Andean deformation. During the compressional events, virtually all the westward drift of South America is accommodated by the tectonic shortening of the Andes, i.e., there is no slab retreat. The calc-alkaline arc remained stable from 26 Ma to the Present in southern Peru and from 26 Ma to 4 Ma in central Peru after which the slab shallowed. Calc-alkaline magmatism is the normal product of a subduction beneath an asthenospheric mantle wedge and is quite independent of the Andean state of stress. In central Peru, back-arc alkaline magmatism appeared when the convergence rate was the highest, i.e., during the Miocene, while in southern Peru, Miocene to Present shoshonitic volcanism was controlled by deep-seated faults. Peraluminous felsic magmatism of the Cordillera Oriental of southeastern Peru was controlled by the underthrusting of the Brazilian shield under the Andes.