Mapping the Moho of the Western Alps by wide-angle reflection seismics

Abstract

A new picture of the Moho beneath the Western Alps could be obtained using wide-angle reflection seismic data. This picture, in addition to its increased sharpness, clearly shows how the Moho deepens down to the root zone of the chain (55 km). A 25–30 km deep reflector with upper mantle characteristics is discovered under the Briançonnais zone where it overlies the autochthonous Moho. This unit does not seem to connect to the so-called “Ivrea body”, situated much shallower and farther into the chain. It supports a hypothetical flaking of the European lithosphere beneath the Western Alps. Farther east, the Moho beneath the Po Plain is clearly mapped too, with a crustal thickness increasing in steps from 25 to 35 km. This P-wave Moho picture is compared with an S-wave picture, and major differences appearing in the middle crust are discussed. Finally, the seismic response of the Moho is investigated, and is shown to be shifted towards low frequencies in the inner zones of the chain. This could partly explain why the ECORS-CROP vertical seismic line, equipped with high-frequency geophones, faltered over the identification of the deep Alpine Moho.