Deformation and veining processes on the subduction zone; example from the Cretaceous Shimanto accretionary complex in Japan

Abstract:

In this study, the authors discuss the relationship between deformation and veining stages in the accretionary complex in order to understand the deformation and fluid flow process during the subduction.

This study deals with the Miyama Formation in the Cretaceous Shimanto accretionary complex located in the Kii Peninsula of Japan. The deformation can be divided three types with four types of vein. Based on the differences, the process can be divided four stage by their cutting relationship as follow. Stage 1; the deformation (D1) in this stage is characterized by the aspect ratio of the deformed clasts with range from 0.1 to 0.4, and pinch-and-swell structure and budinaged structures of the sandstone are developed. The veins (V1) are recognized only within the sandstone, and the veins are cut by muddy matrix. Stage 2; the deformation (D2) is characterized by the aspect ratio is more than 0.4, and is distributed along the unit boundary. The clasts such as sandstone and chert are strongly sheared, and random fabric is often recognized. The vein (V2) in this stage cut the D1, and the V2 and D2 are cut by each other. Stage 3; outcrop-scale faults (D3) cutting D1 and D2 has been progressed in this stage. The vein (V3) is recognized along D3. Stage 4; the vein (V4) cutting D₁ to D₃ is developed in this stage. From the above-mentioned occurrence and data shown by previous studies of the Miyama Formation, process of each Stage interpreted as follows; Stage 1 developed underthrusting, Stages 2 and 3 correspond to underplating of subducted sediments. In addition, it is considered that Stage 4 is in later stage of underplating.

The main components of the V1 to V3 veins are calcite and/or quartz. On the other hand, the V4 vein consists mainly of siderite and ankerite, and contain an abundance of Fe than the other veins. Morphological feature of V1 and V3 veins also show blocky texture suggesting higher rate growth than fracture opening, and V2 vein indicate syn-taxial growth. V4 vein of siderite and ankerite euhedral shape indicate the vein growth under relatively tension stress field. The structural and intra-relationship of these veins suggests the different PT condition as well as the fluids characteistics.