FLYSCH TO MOLASSE TRANSITION IN PERIPHERAL FORELAND BASINS - THE ROLEOF THE PASSIVE MARGIN VERSUS SLAB BREAKOFF

The initiation of continental collision and the inception of periphera l foreland basins occur by the deformation and flexure, respectively, of the inherited passive margin of the foreland plate, During progress ive plate convergence, peripheral foreland basins develop from an unde rfilled flysch stage to a filled or overfilled molasse stage, Classica lly, this flysch to molasse transition is interpreted as recording the migration of the thrust wedge and foreland basin over the hinge line of the inherited passive margin, it is demonstrated that during the de velopment of the North Alpine foreland basin neither inherited paleoba thymetry nor changing lithospheric strength of the underthrust Europea n passive margin played a significant role in the flysch to molasse tr ansition. Sediment supply from the Alps increased at least 30% from th e time of flysch to molasse deposition, At about the same time as the flysch to molasse transition (mid-Oligocene), the inner parts of the m ountain belt experienced accelerated exhumation, uplift of high-pressu re metamorphic rocks, lower lithospheric melting, and the onset of maj or backthrusting, all of which have been linked via a model of slab br eakoff, A further consequence of the model is isostatic surface uplift and erosion, It is proposed that slab breakoff may have been responsi ble for the increased sediment supply that resulted in the flysch to m olasse transition in the North Alpine foreland basin, and that this pr ovides an alternative to the passive margin model.