Role of the deep crustal scale geometry on Western Alps strain partitioning : Insights from S-wave velocity tomography

The recent S-wave velocity tomography undertaken at the scale of the Alps by Nouibat et al. (2021) allows a reappraisal of the deep structure of this mountain belt. These geophysical data highlight the role of crustal geometry in the strain field development observed in the Western Alps. The geophysical imagery shows a standard crustal thickness in the foreland, with slow velocities (<3.6 km.s^-1) in the lower crust. The occurrence of a sharp Moho offset of 5-12 km is detected beneath the External Crystalline Massifs (ECMs). The ECMs do not show any significant crustal thickening in their frontal parts (<35 km), except for the Pelvoux ECM (35-40 km). Beneath the internal zones, east of the Penninic Frontal Thrust, the crustal geometry is more complex with the presence of an European continental slab subducting locally deeper than 80 km beneath the Adria plate. This slab is overlain by a high-pressure metamorphic orogenic prism. The lower part, corresponding to the Ivrea gravimetry anomaly, shows seismic signatures of serpentinized mantle (Vs between 3.8 and 4.3 km.s^-1) whose upper limit is located at 10 km depth below the Dora Maira internal crystalline massif. This new crustal-scale image can be compared to the current deformation pattern, which appears highly partitioned at the scale of the Alpine arc. The internal zones show a transtensional deformation regime, whose activity is distributed along two major seismic lineaments (the ‘Piemontais’ and ‘Briançonnais’ ones). The Alpine European foreland shows a transpressional deformation that is more diffuse and associated with vertical displacements in the ECMs. Beneath the Po plain, the seismic activity is deeper (>40 km), and correlates with a transpressional deformation which is localized along sub-vertical lineaments. The deformation of the orogenic prism appears controlled by a deeper and rigid mantle indenter split in two units by a major subvertical serpentinized structure. The upper unit, which indents horizontally and vertically the crustal orogenic prism, is located between 20 and 45 km depth. The lower unit corresponds to the western boundary of the Adria mantle that pinches directly the European slab. The surface observations and geochronological data suggest that the Moho offstets are superposed on European crustal-scale faults trend inherited from the Variscan orogeny, following the East-Variscan strike-slip system. This structural anisotropy was reactivated during the Alpine orogeny as shear zones in a mainly transpressional regime since about 25-30 Ma, as documented by Ar-Ar data on syn-kinematic mica and U-Pb on monazite. The comparison of current seismicity with the kinematics of exhumed shear zones suggests a continuity of this regime since 25-30 Ma, in response to the Adria plate anticlockwise rotation.