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This paper presents an experimental study on the local bed morphology and grain size characteristics around an impermeable spur dyke.A series of experiments are conducted with different types of sedime... 相似文献
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Remelting of an Andesitic Crust as a Possible Origin for Rhyolitic Magma in Oceanic Arcs: an Example from the Izu-Bonin Arc 总被引：10，自引：0，他引：10
The Izu–Bonin volcanic arc is an excellent example ofan intra-oceanic convergent margin. A total of 1011 chemicalanalyses of 17 Quaternary volcanoes of the arc are reviewedto estimate relative proportions of magmas erupted. Basalt andbasic andesite (SiO2 < 57 wt %) are the predominant eruptiveproducts of the Izu–Bonin arc, and rhyolite (SiO2 >70 wt %) forms another peak in volume. Such rhyolites possesscompositions identical to those of partial melts produced bydehydration-melting of calc-alkaline andesites at low pressure(<7 kbar). Meanwhile, the major element variation of theShirahama Group Mio-Pliocene volcanic arc suite, Izu Peninsula,completely overlaps that of the Quaternary Izu–Bonin arcvolcanoes, and groundmasses of Shirahama Group calc-alkalineandesites have compositions similar to those of Izu–Boninrhyolites. Moreover, phenocryst assemblages of calc-alkalineandesites of the Shirahama Group resemble restite phase assemblagesof dehydration-melting of calc-alkaline andesite. These linesof evidence suggest that the rhyolite magmas may have been producedby dehydration-melting of calc-alkaline andesite in the upperto middle crust. If so, then the presence of large amounts ofcalc-alkaline andesite (3–5 times more abundant than therhyolites) within the oceanic arc crust would be expected, whichis consistent with a recently proposed structural model acrossthe Izu–Bonin arc. The calc-alkaline andesite magmas maybe water saturated, and would crystallize extensively and solidifywithin the crust. The model proposed here suggests that rhyoliteeruptions could be triggered by an influx of hot basalt magmafrom depth, reheating and partially melting the calc-alkalineandesite component of the crust. KEY WORDS: bimodal magmatism; calc-alkaline andesite; oceanic arcs; rhyolite 相似文献
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SHOJI FUJII NORIYUKI NASU ALEC J. SMITH NORIO FUJI YOSHIHIKO MIZUTANI MISABURO SHIMAKURA KENJI KONISHI CHIAKI IGARASHI JUNRYO MURAMOTO TOSHIO TAKEMURA TAKASHI SHIMODA SAM BOGGS JR. KANTARO FUJIOKA HARUNA MAPPA HOTAKA KAWAHATA YONG SAE KONG TAKEO TANAKA 《Boreas: An International Journal of Quaternary Research》1986,15(4):265-277
Remnants of a submerged ancient forest have been found on the continental shelf of NW Japan off the Kurobe alluvial fan in Toyama Bay. The remains, standing tree stumps and roots, are located at depths between 20 and 40 m and ages of 8,000 years B.P. to 10,000 years B.P. have been determined by the 14 C method. Alnus and Salix are the most commonly preserved genera and the trees grew on the now sub-merged seaward margin of the alluvial fan. Pollen analysis permits an estimate of palaeo-temperatures 2°C to 4°C lower than at present. Diatoms give some indication of salinity variations at the time of forest development and suggest brackish water, possible lagoonal conditions. The distribution of the stumps and roots suggests that there were probably two separate forests which were overwhelmed by rising sea-level and flash flood deposits. The present day submarine exposures may be due to recent submarine erosion related to the seaward movement of flood waters which passed from the fan into the upper reaches of the Toyama submarine canyon system. 相似文献
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