Structure of the "diapir" of Champfromier (Jura Mountains, France

Located to the SW of Geneva, the internal Jura mountains often form anticlines which are eroded down to Middle or Upper Jurassic strata. An exceptional anticline core of Triassic age, previously described as a diapir, is exposed in the western part of the Valserine valley close to the Sandezanne river. Detailed mapping, logging and structural analysis of Upper Triassic and Liassic beds have been completed in order to better understand the structure of this area. This complex structure results from two successive thrusting events. The first phase caused a back-thrusting to the East whereas the second resulted in thrusting to the West. These two combined tectonic phases isolated an important hidden volume that explains the topographically high position of the Triassic units described in the Sandezanne valley. This unusual structure is linked to rotational movements induced by the strike-slip Vuache fault.