Kinematics of collisional processes in the Western and Central European Alps: Insights from a synthesis of geological data and new geophysical models

We provide new interpretations of the most recent geophysical models (Vs and Vp tomography mainly) coupled to geological surface information. We show that along-strike significant differences, but also first-order similarities in collision kinematics can be described from the Western to the Central Alps. Moreover, new, precise shortening estimates are obtained, giving some realistic convergence rates since 35 Ma.

In both the Western and Central Alps, after the subduction-collision transition (37-32 Ma), the orogen evolved to a doubly verging wedge with distributed shortening throughout the orogen during a first collision phase (~32-20 Ma) controlling the first mega-sequence of the molasse-type basin. From 20 Ma until recent times, the orogen was structured by localized west- or northwest-verging thrusts in the pro-side below the External Crystalline Massifs controlling the second mega-sequence of the molasse basin. This probably witnesses localization processes in the proximal European crust (i.e., below the Penninic Frontal Thrust) on a 10 Myr timescale. These structures (both distributed and localized ones) root in middle- to lower crustal low velocity (Vs) zones; the low seismic velocity being most probably controlled by fluid circulation, structural anisotropy, and/or metamorphic Alpine paragenesis (amphibolite facies). Balanced cross sections with realistic inherited Mesozoic structures allow locating the different paleogeographical domains at depth and then construction of the pre-collisional geometry.

In the Central Alps, the orogen forms a doubly verging wedge during both phases of collision with a strong amphibolite facies metamorphic imprint in the internal zone. There, the north-alpine foreland basin consists of a thick, large basin recording rather continuous sedimentation. At depth, the crustal root reaches a depth of around 50 km. Below the wedge, the subducting slab in the upper mantle is steep with no clear break-off, but possibly showing an area of attenuation.

In the Western Alps, doubly verging kinematics switch to west-verging kinematics between the two collisional phases and the overall collisional shortening is smaller than in the Central Alps; it is characterized by frontal accretion in the pro-side (while it corresponds to underplating/underthrusting in the Central Alps). As a consequence, the west alpine foreland basin is very segmented and composed of thin sub-basins. At depth, the crustal root is longer than in the Central Alps and underthrusted below the orogen down to at least 70 km. The slab in the upper mantle is moderately East-dipping with a probable break-off at around 120 km depth.

While similarities in terms of deformation localization in both parts of the orogen most likely reflect crustal rheology, the differences allow discussing the influence of both the inherited Mesozoic structure and the kinematics of Adria after the subduction phase.