3-D Modelling of Alpine Mohos in Southwestern Alps

We present and discuss a 3-D geometrical model of the Moho topography in the Southwestern Alps. To achieve this objective, we used the potential of 3-D modelling software (i.e. a 3-D GeoModeller) to combine gravity, seismic and seismological constraints in a same and coherent 3-D space.A new regional Bouguer gravity anomaly map of the Southwestern Alps was calculated and filtered to isolate the Moho signature in the Southwestern Alps. Then, two alternative 3-D models were computed with data from the literature based on seismic and seismological constraints. The first one represents an European lithospheric mantle decoupled from the European orogenic crust which is back-thrusted by the Ivrea body whereas the second 3-D model illustrates the subduction of a 20-km-thick piece of lower continental crust, coupled with the European upper mantle, beneath the Ivrea body. According to the geological knowledge of these units we then assigned a density value to each modelled layer and we computed their gravity effects to compare them with the filtered Moho map of the Southwestern Alps. First of all, the significant discrepancies of the Moho gravity signature generated by these two geometrical models reveal that subduction of continental crust is no more active in the present-day configuration of the Southwestern Alps. Therefore, the first 3-D model was refined by a stochastic 3-D gravity inversion.Based on this processing, our investigations confirm the presence of three superposed Mohos in the Southwestern Alps and underline that:(i) The crust/mantle European boundary is localized around 50 km depth beneath the Argentera-Mercantour massif,(ii) The European lithospheric mantle is decoupled from the European orogenic crust,(iii) The Adriatic mantle (Ivrea body) is split into two distinct units; the upper unit is located at 10 km depth beneath the Dora-Maira massif, and the lower unit extends from 20 to 45 km depth.(iv) These two mantle indenters affect differently the European crust and are responsible for the localization of crustal deformation and for strain partitioning in the Southwestern Alps.