Trace element and isotope characteristics of fault rocks and coseismic fluid-rock interactions in fault zones

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  • ISHIKAWA Tsuyoshi
    Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
  • HIRONO Tetsuro
    Department of Earth and Space Science, Graduate School of Science, Osaka University

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Other Title
  • 断層岩の微量元素組成・同位体組成からみた地震時の流体岩石相互作用
  • ダンソウガン ノ ビリョウ ゲンソ ソセイ ・ ドウイタイ ソセイ カラ ミタ ジシンジ ノ リュウタイ ガンセキ ソウゴ サヨウ

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Abstract

In this paper, we review recent progress on an attempt at evaluating coseismic fluid-rock interactions in fault zones on the basis of trace element and isotope analyses of fault rocks. The slip zone rocks from Taiwan Chelungpu fault at 1 km depth exhibit marked decreases of lithium, rubidium, cesium and 87Sr/86Sr and an increase of strontium relative to adjacent host sedimentary rocks. Model calculations reveal that these trace element and isotope spectra were produced by coseismic fluid-rock interactions at >350℃, which may have caused a dynamic decrease of friction along the fault through thermal pressurization. The slip zone rocks from a major reverse fault in the Boso Emi accretionary complex at 1-2 km depth also show similar evidence for coseismic fluid-rock interactions at high temperatures. For the slip zone rocks from the Shimanto accretionary complex in Kure area, which represent rocks of ancient megaspray fault at 2.5-5.5 km depth, the signals derived from high-temperature fluids overlap with those from melting, indicating coseismic fluid-rock interactions followed by frictional melting. These results demonstrate that high-temperature fluid-rock interactions widely occur during seismic slip and geochemical characteristics of the fault rocks are useful indicators of such coseismic events.

Journal

  • Chikyukagaku

    Chikyukagaku 46 (4), 217-230, 2012

    The Geochemical Society of Japan

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