Analysis of mesoscopic deformation structures in mélange with special respect to the stages of syn- and post- sedimentation : A case study in the Southern Chichibu Terrane in eastern Shikoku, Southwest Japan

Abstract

In this study, the term "mélange" is used as describing several muddy matrical rocks that are characterized by a chaotic, relatively disturved,"block in matrix" fabric. To find a clue to clarify the origin and the mechanism of the mélanges in the Southern Chichibu Terrane in Southwest Japan, the author examined the analyses of the mesoscopic deformation structures of mélanges at Warabiishi coast in eastern end of Shikoku, where widely crops out the Early Cretaceous succession including muddy chaotic rocks with many blocks of limestone. chert and greenstone. The results can be summarized as follows. 1. The deformation stages of the Warabiishi mélange is subdivided. into three stages. 1) The first stage characterized by rupturing of limestone blocks to form jigsaw-puzzle structure under the condition of high confining- and high pore water- pressures possibly caused by mud-diapirism before the succession of the Warabiishi Unit formed. 2) The second stage is the syn-sedimentary deformations which occur mixing by grabity flows and slidings of non- and semi- consolidated deposits along the lower continental slope to form olistostromes and slump deposits with turbidites. 3) The third stage is the formation of shear structus such as right-stepping en échelon shear fractures (Riedel shear fractures with compressive bridges) and clockwise-back-tilted sigmoidal blocks with ductile shear tails by E-W trending left-lateral simple shearing on condition that the sediments are relatively dewatered and consolidated after the sedimentation of the Warabiishi unit has finished. 2. Based on the sedimentary structures and facies analyses, Warabiishi area of the Southern Chichibu Terrane in Early Cretaceous age is situated in southward inclined lower continental slope with submarine fans where the sediments are supplied by sediment gravity flows as such turbidites and olistostromes partly originated in mud-diapears. 3. The faults controling the formation of southward facing imbricate structure of the Southern Chichibu Terrane moved not only as thrusts but also as E-W trending left-lateral simple shearig faults according to the presence of the above Riedel shear fractures and sigmoidal blocks of the third stage in the Warabiishi mélange along the Butsuzo Tectonic Line. 4. The above-mentioned tectonic features in addition to the southward younging of the mélanges age in the Southern Chichibu Terrane are concordant phenomena with the accretional zone settigs of active convergent margin of the plate tectonics model