Role of Lithospheric Delamination in the Uplift of the Central Andean Puna-Altiplano Plateau.

KAY, S.M., COIRA, B. AND MC GLASHAN, N.

San Francisco, California

Congreso; American Geophysical Union, Spring Meeting 2009; 2009

AGU

Delamination of dense lower crust and mantle lithosphere has become a common mechanism to explain late Neogene surface uplift of the Central Andean Puna-Altiplano plateau as well as the mafic to shoshonitic lava flows and giant ignimbrite eruptions in the region. Seismic evidence for delamination has been suggested from tomographic images showing large low velocity anomalies in the mantle wedge under the northern Puna (e.g., Schurr et al., 2006, Tectonophysics). A currently operating 72 station passive seismic array will soon provide images of the upper mantle and lower crust under the southern Puna. A scenario to explain the spatial and temporal evolution of magmatic and structural events from the central Altiplano to the southern Puna is for delamination to occur as the subduction zone steepens after the Juan Fernandez Ridge on the subducting Nazca plate has passed southward. Evidence for shallowing of the subduction zone comes from patterns of magmatism and contractional deformation. Decompression melting of the mantle below a hydrated lower lithosphere and basal crust can produce mafic lavas that ascend into the lower crust where melting occurs producing hybrid magmas. Upward migration of these magmas leaves a gravitationally unstable dense garnet-bearing residue that can delaminate, which in turn generates more mantle melting leading to further eruptions until the lower crust becomes infertile. From chemical mass balance, the large Puna ignimbrites are considered to be near 50:50 mixtures of melted crust and new magma addition from the mantle. Seismic evidence for an irregular Moho boundary supports recent delamination. Crustal shortening and a hot mid-crust contribute to crustal flow which is an additional factor in plateau uplift. The relative contribution of the different uplift mechanisms depends on relative steepening and shallowing of the subducting Nazca plate and the pre- existing state of the continental lithosphere.