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Reinterpretation of seismic profiles and DSDP drilling results, together with dredging data from the Japan Trench and Nankai Trough areas, NW Pacific, shows that the so‐called oceanic crust is composed of PreCambrian continental crust, consisting partly of Proterozoic orthoquartzite. This is supported by paleogeographic evidence, which requires the presence of large continental masses on the present Pacific side of the Japanese islands during Paleozoic‐Mesozoic‐Paleogene times, as source areas for the large volume of sediments carried into the Japanese part of the Tethys. Most of the present NW Pacific was either subaerially exposed or was partly very shallow sea during Paleozoic to Early Mesozoic times, and first became deep sea about the end of the Jurassic. These conclusions require the fundamental revision of plate‐tectonic models for the geological development of the island arcs and trenches in the NW Pacific region.

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