CRUSTAL STRUCTURE OF THE TUAMOTU PLATEAU, 15-DEGREES-S, AND IMPLICATIONS FOR ITS ORIGIN

We investigate the sedimentary and volcanic structure of the Tuamotu P lateau with multichannel seismic, seismic refraction, and gravity data along a ship track crossing the plateau near 15 degrees S. The volcan ic basement of the central portion of the plateau is capped with a 1 t o 2-km-thick sediment layer composed of two compositional sequences. T he uppermost sequence, with semblance-derived P wave velocities of 1.6 -1.9 km/s and thicknesses of 0.2-0.9 km, is composed of pelagic sedime nts. The underlying sequence, with velocities 2.5-3.5 km/s and thickne sses of 0.5-1.5 km, is composed of limestone and volcaniclastic sedime nts. Sonobuoy refraction data show the upper 1 km of the volcanic base ment to have velocities 4.5-5.5 km/s. The gravity data indicate that t he platform is compensated by an elastic lithosphere with effective th ickness 5+/-5 km and that the volcanic thickness is 9-10 km thicker th an normal oceanic crust with a volume of 2.0-2.6x10(6) km(3). The infe rred eruption rates of 0.1-0.13 km(3)/yr are comparable to those of th e Hawaiian and Marquesas island chains but substantially less than tho se of many oceanic plateaus. Radiometric and paleontological ages for the plateau and geomagnetic dates of the surrounding seafloor indicate that the northwestern portion of the plateau formed similar to 600 km off the axis of the paleo-Pacific-Farallon spreading center, on litho sphere of age similar to 10-20 Ma. Linear volcanic ridges and scarps b ounding deep sediment-filled basins, however, are similar to features of oceanic plateaus which formed at or near accretionary plate boundar ies. We suggest that these volcanic ridges and the gross plateau like morphology were formed by magma that was channelled along the lithosph eric discontinuities left behind by a southward propagating rift segme nt of the nearby spreading center. We attribute the formation of the n orthwestern portion of the Tuamotu Plateau to the passage of two hotsp ots during times 50-30 Ma as they migrated beneath the Pacific plate b ut remained west of the Tuamotu propagator.