Vertical movements and material transport during hotspot activity: Seismicreflection profiling offshore La Reunion

The structure of the submerged part of the La Reunion hotspot island is det ermined by a grid of multichannel seismic reflection profiles. The submarin e part of the edifice appears as a poorly stratified wedge of material lyin g above a significant thickness of preexisting sediments and the oceanic ba sement. The dense data coverage has allowed us to derive contour maps of th e top of the basement and of the base of the volcaniclastic edifice, furthe r constrained by coincident wide-angle profiles. The resulting isobath maps reveal new, unsuspected features that could not be deduced from observatio n along a single seismic line since the geometry of these horizons varies s ignificantly from one radial profile to the next. Both maps show a large de gree of heterogeneity in the topography, with no axial nor cylindrical symm etry, indicating that plate flexure is not dominant. A slight depression to ward the island is observed only in the southern area, ahead of the hotspot trace. The lack of angular unconformity in the volcano-sedimentary pile th at covers the oceanic basement firmly establishes the lack of significant v ertical movement and flexure. The base of the edifice is roughly domed, cen tered on the island, with several topographic highs or lows superimposed. T he submarine apron appears as a composite constructional body, spreading by slumping of its flanks. Superficial lenses of laterally transported materi al are observed on the seismic data south of the island, not only to the ea st of the active Piton de la Fournaise volcano. Oceanic sediments trapped b eneath the apron seem undeformed.