Size and aspect ratio distributions of blocks in a mélange of the Shimanto accretionary complex, southwest Japan

Abstract:

Block-in-matrix fabrics in accretionary complex have variety based on their deformation processes. We report characteristics of blocks in the Hioki mélange, which constitutes part of the youngest (late Oligocene to early Miocene) portion of the Shimanto accretionary complex, Shikoku, southwest Japan. The Hioki mélange is regarded as a tectonic mélange which has formed at a few km depth below the surface (e.g., Hibbard et al., 1993; Underwood et al., 1993).

 The ample exposure of the mélange in the study area provides a good opportunity to observe detailed structure of the block-in-matrix fabrics. The matrix, which is partly folded, is composed of dark gray shale and some green tuff and dark brown shale. Foliations of the matrix, which are manifested by cleavages, roughly strike ENE–WSW and steeply dip mostly toward the north. Blocks consist mainly of sandstone. The size of blocks ranges from millimeter to meter in length of major axes. These blocks are divided into brecciated, boudinage, and playted types on the basis of their shapes. Each type of blocks can be observed throughout the study area. We measured size and aspect ratio of blocks that are more than 10 cm in length of major axes. The average and standard deviation of the block size are 28.5 cm and 41.0 cm, respectively. The median of the aspect ratio is 2.0. To compare with the characteristics of blocks, we also measured length of the major axis and the aspect ratio for blocks of a sedimentary mélange (the Cretaceous Ukibuchi Formation), which also constitutes part of the Shimanto accretionary complex. The Ukibuchi Formation consists mainly of dark-gray shale matrix containing sandstone blocks. The average and standard deviation of block size are 12.2 cm and 8.6 cm, respectively. The median of aspect ratio is 1.6. Each parameter is lower than those of blocks in the Hioki mélange.