Arcuate fold and thrust patterns in the Subalpine Chains of Southeast France

The arcuate pattern of folds and thrusts is examined in the Subalpine Chains of Southeast France. Five segments are recognised; (1) a NNE-SSW trending zone between Grenoble and Die, (2) an E-W zone between Die and Sisteron, and (3); (4), & (5) the Arcs de Digne, Castellane, Nice, trending NNW-SSE, E-W, and N-S, respectively. Folds of three ages are known: pre-Senonian (recognised in Dévoluy), Paleocene to early Eocene and late Oligocene to late Miocene. The last phase was accompanied by low-grade metamorphism and cleavage development. Thrusts were formed during the last two phases. Examination of the age of folding and thrusting, directions of tectonic transport and the continuity of structures reveals that segments (1), (2) & (3) are not continuous. Segment (2) has a northward sense of tectonic transport and is thought to be of pre-Senonian and Paleocene age. It is crossed in Dévoluy by the late Oligocene structures which are continuous from segment (3) into segment (1). Segments (3), (4) & (5) are continuous. Their curvature is thought to be primary and to have resulted from gravity gliding. A section through the Digne Thrust Sheet is examined in terms of models of thrusting proposed by Elliott and by Chapple. It is found that the compressional component in Chapple's model was completely dominant in overcoming the resistance to sliding during the Miocene phase, the glacier spreading component having a negligible effect. Other thrusts, in particular those of the Arc de Castellane, do not appear to fit the model. It is thought that gravity gliding may have been the mechanism of thrusting in these areas.