Density inhomogeneities, tectonics, and stresses in the 21 degrees S Andean subduction zone: 1. A geophysical model and tectonics

Most convergent zones of the Earth are conceived in terms of the underthrus ting of the oceanic lithosphere beneath stable or weakly deformed continent s. However, we believe that the Andean subduction zone is a region in which the lithospheric material moves downward on both the oceanic and continent al sides. The movement is strongly asymmetric: the rate estimates are about 7-8 cm/yr for the Nazca oceanic plate and about 1-2 cm/yr for the northwes tward drift of South America. Although the continental lithosphere moves sl ower than the oceanic lithosphere by an order of magnitude, the pulldown of the South American craton lithosphere is a factor that plays a key role in interpreting the origin of the thickened Andean crust. The Andean mountain system develops under conditions of lateral compression and is controlled by large-scale deformations and accumulation of the light crustal material above the sinking root consisting of the denser material of the lower conti nental crust and upper mantle of the South American craton and Nazca subduc ting plate.