In situ pressure–temperature conditions of a tectonic mélange: Constraints from fluid inclusion analysis of syn-mélange veins |
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| Authors: | Y Hashimoto M Enjoji A Sakaguchi G Kimura |
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| Institution: | Department of Natural Environment Science, Faculty of Science, Kochi University, Akebono 2-5-1, Kochi, 780-8520 Japan (email: ),;Department of Earth Science, School of Education, Waseda University, Nishiwaseda 1-6-1, Sinjuku-ku, Tokyo, 169-8050 Japan,;The Institute for Frontier Research of Earth Evolution, JAMSTEC, Yokosuka, 215-0016 Japan and;Institute of Earth Planetary Science, Faculty of Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 113-0033 Japan |
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| Abstract: | Abstract Pressure and temperature (P–T) conditions of mélange formation are estimated from fluid inclusions within “syn‐mélange” veins developed in the necks of boudins of sandstone blocks in the mélange of the Shimanto accretionary complex, south‐west Japan. The mélange records décollement‐zone processes. P–T conditions are in the range of 81 (+15) to 235 (±18) MPa and 150 (±25) to 220 (±31)°C. Assuming a constant fluid‐pressure to lithostatic‐pressure ratio for each data set, we estimate a P–T gradient of between 10.0°C/km (+0.2/?1.5) (lithostatic pressure) and 4.2°C/km (+0.1/?0.9) (hydrostatic pressure) from these results. The estimated lithostatic P–T gradient is much lower than that calculated from the age of the subducting oceanic plate. The estimated P–T conditions suggest that the mélange was formed within the seismogenic zone (hypothesized from thermal modeling), although the deformation mechanism of mélange (i.e. dominant diffusive mass transfer mainly in shale matrix with minor brittle breakage mainly in sandstone blocks) does not show evidence of seismic deformation. In addition, at the time of syn‐mélange vein formation, a shale matrix of mélange has injected into the vein, which indicates a ductile deformation of shale. A possible explanation for this discrepancy is that the mélange was formed during the interseismic period. |
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| Keywords: | accretionary complex fluid inclusion P–T condition seismogenic zone |
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