Transition from pelagic to continental margin settings recorded in the Sorachi Group, Hokkaido, Japan: Reconstruction of arc-back arc system in the Mesozoic NW Pacific.

The Sorachi-Yezo belt in central Hokkaido, northern Japan, is characterized by the occurrence of Jurassic oceanic basalts and Early Cretaceous siliceous and volcanogenic deposits (Sorachi Group) and overlying Cretaceous terrigeneous sediments (Yezo Group). The Sorachi Group is likely to be remnants of an oceanic crust that occupied a part of the Mesozoic NW Pacific Ocean, and is important for reconstructing the plate configuration at that time. We examined the original depositional site and drift history of the group recorded in the stratigraphy and clastic composition of the sediment covering the basalts.

In the Naegawa area, the Sorachi Group consists of two stratigraphic units (S1: basalt lavas, and S2: mainly sedimentary rocks), and S2 is subdivided into four sub-units (S2a-S2d). S2 is conformably overlain by the Yezo Group, whose lower sections comprise Ly1 and Ly2 units. We determined precise sedimentary ages at seven horizons by zircon U-Pb method from S2b (139 Ma) to Ly1 (119 Ma). Tithonian-Berriasian fossils were obtained from the red chert in S2a. In the Nunobe, we divided rocks exposed in this area into N1 unit (HP metabasalt of subduction complex) and structurally overlying N2 (N2a-N2d) sedimentary rocks of Sorachi Group.

S1, S2a, and S2b in the Naegawa area never contain continent-derived clasts suggesting pelagic environments. S2c and S2d are regarded as hemi-pelagic because tuff beds contain accidental Precambrian zircons. Ly1 sandstones consisting mainly of continent-derived clasts were deposited on the Eurasian continental margin. They can be regarded as a kind of oceanic plate stratigraphy, which indicates approaching of an ocean basin to a continent. Therefore the Sorachi Group probably had a convergent relative motion with respect to the Eurasian continent. Geochemistry of volcanogenic clasts in S2b and N2c suggests that there existed an oceanic island arc near the Sorachi-Yezo basin as the sedimentary provenance. Since the S1 basalts have MORB composition, it is probable that the Sorachi Group was a back-arc basin. It is likely that the young Sorachi oceanic plate was the hanging wall, beneath which the old major oceanic plate was being subducted. Therefore, there might have existed an oceanic plate with an arc-backarc system distinct from any of the Eurasia and major oceanic plates of the Pacific.