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Abstract. A 250 km long NE-SW lithospheric transect spanning the 40 km wide island of La R6union and its submarine edifice is derived from lines of air gun shots at sea on either side, along the assumed hotspot trace. Seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers, providing a system of reversed and overlapping observations. Low seismic velocity, and hence density, is found on average for the whole edifice above the oceanic plate. We attribute highvelocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island-crossing segment. Unexpectedly, the main seismic interfaces, top and bottom of the prevolcanic crust, do not show significant flexural downwarping under the island. In addition, clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle. This lithospheric structure provides the first example where underplating occurs beneath an active volcanic island, suggesting a genetic relationship. The underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts. Evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity. In the plate/hotspot model commonly assumed, the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material. Active volcanism rather than plate flexure being the prime target, the long transect presented here is directed SW-NE along the supposed hotspot trace in order to sense the variation in structure of the lithospheric plate from its intact state before, during, and after having been submitted to the magmatic discharge of the mantle plume. The main refraction profile described here was recorded at sea by five OBS on the SW segment of the line of shots and two OBS on the NE segment (Figure 1), as well as on land in between by 32 por- The multiple-receiver recording of seismic shot lines achieved is fundamentally important in wide-angle reflectionrefraction seismics in order to obtain wave propagation along a system of reversed and overlapping profiles. Only such a cross sampling may resolve the variation of velocity with depth (layering) and the lateral variation of velocity (dip of interfaces or lateral heterogeneity within layers) from each other. Trialand-error forward modeling then results in a two-dimensional (2-D) seismic velocity structure consistently but not uniquely accounting for the observations. In such a model, those elements required by data that are critical by their quality or sampling may not stand out from other ones that are less constrained. Therefore the present section is devoted to displaying crucial data and commenting on particular aspects of the structural model they constrain before the derivation of the 2-D velocity model in section 3.The most relevant features, illus...