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

Abstract

VOLCANISM in active continental rift zones^1,2or 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 Hawaii^4–6, 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.