New insight into a subduction-related orogen: A reappraisal of the geotectonic framework and evolution of the Japanese Islands

The geotectonic framework and the evolutionary history of the Japanese Islands need revision in accordance with the various geophysical/geological evidence gathered by new methodologies in the recent years including seismic tomography, vibroseis/ground-breaking seismic experiments, and detrital zircon chronology. These investigations have addressed various themes such as: 1) seismic profile of the crust and mantle beneath the Japanese Islands, 2) high-precision ages of the protoliths of high-P/T metamorphic rocks, and 3) provenance of terrigenous clastics. The results have led to a number of important findings including: 1) detection of a large mass of slab around the mantle boundary layer suggesting the long-term oceanic subduction beneath Japan, 2) confirmation of the subhorizontal piled-nappe structure for the entire crust of Japan, 3) finding a new high-P/T metamorphosed accretionary complex unit that represents the youngest blueschist in Japan, 4) finding of the oldest (Early Cambrian) arc batholith and cover sediments, and 5) the identification of plural arc batholiths which have already been erased from the surface. Based on a synthesis of these new data, this article presents a re-evaluation of the conventional geotectonic subdivision of the subduction-related orogen in Japan, re-definition of the elements and their mutual boundaries, and re-consideration of the geotectonic evolution of the Japanese Islands. In particular, the historical change in provenance suggests that proto-Japan has experienced large-scale tectonic erosion in multiple stages, and the corresponding large amounts of continental crust materials were subducted. For understanding the orogenic growth of Japan during the last ca. 500 million years, the significance of tectonic erosion coupled with continental contraction, as well as the oceanward accretionary growth, requires further attention.