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Micro-tectonic constraints on the evolution of the Barles half-window (Digne nappe, southern Alps). Implications for the timing of folding in the Valensole foreland basin

¹ Laboratoire de Tectonique, Université Pierre et Marie Curie-Paris 6, CNRS UMR 7072, UCP, Case 129, 4 place Jussieu, 75252 Paris, France
² Laboratoire de Géologie, Ecole Normale Supérieure, CNRS UMR 8538, 24 rue Lhomond, 75005 Paris, France

Correspondence: ^* corresponding author: marc.fournier{at}upmc.fr

The "Vélodrome" overturned syncline, at the northern margin of the Cenozoic foreland basin of Valensole in SE France, was formed during the Late Cenozoic at the front of the Digne nappe. Microstructural analyses reveal that mesoscale faulting in the molassic series, from the Oligocene "Molasse Rouge" at the base to the middle to late Miocene "Valensole Conglomerates" at the top, partly occurred before the folding, as layer-parallel shortening: the NNE-SSW-directed compression is recorded by two systems of reverse and strike-slip faults, which formed when the strata were still horizontal and were passively tilted as folding occurred. These data suggest that the Vélodrome folding postdates the deposition of the Valensole Conglomerates and occurred in late Miocene-Pliocene times during the emplacement of the Digne nappe. These results are difficult to reconcile with the interpretation of the Vélodrome as a growth fold progressively formed in 10–15 m.y. during the deposition of the Miocene molasses. Structural data collected in the Barles tectonic half-window enable to reconstruct the evolution of the deformation since the Jurassic. The two main phases of shortening, the pre-Oligocene Pyrenean-Provençal and the Mio-Pliocene Alpine phases, are almost homoaxial with a direction of compression trending N-S for the former and NNE-SSW for the later. A late Eocene-basal Oligocene N-S extensional episode is documented between these two phases, probably in relation with the formation of the western European rift system. The direction of extension of the Liassic rifting of the Alpine Tethys is roughly constrained in the NW-SE quadrant. Paleo-stress field reconstruction brings consistent results at the regional scale and proves to be a powerful tool to decipher the evolution of the deformation in a remarkably complicated tectonic setting.

Key Words: Barles tectonic half-window • Digne nappe • Valensole foreland basin • External western Alps • Stress-field evolution • Growth fold