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LEGENDRE C.; MAURY R. C.; CAROFF M.; GUILLOU H.; COTTEN J.; CHAUVEL C.; BOLLINGER C.; HEMOND C.; GUILLE G.; BLAIS S.; ROSSI P.; SAVANIER D. 《Journal of Petrology》2005,46(9):1925-1962
On the basis of the first systematic mapping of Ua Pou, longknown for its exceptionally abundant phonolites, we estimatethat these rocks cover 65% of the surface of the island whereasmafic lavas cover 27% and intermediate ones 8%. The silica-undersaturatedsuite was erupted in a restricted time span (2·9–2·35Myr), following the emplacement of tholeiites derived from ayoung HIMU-type source at c. 4 Ma. Primitive basanites, derivedfrom a heterogeneous mantle source with a dominant EM II + HIMUsignature, represent likely parental magmas. The series is characterizedby a Daly gap defined by a lack of phonotephrites. We considerthat the most likely model for the origin of evolved lavas ispartial melting at depth of primitive basanites, leaving anamphibole-rich residuum and producing tephriphonolitic magmas.These tephriphonolitic magmas may have evolved by closed-systemfractional crystallization towards Group A phonolites. Threeother groups of phonolites could have been derived from tephriphonoliticmagmas by open-system fractional crystallization processes,characterized respectively by seawater contamination (GroupB), assimilation of nepheline syenite-type materials (GroupC) and extreme fractionation coupled with assimilation of theunderlying oceanic crust (Group D). The prominence of evolvedlavas is a consequence of their origin from partial meltingof mafic precursors followed by crustal contamination. KEY WORDS: Marquesas; French Polynesia; phonolite; partial melting; contamination 相似文献
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Geophysical observations demonstrate that the archipelagic apron surrounding the Marquesan hot-spot volcanoes is derived almost entirely from mass wasting processes. Seismic reflection and refraction data constrain the volume of the apron sediments to approximately 200,000 km3, with thicknesses reaching over 2 km in the deep portions of the moat near the edge of the volcanic edifice. Seismic velocities average 4 to 5 km s–1 in the sediments, and 6 km s–1 at the top of the underlying basement. Single channel seismic profiles show acoustically chaotic cores in the sediments of the apron, which are interpreted as debris flows from mass wasting events. We deduce that the apron is formed by catastrophic collapses that may involve volumes over 100 km3 tens to hundreds of times during the lifetime of a volcano. Comparison with similar data from the Hawaiian Islands yields the result that the total volume of volcanics and their derived sediments along the strike of the chains is only slightly smaller for the Marquesas, implying comparable eruption rates. However, the ratio of sediment to surface volcanic load is much larger for the latter, leading to an overfilled moat in the Marquesas and an underfilled moat at Hawaii. The much larger size of the Hawaiian islands can be explained as the combined effects of a higher thermal swell, loading a stiffer elastic plate, and proportionately less mass wasting. 相似文献
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GREG DENING 《Geographical review》2007,97(2):288-301
ABSTRACT. Two thousand years ago, or thereabouts, a double canoe sailed on a northeast tack (or maybe a southeast tack) from a Homeland (Hawaiki) among the islands of Samoa, Tonga, and Fiji. After a voyage of 7,000 kilometers, which bypassed the many as yet uninhabited islands of the central Pacific (such as Tahiti) and the stretch of the seventy atolls of the Tuamotu that spread umbrellalike across the eastern entry of the Pacific, the canoe landed on islands that the Spaniards in 1595 were to call “the Marquesas.” The descendants of these first settlers call their islands “Fenua'enata” (Land of the People). Here I tell the story of this first beach crossing after what I consider to be the most remarkable voyage of discovery and settlement in all of human history. These first settlers (shall we say a dozen adults?) brought the animals and food plants that would make their island inhabitable. More mysteriously, these voyagers were—in body, mind, and spirit—all that we have come to call “Polynesian” in the great triangle of Hawai'i, Aotearoa (New Zealand), and Rapa Nui (Easter Island). “Sea of Islands” is the name the descendants of this first voyage prefer to call that great triangle. I here celebrate a Sea People's mastery of their Sea of Islands. 相似文献
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Raphaël Paris Simon Falvard Catherine Chagué James Goff Samuel Etienne Pascal Doumalin 《Sedimentology》2020,67(3):1207-1229
X-ray tomography is used to analyse the grain size and sedimentary fabric of two tsunami deposits in the Marquesas Islands (French Polynesia, Pacific Ocean) which are particularly exposed to trans-Pacific tsunamis. One site is located on the southern coast of Nuku Hiva Island (Hooumi) and the other one is on the southern coast of Hiva Oa Island (Tahauku). Results are compared with other techniques such as two-dimensional image analysis on bulk samples (particle analyser) and anisotropy of magnetic susceptibility. The sedimentary fabric is characterized through three-dimensional stacks of horizontal slices (following a vertical step of 2·5 mm along the cores), while grain-size distribution is estimated from two-dimensional vertical slices (following a step of 2 mm). Four types of fabric are distinguished: (a) moderate to high angle (15 to 75°); (b) bimodal low-angle (<15°); (c) low to high angle with at least two different orientations; and (d) dispersed fabric. The fabric geometry in a tsunami deposit is not only controlled by the characteristics of the flow itself (current strength, flow regime, etc.) but also sediment concentration, deposition rate and grain-size distribution. There is a notable correlation between unimodal high-angle fabric – type (a) – and finely-skewed grain-size distribution. The two tsunami deposits studied represent two different scenarios of inundation. As demonstrated here, X-ray tomography is an essential method for characterizing past tsunamis from their deposits. The method can be applied to many other types of sediments and sedimentary rocks. 相似文献
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The classic hotspot hypothesis [Morgan, W. J., 1971. Convection plumes in the lower mantle. Nature 230, 42–43], which posits that linear volcanic chains are traces of fixed plumes in the mantle on moving lithospheric plates, was instrumental in elevating the plate tectonics paradigm in the 1960s into a modern Earth Science theory. The hypothesis itself, however, remains conjectural because many of its predictions, particularly the simple age-progressive type of volcanism, are not observed in many linear volcanic chains. As an alternative explanation, it is proposed that linear volcanic chains are formed through magmatism along pre-existing lines of weakness such as transform zones and old sutures, or along cracks created by stresses on lithospheric plates. The Marquesas linear volcanic chain in south-central Pacific has geologic features that are consistent with some of the predictions of both hypotheses. To better constrain the origin of this volcanic chain, we collected major and trace element and Sr, Nd, Pb, and He isotopic data from several Marquesan lavas. Our new analyses combined with literature data classify the samples into the well established tholeiitic to mildly alkalic, low 87Sr/86Sr, high 143Nd/144Nd, shield-building volcanic phase lava group and highly alkalic, high 87Sr/86Sr, low 143Nd/144Nd, post-shield phase group. Lead isotopes show generally higher 206Pb/204Pb ratios and suggest evidence of crustal assimilation for the shield-building phase lavas, consistent with the argument that the shield-building phase volcanism has a lithospheric source component. On the other hand, post-shield phase lavas that are predicted to represent the true composition of the mantle source by the hotspot hypothesis have higher 3He/4He ratios and these are coupled to other geochemical tracers. Thus our results show that the Marquesas volcanic chain, similar to many other linear volcanic chains, has a high 3He/4He component in its mantle source. The presence of such a distinct source component cannot be easily explained by dispersed upper mantle heterogeneities, but provides a powerful constraint for the hotspot origin of many linear volcanic chains. 相似文献
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