Two deformation phases and two uplifting stages of the Jurassic Mizunashigawa metamorphic rocks, Ashio Belts, central Japan.

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  • ジュラ紀付加体足尾帯，水無川変成岩類における２つの変形時相と２段階の上昇過程
  • ジュラキ フカタイ アシオタイ ミズナシガワ ヘンセイガンルイ ニ オケル 2ツ ノ ヘンケイジソウ ト 2 ダンカイ ノ ジョウショウ カテイ

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Abstract

The Mizunashigawa metamorphic rocks considered to be the deformed low grade metamorphic rocks of the Ashio Belt regarded as Jurassic accretionary complex show excellent overprinting relationships of deformed minor and micro-structures of ductile to brittle deformation. The results of deformation facies analysis of these structures represent two regular deformation sequences in time and space. In the temporal sequence, deformation grade decreases with time from sheath-intrafolial fold facies to kink band facies. In the spatial sequence, deformation grade decreases from zone I at the axial part of a major syncline to zone IV at the western limb, which are defined by assemblages of the deformed minor and micro-structures. Furthermore, temporal deformation sequence can be divided into two deformation phases depending on each stress field induced from the deformed minor structures.Two regular deformation sequences and two deformation phases suggest the emplacement process of the Mizunashigawa massif from the deeper to the shallower in the subduction zone, which can be elucidated by the earlier deformation with high mean ductility formed under the sinistral shear regime in response to NE directed movement of the oceanic plate (phase 1), and the later deformation with low mean ductility formed under dextral shear regime, NE-ENE directed movement of Hida Nappe (phase 2). Phase 1 and Phase 2 deformations might have resulted from the oblique subduction of the oceanic plate and the oblique up-thrusting being reactivation of Hida Nappe, respectively.

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