Kinematic models of deformation at the front of the Alps: new data from map-view restoration

Abstract : Map-view restoration of the frontal part of the French Alps was done in order to test two hypotheses about the propagation of deformation. We discussed two models of the ‘rooting' of the Jura-Molasse basal detachment either along basement thrusts beneath the External Basement Massifs (EBM-rooting model) or along basal thrusts of the Subalpine Chains above the External Basement Massifs (SAC-rooting model). The different sequences of deformation are described for the two models with their kinematics and geochronological constraints. The geochronological data available on the uplift timing of the External Basement Massif are then discussed. Due to their range of uncertainty, geochronological data on the exhumation of the External Basement Massifs are not decisive to discriminate between the two models. Using either the maximum or the minimum age in the range of their uncertainty allows fitting the timing constraints of both models. Interpretation of map-view restoration reveals a weakness of the EBM-rooting model linked to the fact that a sharp virgation is predicted between the two main cross sections. This does not discard this model as such virgation may be linked to paleostratigraphic or tectonic effects but this must be taken into account when using this model. Structural data are also discussed as field geology on the mechanism of uplifting of the external basement massif or geophysical data as deep seismic lines shot in the frame of the ECORS and NRP20 research programs. Such data favor one or the other model without discarding one of them. We conclude that, with the available data, both models seem possible and that the less popular one (SAC-rooting model) shows the more coherent restoration field, the reason why we favor it at the present time.