Reef-rim structure and building history, Rangiroa, an uplifted Atoll, French Polynesia: The role of morphotectonics and extreme marine hazard events
Résumé
Reconstructing the responses of low-lying, atoll-rim islands to future environmental changes due to global warming requires a robust knowledge of the building history of reef rims through time. In the northwest of Tuamotu Archipelago, atolls have experienced opposite vertical motions (uplift versus subsidence) in response to differential tectonic constraints undergone by the underlying oceanic crust. As a result, during Pleistocene high sea levels, some atolls remained emerged whereas others remained flooded. Based on two electrical resistivity tomography profiles across the northern rim-island of the uplifted Rangiroa Atoll and on U/Th dating of corals clasts collected from unconsolidated sand-rubble deposits within two wells through the same island area, the internal structure of the atoll-rim and its mode and timing of building are tentatively reconstructed. The irregularly shaped, upper boundary of the basal resistive unit is interpreted as residual, pinnacle-shaped reliefs of Miocene age. At the seaward rim side, the uppermost highly resistive layers, an about 5 m-thick, were assumed to be reef-related, partly consolidated material emplaced during the Holocene. Thicker resistive layers that infill cavities in the Miocene basement, in the central part of the atoll rim, refer to the fresh water lense in porous, more or less consolidated material (phosphorites?) presumably emplaced during the Mio-Pliocene. In northern Rangiroa, the age of the oldest coral clasts trapped into the atoll-rim islands ranges between about 11,000 and 8000 calendar years when sea level was approximately 60 to 20 m below its present position. This suggests that coral-clast supply from the adjacent forereef slopes was triggered by extreme wave-surge events (presumably, tsunamis generated by island flank failures). The earliest island initiation phase is thought to start by approximately 6000 cal. yr and to be achieved by about 3000 cal. yr. By constrast, on the nearby subsiding Takapoto Atoll, the rim island began to initiate not earlier than 2500 cal. yr. At Rangiroa, morphotectonics is assumed to have played a major role in controlling the internal rim architecture and, associated to extreme marine hazard events, to have promoted the earlier settlement of low-lying, rim islands through an earlier infilling of accommodation space as compared to nearby subsiding atolls.