Late-Stage Alpine Continent-Continent Collision: Implications from Exhumation of the External Crystalline Massifs

Late stage Alpine collision is the result of collision of the European and Adriatic continental plates. For this stage, particularly the External Crystalline Massifs (ECM) and their forelands provide information on deformation style/kinematics, exhumation history and geodynamic driving forces. Using the ECMs as marker for the non-thinned European passive continental margin, the margin’s paleogeography with its curved geometries controls in parts later compressional tectonics. During closure of the Valaisan in Eocene times, a major NW-SE trending sinistral transfer zone must have acted as lateral ramp between the NNW migrating Penninic front and the westerly situated European margin units (Argentera and Maritime Alps, stage 1 of Schmid et al. 2017). Hence, first transpressive movements were documented by transpressional strike-slip faults in the case of Argentera and southern Belledonne s.l. Massifs. The SW-NE trending ECMs became affected during a first stage of horizontal tectonics in Oligocene, when first the margin sediments were scrapped off their substratum (Helvetics, Chaînes Subalpines, Dauphinois) followed by thick-skinned thrusting. Transport directions gradually changed from W to N from the Western towards the Eastern ECMs. This spread in transport direction is the consequence of an Alp-internal vertical uplift (Internal Crystalline Massifs (ICM), Lepontine dome) induced by indentation of Adriatic mantle into European lithosphere (stage 2 of Schmid et al. 2017). With further down-bending of the European crust, a major change to a vertical tectonic deformation style occurred in Mid to Late Miocene. Steep reverse faults in the ECMs, in parts with oblique slip components (Mont Blanc), witness this stage with its enhanced vertical rock uplift component. The latter is most pronounced in the Aar Massif and gradually decreases towards the West (Belledonne s.l.). With continuous ICM exhumation in Late Miocene, deformation style switches again to horizontal tectonics, leading to ‘en bloc’ exhumation above basement thrusts of all massifs (Belledonne to Aar Massifs). Progressive shortening induced thrust propagation into the foreland sediments as well as the Jura mountains. In the case of the Argentera Massif, oroclinal bending probably led to a substantial anticlockwise rotation, which goes in hand with the rotation of the entire SW Alpine arc (stage 3 of Schmid et al. 2017). In a geodynamic context, the ‘Adriatic push model’ could explain aforementioned stages of classical horizontal tectonics. Not so, however, the observed severe components of vertical tectonics in the case of ECMs. Here, lower crustal delamination, with a loss of lithospheric negatively buoyant forcing and consequent strong increase in positive buoyancy explains generation and activity of steep reverse faulting in the ECMs. In this context, the ‘orogeny slab rollback’ model provides a physically more consistent framework to explain the observed deformation sequences of late-stage continent-continent collision.

Schmid, S.M., Kissling, E., Diehl, T., van Hinsbergen, D.J., Molli, G., 2017. Ivrea mantle wedge, arc of the Western Alps, and kinematic evolution of the Alps–Apennines orogenic system. Swiss Journal of Geosciences 110, 581-612.