Seismic imaging of hotspot-related crustal underplating beneath the Marquesas Islands

Abstract

VOLCANISM in active continental rift zones1,2or on rifted continental margins3is frequently associated with unusually high lower-crustal seismic velocities, indicating the presence of large igneous intrusions at the base of the crust. The only comprehensive investigation of crustal underplating at an oceanic hotspot, beneath Hawaii4–6, has yielded controversial results7. Here we report the results of seismic refraction experiments across the Marquesas Islands hotspot trace, which show that the island chain is underlain by crust 15–17 km thick, and that a large lower-crustal region (275 km wide and 2–8 km thick) has seismic velocities (7.3–7.75 km s−1) indicative of crustal underplating. Wide-angle reflections occur near the base of the normal lower-crustal velocities and at the base of the underplating complex; we interpret these reflectors as the relict pre-hotspot Moho and the current post-hotspot Moho, respectively. The high-velocity material may be purely intrusive or may consist of a mixture of intrusive and preexisting rocks. The volume of the high-velocity body is impressive, amounting to nearly twice the volume of volcanics erupted at the surface.