Jurassic accretion tectonics of Japan

Abstract

Abstract The Jurassic accretionary complex and coeval granites in Japan represent remnants of the Jurassic arc‐trench system developed between the Asian continent and Pacific Ocean. The Jurassic accretionary complex occurs as a large‐scale nappe that is tectonically sandwiched between the overlying pre‐Jurassic nappes and underlying post‐Jurassic nappes. By virtue of new research styles (microfossil mapping and chronometric mapping) the following new views of the Jurassic accretionary complex in Japan, that suggest those for on‐land exposed ancient accretionary complexes in general, have been obtained: (i) the accretion age of the Jurassic accretionary complex ranges over ∼ 80 million years from the latest Triassic to earliest Cretaceous according to a reconstructed stratigraphy of component rocks (oceanic plate stratigraphy); (ii) the accretionary complex is subdivided into several nappe units, each characterized by unique oceanic plate stratigraphy; (iii) a tectonically downward‐younging polarity is observed in the piled nappes; (iv) the Jurassic accretionary complex is composed of coherent‐type and chaotic‐type units, the former retaining the primary accretionary structures while the latter are characterized by collapsed and secondarily mixed materialslfabrics derived from the former; (v) the chaotic‐type units predominate in volume over the coherent‐type units; (vi) the accretionary complex suffered from a regional low‐grade metamorphism (up to the lower greenschist facies) within ∼10–20 million years after the accretion timing; and (vii) the lateral extent of the Jurassic accretionary complex in East Asia is intermittently traced from the Koryak mountains in Russia to North Palawan in the west Philippines for ∼6000 km. Discussion focuses on (i) the low preservation ratio of the coherent‐type units to the chaotic‐type units with respect to frequent subduction erosion by seamount subduction; (ii) absence of the Franciscan‐type melange, suggesting sedimentary mixing origin for the chaotic‐type unit; (iii) a growth rate of the Jurassic accretionary complex compatible to modern analogues; and (iv) the total volume of the Jurassic accretionary complex in Japan with respect to the most likely terrigeiious elastics source along the 250 Ma continent‐continent collision suture in central China (between the Sino‐Korean and Yangtze blocks).