生駒断層帯周辺における露頭規模での脆性変形構造  [in Japanese] Meso-scale brittle deformation structures in and around the Ikoma fault zone  [in Japanese]

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Abstract

生駒断層帯において,白亜紀花崗岩類中に発達する露頭規模の断層群に対する構造地質学的解析を行った.断層群のスリップデータに基づく多重逆解法により求められた古応力場は,南北引張の正断層型であった.また,断層コアのガウジから分離したイライトのK-Ar年代は,45.2±1.0Ma(粒径0.2-0.4µm)および46.0±1.1Ma(粒径0.4-1.0µm)となった.各粒径区画におけるK-Ar年代値や低温イライトの含有率に有意な差が認められなかったため,母岩の黒雲母K-Ar年代を用いて砕屑性高温イライトの影響を考慮した結果,断層コア形成およびその後の熱水変質による年代として~45-30Maを得た.これらの結果は,生駒断層帯における白亜紀花崗岩類中に発達する露頭規模の断層群の形成が始新世後期〜漸新世前期の伸長テクトニクスに関連したものであることを示唆する.

We report the results of field observations and microstructural analyses of meso-scale faults that cut Cretaceous granitoids along and near lineaments of the Ikoma active fault zone. Based on measurements of the slip direction on fault planes, a paleostress of N-S extension is determined using the multiple inverse method. This stress field differs from the present-day regional stress field of predominant E-W compression, inferred from the seismic data inversion and the multiple inverse method applied to the active faults. Illite crystals of various sizes collected from a fault gouge, give K-Ar ages of 45.2±1.0 Ma for the 0.2-0.4 µm fraction and 46.0±1.1 Ma for the 0.4-1.0 µm fraction. High- and low-temperature components of illite are quantified by decomposing X-ray diffraction patterns to extrapolate the authigenic and detrital end-members. The ratios of low-temperature illite (1M<sub>d</sub>) to high-temperature illite (2M<sub>1</sub> and 1M) from a fault gouge are indistinguishable from one another. Based on the relative K-Ar ages of illite composites and of biotites from the host granitoid, the upper age limit of fault gouge formation is ca. 30 Ma. These results suggest that formation of the fault gouge occurred between ~45 and 30 Ma, indicating that the meso-scale faults in basement rocks formed by N-S extension during the late Eocene to early Oligocene, rather than active tectonics under E-W compression.

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 122(2), 61-74, 2016

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130005157154
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • ISSN
    0016-7630
  • NDL Article ID
    027162898
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  J-STAGE 
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