Seismic and Magmatic Evidence for Continental Lithospheric Evolution on the Andean Margin beneath the Central Volcanic Zone (CVZ) Arc and Southern Puna Plateau (25°S-28°S) since 8 Ma

KAY, S.M.; LIANG, X.; SANDVOL, E.; HEIT, B.; MULCAHY, P.; CHEN, C.; CALIXTO, F.; YUAN, X.; COIRA, B.L.; BROWN, L.D.

San Francisco, California

Congreso; AGU's Fall Meeting; 2011

AGU

Seismic and Magmatic Evidence for Continental Lithospheric Evolution on the Andean Margin beneath the Central Volcanic Zone (CVZ) Arc and Southern Puna Plateau (25°S-28°S) since 8 Ma: The Central Andean margin has been proposed as a site of crustal destruction by forearc subduction erosion and by delamination or founding of thick continental crust as well as an area of crustal addition and reworking by arc magmatic processes. These processes are supported both by the magmatic record and by new slab and crustal earthquake distributions and tomographic and receiver function images from the 2007-2009 southern PUNA passive seismic experiment. The principal evidence for forearc subduction erosion comes from an ~50 km eastward displacement of the arc front at ~27°S to 28°S at 8 to 3 Ma as high Sm/Yb magmas erupted on the hinge of the shallowing slab to the south. The seismic images in this region show a sharp transition to a low Qp anomaly at 50 to 100 km in the southernmost CVZ arc at 27°S that shallows to < 50 km at 26.5°S in a region of a shallow seismic swarm. The underlying high Qp anomaly above the slab can be interpreted as a subduction channel. Further north, intermediate Qp velocities occur above the slab. In the backarc, the chemistry and distribution of the giant 6 to 2 Ma Cerro Galan ignimbrite eruptions and the < 7 Ma mafic calc-alkaline and intraplate like mafic lavas have been attributed to delamination of thickened mafic crust as a shallow subduction zone steepened after 7 Ma. The seismic tomographic images support this interpretation with Qp images in the Cerro Galan region showing an ~ 100 km long low Qp velocity anomaly to a depth of ~100 km and an ~ 100 km long west dipping high Qp anomaly at a depth of 100 to 150 km just to the east being interpreted as the delaminated crust and lithosphere. A seismic swarm at < 15 km beneath Cerro Galan supports the low Qp anomaly and receiver functions confirm high crustal velocities at depths of >75 km. A low Qp crustal to mantle anomaly to the south in the most concentrated region of young intraplate-like mafic lavas supports decompression mantle melting above an expanded mantle wedge.