STRUCTUAL EVOLUTION OF MÉLANGE UNIT IN THE UPPER CRETACEOUS SHIMANTO BELT, WAKAYAMA PREF., JAPAN

The Shimanto Belt is distributed along the Pacific side of southwest Japan. It is widely accepted that is preserved the earliest deformation of accretionary processes of oceanic plate subduction in the circum Pacific region. The Shimanto Belt comprises alternative units of coherent flysch and tectonic mélange unit. The Hidakagawa Subbelt of the northern part Shimanto Belt was indicated that the successions of Cretaceous to Tertiary age by radiolarian biostratigraphy. Mélange unit in Miyama Accretionary Complex, the upper Cretaceous Hidakagawa Subbelt was studied by means of regional field mapping, sedimentary facies and structural analyses. Using these methods structural style of sheared argillaceous rocks and flysch units were delineated.

 The mélange unit is composed of a sheared argillaceous “matrix” in which various sizes of tectonic lenses are incorporated. The tectonic lenses are intensely disrupted sandstone, mudstone, varicolored shale. The mélange structure seem to indicate the sinistral sense of shear due to an asymmetric orientation of the tales of the lenticular sandstone surrounded by scaly shale matrix and asymmetric varicolored shales. The asymmetric structure is characterized by boudin structure, pinch & swell structure and composite planar fabric. The deformation of sheared soft sediments are composed of highly sheared with layer of Y directions (Y-layer) and layer of P directions (P-layer oblique to Y-layer). A remarkable fact regarding the Y-layer is that composite planar fabric, Y-shears and P-foliations are developed by the deformation of sheared soft sediments. A comparative study between the first deformation structure and the later structures be able to present origin of mélange and mélange unit consist of sinistral sense of shear by the deformation of soft sediments.