Classic swiss clastics (flysch and molasse) The alpine connection

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Abstract
Flysch and molasse are discussed in the light of alpine geodynamics. They are pre-collision and post-collision orogenic clastics which accumulated in basins under different geodynamic controls. We propose that in the case of the Alps, their succession records the change in geodynamics from pre-collision inversion of shallower extensio-nal structures, to post-collision inversion of one or more deep-seated features. The classical flysch of the Prealps, lying within a pile of nappes at the front of the Western Alps, are invariably turbidite deposits. Flysch has therefore acquired a sedimentological connotation over the years, and this has been emphasized over the last few decades. Turbidite facies were also laid down in marginal and foreland locations during and a little after the collision between the South and North Tethyan Alpine margins, and this has obscured the possible deeper signification of flysch and molasse. Geodynamic regimes dictate the subsidence behaviour of basins, so by use of geohistory analysis, the time and place of the onset of Molasse basin development may be located. This was at the southern margin of the Helvetic belt, from the start of the Oligocene. Along the Alpine traverse of Western Switzerland, the change in regime from flysch to molasse (i.e. from trench and forearc or retro-arc, to foreland basin deposits) suggests that a major deep-seated inversion structure was situated near the Helvetic-Ultrahelvetic boundary.

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