UNDER THE ALTIPLANO The xenolith sotry

QUESTIONS  When did it get thick and high?  How much magmatic versus thickening?  Mechanism of thickening  Thermal structure  Depth to basement

THE ALTIPLANO — The America ’ s Roof The Altiplano plateau in Bolivia is the second largest plateau on Earth. It has an average eleva-tion of nearly 4000 m, a crustal thickness of at least 65 km, and covers an area of over 600,000 km 2. The plaetau is part of the central Andean mountain belt associated with the subduction of the Nazca plate beneath the South American plate. Cenozoic thickening of the Altiplano crust has resulted in a decrease of the average density of the continent-al lithosphere in that part of the world, which in turn explains its high topography.

TYPES OF THICKENING  1 Sedimentation  2. Shortening ( “ tectonic ” )  3-5. Magmatism ( “ magmatic ” ) Sedimentation is significant in the Altiplano (up to 15 km!) but most is attributed to crustal shortening. Magmatism also present, difficult to quantify, but probably not the main cause.

CONCLUSIONS  Shortening started early – 45 Ma Incaic  Crust heated since  Metaseds of Triassic age are at 30 km deep today and lost partial melt  Bio-monzodiorite added from deeper (lower crust)  Young (essentially zero age) plutons at 6 kbar depth  Magmatic contributions seem significant  Crust is overall less isotopically evolved than surface shows, mainly due to new magmatic additions