Structure of Reunion Island (Indian Ocean) inferred from the interpretation of gravity anomalies

Reunion is a volcanic edifice whose origin is related to a hot spot in the Indian Ocean. Only 3% of its volume is emergent. Many geological and geophy sical studies were carried out on Reunion Island during the 1980's but few of them allow study of the internal structure of the edifice. Several gravi ty surveys have been carried out on the island since 1976 and we have compi led the available data set. The lack of data on the western side of the isl and led us to conduct a regional survey in 1993 to obtain a more homogeneou s distribution of the stations. Computation of Bouguer anomalies for differ ent correction densities accounts for the variable density of the rocks con stituting the edifice and provides a distribution of gravity anomalies inte rpreted as dense bodies of intrusive rocks inside the edifice. Two very lar ge intrusive complexes can be unambiguously recognised: one beneath Piton d es Neiges and one beneath the Grand Brule area. Both have been penetrated b y geothermal exploration drill holes and the first is also known from outcr op observations. 2.5D simple models were constructed to reveal the geometry and extent of the buried intrusives. They are deeply rooted, extending sev eral kilometres below sea level, and extensive (20-25 km long and 10-13 km wide for the Piton des Neiges complex, 12-15 km long and some kilometres wi de for the Grand Brule complex). The development of such complexes implies that the activity of the two volcanic centres was long lasting and remained stable while the volcanoes were growing. The Grand Brule complex has been interpreted as relies of an old volcano named Alizes Volcano. The interpret ation of the gravity maps suggests the presence of a ridge of dense rocks t o the North of the axis joining the centres of Piton des Neiges and Piton d e la Fournaise volcanoes. By analogy with the other structures, 2.5D models show that this structure would culminate between 0 and 1 km below sea leve l and be 15 km wide. This complex induces a maximum anomaly in Takamaka Val ley and we thus propose to name it Takamaka Volcano. No geological evidence of the nature of these dense rocks is available but the ridge coincides wi th structures revealed by magnetic and seismic data. Interpretation of the Bouguer anomaly maps suggests that the inner gravity structure of Piton de la Fournaise is not characterised by the presence of a voluminous dense bod y but probably by more restricted concentrations of dense rocks. Some struc tures can be recognised: along the present NE and SE rift zones and in the previous central part of Piton de la Fournaise to the West of the present s ummit. The recent eastward migration of the centre of activity of Piton de la Fournaise accounts for the lack of a large positive anomaly beneath the active craters. (C) 1999 Elsevier Science B.V. All rights reserved.