The crustal structure beneath the Central Andean forearc and magmatic arc as derived from seismic studies — the PISCO 94 experiment in northern Chile (21°–23°S)

Abstract

In this study, we present an interpretation of seismic refraction profiles from the PISCO 94 experiment in northern Chile. As the PISCO experiment was a combined active and passive seismological study, we also discuss results of the passive part in the context of the seismic refraction model. Previous seismic refraction and gravimetric studies indicate a maximum crustal thickness of about 70 km beneath the Pre- and Western Cordillera. The new seismic refraction data lead to a differentiated image of the Andean crust which shows strong varying characteristics. The crustal discontinuities (up to five are detected) dip from W to E. The upper crust has a thickness of 18 km (Precordillera) to 23 km (magmatic arc) underlain by the recent middle crust down to 35–45 km where the velocity increases to about 7 km/s at its base. This crustal level is interpreted as old continental lower crust and its base as blurred continental (paleo) Moho. Beneath the Precordillera, a strong discontinuity at 70 km depth with a velocity increase to about 8 km/s was detected, interpreted as the recent geophysical Moho. For the magmatic arc, this deep discontinuity could not be found by active seismic measurements. The tomographic models of the seismological studies, in general, confirm the seismic refraction results. Anomalously high v p/ v s ratios in the deeper part of the forearc indicate a hydrated mantle wedge consisting of serpentine and amphibole-bearing peridotite and the 70 km discontinuity is interpreted as the boundary between these two different stages of the hydrated mantle wedge. A zone of high attenuation (Qp) and high v p/ v s ratios beneath the magmatic arc coincides with the low velocity zones and indicates partially molten rocks from a depth of 20 km down to the asthenospheric wedge.