MISSING OCEAN PLATE STRATIGRAPHY: A KEY TO UNDERSTANDING CONTINUOUS VERSUS DISCONTINUOUS ACCRETIONARY COMPLEX FORMATION IN THE PALEOZOIC AND MESOZOIC RECORD OF SOUTHWEST JAPAN

Subduction – accretion processes occurred to form the basement of Japan since about 500 Ma. Oceanic plate subduction is a major tectonic process driving arc volcanism and uplift of active continental margins that in turn provide clastic sediments to a trench. Such sediments are commonly preserved as turbidites or their metamorphic equivalent, psammitic schist, within ancient accretionary complexes. If these processes and feedback mechanisms operate continuously, we would expect to see uninterrupted accretionary complex formation – growth. The modern Japanese Islands are lacking a record of Early to Middle Paleozoic accretionary complex construction in their Paleozoic through Neogene history. The SW Japan accretionary complexes mainly comprise in an ascending order Ocean Plate Stratigraphy (OPS) of various ages, including basalt – limestone of seamount origin, pelagic ribbon chert, hemipelagic siliceous shale and trench turbidites. Such stratigraphy becomes generally younger toward structurally lower parts. There are, however, some gaps of Ocean Plate Stratigraphy have been known from the Microbiostratigraphy of pelagic-hemipelagic rocks has shown the apparent occurrence of major gaps in OPS between the Late Paleozoic and Jurassic, and then between the Jurassic and Late Cretaceous accretionary complexes. However, recent U-Pb detrital zircon dating of these accretionary complexes has revealed that psammitic schists in the Suo metamorphic complex contain Early to Middle Triassic detrital zircons, whereas psammitic schists in the Sambagawa metamorphic complex contain Early Cretaceous detrital zircons. These new findings recover the “missing ages” in the accretionary complex history of Japan, and indicate continued subduction with intermission of accretion during those time intervals of Triassic and early Cretaceous. This discovery points out the significance of exhumation processes in accretionary complex development through time that may be driven by tectonic erosion, oceanic ridge subduction, and volcanic arc and/or oceanic plateau collision.