Late stages of continent-continent collision: Timing, kinematic evolution, and exhumation of the Northern rim (Aar Massif) of the Alps

Herwegh, Marco; Berger, Alfons; Glotzbach, C.; Wangenheim, C.; Mock, Samuel; Wehrens, Philip; Baumberger, Roland; Egli, Daniel; Kissling, E. (2020). Late stages of continent-continent collision: Timing, kinematic evolution, and exhumation of the Northern rim (Aar Massif) of the Alps. Earth-science reviews, 200, p. 102959. Elsevier 10.1016/j.earscirev.2019.102959

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The Alpine front’s External Crystalline Massifs represent exhumation of mid-crustal basement during late-stage continent-continent-collision. We unravel the geodynamic evolution of the Northalpine front (Aar Massif) by combining structural and low-T thermochronometric data. A crustal-scale tectonic restoration (Neogene-present) and estimation of associated changes in exhumation rates become possible. The European continental margin experienced multiple switches from horizontal- to vertical- and again to horizontal-dominated tectonics. Early thrust-related inversion of rift basins is followed by a switch to rapid vertical exhumation, with deformation along steep reverse/normal faults. This vertical tectonics related to a positive buoyancy force of the European crust induced by lower crustal delamination of the Aar Massif during an orogenic rollback process of the European lithosphere (~22-20 Ma). Differential rock uplift rates of up to 1.3-1.6 km/Myr resulted, which decreased northwards to 0.1 km/Myr. Reducing buoyancy forces (isostatic compensation) led to a horizontal tectonic style again. Persisting compressional forces induced an orogen-scale strain partitioning with dextral transpressive faults in the south and simultaneous crustal-scale northwest-directed thrusting in the north. ‘En-bloc’ exhumation of the entire Aar Massif in the thrusts’ hanging wall occurred between 13-5 Ma and 5-0 Ma at rates of 0.5-0.9 km/Myr and 0.6-0.8 km/Myr, respectively. It is this late-stage thrusting in combination with surface erosion, which shaped today’s prominent Northalpine front. Switches between horizontal and vertical tectonics might generally be relevant for late continent-continent-collision stages. Here buoyancy forces of non-thinned continental crust entering the subduction zone cause a strong vertical tectonic deformation style instead of classical thrust tectonics.

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