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ABSTRACT

We describe the tectono-sedimentary evolution of a Middle Jurassic, rift-related supra-detachment basin of the ancient Alpine Tethys margin exposed in the Central Alps (SE Switzerland). Based on pre-Alpine restoration, we demonstrate that the rift basin developed over a detachment system that is traced over more than 40 km from thinned continental crust to exhumed mantle. The detachment faults are overlain by extensional allochthons consisting of upper crustal rocks and pre-rift sediments up to several kilometres long and several hundreds of metres thick, compartmentalizing the distal margin into sub-basins. We mapped and restored one of these sub-basins, the Samedan Basin. It consists of a V-shape geometry in map view, which is confined by extensional allochthons and floored by a detachment fault. It can be restored over a minimum distance of 11 km along and about 4 km perpendicular to the basin axis. Its sedimentary infill can be subdivided into basal (initial), intermediate (widening) and top (post-tectonic) facies tracts. These tracts document (1) formation of the basin initially bounded by high-angle faults and developing into low-angle detachment faults, (2) widening of the basin and (3) migration of deformation further outboard. The basal facies tract is made of locally derived, poorly sorted gravity flow deposits that show a progressive change from hangingwall to footwall-derived lithologies. Upsection the sediments develop into turbidity current deposits that show retrogradation (intermediate facies tract) and starvation of the sedimentary system (post-tectonic facies tract). On the scale of the distal margin, the syn-tectonic record documents a thinning- and fining-upward sequence related to the back stepping of the tectonically derived sediment source, progressive starvation of the sedimentary system and migration of deformation resulting in exhumation and progressive delamination of the thinned crust during final rifting. This study provides valuable insights into the tectono-sedimentary evolution and stratigraphic architecture of a supra-detachment basin formed over hyper-extended crust.