Late Paleozoic-Mesozoic tectonic evolution of SW Japan: A review - Reappraisal of the accretionary orogeny and revalidation of the collisional model

This paper makes a review of the interpretations of the tectonic evolution of SW Japan during the last three decades. In the late 1970s, the dominant model was the so-called "Pacific-type orogeny", emphasizing the purported absence of nappes and the contrast with the alpine chains, and interpreting the evolution as due to a steady oceanic subduction since the Paleozoic time. In the 80s, the discovery of the actual structure made of a pile of large thrust sheets led authors to propose collisional models, involving the intermittent underthrusting of buoyant blocks like micro-continents. At the same time, the use of high-resolution biostratigraphy allowed several authors to recognize ancient accretionary wedges, with a reconstructed ocean plate stratigraphy of individual accreted units, especially in the Tanba and Shimanto zones. Also, precise radiometric dating permitted the distinction of metamorphosed units, especially in Sanbagawa and Shimanto belts. As a result of these new data, since the 1990s, the plate tectonic interpretation of the history of the Japanese Islands was revised by Japanese scientists and presented again in terms of accretionary processes linked to a steadily oceanic subduction, with an episodic ridge subduction: the so-called "Miyashiro-type orogeny". The review of different data leads to the following conclusions. The structure of SW Japan is made of a pile of sub-horizontal nappes, polydeformed, with a geometry similar to the one encountered in collisional orogens. The geodynamic mechanisms advocated for the tectonic building within the accretionary orogeny concept (Miyashiro-type orogeny) are inappropriate. A permanent oceanic subduction with the intermittent "collision" (actually subduction) of an active ridge or seamount chain is unable to build such structures, as this process induces in fact an acceleration of the tectonic erosion and collapse of the upper plate; the underthrusting of a micro-continent or mature arc is likely needed. The exhumation story of Sanbagawa HP schists suggests the setting of a continental subduction. The petrological and new geochemical data from the literature strongly support the existence, beneath the nappes of accretionary complexes, of continental bodies showing affinities with South China, from which they were once separated. The episodic collision, underthrusting, of such blocks was responsible for the tectonic piling. Tectonic erosion plaid likely a major role in removing material during the intervening subduction stages. A revised geodynamic model, implying the collision of the Honshu, South Kitakami-Kurosegawa, and Shimanto Blocks, is proposed for explaining the three orogenic crises which took place respectively at around 240, 130, and 80-60 Ma ago in SW Japan. The paleogeographic position and affinity of the Hida block with surrounding units, in the hinterland, are still unclear. More work is needed to solve this question. (c) 2012 Elsevier Ltd. All rights reserved.