Temporal change in subducted oceanic crusts along the NE Asian margin in the Mesozoic: Insights from accreted oceanic rocks in NE Japan.

Because the Pacific plate and its counterpart plates have been moving northwestward since the Mesozoic, great uncertainties of oceanic plate paleogeography exist between the NE Asian margin and the remaining parts of the Pacific plate rewound to the south-central Pacific in the Mesozoic. Recent progress of plate reconstruction by improved reference frames and tomography are approaching to fill the great vacancy, however, the fully comprehensive reconstruction might be achieved when such a model is in agreement with, or constrained by geological records. Fragments of oceanic basalts and sediments tectonically accreted to active continental margin could provide reliable constraints on basement ages, topography, geochemistry, and geographical position for reconstruction of the lost oceanic plates. When temporal change in these characteristics is traced in a single area, the data set can be a pin for every snapshot of a time-resolved plate reconstruction. This paper tries to show a current version of the summarized temporal change in subducted oceanic plate natures based on accreted oceanic rocks in NE Japan.

From mid-Cretaceous to early Paleogene, the age of subducted oceanic crust became progressively younger, and MORB basalts erupted and intruded in Paleogene trench sediments. Because this sequence is compatible with plate reconstructions in which the Izanagi-Pacific ridge approached to and was finally subducted beneath the Eurasian margin, the oceanic crust subducted in this period could be assigned to the Izanagi plate. In the Late Jurassic to Early Cretaceous, at least two distinct oceanic basins of differing ages are assumed to have coexisted. One is >100 my old oceanic crust, which left Triassic and Permian limestones and cherts. The other is a Late Jurassic oceanic basin with MORB basement (Sorachi Group) and ophiolites with island arc characteristics (including boninites) with similar ages. Some of the arc rocks are overlain by either pelagic or hemipelagic sediments. These lines of evidence suggest the existence of an intraoceanic convergent plate boundary(-ies), which terminated at a triple junction(s) along the Eurasian margin. The older basin could have comprised parts of the Izanagi plate, which was probably subducted beneath both the Eurasia plate and the younger oceanic basin taking structural relations of ophiolites and accretionary complexes into account.