158 ARGENTINA and down to 46 300 S, large isolated patches of volcanic rocks represented the main arc along the Andes These volcanic rocks, termed the Southern Volcanic Zone of the Andes, have two distinct segments The northern one corresponds to andesites, dacites, and minor basalts, which erupted through large stratovolcanoes, such as the Tupungato, Marmolejo, and San Jose´ The southern segment is represented by basalts, basandesites, and scarce dacites and rhyolites, which erupted through several Late Cenozoic volcanoes The difference in composition is closely related to the thickness of the continental crust along the Andean chain; this varies from over 60 km north of 33 S down to 42 km at 46 S South of 46 300 S there is a volcanic gap in arc volcanism; this is indicated by isolated adakite outcrops such as Cerro Pampa and Puesto Nuevo, which are related to exceptional oceanic slab melting of young oceanic crust prior to the subduction of an oceanic seismic ridge Further south, from 48 S to 52 S, there are five small isolated volcanoes formed of basalts and basandesites; these (including the Lautaro, Aguilera, and Monte Cook volcanoes) constitute the Austral Volcanic Zone of the Andes All of these rocks have an adakitic signature that indicates small partial melting of the oceanic slab superimposed onto the asthenospheric wedge magmas Fold-and-Thrust Belts and Their Synorogenic Deposits The eastward migration of volcanic activity was coeval with the deformation, uplift, and development of synorogenic deposits The eastern slope of the Andean chain records a series of discontinuous foreland basins, with thick sequences of continental deposits up to more than 10 000 m thick The fold-and-thrust belts vary in style and kinematics from north to south and were controlled by the segmented nature of the subduction zones and the previous Palaeozoic and Mesozoic geological history Sub-Andean fold-and-thrust Belt The thin-skinned sub-Andean Belt, which moved on Late Ordovician, Silurian, and Devonian shales, has open folds and thrusts The area was covered by marine deposits about 13.5 million years ago, showing that deformation in the sub-Andean Belt is later than Middle Miocene The foreland basin deposits reach over 6000 metres The syngrowth strata of Late Pliocene and Pleistocene age, as well as Global Positioning System (GPS) data and earthquake locations, indicate active tectonics in the thrust front The Cenozoic orogenic shortening rate was of the order of 6.7–6.9 mm year 1, but probably with periods of higher activity during Late Miocene and Plio-Quaternary times Santa Ba´rbara fold-and-thrust Belt The Santa Ba´ rbara Belt is controlled by tectonic inversion of basement faults, and therefore shortening is less important than in the Sub-Andean Belt Fault vergence is towards the west, controlled by the polarity of Mesozoic extension Sag and synrift sequences of Cretaceous and Palaeogene age have marine deposits of Maastrichtian–Palaeogene age with a Pacific provenance Most of the synorogenic deposits have been preserved in intermontaneous basins, up to 3-4 km thick Sierras Pampeanas Belt The Sierras Pampeanas Belt is a region of uplifted basement in which crystalline basement of Precambrian–Early Palaeozoic age is widely exposed The reverse faults that bound the basement blocks have a dominant west vergence and are controlled by previous crustal sutures inherited from the Early Palaeozoic tectonics This sector coincides with the subhorizontal subduction segment The synorogenic deposits record two different stages The older stage was an open-to-the-east large foreland basin associated with uplift and shortening in the main Andes The second stage, which is characterized by a broken foreland, has the largest subsidence Consequently, basins with more than 10 000 m of Late Miocene–Pliocene deposits have accumulated in a continental environment Cordillera Principal Belt The Cordillera Principal Belt comprises (1) a northern sector (30 –32 S latitude) characterized by the tectonic inversion of basement blocks where Late Palaeozoic–Triassic volcanic and plutonic rocks are exposed, (2) a central sector (32 –33 300 S latitude) where thin-skinned deformations of Jurassic and Early Cretaceous marine deposits are widely exposed, and (3) a southern sector (33 300 –37 S latitude) where inversion tectonics occurred again All of these belts were deformed in Late Cenozoic times, resulting in accumulation in the foothills of up to 3000-m-thick synorogenic deposits of continental sequences The southern sector is linked to the Neuque´ n Embayment, a wide retroarc basin that developed during Early Mesozoic times and which covered most of the adjacent extraAndean platform Patagonia fold-and-thrust Belt The Patagonia Belt also has two different segments The northern segment (37 –46 S latitude) has a mild basement uplift with only local Cenozoic depocentres, because the main phase of uplift was produced in Cretaceous times South of 46 300 S latitude, thick foreland basin sequences constitute the Austral or Magallanes retroarc Basin The uplift and subsequent deposition