C. Dorbath et F. Masson, Composition of the crust and upper-mantle in the Central Andes (19 degrees30 ' S) inferred from P wave velocity and Poisson's ratio, TECTONOPHYS, 327(3-4), 2000, pp. 213-223
The present-day crustal structure of the Andean chain results from deformat
ion processes and magmatic activity since the onset of subduction. One line
of investigation to resolve questions about the mechanism of crustal thick
ening is to get information about the composition of the crust and upper-ma
ntle by imaging the structures in velocity and Poisson's ratio (or V-p/V-s)
. A high density seismic network was operated during six months in 1994 acr
oss the whole Andean chain. The P and S travel times of the best located lo
cal earthquakes, mainly situated in the subduction zone, have been inverted
for V-p and V-p/V-s using Thurber's 3D iterative simultaneous inversion me
thod. The resulting models, giving short-scale information about the crusta
l and upper-mantle composition, provide new limits on the respective involv
ement of crustal shortening and magmatism in the thickening of the Andean c
rust.
Beneath the Western Cordillera, the seismic properties are well explained b
y typical active volcanic are processes, and the thickening of the crust is
explained by magmatic addition. Conversely, beneath the Central Altiplano
Basin and the Eastern Cordillera, our results preclude significant volume o
f magmatic addition from the mantle and reinforce crystal shortening as the
main thickening process. Moreover, cratonic lower crust and subcontinental
lithospheric upper-mantle are observed under the Eastern Cordillera up to
67 degreesW, evidence of the underthrusting of the Brazilian craton, which
drives the crustal shortening. Beneath the Central Altiplano Basin, a thin
lithospheric mantle is observed above a well marked asthenospheric wedge wh
ich extends in depth below the western part of the Eastern Cordillera. Thus
partial melt is observed in the shallow mantle west of the Western Cordill
era up to the Brazilian craton. (C) 2000 Elsevier Science B.V. All rights r
eserved.