MORPHOSTRUCTURAL STUDY AND TYPE OF VOLCANISM OF SUBMARINE VOLCANOS OVER THE PITCAIRN HOT-SPOT IN THE SOUTH-PACIFIC

Undersea volcanoes found at about 80 km southeast of the island of Pitcairn, are believed to be the manifestation of a hot-spot activity located near 129-degrees-30'W 25-degrees-10'S, along the strike of the Mururoa-Gambier-Pitcairn volcanic alignment. Hydrothermal activities and recent volcanic flows were observed on the two largest (20 km in basal diameter) and shallowest (60 m and 450 m depth) volcanoes. More than twenty other smaller volcanic edifices (< 500 m in height) were mapped during a Seabeam survey covering an area of about 8000 km2. The edifices from the Pitcairn region are conical with a low degree of flatness (summit/basal diameters ratio < 0.25), and consist of fresh alkali-enriched lava flows. The other truncated edifices with a high degree of flatness (> 0.25) made up of ancient MORB-type tholeiitic rocks are inferred to be inherited from the EPR axial regions. The shallow volcanic activity which occurred on the two largest edifices are classified as: (1) reactive eruptions, with hydromagmatic activities, giving rise to volcanic ejecta, bombs, xenoliths, and ash; and (2) quiet eruptions which formed pillows and/or lobated lavas, and large massive flows. Intrusives (dyke and sill) were observed, cutting through the volcanic ejecta near the summit (< 500 m depth) of the seamounts. The general structural orientations of the rift zones recognized from the bathymetry of individual volcanoes (N160-degrees-180-degrees, N80-degrees, N30-degrees and N120-degrees) are comparable to those observed in the Society and Austral hot-spot regions. These orientations were inferred as corresponding to the structural discontinuities of the ancient oceanic crust, and to the regional stress field of the Pacific plate. Rock samples from the Pitcairn region consist of alkali-basalts, basanites, trachyandesites, and trachytes which are closer in chemical composition to some of the volcanics from the Society rather than to those from the Austral hot-spot regions.