Andean crustal and upper mantle structure

Abstract

Phase and group velocities of Rayleigh and Love waves have been used to derive models of the structure of the crust and upper mantle beneath southern Peru, Bolivia, and northern Chile. A three-dimensional model of crustal structure has been obtained that shows crustal thickness varying from about 11 km (including water layer) in the ocean basin to 30 km along the coast to more than 70 km beneath the western cordillera and western part of the altiplano. The crust thins eastward and beneath the eastern cordillera is only 50–55 km thick. The crust beneath the crest of the Andes appears to thin to the north and south of the altiplano region, and the maximum crustal thickness in those parts of the Andes not associated with the altiplano may be on the order of 55–60 km. Mean crustal velocities within this region of the Andes are characteristically low; typical values of mean P- and S-wave velocities are ∼6.2 km/sec and 3.45 km/sec, respectively. No significant low-velocity zone for shear waves has been found in the upper 150 km of the mantle beneath the continental area studied, although subcrustal velocities for the entire region are somewhat low. The lowest uppermost mantle velocities measured are for the region between La Paz, Bolivia, and Huancayo, Peru. Here the upper mantle shear velocity corresponds to the axis of the high electrical conductivity anomaly that has been found in the Andean region. Beneath the oceanic areas, a major shear-wave low-velocity zone is required to satisfy phase- and group-velocity observational data. The top of this zone is at a depth of 50–60 km, and the low velocities extend to a depth probably in excess of 200 km. There is a slight suggestion that the upper boundary of the low-velocity zone may be dipping toward the continent.