Emplacement Mechanism of the Older Ryoke Granites in the Yanai District, Southwest Japan, with Special Reference to Extensional Deformation in the Ryoke Metamorphic Belt

The Ryoke metamorphic rocks in the Yanai district, southwest Japan show geological and rock structures produced by three different phases of ductile deformation. During the first phase (D1), a distinct foliation parallel to lithologic layering was formed under the thermal peak conditions during the low-pressure facies series Ryoke metamorphism, which is ascribed to the sheet-like intrusion of the older Ryoke granites. The second phase deformation (D2) led to the formation of mylonitic shear zones and nappes. The deformation of the third phase (D3) was responsible for the formation of the upright folds with E-W trending axes.
In the metamorphic rocks of the Tsuzu area, which is placed in the northern part of the Yanai district, are often found melt-filled fractures of minor scales, which cut across their foliation. The deformation related to the formation of these melt-filled fractures resulted commonly from the foliation parallel extension. The overall movement picture inferred from the melt-filled fractures appears to be of the shear sense for the top to the north. The formation of the melt-filled fractures was responsible for the intrusion of the older Ryoke granites. Asymmetric textures such as extensional crenulation cleavage (ECC) and rotation of porphyroblasts, which grew under the thermal peak of the low-pressure facies series metamorphism, are also formed in the metapelites. The shear sense read from the asymmetric textures is the top to the north, though fairly dispersed. This is harmonic with the overall movement picture inferred from the melt-filled fractures. Therefore it can be said that the overall movement picture of the D1. deformation of the metamorphic rocks in the Tsuzu area during and immediately before the intrusion of the older Ryoke granites appears to have been of the same style of extension tectonics. Consequently, the D1 deformation occurred under extensional stress regime and the older Ryoke granites intruded in extensional fracture zones at intermediate crustal depths. The nappes of the metamorphic rocks and older granites were formed during the D2 deformation probably under compressional stress regime.