Kinematics of mylonitic rocks along the Median Tectonic Line, Akaishi Range, central Japan

Abstract

The Ryoke Belt along the Median Tectonic Line (MTL) in the Akaishi Range is composed of quartz diorite, granodiorite and metasedimentary rocks in amphibolite facies. Microstructural evidence reveals that the quartz diorite and most of the metasedimentary rocks in the study area suffered a ductile shear deformation or a mylonitizaton. The shear surface is subhorizontal and the shear direction of the hanging wall trends N-S, based on the attitude of foliation and lineation of mylonitic rocks. Shear sense is deduced from microstructures such as mica “fish” and asymmetric tails on porphyroclasts; they indicate relative southward displacement of the hanging wall. Comparison of deformation features in the described mylonites with those in fault rocks along the MTL, distribution of the mylonites, and kinematics of the mylonites and the MTL lead to the conclusion that the mylonites were mostly formed in a subhorizontal shear zone which was then exposed due to a dip-slip component of faulting on the MTL.