Soulèvement et tectonique de l'île d'Efaté (Vanuatu) arc insulaire des Nouvelles-Hébrides, au cours du Quaternaire récent. Datations de terrasses soulevées par la méthode

Abstract The uplift of Vanuatu (formerly New Hebrides) is related to the subduction of the Australian plate beneath the Pacific plate. The tropical latitude of these volcanic islands favours important Quaternary growth of coral reefs. During the periodic uplifts, several terraces emerged, forming typical features of the landscape. When eustatic sea-level rise is compensated by the uplift, relative sea level is therefore constant and favours development of terraces. Efate, situated near the centre of Vanuatu Archipelago, is related to this series of uplifts and reef crest facies lie discordantly on a volcanic substratum. Thirty five new U Th datings of corals in growth position confirm high eustatic sea levels at about 82,000, 103,000 and 125,000 yrs B.P., and perhaps at 140,000 yrs B.P. The amplitude of the vertical movements is estimated quantitatively from a geomorphological study of the terraces, complemented by the dates and the positions of the corals. A calculation of the average uplift rates, permits the distinction of four major areas on the western part of Efate; the numerous faults in these areas also favour minor, local uplifts. The northwestern area is affected by a differential uplift which, at about 180,000 yrs B.P., formed a broad terrace sloping northward from 80 to 50 m. The younger (about 103,000 and 125,000 yrs B.P.) and the older (300,000 yrs B.P.) terraces are narrower. Generally, the uplift was not uniform during any particular stage of sea level. The average uplift rate here ranges from 0.2 to 0.6 mm/yr. The western area is the best known and shows four prominent major Pleistocene terraces. Their ages range from 103,000 to 300,000 yrs B.P., but the latter age is uncertain due to the method used and to the high rate of calcification of the samples. Most of the faults are radially oriented and perpendicular to the terraces forming Samoa Point. The average uplift rate in the western area is 0.9–1 mm/yr more than in the north. The southwestern area displays two remarkable series of terraces ranging from 50 to 110 m in altitude and dating from 86,000 to 130,000 yrs B.P. The average uplift rate is 0.8 mm/yr. The southern area , close to Port Vila, is very low in altitude (as low as 50 m) and bears three terraces dated at 40,000, 85,000 and 125,000 yrs B.P. The uplift rates were 1 mm/yr from 40,000 yrs B.P. to the Present, but before this time were 0.3 mm/yr. Altitudes as a function of age display a remarkable clustering. The levels given by a frequency diagram confirm the stages of sea level previously defined by other authors. These results also coincide with the curve of changes in sea level. The different average uplift rates between high stages in sea level are shown and appear to be variable. They change from one area to the other, comparatively slowly between 103,000 and 10,000 yrs B.P. and generally faster from 10,000 yrs B.P. to the Present (a possible effect of a recent, local, tectonic uplift). These periodic uplifts are less important here than in the northern islands of Santo and Malekula, which are affected by the subduction of d'Entrecasteaux Ridge.