ANCIENT SHORELINES AND QUATERNARY VERTICAL MOVEMENTS ON RURUTU AND TUBUAI (AUSTRAL ISLES, FRENCH-POLYNESIA)

Rurutu is a basaltic island surrounded by raised limestone blocks, reaching over 100 m in elevation, forming high vertical cliffs. Marine notches, cut on the cliffs at several levels, mark the position of former shorelines. Tubuai, located some 200 km east of Rurutu, is an island with low coasts only, suggesting a long-term slow subsidence. In this paper, marks of ancient sea levels are investigated and interpreted taking into account the present marine zonation and the regional eustatic variations. At Rurutu, morphological marks of Holocene emergence reach +1.7 m in altitude and traces of sea-level stands are visible also near +1.2 m and +0.6 m. At Tubuai, the maximum Holocene emergence observed does not exceed 0.65 +/- 0.1 m. It is inferred that Rurutu has been uplifting at the average rate of 0.05 to 0.10 mm/yr since the early Pleistocene and at about 0.17 mm/yr in the Holocene. Tubuai, on the other hand, is affected, like Tahiti and Moorea, by a subsidence movement slightly faster than in most Tuamotu atolls.