The Peruvian segment of the Andean Cordillera represents the paradigm of the Andean type of subduction, whereby the oceanic Nazca plate subducts the ensialic South American plate. This plate has developed along its western margin a considerable crustal thickening of as much as 70 km, leading to an attendant cordilleran uplift of nearly 4,000 meters above sea level (m.a.s.l.).
The Andean Cordillera is the result of three major geodynamic cycles: Precambrian, Paleozoic to Early Triassic, and Late Triassic to present. The last cycle commenced with the opening of the South Atlantic in the Triassic and includes a first phase of Late Triassic to Early Senonian, Mariana-type subduction, which was basically extensional and of crustal attenuation. During this phase, the cordilleran belt was the site of major shelf sedimentation, bordered on the west by island arc volcanism or a marginal volcanic rift.
In the Early Senonian, a profound geodynamic change led to the Andean-type of subduction, marine withdrawal, and emergence of the Cordillera. This phase was characterized by the recurrence of compressive pulses and the presence along the continental margin of a magmatic arc with intense plutonic and volcanic activity. During this phase, a sequence of compressive episodes: Peruvian (84-79 Ma), Incaic I (59-55 Ma), Incaic II (43-42 Ma), Incaic III (30-27 Ma), Incaic IV (22 Ma), Quechua I (17 Ma), Quechua II (8-7 Ma), Quechua III (5-4 Ma), and Quechua IV (early Pleistocene) formed three major, successive, and eastward-shifting fold and thrust belts: Peruvian (Campanian), Incaic (Paleocene-Eocene) and sub-Andean (Neogene). In general, the compressive pulses affected the entire mobile belt, but were particularly focused on the fold and thrust belts. They resulted in crustal thickening and uplift which was followed by periods of relative quiescence when well-developed erosional surfaces were formed, the most distinctive of which is the Puna surface, generated about 17 m.y. ago. The compressive pulses interrupted longer periods of extension during which the magmatic arc was particularly active, and which were also characterized by the development of fore-arc basins, intermontane grabens, and the great eastern foreland basin. All along this process, however, there were some persistent features, such as the continued presence of the magmatic arc, the Marañón arch, and the eastern foreland basin. The western margins of the Incaic and sub-Andean fold and thrust belts are considered to represent megafaults, deeply rooted into the ductile region, and along which the shortening experienced by the compression of the belt was absorbed.
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Geology and Ore Deposits of the Central Andes
Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages:
1. Normal subduction extended into Eocene time.
2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera.
3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle.
4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab.
5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner