New insight into a subduction-related orogen: A reappraisal of the geotectonic framework and evolution of the Japanese Islands Yukio Isozaki a, ⁎, Kazumasa Aoki b,1 , Takaaki Nakama b , Shuichi Yanai c a Department of Earth Science and Astronomy, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan b Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan c Japan Geo-communications, Yotsuya, Tokyo 160-0004, Japan abstract article info Article history: Received 22 October 2009 Received in revised form 20 February 2010 Accepted 24 February 2010 Available online 10 March 2010 Keywords: Subduction Accretion Blueschist Tectonic erosion Orogeny 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. © 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction The Japanese Islands form a bow-shaped chain of islands over 3000 km in length along the eastern margin of Asia. Among the many modern island arc systems in the world, the Japanese Islands likely represent one of the best analyzed examples because several significant and classic concepts in geology and geophysics were proposed on the basis of direct observations from Japan. These include the Wadati (-Benioff) plane (Wadati, 1935), volcanic front (Sugimura, 1965), paired metamorphic belts (Miyashiro, 1961), Pacific-type orogeny (Matsuda and Uyeda, 1971), structure of modern/ancient accretionary complexes (Kanmera, 1976; Taira et al., 1989; Matsuda and Isozaki, 1991), anatomy of accretionary orogen (Isozaki and Maruyama, 1991; Isozaki, 1996), and wedge extrusion of high-pressure metamorphic unit (Maruyama et al., 1996), among other themes. Ever since Naumann (1885), the nearly 150 year-long intensive geologic mapping of the surface geology clarified that the majority of the Japanese crust is composed of strongly deformed sedimentary rocks (dominant sandstone and mudstone with minor volcanics, chert, and limestone), metamorphic rocks (crystalline schist, gneiss), and granitic plutons that penetrated the former two (e.g., Geological Survey of Japan, 2003). Such intense deformation, associated regional metamorphism, and granitic intrusion confirm that the Japanese Islands belong to a segment of a long-existing old orogenic belt, as traditionally discussed by many geologists in terms of geosyncline or of plate tectonics. In plate tectonic terminology, the active magma- tism, high seismicity, and frequent crustal faulting in modern Japan prove that the islands currently form an active subduction-related orogenic front. From a historical viewpoint, the geological studies in Japan prior to the 1980s are categorized in the “non-science phase” and “colonial science phase” in the framework of typical pattern in transplanting scientific ideas from Western Europe to the rest of the world (Basalla, 1967). However, there were several exceptional pioneering geologists (e.g., Miyashiro, 1961; Matsuda and Uyeda, 1971; Uyeda and Miyashiro, Gondwana Research 18 (2010) 82–105 ⁎ Corresponding author. E-mail address: isozaki@ea.c.u-tokyo.ac.jp (Y. Isozaki). 1 Currently at Department of Earth Science and Astronomy, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan. 1342-937X/$ – see front matter © 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2010.02.015 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr