The structure and kinematics of an imbricate stack of oceanic rocks in the Jurassic accretionary complex of Central Japan: an oblique subduction model

Abstract

The Jurassic accretionary complex of the Mino Belt in the Takayama area, Central Japan includes a major unit characterized by abundant bedded chert and basalt slabs. This unit has an imbricate structure, represented by the repetition of stratigraphic layers composed of sandstone, black mudstone, felsic tuff, bedded chert, siliceous claystone, limestone and basalts. Shear zones subparallel to the trend of the slabs occur in the siliceous claystones and basalts. Deformation structures in the shear zones suggest that phyllosilicate-rich layers within the accretionary complex were preferentially weakened due to grain size reduction by cataclasis and pressure solution-accommodated deformation. Restored shear directions in the siliceous claystones and basalts indicate dextral kinematics subparallel to the trench axis. In contrast, mesoscopic asymmetric folding of the bedded chert, indicated from the restored shear, was oblique or normal to the trench axis. To interpret the two distinct shear directions, a model is proposed in which the constituent rocks of this unit were obliquely subducted dextrally along the East Asian continental margin. Due to the oblique subduction, slip vectors were apparently partitioned into trench-orthogonal components in the shallower part of the subduction zone and trench-parallel components in the deeper part.