Accretion and exhumation structures formed by deeply subducted seamounts in the Kamuikotan high-pressure/temperature zone, Hokkaido, Japan

[1] The Lower Cretaceous Iwashimizu complex in the southern Kamuikotan zone, Japan, was formed by accretion of seamounts, which subducted to the lawsonite-albite to blueschist facies. The complex is a pile of four nappe units. Each unit comprises a duplex with horses of seamount volcanics sedimentary rocks and ramp shear zones, suggesting a thin-skinned peeling of subducted seamounts. Each unit-bounded fault is regarded as the floor thrust of an overlying duplex and the roof thrust of an underlying duplex, reflecting a downward progression of underplating accretion. The structurally upper Pirashuke unit was metamorphosed at higher pressure than the structurally lower units. This inverted arrangement of metamorphic grade can be explained by shallowing of progressively younging subducted slab. Structurally upper units were exhumed synchronously with growth of the structurally lower units. This synaccretionary exhumation can be explained by repetition of undercritical duplex-forming compression and overcritical extensional unroofing during seamounts subduction.