TECTONO-SEDIMENTARY DEFORMATION OF PLIOCENE BOSO TRENCH SLOPE, JAPAN

The Pliocene Chikura Group in the Boso Peninsula, south of Tokyo, is composed of many fold and thrust structures in E-W trend. It is developed in the Paleo-Sagami trough and trench slope, close to the Boso TTT-type triple junction.The structural geology, stratigraphy and sedimentology revealed that the lowermost Shirahama Formation and the upper, Shiramazu (NE part), Mera (SW part) and other younger formations are structurally controlled by the N-S compression during forearc development. The Shirahama Formation, mainly characterized by pyroclastic deposits derived from the Izu volcanic arc at the base, is interfingered gradually upward with the Shiramazu Formation of turbiditic sequence of interbedded siltstone and scoriaceous sandstone. Significant deformation structures such as meso-scaled thrust-related anticlines, small scale thrusts, folds and contractional duplexes along many basal detachment faults, which are associated with methane-supported biocommunities, Calyptogena, are developed. Associated synsedimentary structures such as contorted beds, liquefied beds and debris flow units are also recognized, all of them occur soon after sedimentation. Indicators for paleoslope direction such as convolute and cross laminations from the Shiramazu Formation show SW, whereas NE in the Mera Formation, suggesting that these formations are deposited in the trench slope basin, the Shiramazu Formation in the SW dipping slope and Mera Formation in the NE dipping slope. From the geometry of thrust-related anticlines and duplexes, these deformed structures are considered to be piled up at the toe of a large scale gravity driven sedimentary stack, concurrently occurring with a large scale thrust at the edge of the basin. Thus the whole Chikura Group is thrust-bounded trench slope basin in the context from trench to trench slope dynamics, in which a fold and thrust belt is ongoing with sedimentation and deformation.