Tectonics of the Mont Tendre and Mont Risoux (Internal Jura Mountains): mapping and 2D kinematic forward modelling

This study focuses on a new geological cross-section and forward model of the Mont Tendre and Mont Risoux anticlines within the Haute Chaine of the Jura fold-and-thrust belt. Thes results are based on fieldwork, remote sensing, data compilation of previous studies and geological maps, as well as balanced and forward section modelling. We propose a refined understanding of the wedge geometry, especially the near top basement surface. We identify a large-scale structural tectonic unit with an important lateral continuity, formed by the main regional Vallée de la Saine thrust. This thrust has a 15 km long flat in the Cretaceous units and accommodates a displacement of 17 km, which lead to the formation of the Mont Risoux nappe. The importance of major thrusting in the clay-rich Middle Cretaceous series is recognised as a corollary of the structural modelling. The interpretation of the Risoux-1 deep well has led to the determination of the significant secondary décollement levels in the clay-rich Lower to Middle Jurassic series that lead to the formation of important flats, fishtail and pop-up geometries in structurally higher levels. The activation of the Vallée de Joux frontal thrust resulted in the formation of the Mont Tendre anticline, which lead to the dissection of the Mont Risoux nappe into a sub-unit in the SE, the Mont Tendre nappe. The formation of the Vallée de Joux thrust was followed by a backstepping sequence and a locally oscillating thrust sequence. This detailed interpretation of the sequential development, highlighting the importance of top to the south thrusting (backthrusting), confirms the mechanics of a low-tapered wedge with the décollement in the weak evaporites promoting backthrusting. Moreover, the tectonic nappes demonstrate the lateral structural subdivision by primary thrusts, which have a NE-SW orientation, and by the primary strike-slip faults, which have a NNW-SSE and ESE-WNW orientation. We propose a kinematically viable solution for the activation of the Vallée de la Saine thrust and the formation of the Mont Tendre anticline, which were active between 14 Ma and 4 Ma and accommodated a shortening of some 20 km.