Anatomy and genesis of a subduction‐related orogen: A new view of geotectonic subdivision and evolution of the Japanese Islands

Abstract

Abstract The Japanese Islands represent a segment of a 450 million year old subduction‐related orogen developed along the western Pacific convergent margin. The geotectonic subdivision of the Japanese Islands is newly revised on the basis of recent progress in the 1980s utilizing microfossil and chronometric mapping methods for ancient accretionary complexes and their high‐P/T metamorphic equivalents. This new subdivision is based on accretion tectonics, and it contrasts strikingly with previous schemes based on‘geosyncline’tectonics, continent‐continent collision‐related tectonics, or terrane tectonics. Most of the geotectonic units in Japan are composed of Late Paleozoic to Cenozoic accretionary complexes and their high‐PIT metamorphic equivalents, except for two units representing fragments of Precambrian cratons, which were detached from mainland Asia in the Tertiary. These ancient accretionary complexes are identified using the method of oceanic plate stratigraphy. The Japanese Islands are comprised of 12 geotectonic units, all noted in southwest Japan, five of which have along‐arc equivalents in the Ryukyus. Northeast Japan has nine of these 12 geotectonic units, and East Hokkaido has three of these units. Recent field observations have shown that most of the primary geotectonic boundaries are demarcated by low‐angle faults, and sometimes modified by secondary vertical normal and/or strike‐slip faults. On the basis of these new observations, the tectonic evolution of the Japanese Islands is summarized in the following stages: (i) birth at a rifted Yangtze continental margin at ca 750–700 Ma; (ii) tectonic inversion from passive margin to active margin around 500 Ma; (iii) successive oceanic subduction beginning at 450 Ma and continuing to the present time; and (iv) isolation from mainland Asia by back‐arc spreading at ca 20 Ma. In addition, a continent‐continent collision occurred between the Yangtze and Sino‐Korean cratons at 250 Ma during stage three. Five characteristic features of the 450 Ma subduction‐related orogen are newly recognized here: (i) step‐wise (not steady‐state) growth of ancient accretionary complexes; (ii) subhorizontal piled nappe structure; (iii) tectonically downward‐younging polarity; (iv) intermittent exhumation of high‐P/T metamorphosed accretionary complex; and (v) microplate‐induced modification. These features suggest that the subduction‐related orogenic growth in Japan resulted from highly episodic processes. The episodic exhumation of high‐P/T units and the formation of associated granitic batholith (i.e. formation of paired metamorphic belts) occurred approximately every 100 million years, and the timing of such orogenic culmination apparently coincides with episodic ridge subduction beneath Asia.