HR: 0830h
AN: T31F-0897    [PDF]
TI: Mid-Cretaceous Exhumation of the Early Cretaceous Kamuikotan Blueschists in Hokkaido, Japan
AU: * Ueda, H
EM: ueta@mvf.biglobe.ne.jp
AF: Department of Geology, Faculty of Science, Niigata University, Ikarashi ni-no-cho 8050, Niigata, 950-2181 Japan
AU: Kawamura, M
EM: mkawa@strati.sci.hokudai.ac.jp
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, North 10 West 8, Sapporo, 060-0810 Japan
AB: Unconformity and fluvial deposits containing blueschist clasts in a Cretaceous forearc basin sequence indicate that a part of the Kamuikotan high-P/T metabasites (Iwashimizu Complex: Valanginian-Barremian) was exhumed in the mid-Cretaceous (Aptian-Albian). The Iwashimizu Complex consists mainly of metabasites originated from subducted seamounts, accompanied by subordinate chert and turbidites. The complex is a pile of nappe units, each of which comprises a duplex of metabasite slices. Metamorphic grade decreases structurally downward from the blueschist facies to the lawsonite-albite facies. Metabasite clasts in the forearc basin were derived limitedly from the epidote-bearing upper unit, and they never contain lawsonite, the characteristic mineral in the lower unit. White-mica K-Ar ages of the structurally lowest units (Watanabe et al., 1994) are coeval to the unconformity event, suggesting that the exhumation of the upper unit and metamorphism of the lower unit contemporarily occurred. The most reasonable explanation may be that the structurally upper blueschist unit was rapidly jacked-up by subsequent underplating accretion of structurally lower unit during the seamounts subduction. Decrease of metamorphic grade structurally downward can be explained by the decrease of subducted slab dip, as suggested by younging of subducted oceanic crust based on fossil ages of oceanic sediments. The Iwashimizu Complex was originally overlain by the Jurassic Lower Sorachi Ophiolite, based on tectonostratigraphic comparison with the area where the blueschists were not exhumed in the mid-Cretaceous. The absence of detritus of the ophiolite origin suggests that the ophiolite must have been removed tectonically, not by sedimentary erosion, during the exhumation. The contrasting tectonism, i.e. structural thickening (duplex) and unroofing (presumable extension), can contemporarily occur during repetitive seamounts subduction, because of the forearc thickening in front of subducting seamounts, coupled with extensional failure in rear of them.
DE: 8105 Continental margins and sedimentary basins
SC: Tectonophysics [T]
MN: 2003 Fall Meeting