2012 Volume 121 Issue 6 Pages 1081-1089
In response to comments by Ishiwatari and Tsujimori (2012) on the position of the geotectonic boundary between the North China and South China blocks in easternmost Asia, i.e., in Japan, some additional remarks are presented here to supplement our previous article (Omori and Isozaki, 2011). The 230 Ma ultrahigh-pressure metamorphic belt (the Dabieshan-Sulu collision suture) extends from Shaanxi province to the Shandong peninsula in mainland China, but is not recognized to the east on the Korean peninsula and in Japan. There are two contrasting interpretations of the further extension; (1) a continuity to the east through the Korean peninsula and Japan (Omori and Isozaki, 2011); and (2) a large-scale deviation south to the Ishigaki Island of the Ryukyus and a connection to the 210 Ma high-pressure Suo metamorphic belt in the northern Kyushu and Chugoku districts of SW Japan (Ishiwatari and Tsujimori, 2012). There are two essential differences between the interpretations; a) assuming the eastern margin of South China block per se extends to NE Japan (Omori and Isozaki, 2011) or up to the Shandong peninsula and no further to the east (Ishiwatari and Tsujimori, 2012); and b) assuming two parallel-running geotectonic boundaries (collision suture and oceanic-subduction zone) (Omori and Isozaki, 2011) or one boundary (collison suture changing laterally into an oceanic-subduction zone) (Ishiwatari and Tsujimori, 2012) in Triassic East Asia. Regarding the geotectonic framework around Japan, the northern Kyushu and Chugoku districts belong to the South China block according to the former, whereas they belong to the North China block together with the eastern half of East China Sea and the entire Korean peninsula according to the latter. The traditional paleo-biogeographical data of Paleozoic fauna and the recently dated detrital zircon ages of the Neoproterozoic from non-metamorphosed Silurian, Devonian, and Carboniferous fore-arc sediments prove that Paleozoic Japan was located along the active margin of South China. In addition, NE Japan with the Paleozoic fauna having a South China affinity represents the eastern extremity of the South China block as hitherto known. These potentially require a continuation of the collision suture between the two China blocks to NE Japan. These facts and discussion clearly deny the unusual assumptions of Ishiwatari and Tsujimori (2003). Instead, the following conclusions are confirmed; (1) the South China block extends to NE Japan, (2) the 230 Ma UHP-bearing collision suture between the two China blocks continues to NE Japan; and (3) Triassic East Asia had two parallel-aligned major plate boundaries, i.e., collision suture and the oceanic trench.
