Reconstructing Jurassic‐Cretaceous Intra‐Oceanic Subduction Evolution in the Northwestern Panthalassa Ocean Using Ocean Plate Stratigraphy From Hokkaido, Japan

Plate reconstructions of the Panthalassa Ocean typically portray a simple system of diverging plates surrounded by active margins, yet geological and seismic tomographic records demonstrate that intra-oceanic subduction existed. Here, we reconstruct the plate tectonic evolution of the pre-Cretaceous intra-oceanic Oku-Niikappu island arc, remnants of which are exposed on Hokkaido, Japan. This arc formed at a Jurassic subduction zone separating two oceanic plates: the Izanagi Plate and the here proposed ‘Izanami’ Plate. The Oku-Niikappu arc was previously shown to have gone extinct in an intra-oceanic setting, was subsequently (hyper)extended, and overlain by Berriasian cherts. The extinct arc remained on the Panthalassa ocean floor for ∼45 Myr until its ∼100 Ma accretion to Hokkaido, revealing an original position far from the continental margin and likely above the previously identified Telkhinia slabs. We show that arc extinction coincided with a northwestern Panthalassa plate reorganization recorded by a ∼30° change in spreading direction, and that extinction and subsequent extension of the arc is straightforwardly explained by subduction of the Izanami-Pacific ridge followed by continued divergence between the Izanagi and Pacific plates. From our reconstruction, it follows that the outer zone of Japan, to which the accretionary complex in which the Oku-Niikappu Complex resides belongs, was separated from the inner zone by a back-arc basin during the Early to mid-Cretaceous. This study illustrates the value of accretionary orogens in the development of plate reconstructions of lost oceanic plates and ancient continental margins, particularly when combined with seismic tomographic and marine geophysical data sets.