WESTERN PREALPES-MEDIANES-ROMANDES - TIMING AND STRUCTURE - A REVIEW

Between the original position and their present day location as klippe n, the Prealpes Medianes underwent a complex history of paleotectonics and alpine tectonics. Due to the opening of the Piemont ocean the Bri anconnais sedimentation realm of the Prealpes Medianes evolved as a ri m basin of the northern passive margin during Jurassic to Eocene times . Different paleotectonic features (normal faults, synsedimentary grow th structures, inversion structures) developed and were active above a basal detachment in evaporitic layers. The tectonic movements were a consequence of thermal events in the crust. Isolated from the Iberic c ontinent at the end of the Late Cretaceous, the Brianconnais exotic te rrain was incorporated into the accretionary prism of the closing Piem ont ocean and the incipient alpine orogeny during the Lutetian-Bartoni an. The Prealpes Medianes were detached from their homeland during the Bartonian-Priabonian and were transported onto the foreland. The tect onic style is one of a thin-skinned foreland fold and thrust belt. Fau lt associated fold development above a main decollement, together with internal deformation, represent the Prealpes Medianes main structural features. The very low-grade metamorphic conditions have their origin in the heat flux induced by tectonic burial by overriding nappes in t he accretionary prism. After having been transported on top of the dev eloping Helvetic nappes the Prealpes were emplaced in their present da y position in front of the Alpine mountain belt during Oligocene times . Post-emplacement and out of sequence thrusting, possibly younger tha n Oligocene, is observed and can be related to thrusting in the sedime ntary substratum and the basement.