Structure of the Tectonic Front of the Western Alps: Control of Fluid Pressure and Halite Occurrence on the Decollement Processes

This study focuses on the respective role of (1) the occurrence and the nature of evaporitic layers and (2) the fluid pressure conditions on the decollement processes and the structure of the foreland fold and thrust belt of the Western Alps. The decollement in the Jura Mountains is predominantly localized in halite‐bearing layers. Anhydrite‐bearing units which are not associated to halite are not hosting major decollement. The shift of decollement level between Mid‐Triassic halite unit and Upper Triassic halite unit has induced local tectonic subtractions and, elsewhere, tectonic duplications at depth. Available fluid pressure measurements show that fluids are not overpressured in the Jura. Even below the salt decollement, they remain in hydrostatic conditions. South of the Jura, the absence of halite is correlated with no efficient role of the Triassic layers in terms of decollement. The available pressure measurements show that the decollement is associated with high overpressure. Because of the low friction of halite, the Jura thrust wedge shows a narrow angle (3°–4°). The relatively high friction behavior of the decollement south of the Jura is responsible for stacks of tectonic units associated with a relatively wide angle of the tectonic wedge (12°–13°). The structural change between the Chartreuse and Vercors massifs is not controlled by the properties of the decollement but by the change of thickness of the sedimentary pile involved in the tectonic wedge. The change of thickness is controlled by paleogeographic heritage during Jurassic and Cretaceous times. Plain Language Summary This study presents seismic reflection and drilling data which show the subsurface nature and structure of the Jura mountains and the western parts of the Alps in France. These data illustrate how the formation of the Alps is associated with the deformation of rocks at depth. The deformation processes are not homogenous in this area, and they are controlled by the local presence of halite (salt) layers which are able to be deformed very easily within the Jura mountains. In the front of the western Alps is the fluid pressure which is mostly the triggering factor that favors rock deformation. Key Points The structure of the front of the Alps is controled by the occurrence of halite and fluid pressure condition Shift of decollement level induces tectonic substraction or duplication Change in the thickness of the sediments involved in the tectonic wedge induces differential p