Formation of the Yakuno ophiolite; accretionary subduction under medium-pressure-type metamorphic conditions

摘要: Abstract The notion that the Yakuno ophiolite and overlying Maizuru Group represents an accretionary prism formed during Permian evolution of Japan on eruptive sequence, association hemipelagic mudstone with silicic tuff, exotic fossiliferous limestones derived from previously accreted sea-mounts, upward coarsening sequences terrigenous sandstone conglomerate, mildly deformed Triassic forearc basin formations. most important indicator, however, is seaward imbrication repetition observed in both itself. D1 deformation structures include axial–planar foliations (pressure-solution cleavage for granulite–amphibolite metamorphic layering ophiolite), flattening type strain, symmetric pressure shadows fringes, isoclinal folds showing thrust faulting at their overturned limb. exceptional asymmetry indicates seaward-directed shearing near thrust, while zone are explained by off-scraping, above basal decollement. later Jurassic D2 kink fold structure includes a first-order asymmetric brittle its limb, more-or-less coeval M2 retrograde metamorphism. Medium-pressure M1 prograde metamorphism produced granulite amphibolite, Group, formation illite along pressure-solution mudstones. grade controlled stratigraphic relationships appears typical ocean floor regions. However, there was only one episode which occurred contemporaneously off-scraping. Given subduction zones normally characterized high P/T regimes, history to reflect relatively unusual conditions. Such thermal gradients may plausibly approach young, hot oceanic plate continued subducting beneath Japanese arc. Accordingly, probably trench–trench–ridge triple junction.