Download PEUXENOS

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 km2. 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