Striking differences in lithospheric structure between the north- and south-western Alps: insights from receiver functions along the Cifalps profiles and a new Vs model

The CIFALPS receiver-function (RF) profile in the southwestern Alps provided the first seismological evidence of continental subduction in the Alps, with the detection of waves converted on the European Moho at 75-80 km depth beneath the western edge of the Po basin (Zhao et al., 2015). To complement the CIFALPS profile and enhance our knowledge of the lithospheric structure of the Western Alps, we installed CIFALPS2, a temporary network of 55 broadband seismic stations that operated for ~14 months (2018-2019) across the North-Western Alps (Zhao et al., 2018). The CIFALPS2 line runs from the Eastern Massif Central to the Ligurian coast, across the Mont-Blanc and Gran Paradiso massifs and the Ligurian Alps. Seismic stations were installed along a quasi-linear profile with a spacing of 7-10 km.

We will show 2 receiver-function CCP (common-conversion point) depth-migrated sections along the CIFALPS2 profile, the first one across the Alps, and the second one across the Ligurian Alps and the Po basin. The time-to-depth migration of RF data is based on the new 3-D Vs model of the Greater Alpine region derived by Nouibat et al. (2021) using transdimensional ambient noise tomography on a large dataset including the AlpArray seismic network. Depth sections across the Vs model are also useful for interpreting the RF CCP sections as they have striking similarities.

The images of the lithospheric structure of the NW Alps along CIFALPS2 are surprisingly different from those of the SW Alps along CIFALPS. The deepest P-to-S converted phases on the European Moho are detected at 60-65 km depth beneath the Ivrea-Verbano zone, that is 15 km less than on CIFALPS. The negative polarity converted phase interpreted as the base of the Ivrea body mantle flake on the CIFALPS section is still visible on CIFALPS2, but with a lower amplitude. The RF section confirms the existence of a jump of the European Moho of ~10 km amplitude in less than 10 km distance, which is located within a few km from the western boundary of the Mont Blanc external crystalline massif. All these observations are confirmed by the Vs model that also displays a less deep continental subduction than on CIFALPS, weaker S-wave velocities in the Ivrea body wedge, and the jump of the European Moho.

The Moho beneath the Ligurian Alps is detected at 25-30 km depth both on the RF and on the Vs depth sections. Moving northwards, this Ligurian Moho is separated from the Adriatic Moho by a puzzling S-dipping set of P-to-S converted waves with negative polarity. The crust of the Ligurian Alps is characterized by a set of north-dipping negative-polarity converted waves at 10 to 20 km depth beneath the Valosio massif, which is a small internal crystalline massif of (U)HP metamorphic rocks located north of Voltri. The similarity of this set of negative-polarity conversions to the one observed beneath the Dora Maira massif on the CIFALPS profile suggests that it may be a relic of the Alpine structure overprinted by the opening of the Ligurian sea.