Ordovician Japan: geotectonic setting and palaeogeography

Abstract

Ordovician Japan formed a mature arc-trench system developed along the Paleo-Pacific (Panthalassa) margin of the Greater South China (GSC) continental block. GSC consists of South China, East China Sea, SW-NE Japan, and Khanka-Jiamusi-Bureya megablock in the Far East; thus in total twice as large as the South China components by itself (Yangtze and Cathaysia). The Ordovician crust of Proto-Japan comprised coeval arc-related rocks, such as granitoids, supra-subduction zone ophiolite, and fore-arc basin strata, although most of them were considerably fragmented. The Ordovician and middle-late Paleozoic fossils from Japan are highly limited but suggest that Proto-Japan was positioned in the low-latitude domains probably of the Paleo-Pacific Ocean in connection to Paleo-Tethys. GSC became separated from Rodinia in the Neoproterozoic, and its Proto-Japan segment evolved as a collision-free subduction margin for nearly 500 Myr since the mid-Cambrian. The GSC framework provides critical constraints to paleogeographical reconstruction of circum-Pacific continental blocks. First, the Cambro-Ordovician GSC should have been isolated from Australia/India/East Antarctica of East Gondwana, by a relatively wide ocean domain for keeping “subduction potential”. Second, the Cathaysian margin of GSC should have faced to an extensive ocean without major continents since the Cambrian. The Paleo-Pacific is the only possible candidate for this.

LateTriassic A‐type granite boulders in Lower Cretaceous conglomerate of theHida belt, Japan: Their origin and bearing on theYamatotectonic line inFar East Asia

“…The new zircon ages confirmed that the East China Sea, SW-NE Japan, and Khanka block primarily, at least since the Cambrian, formed northeastern extensions of the South China block, in particular, the Cathaysian part on the Pacific side (Isozaki, 2019(Isozaki, , 2023Isozaki et al, 2017;Yamamoto et al, 2022). In this newly proposed framework named Greater South China (GSC; Isozaki et al, 2014), the KJB megablock in Far East Asia corresponds to a northernmost segment of GSC, although their present configuration and relative positions were drastically modified by later tectonics, which was culminated by the Miocene opening of the Japan Sea.…”

“…More research is needed to confirm the full spectrum and details of YTL, its geotectonic significance is potentially more than a simple divide between two neighboring granitoid provinces, because YTL likely represents a prominent boundary between two major orogens in Far East Asia, that is, EW‐trending CAOB and NS‐trending GSC‐Nipponides (Figures 1 and 7), as emphasized by Isozaki (2019, 2023). More details on the regional tectonics of relevant areas will be discussed elsewhere.…”