Physical and Mechanical Properties of the Tertiary Sedimentary Rocks in the Shimanto Belt in Shikoku.

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In recent years, detailed study of trench and forearc geologic processes have become increasingly important to understand the crustal evolution of continents. Geological and paleontological investigations of the onland area in the Shimanto Belt have demonstrated its origin of subduction-related accretionary process. However, not much structural studies that disclose structural features anticipated in a major thrust belt have been done until the present.The Upper Shimanto Supergroup in the Muroto Peninsula and the Hata Peninsula has been studied on the point of the structural geology. As a result, it is revealed that the structural features of accretionary complex in the Upper Shimanto Belt is characterized by that of the large thrust belt, such as multi-stacking faults, duplex and imbricated structures. Various classes and styles of folds basically provide example of folds in a single thrust sheet.The Upper Shimanto Supergroup in Shikoku is structurally divided into the coherent unit a … More nd the melange unit. The coherent unit is characterized by a turbidite sequence of weakly deformed, alternated beds of sandstone and mudstone. It is subdivided into two units of the offscraped accretionary complex and the slope sediments overlying on it. The mixing process of the melange can be regarded as the result of large accumulations of gravitational slides in the shallow level of the accretionary prism or at the underplating process in the deeper levels of the accretionary prism.Sandstones of these units are examined on the point of the physical and mechanical properties. The sandstones of the slope sediments have lower densities, lower P-wave velocities and higher prosities than those the accretionary complex. While sandstone blocks in the melange unit have physical values between those of the accretionary complex and slope sediment units. On the contrary, Young's modulus of sandstone in the melange unit is well higher than previous two units. The triaxial compression test indicates that the deformational mode of the slope sandstones changes to the those of ductile flow in the field more than 50MPa C.P., whereas those of the accretionary sandstone is still brittle failure in the field more than 200MPa C.P. Two types of texture are differentiated in the blocks of the melange sandstone. One is the cataclastic deformation characterized by the "web structure" and the other is the soft sediment deformation which was caused by the liquefaction and mixing with a surrounding pelitic matrix. Viewed in the light of physical and mechanical properties, the Miocene melange unit can be regarded as the deformed olistostrome in the slope basin which contains sandstone blocks derived from both the accretionary basement and the slope sediments itself. Less