Quantifying Miocene thin- and thick-skinned shortening in the Baous thrust system, SW French Alpine Front

Abstract

At the southernmost front of the southwestern French Alps, the Baous thrust system is a key marker of Alpine deformation. Based on detailed surface data and a balanced, sequentially restored cross-section, this study shows that the SSW-verging Baous thrust is rooted in a deep-seated basement thrust, kinematically linked to shallower thrusts detached within Triassic evaporites. This basement thrust reactivates an inherited, north-dipping Jurassic extensional fault, which controls the development of the Saint-Barnabé anticline in the north. To the south, the Courmettes structures display fault-bend fold geometries and associated shortcut thrusts. Restoration constrains a total horizontal shortening of 5 km (31.4 %), partitioned between the sedimentary cover (43 %, ∼2.1 km) and the basement (57 %, ∼2.9 km), reflecting both thin- and thick-skinned tectonics. The Courmettes-Vence syn-tectonic basin, located in the footwall, provides key temporal constraints on thrust development. Six deformation stages, identified through sequential restoration between the Burdigalian and Messinian, highlight increasing shortening rates: 0.17 km/Myr (Burdigalian-Tortonian), 0.37 km/Myr (Tortonian), and 1.16 km/Myr (Messinian). Kinematic relationships indicate a primarily in-sequence deformation propagation toward the Alpine foreland: initial basement-involved thrusting was followed by activation of thin-skinned thrusts, then overprinted by out-of-sequence reactivation of the basement thrust. The proposed kinematic model is consistent with recent onshore and offshore seismic interpretations, supporting its robustness and relevance for understanding the tectonic evolution of the southwestern Alpine front. These results emphasize the role of inherited extensional structures (here Jurassic in age) in shaping fold-and-thrust belts and controlling thrust geometry, deformation style, and present-day distribution of seismicity.