Reappraisal of the accretionary orogeny and revalidation of the collisional model

9 10 This paper makes a review of the interpretations of the tectonic evolution of SW Japan 11 during the last three decades. In the late 1970s, the dominant model was the so-called 12 ―Pacific-type orogeny‖, emphasizing the purported absence of nappes and the contrast with 13 the alpine chains, and interpreting the evolution as due to a steady oceanic subduction since 14 the Paleozoic time. In the 80s, the discovery of the actual structure made of a pile of large 15 thrust sheets led authors to propose collisional models, involving the intermittent 16 underthrusting of buoyant blocks like micro-continents. At the same time, the use of high17 resolution biostratigraphy allowed several authors to recognize ancient accretionary wedges, 18 with a reconstructed ocean plate stratigraphy of individual accreted units, especially in the 19 Tanba and Shimanto zones. Also, precise radiometric dating permitted the distinction of 20 metamorphosed units, especially in Sanbagawa and Shimanto belts. As a result of these new 21 data, since the 1990s, the plate tectonic interpretation of the history of the Japanese islands 22 was revised by Japanese scientists and presented again in terms of accretionary processes 23 linked to a steadily oceanic subduction, with an episodic ridge subduction: the so-called 24 ―Miyashiro-type orogeny‖. The review of different data leads to the following conclusions. 25 The structure of SW Japan is made of a pile of sub-horizontal nappes, polydeformed, with a 26 geometry similar to the one encountered in collisional orogens. The geodynamic mechanisms 27 advocated for the tectonic building within the accretionary orogeny concept (Miyashiro-type 28 orogeny) are inappropriate. A permanent oceanic subduction with the intermittent ―collision‖ 29 (actually subduction) of an active ridge or seamount chain is unable to build such structures, 30 as this process induces in fact an acceleration of the tectonic erosion and collapse of the upper 31 plate; the underthrusting of a micro-continent or mature arc is likely needed. The exhumation 32 story of Sanbagawa HP schists suggests the setting of a continental subduction. The 33 petrological and new geochemical data from the literature strongly support the existence, 34 *Manuscript Click here to view linked References in su -0 06 99 98 8, v er si on 1 30 M ay 2 01 2 Author manuscript, published in "Journal of Asian Earth Sciences Accepted manuscript (2013) 46 p." DOI : 10.1016/j.jseaes.2012.04.023