INVESTIGADORES
COIRA beatriz lidia luisa
capítulos de libros
Título:
Shallowing and steepening subduction zones, continental lithospheric loss, magmatism and crustal flow under the Central Andean Altiplano-Puna plateau.
Autor/es:
KAY, S.M. AND COIRA, B.
Libro:
Backbone of the Americas: Shallow Subduction, Plateau Uplift and Ridge Collision
Editorial:
The Geological Society of America
Referencias:
Lugar: Boulder, Colorado; Año: 2008; p. 229 - 258
Resumen:
Integrated magmatic, structural and geophysical data provide a basis for modeling the Neogene lithospheric evolution of the high central Andean Puna-Altiplano plateau. Reconstruction of three transects south of the Bolivian orocline in the Altiplano and Puna shows processes in common, including subduction with relatively shallow and changing slab dips, crustal shortening, delamination of thickened lower crust and lithosphere, crustal melting, eruption of giant ignimbrites and deep crustal flow. Temporal similarities in events in the three transects can be correlated with changes in the rate of westward drift of South America and slab rollback. Temporal differences between the three transects can be attributed to variations in Nazca plate geometry in response to southward subduction of the aseismic Juan Fernandez Ridge. Subduction of the north-south arm or the ridge can explain an Oligocene flat-slab under the Altiplano, and subduction of a northeast arm of the ridge can explain a long period of relatively shallow subduction with local steepening and shallowing. Major episodes of ignimbrite eruption and delamination occur over steepening subduction zones as the ridge passes to the south. Late Miocene to Recent delamination of dense lithosphere is corroborated by published seismic images. The southern Altiplano transect (17°-21°S) is notable for high structurally complex western and astern cordilleras flanking the Altiplano basin whose eastern border is marked by late Miocene ignimbrites. The broad Subandean fold-thrust belt is to the east. The Neogene evolution can be modeled by steepening of a shallowly subducting plate leading to mantle and crustal melting that produced widespread volcanism including large ignimbrites. Major uplift of the plateau at 10-6.7 Ma is dominantly a response to crustal thickening related to Subandean shortening and peak lower crustal flow into the Altiplano from the bordering cordilleras as the ignimbrites erupt; and partly a response to delamination along the eastern Altiplano border. A smaller ignimbrite volume than in the northern Puna suggests the Altiplano lithosphere never reached as high a degree of melting as to the south. An Oligocene flat-slab stage can explain extensive Oligocene deformation of the high plateau region. The northern Puna transect at ~ 21-24°S is notable for voluminous ignimbrites (> 8000 km3) and a narrower Subandean fold-thrust belt that gives way southward to a thick-skinned thrust belt. The evolution can be modeled by an early Miocene amagmatic flat-slab whose steepening after 16 Ma leads to mantle melting that culminated in widespread ignimbrite eruptions beginning at 10 Ma, peaking in the backarc at ~ 8.5-6 Ma, restricted to the near arc by 4.5 Ma and ending by 3 Ma. The formation of eclogitic residual crust caused periodic lower crustal and lithospheric mantle delamination. Late Miocene uplift is largely due to crustal thickening in response to crustal shortening, magmatic addition and delamination. Crustal flow played only a minor role. The high degree of mantle and crustal melting can be explained as a response to steepening of the early Miocene flat-slab. The southern Puna transect at ~ 24° to ~28°S is notable for eastward frontal arc migration at 8-3 Ma, intraplateau basins bounded by high ranges, long lived Miocene stratovolcanic-dome complexes, voluminous 6-2 Ma ignimbrites, 7-0 Ma backarc mafic flows and the latest Miocene uplift of the reverse faulted Sierras Pampeanas ranges to the east. The evolution can be modeled by a moderately shallow slab producing widespread volcanism with subsequent steepening by 6 Ma leading to delamination of dense lithosphere culminating in the eruption of the voluminous Cerro Galan ignimbrite at 2 Ma.