SEISMIC IMAGING OF HOTSPOT-RELATED CRUSTAL UNDERPLATING BENEATH THE MARQUESAS ISLANDS

VOLCANISM in active continental rift zones(1,2) or on rifted continent al margins(3) is frequently associated with unusually high lower-crust al seismic velocities, indicating the presence of large igneous intrus ions at the base of the crust. The only comprehensive investigation of crustal underplating at an oceanic hotspot, beneath Hawaii(4-6), has yielded controversial results(7). Here we report the results of seismi c refraction experiments across the Marquesas Islands hotspot trace, w hich 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) h as seismic velocities (7.3-7.75 km s(-1)) indicative of crustal underp lating. Wide-angle reflections occur near the base of the normal lower -crustal velocities and at the base of the underplating complex; we in terpret these reflectors as the relict pre-hotspot Moho and the curren t post-hotspot Moho, respectively. The high-velocity material may be p urely intrusive or may consist of a mixture of intrusive and preexisti ng rocks. The volume of the high-velocity body is impressive, amountin g to nearly twice the volume of volcanics erupted at the surface.