相模湾北縁部，姥島群島の三浦層群の層序と構造

─火砕岩鍵層の追跡による複合デュープレックス構造と断層伝播褶曲─

抄録

 Mapping was carried out of well exposed folded and faulted strata in the Ubajima Islands off Chigasaki City, Kanagawa Prefecture, central Japan of the northernmost part of Sagami Bay, where the present Philippine Sea plate subduction boundary extends beneath the central part of the North America plate. The strata were dated with radiolarian fossils as middle to late Miocene, and correlated with part of the Misaki Formation of the Miura Group of the southern Miura Peninsula. The strata are composed of dominant basaltic pyroclastic or volcaniclastic sedimentary rocks with sporadic felsic tephra layers and rare silty sedimentary rocks, and were defined as the Ubajima Formation. Detailed field mapping of the Ubajima Formation verified that the strata are deformed in a complicated way into duplex structures having two stages. The first stage is characterized by bedding parallel to subparallel thrust faults in the SW direction of the horizontal reference frame. This might be of the antiformal stack type known by many parallel repetitions of the same horizons. At the second stage, the same parts are further duplicated into other antiformal stack-type duplex structures, forming multi-duplex structures. Finally those structures are folded and faulted by a fault-propagation fold system that has strong shear zones at propagation fault zones. The Ubajima Formation is thought to be deposited on the forearc area of the Izu volcanic arc, then accreted to the Honshu side by accretionary prism formation. These structures represent good examples of accretionary prism toes along subduction zones.