Plate kinematics and tectonics leading to the Alpine belt formation; A new analysis

We use the paleogeographic reconstructions of the Tethyan belt made during a recent French-Soviet cooperative program (Dercourt and others, 1985, 1986) to consider the relationship between plate kinematics and plate tectonics. These reconstructions are based on kinematic and paleomagnetic syntheses. They enable us to obtain a quantitative estimate of the total amount of shortening along the Tethyan belt, both in subduction and in collision. We propose that, during Mesozoic time, the basic system was a three-plate system that involved the northward migration of a mid-ocean ridge across the ocean, its subduction beneath Eurasia, and the subsequent formation of a new Rift along the African margin. This might explain consecutive northward migrations of microcontinents across the Tethys. Our quantitative evaluation of continental collision indicates that about half the total amount of continental crust involved must have disappeared into the mantle. We suggest that this is because only the upper brittle part of the crust is involved in the stacking of nappes, whereas the lower ductile crust disappears into the mantle. As a result, the Tethyan continental collision resulted in a 75-m lowering of sea level. A discussion of the paleo-stress trajectories over the West European platform since the Eocene shows that Oligocene time was dominated by east-west extension related to relative motion between western Europe and central Eurasia. This relative motion is confirmed by the difference between the plate kinematics deduced from Arctic magnetic lineations and those deduced from North Atlantic magnetic lineations. The paleo-stress trajectories are remarkably parallel over most of the West European platform at all stages. However, since latest Miocene time, there is an outward fanning of the compressive stress trajectories in a zone 300 to 150 km wide, along the forefront of the Alps. The significance of this zone of fanning stress trajectories is not clear.