The Grès Singuliers of the Mont Blanc region (France and Switzerland): stratigraphic response to rifting and crustal necking in the Alpine Tethys

The Gres Singuliers unit represents an anomalous occurrence of a siliciclastic-dominated sedimentary system typifying a restricted geographic area around the Mont Blanc massif. Deposited during Early Jurassic rifting, this unit was influenced by the tectonic processes responsible for its development. This contribution integrates and reconciles sedimentological, stratigraphic and tectonic data and discusses the tectono-sedimentary evolution of the Mont Blanc basement and its autochthonous sedimentary cover. Based on depositional facies, a petrographic and detrital zircon provenance analysis, we propose that the Gres Singuliers unit is mainly derived from erosion of the local basement and pre-rift sedimentary cover. Furthermore, recognition of Jurassic cataclasites and black gouges capping the Mont Blanc basement confirms the hypothesis that the Mont Blanc domain formed an extensional core complex. The source to sink relationship between the Mont Blanc detachment system and the Gres Singuliers unit, as well as timing and location within the Alpine rift system, allows us to interpret this unit as the syn-tectonic sedimentary response to crustal necking, responsible for the onset of localized severe crustal/lithospheric thinning in the European margin of the Alpine Tethys rift system. The main result of this study was to show that the exhumation and uplift of basement during crustal/lithospheric necking produced a new source area for clastic sedimentary systems. Therefore, the local occurrence of a siliciclastic unit similar to the Gres Singuliers along passive margins may be symptomatic of necking zones in other rift systems.