Transpressional tectonics of the Mineoka Ophiolite Belt in a trench–trench–trench‐type triple junction, Boso Peninsula, Japan

Abstract

Structures developed in metamorphic and plutonic blocks that occur as knockers in the Mineoka Ophiolite Belt in the Boso Peninsula, central Japan, were analyzed. The aim was to understand the incorporation processes of blocks of metamorphic and plutonic rocks with an arc signature into the serpentinite mélange of the Mineoka Ophiolite Belt in relation to changes in metamorphic conditions during emplacement. Several stages of deformation during retrogressive metamorphism were identified: the first faulting stage had two substage shearing events (mylonitization) under ductile conditions inside the crystalline blocks in relatively deeper levels; and the second stage had brittle faulting and brecciation along the boundaries between the host serpentinite bodies in relatively shallower levels (zeolite facies). The first deformation occurred during uplift before emplacement. The blocks were intensively sheared by the first deformation event, and developed numerous shear planes with spacing of a few centimeters. The displacement and width of each shear plane were a few centimeters and a few millimeters, respectively, at most. In contrast, the fault zone of the second shearing stage reached a few meters in width and developed during emplacement of the Mineoka Ophiolite. Both stages occurred under a right-lateral transpressional regime, in which thrust-faulting was associated with strikeslip faulting. Such displacement on an outcrop scale is consistent with the present tectonics of the Mineoka Belt. This implies that the same tectonic stress has been operating in the Boso trench-trench-trench-type triple junction area in the northwest corner of the Pacific since the emplacement of the Mineoka Ophiolite. The Mineoka Ophiolite Belt must have worked as a forearc sliver fault during the formation of a Neogene accretionary prism further south.

Depositional setting, provenance, and tectonic-volcanic setting of Eocene–Recent deep-sea sediments of the oceanic Izu–Bonin forearc, northwest Pacific (IODP Expedition 352)

“…The IzuBonin-Mariana arc indented the pre-existing active margin of the Eurasian plate, which would be expect to result in right-lateral (tectonic escape) structures along the northwest margin of the indentor (Koyama and Kitazato 1988;Figure 19(c)). Field evidence indicates that right-lateral strike-slip/transpression, exemplified by Riedel shear fabrics, did indeed dominate the Miocene-recent structural development of preexisting accretionary melange (Mineoka melange) (Ogawa and Takahashi 2004;Mori and Ogawa 2005;Mori et al 2011).…”

“…The Eurasian plate active margin including older accretionary mélange (Mineoka mélange) and forearc basin sediments (Shimanto Group) was covered by heterogeneous clastic sediments (Sakuma Group) during the mid-Miocene (c. 15 Ma), as indicated by the presence of the large foraminifer, Lepidocyclina sp. (Mori and Ogawa 2005;Ogawa and Sashida 2005;Ogawa et al 2008). Basic tuffaceous rocks that are interpreted to have been derived from the Izu-Bonin arc first appear in the Honshu forearc basin in the Boso Peninsula (Amatsu Formation) during the early late Miocene (13-12 Ma) Saito 1997, 1999).…”

Cretaceous to Miocene NW Pacific Plate Kinematic Constraints: Paleomagnetism and Ar–Ar Geochronology in the Mineoka Ophiolite Mélange (Japan)

“…Arai et al., 1991). The overall highly fragmental 'mélange'‐like occurrence may be related to its emplacement mechanism at a trench–trench–trench triple junction (Mori & Ogawa, 2005). The fragments of oceanic lithosphere rock and metamorphic rocks crop out as tens to hundreds of meters sized blocks within a serpentinite distributed all over the MOM (Figure 2).…”