Horizontal shortening control of Middle Miocene marine siliciclastic accumulation (Upper Marine Molasse) in the southern termination of the Savoy Molasse Basin (northwestern Alps/southern Jura)

Between the southern Jura and the Subalpine Chains (Bornes, Bauges, and Chartreuse Massifs), the Burdigalian-Langhian-Serravalian tidal siliciclastic ‘Upper Marine Molasse’ (UMM) outcrops in N-S synclines between N-S-trending ramp anticlines separating southward-narrowing synclines. A surface survey and an interpretation of transverse seismic-reflection profiles across the latter confirm that they represent progressively individualized and separated sedimentation areas. Combined seismic configuration and surface geometry show westward migration of depo-axes, and coeval progressive steepening close to each ramp, upon blind thrusts. Several thick earthquake-disturbed layers offer further evidence of a syntectonic sedimentation. A regime of flexure-subsidence prevailing during the Chattian-Aquitanian (the so-called Lower Freshwater Molasse, LFM) was overprinted by thin-skin shortening. The latter, combined with Burdigalian-Langhian higher eustatic sea-level, favoured the accumulation and the preservation of abundant tidal bedforms and sequences, which are generally poorly preserved in stable settings.