DISTRIBUTION PATTERN OF OPHIOLITIC AND METAMORPHIC ROCK BLOCKS IN THE FAULT ZONE OF THE MINEOKA BELT, BOSO PENINSULA, NW PACIFIC

Mineoka ophiolite is a so-called mélange, composed of various rock blocks in fault zones, resting around the trench-slope break position since middle Miocene till present, now working as a forearc sliver fault along the oblique subduction boundary of the Sagami trough, NW Pacific, where is the Izu volcanic arc collision zone to the Honshu arc. It may extend from the southern boundary of the Shimanto belt in the Kanto Mountains through the Miura and Boso Peninsulas, all the way to the Boso TTT-triple junction. Blocks in the fault zones are composed of dominant ophiolitic rocks, ranging from harzburgitic serpentinite, gabbro-dolerite, basalt pillow lavas (mostly MORB in composition with some IAT and WPB), limestone-chert, to tonalite, with considerable amount of Miocene clastic rocks both from the Izu volcanic island arc, Honshu continental crust and ophiolitic derivatives. Some parts of the Shimanto Supergroup slabs of Eocene age (turbidite and siliceous mudstone) are also included. Another important component, although small and minor, are metamorphic rocks of greenschist to amphibolite facies as schist, suggesting a product in a subduction zone. We mapped the distribution pattern of those rocks, particularly small, meter to hundred meters-size blocks as knockers in the fault zones, and analyzed the tectonic implication for the present forearc sliver faults. Basaltic rocks are dominant in the eastern part of the Boso Peninsula, whereas gabbro, tonalite, and alkali basalt are dominant in the western part. We concluded that the overall fault patterns coincide with Riedel shear of dextral sense, with R1 and P shears, well suggesting the transpressional regime, further implying the effect of the present dextral-transpression along the Sagami rough oblique subduction boundary. Several deformation phases within and along the boundaries of those blocks indicate several different types of shearing work for the deformation during the first and second emplacement from the subduction zone to the arc collision zone around the triple junction.