HR: 11:35h
AN: S41F-06    [PDF]
TI: The Role of Flat Subduction in Andean Evolution
AU: * James, D E
EM: james@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd., N.W., Washington, DC 20015 United States
AU: Sacks, I S
EM: sacks@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd., N.W., Washington, DC 20015 United States
AB: Transient changes in 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, have strongly influenced the tectonomagmatic development of the Andes. In the central Andes, the processes of normal-to-flat-to-normal subduction developed in several stages starting with normal subduction. The transition from normal to flat subduction coincides with cessation of main arc magmatism and extensive crustal deformation in the eastern altiplano and eastern cordillera. During flat subduction, water from the slab infiltrates and hydrates the overlying continental lithosphere, resulting in advective cooling manifest by low surface heat flow. Lithospheric hydration is concentrated not only in the usual forearc region but also within the inner arc, particularly in the zone of resubduction where amphibole breaks down and the slab dips steeply into the mantle. The transition from flat to normal subduction brings 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 produces widespread melting of both mantle and crust beneath the eastern ranges coupled with ductile deformation, which in the central Andes was associated with oroclinal formation. The magmatic activity and orogenic uplift accompanying the transition from flat to normal subduction began near the zone of resubduction (the inner arc) and broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab. The cycle is complete when westward broadening of volcanic activity culminates in a resumption of calc-alkaline volcanism along the main volcanic arc. The crust beneath the main arc may be thickened and uplifted by the intrusion or underplating of mantle-derived magmas. Eruptive activity in the inner (eastern) arc, much of it anatectic and correlated with periods of crustal deformation, gradually died out, while the thermally thinned and weakened lithosphere of the eastern cordillera and subandean belt formed a ductilized zone in which compressive stresses were concentrated in Neogene time. The tectonic collapse of the inner arc is correlated with the Neogene formation of the Bolivian orocline. By contrast, the western cordillera and the western altiplano, underlain by the Arequipa-Antofalla cratonic block, underwent relatively minor deformation.
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
DE: 8102 Continental contractional orogenic belts
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 9360 South America
SC: Seismology [S]
MN: 2003 Fall Meeting