Composition of the crust and upper-mantle in the Central Andes (19 degrees30 ' S) inferred from P wave velocity and Poisson's ratio

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.