Eastward subduction of the Nazca Plate beneath the South American Plate has been ongoing for the past 50 million years, with a rate of about . It has generated the Altiplano-Puna high plateau, which with a length of and a width of is the second largest such plateau in the world. The northern Altiplano has a large central basin and a thick underlying crust, while the Puna has a rugged landscape with a number of intervening basins. The Altiplano-Puna high plateau is undergoing horizontal extension, perhaps due to the weight of the high plateau overcoming the compressional tectonic forces, due to delamination of the crust inducing uplift or in the case of the southern Puna by southward shearing of the plateau. In the Puna, extension appears to have begun between 10 and 5 million years ago. A basin formed at Pasto Ventura during the Miocene and was filled with thick sedimentary deposits; it is the only exposed basin in the Puna.