HYDROGRAPHIC CHANGES AND PLIOQUATERNARY V OLCANISM IN THE DRAINAGE BASINS OF THE LOIRE AND ALLIER RIVERS (MASSIF-CENTRAL, FRANCE)

Hydrographic changes and Plio-Quaternary volcanism in the drainage bas ins of the Loire and Allier rivers (Massif Central, France). Geometric analysis was used to determine the paleogeography of the valley cours es in the upper drainage basins of the Loire and Allier rivers, and to define their relations to regional volcanism and associated sedimenta tion dynamics. The chronology of this scheme is based on the integrati on of new radiometric data and the study of tephrochronological indica tors. According to these data, there is no evidence for a Plio-Pleisto cene course of the Loire towards the Allier. On the contrary, a probab le S-N water course of the river is fossilized by the Deves basalts up stream from the basin of Le Puy. in the upper Allier drainage basin th ere are insufficient arguments to prove that the Allier ever flowed th rough the Paulhaguet basin during Pliocene. However, our data confirm the existence of an old N-S course of the Senouire, now dammed by the Ple-du-Roi volcano. It explains the sedimentation in the basin and the change of direction of the course to the north. Further north, Plioce ne paleogeography of the Allagnon, buried under the Cezallier basanite s, is examined. On the eastern slopes of the Mont Dore, important chan ges in courses are evident within the Quaternary network of the tribut aries of the Allier (Couzes). Finally, further east, the 4 Ma complex evolution of the Allier course can be described precisely, using volca nic chronological indicators. Our results illustrate the direct role p layed by volcanism in the changes of the river courses and its importa nt role in the fluvio-lacustrine sedimentation. The part played by tec tonics seems to be less important than that of the climatic dynamic.