MODEL FOR THE STRUCTURAL EVOLUTION OF THE VOLCANOS OF REUNION ISLAND

Reunion Island is an intraplate volcano linked with the hot spot activ ity that is also responsible for the volcanism of Mauritius and of the Mascarene Plateau. It is composed of two main volcanic complexes, the Piton des Neiges and the still active Piton de la Fournaise. The latt er exhibits a special morphostructural evolution characterized by succ essive nested U-shaped calderas that open seaward. We propose an analy sis of this evolution which permits us to deduce a model which can be extended to the general history of the island. From the morphology of the youngest caldera on the one hand and from the K-Ar data on the cli ffs corresponding to the successive rims on the other we establish the frequency of landslide collapses and estimate the volumes emitted dur ing each successive phase. This yields evidence of an increasing diseq uilibrium of the southeastern flank of Piton de la Fournaise. Indeed, through time the frequency of collapses increases and collapse volume decreases. We model the phenomenon with an exponentially smoothed sinu soidal curve, with time on the abscissa and eruptive output on the ord inate. Each maximum on the curve corresponds to the successive slumps and the area of each arch represents the volume erupted during each ph ase. This can be extrapolated to the earlier volcanic history of Reuni on, setting the first collapse at about 1.8 Ma, which affected the mai n part of the present island, preserving only the Massif de la Montagn e at its north-northwestern end. This explains the breccias observed i n the deep erosional features of the massif of the Piton des Neiges an d takes into account the lack of lavas with dates between 1.8 and 1.1 Ma. Comparison between the curve of emissivity and the total volume es timated for the whole island volcano yields an age of 5 Ma for the beg inning of the activity. This age coincides with the end of the main vo lcanic activity in Mauritius.