VOLCANISM AND ARCHIPELAGIC APRONS IN THE MARQUESAS AND HAWAIIAN-ISLANDS

Geophysical observations demonstrate that the archipelagic apron surro unding the Marquesan hot-spot volcanoes is derived almost entirely fro m mass wasting processes. Seismic reflection and refraction data const rain the volume of the apron sediments to approximately 200,000 km3, w ith thicknesses reaching over 2 km in the deep portions of the moat ne ar the edge of the volcanic edifice. Seismic velocities average 4 to 5 km s-1 in the sediments, and 6 km s-1 at the top of the underlying ba sement. Single channel seismic profiles show acoustically chaotic core s in the sediments of the apron, which are interpreted as debris flows from mass wasting events. We deduce that the apron is formed by catas trophic collapses that may involve volumes over 100 km3 tens to hundre ds of times during the lifetime of a volcano. Comparison with similar data from the Hawaiian Islands yields the result that the total volume of volcanics and their derived sediments along the strike of the chai ns is only slightly smaller for the Marquesas, implying comparable eru ption rates. However, the ratio of sediment to surface volcanic load i s much larger for the latter, leading to an overfilled moat in the Mar quesas and an underfilled moat at Hawaii. The much larger size of the Hawaiian islands can be explained as the combined effects of a higher thermal swell, loading a stiffer elastic plate, and proportionately le ss mass wasting.