Evolution des volcans en systeme de point chaud : ile de tahiti, archipel de la societe (polynesie francaise)
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Abstract:
Les datations k-ar realisees sur tahiti-nui (ile principale de tahiti, archipel de la societe) montrent une activite volcanique comprise entre 1,37 et 0,19 ma (vitesse moyenne de construction de l'edifice aerien de 0,38 cm/an). Cette construction se fait en 3 phases: un edifice bouclier se developpe jusqu'a 0,87 ma (vitesse moyenne d'empilement de 1 mm/an). A 0,87 ma, se produit un effondrement des flancs nord de l'edifice, provoque par la rift-zone mise en evidence dans le centre de l'ile par la teledetection, les observations de terrain et la gravimetrie. A partir de 0,87 ma, un volcan bouclier secondaire se construit dans la depression laissee par l'effondrement (vitesse moyenne d'empilement de 1 cm/an). Des 0,64 ma, les produits du volcan bouclier secondaire qui ont comble la depression viennent en remplissage des vallees du sud de l'edifice. Apres une periode de repos de 0,46 a 0,23 ma, des laves post-erosionnelles sont emises dans le nord de tahiti-nui. Au cours de la construction de tahiti-nui, les laves emises evoluent progressivement vers des termes de plus en plus alcalins. Cette evolution implique deux sources isotopiquement distinctes: une source 1, saturee en silice avec des rapports th/ta deux fois superieurs a ceux des morb et une composante em ii predominante, et une source 2 sous-saturee en silice, enrichie en ta (rapports th/ta plus proches de ceux des morb) avec une plus grande contribution du pole morb. Les datations des laves sous-marines des edifices de la zone de point chaud montrent une activite se developpant dans la zone depuis au moins 980 ka. Ceci implique une activite simultanee sur tahiti et dans cette zone pendant au moins 500 ka. La zone de point chaud a donc diminue de diametre (de 200 a 140 km). A l'interieur de la zone, le volcanisme migre vers le nord-ouest alors qu'il a migre vers le sud-est entre la construction de tahiti et mehetia (vitesse de migration de 10,8 cm/an)Cite
Mantle plume
Pacific Plate
Hotspot (geology)
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The main motivation for Integrated Ocean Drilling Program Expedition 310 to the Tahitian Archipelago was the assumption that the last deglacial sea‐level rise is precisely recorded in the coral reefs of this far‐field site. The Tahitian deglacial succession typically consists of coral framework subsequently encrusted by coralline algae and microbialites. The high abundance of microbialites is uncommon for shallow‐water coral reefs, and the environmental conditions favouring their development are still poorly understood.
Environmental change
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The internal structure and growth pattern of Tahiti reefs over the last 14 ka is reconstructed using sedimentological, morphological and palaeobiological data coupled with radiometric dates in drill cores through the modern barrier reef. Flooding of the volcaniclastic deposits or the karst surface of a Pleistocene reef started at ≈ 14 ka BP, and coral growth began shortly after inundation. The sequence in the Tahiti barrier‐reef edge has formed predominantly through long‐term keep‐up growth controlled by stable environmental conditions, while the adjacent backreef deposits did not start to accumulate before sea‐level stabilization, around 6 ka. The dominance of Porites communities and the coeval occurrence of branching gracile Lithophyllum in the lowermost part of the postglacial reef sequence (14–11 ka) suggest the prevalence of uniformly moderate‐ to low‐energy conditions and/or growth in slightly deeper waters all over the drilled area during the early reef stages. During the last 11 ka, the reef frameworks developed in a high‐energy environment, at maximum water depths of 5–6 m, and were dominated by an Acropora robusta/danai–Hydrolithon onkodes association; the local interlayering of other coralgal assemblages (dominated by tabular Acropora or domal Porites ) reflects distinct diversification stages, resulting either from the palaeotopographic control of the substrate or from slight and episodic environmental changes.
Acropora
Porites
Fringing reef
Dominance (genetics)
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The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW‐SE, parallel with the general trend of the Society Islands hotspot track. Rocks from this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K‐Ar radiometric dating we have established the age of volcanic activity. The oldest lavas ( ∼1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and a late‐stage, valley‐filling phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO 2 , evolved basalts (Mg number (Mg# = Mg/Mg+Fe 2+ ) 42–49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene‐phyric alkali basalts (Mg# 60–64), while the later valley‐filling lavas are basanitic (Mg# 64–68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+clinopyroxene+spinel). Isotopic compositions also change systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield‐building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K‐Ar ages and magnetostratigraphic boundaries allow high‐resolution age assignments to this ∼0.7‐km‐thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0.9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield‐building stage of volcanism, assuming that it was in equilibrium with Fo 89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25 to 50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognize a quasi‐periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25±10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield‐building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB‐like. We propose that the Society plume is radially zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal and compositional radial gradient ensures that greater proportions of melting occur over the plume center than its margins. The changing composition of Tahitian magmas results from lithospheric motion over this zoned plume. Magmas erupted during the main shield‐building stage are derived mainly from the hot, incompatible element‐enriched central zone of the plume; late‐stage magmas are derived from the cooler, incompatible element‐depleted, viscously coupled sheath. A correlation between Pb/Ce and isotope ratios suggests that the Society plume contains deeply recycled continental material.
Xenolith
Shield volcano
Incompatible element
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Indo-Pacific
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Shield volcano
Trace element
Radiogenic nuclide
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Coralline algae
Pocillopora damicornis
Porites
Indo-Pacific
Acropora
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We present evidence for an ancient and voluminous subaerial landslide of the southern flank of Tahiti, Society Islands. During a marine geophysical survey in 1996, submarine mass wasting deposits were mapped as far as 60 km from the southern shore of the island. Acoustic imagery reveals a surface of 2950 km² of debris avalanche and hummocky terrain. Based on bathymetric data, the volume of the debris is estimated to be 1150 km³. In comparison with Hawaiian and other oceanic island landslides, it can be classified as a giant, rapid and cataclysmic event. Tahiti morphology and the distribution of volcanic ages over the island strongly suggest that the slope failure initiated near the top of the southern island flank, between 650 and 850 ka. The landslide scar was subsequently filled by eruptions. The estimated volume of the subaerial removed material exhibits a large discrepancy with the volume of the submarine deposits that can be explained by recurrent slide events.
Subaerial
Mass wasting
Submarine landslide
Flank
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