Revisiting the geodynamical history of the so-called uplifted atolls, north-west Tuamotu, French Polynesia, central South Pacific

Abstract

In the Tuamotu Archipelago, central South Pacific, some north-western atolls – e.g. Makatea, Mataiva, Tikehau, Rangiroa, Kaukura, Niau, and Anaa – exhibit raised reef limestones, locally at unusual elevations above present sea level. These islands were formerly interpreted to have been primarily uplifted by a lithospheric flexure generated by the load of nearby Society high volcanic edifices (Tahiti, Moorea). A reappraisal of previous field data and additional uranium-series, ESR and strontium isotope dating of the exposed reef limestones reveal the flexure-induced load model failed to account for the tectonic evolutionary history of the 7 uplifted north-west Tuamotu atolls, mentioned above. A new geodynamical scenario is proposed, implying the episodic thermal rejuvenation of the Society hotspot swell, thus generating an asthenospheric bump. At Makatea, Mataiva, Tikehau, Rangiroa and Kaukura, the basements and reef-rims were known to be Miocene in age. At Makatea, the closest island to the bump centre, the upper plateau peaks at present at 113 m above present mean sea level (pmsl), while on the other atolls, in peripheral positions, remnants of reef limestones are at 2.5 to 12 m above pmsl. At Niau, the atoll reef-rim, made up of a Pliocene core, locally overlapped by Late Pleistocene reef units, peaks at +7.5 m. At Anaa, the reef-rim is strictly composed of Late Pleistocene reef deposits at elevations of about +4–5 m. Makatea, Mataiva, Tikehau, Rangiroa and Kaukura, resting on the Pacific plate, have shifted northwestwards and as such entered the influence zone of the hotspot swell, approximately two million years (Ma) ago. The hotspot reheating phase that generated Moorea and Tahiti edifices, resulted in the uplift of Makatea – at estimated long-term rates of about 0.16 to 0.20 mm/yr – and of the nearer atolls. By contrast, the most south-eastern ones (Niau and Anaa), at that time, located further south-east from the hotspot swell, are assumed to have continued subsiding according to expected standard crust cooling conditions. From approximately 0.5 Ma to 0.3 Ma, due to their continuous motion northwestwards, the most north-western islands ended up escaping the hotspot influence and started to re-subside. At the same time, Niau and Anaa Atolls, as subjected increasingly to the hotspot reheating effects, presumably in relation to the activity of sub-aerial Tahiti-Iti and Mehetia volcanoes, stopped subsiding and presumably were slightly uplifted prior to being stabilized. This is supported by the occurrence of Last Interglacial high-stand reefs at present-day elevations close to their presumed eustatic position. The uplift-stabilization-subsidence resumption scenario is consistent with a northwestward motion of islands carried away on a treadmill, i.e. the drifting Pacific plate, running over a rejuvenated mantle plume.