ALPINE COLLISION TECTONICS AND THEIR DRIVING FORCES

This presentation aims to link deep crustal and lithosphere-asthenosphere structure with Tertiary orogenic evolution and with current strain and stress fields to derive a geodynamic model of Alpine collision tectonics. A number of peculiarities distinguish Alpine orogeny from other and they left their specific marks clearly visible in current deep lithosphere structure and tectonics. Presently, the Alps exhibit an uplift rate of about 2mm/y, a similar erosion rate and in relation to this vertical motion a very slow convergence rate of only 1-2mm/y. This slow convergence corresponds with relatively short lithosphere mantle slabs that exert only limited pull. Geometry of the two slabs and lower crustal indentation structure in the Central and Western Alps suggests the subduction-collision zone is largely locked.

The European lithosphere slab is obviously denser than asthenosphere and is to weak to elastically support its own weight. Rather, it hangs beneath the Central Alpine crustal root adding to the isostatic load of the orogen. After oceanic slab broke off collision continued further by delaminating European mantle lithosphere and wedging of continental crust. Once continental crust has been sheared off, the denser mantle lithosphere sinks in the asthenosphere again starting to roll back since it remains attached to the European plate. This roll back delamination process is very slow since most of the original slab weight has been lost due to break off. Recent studies about Alpine isostasy confirmed earlier findings that the central Alps have a much to large crustal root compared with the topographic load and in relation to a to positive Bouguer gravity anomaly. As we know from seismic tomography results, the buoyancy of the large crustal root is compensated by the topographic load and by the load of the mantle lithosphere slab still attached to Europe in the western and central Alps. The peculiar stress distribution in and along side the orogen may be interpreted as the result of slow continued convergence of Europe and Adria in combination with isostatic rebound of the axial orogenic region due to erosional unloading of topography and shifting of the load exerted by the mantle lithosphere slab due to progressive NW migration of crustal delamination.