Messinian-Zanclean canyons in the Digne nappe (southwestern Alps): tectonic implications
Résumé
Mapping thick conglomerates in the subalpine chains around Digne allows reconstruction of a deeply incised Messinian-Pliocene paleo-drainage network. It shows that the main rivers (Bléone, Bès, Mardaric, Eaux-Chaudes) have existed since at least the Messinian. Pollen analysis reveals that the Pliocene-Early Quaternary continental infill of these canyons did not start at 5.3 Ma, like in the Zanclean rias, but in the Late Pliocene (between 3.4 and 2.6 Ma). This 2 Myrs delay results in part from the timing of the retrogradation of the alluvial wedge, and in part from the inland shift of the river mouths that accompanied the Zanclean flooding of Messinian canyons. This shift corresponds to a shortening of hundreds of kilometers of the river profiles that caused the river to gain more erosive energy. This strong Zanclean erosion is contemporaneous with the main infill of the rias downstream. We describe sections of paleo-canyons that were uplifted, tilted, overthrusted or truncated by strike-slip fault. The geometry of this dissected drainage network provides new constrains on the tectonic evolution of the southern Subalpine chains. An about 2.3 km strike-slip separation of the Bléone River paleo-canyon confirms that the front of the Digne nappe is an oblique ramp of the South-verging thrust system of the 'Arc de Castellane'. This nappe moved South-Southwestward along the N015 trending tear fault. Fault striation analysis in the Late Pliocene-Quaternary conglomerates shows that this displacement was characterized by ENE-WSW compression, along the Digne oblique ramp, consistent with its dextral slip. Most of the Digne nappe displacement (~20 km) occurred prior to the Late Pliocene as shown by the La Bonnette section of the Bléone paleo-canyon that incises a block of the overthrusted "residual basin". It was followed by ~2.3 km southward displacement along out-of-sequence faults during the Late Pliocene and the Quaternary. This out-of-sequence reactivation of the Digne nappe is contemporaneous with the formation of the Barles Half-window nappe anticline as shown by the 345 m uplift of the l'Escuichière paleo-canyon. The reconstructed paleo-drainage network attests for the tectonic activity of the Barles nappe anticline, the Bès tear fault, the Saint Benoît tear faults and the Digne nappe during the Pliocene and the Quaternary.
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