Abstract
Cretaceous forearc basin deposits of the Nitarachi-Yezo Sequence overlie the high-P/T metamorphosed Iwashimizu accretionary complex in the Mitsuishi River area of the southern Kamuikotan Zone. In the sequence, the Albian Utafue Formation (Middle Yezo Group) unconformably overlies the Valanginian-Barremian Iwashimizu Complex. The basal part of the Utafue Formation comprises a fluvial sequence, which contains greenstone clasts with high-P/T metamorphic minerals such as Na-amphibole and Na-pyroxene. The conglomerate also contains Valanginian-Barremian mudstone clasts derived from the Nitarachi-Yezo Sequence. The fluvial conglomerate clearly shows that the Iwashimizu Complex and lower part of the Nitarachi-Yezo Sequence were exposed subaerially in the Albian and were supplying clasts to the forearc. The Iwashimizu Complex, the high-grade part of which was metamorphosed at a depth of over 20km, is thus concluded to have been rapidly exhumed within ca. 20Ma after peak metamorphism. The complex was exhumed as a coherent mass, not as a serpentinite diapir. The exhumation is inferred to have resulted from lifting by underplating of abundant seamount volcanics, and by increased buoyancy of subducted oceanic lithosphere, which became younger during the Cretaceous.
