The Grand Brule exploration drilling: New data on the deep framework of the Piton de la Fournaise volcano. Part 1: Lithostratigraphic units and volcanostructural implications

Abstract

The sinking of a deep drill hole (3003.5 m) for geothermal exploration in the eastern part of the Piton de la Fournaise volcano on Réunion Island has revealed the underlying lithostratigraphy. Six principal sequences have been distinguished: (a) 0–440 m, subaerial basaltic flows; (b) 440–556 m, a transitional series characterized by an alternation of subaerial basalts and submarine deposits (vitric basalt and hyaloclastic breccia); (c) 556–616 m, submarine basalts (lavas and hyaloclastites); (d) 616–890 m, subaerial basalts; (e) 890–1010 m, submarine basaltic deposits with abundant argillized hyaloclastites; (f) 1010–3003.5 m a layered intrusive complex comprising: 1010–1129 m, sheets of gabbro injected into the lavas and hyaloclastites; 1129–1920 m, a zoned intrusion of olivine gabbro and dolerite; 1920–3003.5 m, stratified cumulate gabbro-wehrlite-dunite.The occurrence of the first submarine deposits at 440 m (268 m below present sea level) and the intercalation with them of subaerial basalts down to 890 m (728 m below present sea level) tend to support a hypothesis of sliding into the sea during subsidence of the eastern flank of the Fournaise massif.The intrusive complex intersected from 1010 m down is regarded as a former shallow magma chamber. A large positive gravimetric anomaly (40–50 mgal) centered over Grand Brûlé and attributed to the intrusive complex indicates that its roof forms an ellipse 3 × 20 km, elongated N-S.Stratigraphic and morphological-structural evidence (on land and on the sea-floor) and the results of the mineralogical study of this very large magma chamber suggest that it belongs to an ancient edifice (proto-Fournaise) whose activity preceded the construction of the present Fournaise and ended at about 0.6 Ma. On land, the evidence for this earlier edifice is to be found in the zeolitised basalts and differentiated lavas (hawaiites) forming the basement to the Phase 1 lava flows ( < 0.35 Ma) of the present Fournaise and found in the deepest valleys cut into the volcano. On the sea floor, an E-W elongated topographic high may be a vestige of the ancient edifice.Certain subsidence structures on the present Fournaise are interpreted as resulting from settling of the proto-Fournaise volcano. The destabilization of the east flank (Grand Brûlé area) of the currently active volcano and the location of the NE and SE rift zones could be attributed to the same phenomenon.