CLIMATIC AND TECTONIC CONTROLS ON LATE QU ATERNARY REEF GROWTH IN NEW-CALEDONIA

Sedimentological and stratigraphic analysis of about 40 sub-surface co res drilled through the reefs of New Caledonia (especially fringing re efs) provides valuable data on the processes of reef recolonization fo llowing the last postglacial sea-level rise, and on the vertical tecto nic behaviour of the island over the past 125,000 years (i.e. the late Pleistocene period). Holocene reefs in New Caledonia are not older th an 8.5 ka, most probably because the sea surface temperature around th e island (Lat. 19-23 degrees S) was formerly too cold to allow signifi cant reef development. This suggests that the intertropical belt had m arkedly narrowed during the last glacial period, but, following the de glaciation, the southern tropical limit shifted progressively southwar d to include New Caledonian latitudes. The fringing reef which develop ed during the last interglacial high sea-level stand, 125 ka ago, is t oday uplifted and lies along some 30 km of coast in the area of Tara/Y ate, in the SE of the island. There, it reaches a maximum elevation of 10 m, while the present-day barrier reef is deeply submerged (around -15 to -20 m). Near Hienghene (east coast), a double system of two not ches is markedly deformed by a bulge, but is much more localized (3 km long) than in the Yate area, with a maximum uplift of 13 m of the upp er double notch system (interpreted as having formed during the last i nterglacial event). Relics of the 125 ky fringing reef are emergent at various locations in the Bourail region (west coast). However, their altitudes are lower than that generally admitted (+6 m) for their cons truction at 125 ka, thus most probably reflecting a slight subsidence of the area. Elsewhere, the 125 ka fringing reef underlies the Holocen e reef: in the SW of the island, in particular, the Holocene - Pleisto cene unconformity is observed at -6 m. In areas of higher subsidence r ates, such as the NW or NE of the island, the 125 ka fringing reef may be more deeply buried. In that case, the Holocene reef rests directly on a metamorphic or sedimentary substratum. Within the barrier reef b uild-up itself, the 125 ka reef flat is overlain by a Holocene sequenc e, whose thickness depends on local subsidence rates. The overall sett ing of the 125 ka reef around New Caledonia reflects an increasing - t hough irregular - tendency for island subsidence northward and south-w estward from a relatively more stable, possibly slightly uplifting, ce ntral zone, that includes the Bourail area to the west and the Yate/Ta ra and Hienghene areas to the east. Subsidence rates increase even mor e markedly seaward, as, for instance, in the Noumea lagoon or at the Y ate barrier reef. The New Caledonian ridge seems to have undergone con tinuous warping over the past 125,000 years. This longitudinal and tra nsversal warping is, in fact, characteristic of the vertical tectonic behaviour of the ridge since Miocene times. The observation of notches , raised beach-rocks or coral reefs (dated at around 5,500 yr) uplifte d up to 1 to 1.5 m above MLWS reflects the existence of a hydro-isosta tic rebound. Traces of this rebound disappear in areas of high subside nce rate, illustrating the action of local tectonics.