Geological comparative studies of Japan Arc System and Kyushu-Palau Arc

Based on the published data of structure geology, geochronology, petrology and isotope geochemistry, the authors of this paper have conducted studies on the tectonic evolution history of Japan arc system and Kyushu-Palau ridge (KPR). The studies show that the initial Japan arc system was resulted from the subduction of ancient Pacific plate beneath Eurasian Plate in Permian. It was part of an Andean-type continental volcanic arc which occurred in the offshore in the east of Asian during late Mesozoic era. The formation of tertiary back-arc basin (Japan Sea) resulted in the fundamental tectonic framework of the present arc system. Since Quaternary the system has been lying at E-W compression tectonic setting due to the eastward subduction of Amur Plate. It is expected that Japan arc system will be juxtaposed with Asian continent, which is similar to the present Taiwan arc system. The origin of Philippine Sea Plate (PSP) is still in debate. Some studies argued that it is a trapped oceanic crust segment, while the others insisted that it is a back-arc basin accompanied with ancient IBM arc. However, it is all agreed that the tectonic evolution of PSP started since 50 Ma, i.e., PSP has drifted from the site around equator at 50 Ma to the present site, and the subduction of PSP along Nankai trough-Ryukyu Trench beneath the Japan arc system during 6-2 Ma led to the formation of the present Ryukyu arc system. Of the PSP, the KPR has been found with the oldest rocks formed at 38 Ma. Combining with its geochemical characteristics of oceanic arc tholeiite, it is suggested that KPR is an intraoceanic volcanic arc, more specifically, a relic arc (i.e., rear arc of the ancient IBM) after rifting of ancient IBM. In addition, Amami-Daito province is of arc tectonic affinity, but has been affected by mantle plume. Therefore, based on their respective tectonic evolution history and geochemical characteristics of rock samples, it is inferred that there is no genetic relationship between Japan arc system and KPR. It is noted that rocks reflecting continental crust basement feature have been collected on the northern tip of KPR, which may be related to the process of KPR accreting on Japan arc, but the arc-continent accretion process are still at initial stage of modern continental crust accretion model. However, due to the scarcity of data of the northern tip of KPR, crustal structure of this location and its adjacent Nankai trough need to be further constrained by geophysical studies in the future.