Crustal structure of the Tuamotu Plateau, 15°S, and implications for its origin

Abstract

We investigate the sedimentary and volcanic structure of the Tuamotu Plateau with multichannel seismic, seismic refraction, and gravity data along a ship track crossing the plateau near 15°S. The volcanic basement of the central portion of the plateau is capped with a 1 to 2‐km‐thick sediment layer composed of two compositional sequences. The uppermost sequence, with semblance‐derivedPwave velocities of 1.6–1.9 km/s and thicknesses of 0.2–0.9 km, is composed of pelagic sediments. The underlying sequence, with velocities 2.5–3.5 km/s and thicknesses of 0.5–1.5 km, is composed of limestone and volcaniclastic sediments. Sonobuoy refraction data show the upper 1 km of the volcanic basement to have velocities 4.5–5.5 km/s. The gravity data indicate that the platform is compensated by an elastic lithosphere with effective thickness 5±5 km and that the volcanic thickness is 9–10 km thicker than normal oceanic crust with a volume of 2.0–2.6×l06km3. The inferred eruption rates of 0.1–0.13 km3/yr are comparable to those of the Hawaiian and Marquesas island chains but substantially less than those 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 –600 km off the axis of the paleo‐Pacific‐Farallon spreading center, on lithosphere of age ∼10–20 Ma. Linear volcanic ridges and scarps bounding deep sediment‐filled basins, however, are similar to features of oceanic plateaus which formed at or near accretionary plate boundaries. We suggest that these volcanic ridges and the gross plateau like morphology were formed by magma that was channelled along the lithospheric discontinuities left behind by a southward propagating rift segment of the nearby spreading center. We attribute the formation of the northwestern portion of the Tuamotu Plateau to the passage of two hotspots during times 50–30 Ma as they migrated beneath the Pacific plate but remained west of the Tuamotu propagator.