Flysch to molasse transition in peripheral foreland basins: The role of the passive margin versus slab breakoff

Abstract

The initiation of continental collision and the inception of peripheral foreland basins occur by the deformation and flexure, respectively, of the inherited passive margin of the foreland plate. During progressive plate convergence, peripheral foreland basins develop from an underfilled flysch stage to a filled or overfilled molasse stage. Classically, 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 development of the North Alpine foreland basin neither inherited paleobathymetry nor changing lithospheric strength of the underthrust European passive margin played a significant role in the flysch to molasse transition. Sediment supply from the Alps increased at least 30% from the time of flysch to molasse deposition. At about the same time as the flysch to molasse transition (mid-Oligocene), the inner parts of the mountain belt experienced accelerated exhumation, uplift of high-pressure metamorphic rocks, lower lithospheric melting, and the onset of major backthrusting, all of which have been linked via a model of slab breakoff. A further consequence of the model is isostatic surface uplift and erosion. It is proposed that slab breakoff may have been responsible for the increased sediment supply that resulted in the flysch to molasse transition in the North Alpine foreland basin, and that this provides an alternative to the passive margin model.