Spatial distribution of hotspot material added to the lithosphere under La Réunion, from wide-angle seismic data

Wide-angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three-dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of La Réunion. The submarine part of the edifice has fairly low seismic velocities, without evidence for intrusives. An upper unit with a velocity-depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope. Between this unit and the top of the oceanic plate, imaged by normal incidence seismic reflection, a more homogeneous unit indicated by shadow zones on several wide-angle sections may correspond to lavas of a different nature, extruded underwater in the earlier phase of volcanism. Coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part, with limited evidence for some intracrustal intrusion. Deeper in the lithosphere, the presence of a layer of intermediate velocity between the crust and mantle is firmly established. It is interpreted as resulting from the advection of hotspot magmatic products, possibly partially molten, and of a composition for which the crust is a density barrier. The extensive wide-angle coverage constrains the extent of this body. It does not show the elongated shape expected from plate drift above a steady hotspot supply. Alternative propositions can hence be considered, for example, that La Réunion is caused by a solitary wave of hotspot material or by a young hotspot. The size of the underplate, 140 km wide and up to 3 km thick, corresponds to less than half the volume of the edifice on top of the plate. Copyright 1999 by the American Geophysical Union.