Recognition of Alpine-Type Tectonics and Its Re-Interpretation as Subduction Tectonics In Japan

Japan is now one of the type localities of plate tectonics, in particular for subduction geology & geophysics. However, in the early days of the geological sciences in the late 19th century, most of the ideas were imported from Europe so that the pioneers in Japan tried to apply the concept of nappe tectonics with flysch, molasse and other terms from the Alpine geology to the geology of Japan. Some of these ideas fit the real geology, e.g. large-scale low angle thrusts, thick piles of sediments just before and after the Mesozoic Akiyoshi and Sakawa orogeneses. But, such thrust slices are now considered as the deep expression of high-pressure metamorphic rocks in the accretionary prisms. Findings of asymmetrical oceanward polarities and vergence of metamorphic & deformed rocks (including paired metamorphic belts) have led researchers to believe that most geological features previously recognized as the products of Alpine tectonics are in fact Pacific-type. Clastic rocks that are rich in island arc components, in either case arkose or greywacke with quartz, feldspar (plagioclase), andesitic-rhyolitic volcanic rock fragments, are thick trench-fill turbidites, rapidly deposited in a short, restricted period of time (Late Cretaceous, middle Eocene, and Miocene-Pliocene-Quaternary). These clastic materials were supplied from the penecontemporaneous collisional orogenies in the vicinity, transported along the trench axial channel, then incorporated into the accretionary prisms. The best example is the late Neogene-Quaternary Nankai & Miura-Boso prisms under the sea and on land (respectively), whose clastics have been derived from the Izu arc collision to the west in the Nankai case and to the east in the Miura-Boso along the Philippine Sea plate subduction boundary. Therefore, the subduction tectonics may reflect some collisional orogenesis as for the thick clastic deposit supply to the trench wedges to be interpreted as molasse.