Dating of a fault zone distributed in the South Kyushu shear zone based on fission-track thermochronology and U-Pb dating

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  • フィッション・トラック熱年代解析およびU-Pb年代測定に基づいた南九州せん断帯に分布する破砕帯の活動時期
  • フィッション ・ トラック ネツ ネンダイ カイセキ オヨビ U-Pb ネンダイ ソクテイ ニ モトズイタ ミナミキュウシュウ セン ダンタイ ニ ブンプ スル ハサイタイ ノ カツドウ ジキ

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Abstract

<p>Fission-track (FT) and U-Pb thermochronometries were applied to rock samples collected from the South Kyushu shear zone for constraining when the zone was active its timing of activity. Two samples were collected from thermally undisturbed portions of the Shibi-san Granodiorite, and one sample was collected along an aplite dike which was cut by a fault zone. The samples yield zircon U-Pb ages of 14.0-13.5 Ma, zircon FT ages of 14.7-13.7 Ma, and apatite FT ages of 12.6-10.0 Ma. were obtained from the three samples. The ages and thermal inversion results show that the Shibi-san Granodiorite intruded into a relatively shallow depth at 14.0-13.5 Ma and cooled below the partial annealing zone of the apatite FT system (60-130℃) within a few million years. The Hirabae and Kusubae rock types of the Shibi-san Granodiorite have similar formation ages and cooling histories. Based on meso-microscopic observations of various directions of sense of shear in the fault zone, the aplite dike was displaced multiple times during and immediately after its intrusion, at temperatures of more than ~200℃. Apatite FT data show that dike intrusion and the series of fault movements occurred at 10.0±0.8 Ma. The fault movements seem to be related to multiple tectonic movements at ca. 10 Ma, rather than to fracturing during the later stage of intrusion and cooling of the Shibi-san Granodiorite body at 14.0-13.5 Ma. Applying a similar approach to the study of further minor faults may provide more detailed understanding of the tectonics of the shear zone over geologic time.</p>

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