The structural setting and palaeogeographical evolution of the Grès d’Annot Basin

Abstract

Abstract A new kinematic model is derived for part of Haute Provence, which recognizes that Alpine deformation, involving both the Mesozoic and sub-Mesozoic section, has been active since the onset of Tertiary deposition. This model places the Grès d’Annot basin in an overall compressional setting, dominated by SW-directed Alpine thrusting, during deposition of the Tertiary basin fill sequence. Evidence is presented that one block of Palaeozoic material (Barrot) was strongly uplifted and deeply eroded prior to the onset of Tertiary deposition in the area. Local thin-skinned extension and strike-slip are observed, but these are secondary components, which are interpreted to be part of the overall thrust system. The region was affected by three different orogenic events. Pyreneo-Provençale thrusting established a structural fabric that influenced later, syndepositional thrust geometries. The Late Eocene to Early Oligocene Grès d’Annot Basin was a structured foreland basin created by the loading effects of the Alpine orogeny. The supply of siliciclastic sediment to the basin was controlled by tectonic events far to the south, in a subduction-related orogeny that extended from East Iberia to SE France. SW-directed Alpine compression within the basin produced significant basin-floor topography, which developed immediately prior to, and during, the Nummulitic transgression. The evolving palaeo-relief of the basin is recorded in thickness and facies variations of the older Tertiary lithostratigraphical units (Poudingues d’Argens, Calcaires Nummulitiques and Marnes Bleues), and by the onlap geometry of the Grès d’Annot turbidites. This allows us to define a set of palaeogeographical maps of the basin throughout its evolution, linked to a set of sequentially restored structural sections. This basin floor topography was complex, because Alpine thrusting overlapped the older east-west striking structures, and it created a set of partially isolated sub-basins. Major early compressional structures partitioned the basin, exerting strong control on all the Tertiary depositional systems, including the Grès d’Annot turbidite systems.