Microstructure and metamorphy: phase of argillaceous rocks of Chichibu Belt.

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Duplex and out-of sequence thrust can be regarded as the important internal structures of the accretionary complex. In the western part of the Kanto Mountains, the Hamadaira Group of the Southern Chichibu Belt is well distributed. Duplexes are frequently observable at various scales ; duplex has three scale classes, from several centimeters to several meters, from several tens to several hundreds of meters, and from several hundreds of meters to several kilometers. The strata of the investigated area have been duplicated and thickened by duplexes. On the other hand, three out-of-sequence thrusts, the Osutaka-yama, Mikuni-yama, and Ryokami-yama Thrusts, are recognized in the investigated area. These thrusts truncate in-sequence thrusts, and disturb the paleothermal structure of the investigated area. Estimating from the 1C values of argillaceous rocks, it is inferred that the vertical displacement of hanging wall of the Mikuni-yama Thrust corresponds to.2. 2 km in the eastern halfofthis thrust, and is 0.5 km in the western half. Asymmetrical structures, that is, asymmetric trail, S'C structure and duplex are developed in the Hamadaira Group. The slip senses could be determined from these structures : In the investigated area, the deformation pbfese, which is defined by the slip sense, is classified into five phases. Firstphase is top to WSW-SW, second is top to SW, third is top to SSE-SE, and fourth is top to SSW. The slip sense of the phases had been changed twice at least from second phase to fourth one. As the slip sense, which was formed by thrusting in the ancient accretionary complex, was in close association with the relative motion between ancient plates, these slip senses may indicate the ancient plate motion of Izanagi Plate from Middle Jurassic to Early Cretaceous.