Transportation of H<sub>2</sub>O in the NE Japan Subduction Zone as Inferred from Seismic Observations

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  • HASEGAWA Akira
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • NAKAJIMA Junichi
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • KITA Saeko
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • TSUJI Yusuke
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • NII Kyohei
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • OKADA Tomomi
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • MATSUZAWA Toru
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • ZHAO Dapeng
    Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University

Bibliographic Information

Other Title
  • 地震波でみた東北日本沈み込み帯の水の循環
  • 地震波でみた東北日本沈み込み帯の水の循環--スラブから島弧地殻への水の供給
  • ジシンハ デ ミタ トウホク ニホン シズミコミタイ ノ ミズ ノ ジュンカン スラブ カラ トウコ チカク エノ ミズ ノ キョウキュウ
  • ―スラブから島弧地殻への水の供給―
  • Supply of H<sub>2</sub>O from the Slab to the Arc Crust

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Abstract

 Transportation of H2O from the slab to the arc crust by way of the mantle wedge is discussed based on seismic observations in the northeastern Japan subduction zone. A belt of intraslab seismicity, perhaps caused by dehydration of eclogite-forming phase transformations, has been found in the Pacific slab crust at depths of 70-90 km parallel to iso-depth contours of the plate interface, showing the major locations of slab dehydration. H2O thus released from the slab may be hosted by serpentine and chlorite just above the slab and is dragged downward. DD seismic tomography detected this layer of serpentine and chlorite as a thin S-wave low-velocity layer. Serpentine and chlorite thus brought down to a depth of 150-200 km should decompose there. H2O released by this dehydration decomposition is then transported upward and encounters the upwelling flow directly above, which perhaps causes partial melting of materials within the upwelling flow. Seismic tomography studies have clearly imaged this upwelling flow as an inclined sheet-like seismic low-velocity zone at depths of 30-150 km in the mantle wedge subparallel to the subducted slab. This upwelling flow finally meets the Moho below the volcanic front, and melts thus transported perhaps stagnate directly below the Moho. Some of them further migrate into the crust, and are also imaged by seismic tomography as low velocity areas. Their upward migration and repeated discharge to the surface form the volcanic front. Seismic tomography study of the mantle wedge further revealed along-arc variations of the inclined low-velocity zone: very low velocity areas appear periodically every ∼80 km along the strike of the arc in the backarc region of northeastern Japan above which clustering of Quaternary volcanoes and topography highs are located, suggesting that melts could segregate from these very low velocity areas in the upwelling flow and rise vertically to form volcanoes at the surface in the backarc region.

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