The western part of the Pacific plate, where the oceanic crust is of Jurassic age, has undergone exceptional volcanic activity during the Cretaceous. It has produced numerous volcanoes (oceanic islands, guyots, seamounts) as well as very thick accumulation of volcanogenic material in the deep basins. The Mesopac cruise of the R/V Jean Charcot in the Nauru Basin obtained multichannel seismic profiles for the first time in these regions. This technique aimed at estimating the thickness of the cretaceous volcanics and locating the top of the underlying Jurassic crust. The results, when compared with those of DSDP drill holes, demonstrate that the abnormally shallow depth of the Nauru Basin results from an accumulation of flows and sills that constitute a "volcanic complex" at least 2.4 km thick, and not from a thermal uplift of the lithosphere.