Young Marquesas volcanism finally located

The Marquesas Island chain in Polynesia is quite unusual because the alignment of the islands on the Pacific oceanic plate (N40°W) does not follow the plate motion in the region (N65°W). The exact location of the active hotspot is unknown but has been predicted to underlie the Marquesas Fracture Zone Ridge. Nevertheless, no concrete evidence exists. Here, we document the occurrence on this ridge of fresh tephrites dated at ~ 92 ka by the 40Ar39Ar method. The lavas dredged on a small seamount have trace element contents and Sr, Nd, Pb isotopic compositions typical of the southwest Marquesas Islands, the Fatu Hiva group. This discovery demonstrates that the Marquesas plume is still active and it puts new constraints on its present location. It also supports McNutt et al.'s (1989) interpretation of the Marquesas Fracture Zone Ridge as a very young volcanic construction underlain by a hotspot. We suggest that the present location of the Marquesas plume is under the ridge, at its intersection with the isotopic divide known along the Marquesas chain. We attribute the presence of young volcanic products 190 km southwest of this location to preferential magma flow along the Marquesas Fracture lithospheric weakness zone. We also suggest that the puzzling general direction of the archipelago is the consequence of a persistent low magma flux over the past 5 Ma that could only find its way to the surface through multiple weak zones in the Pacific plate.