Osamu Kinoshita

Migration of igneous activities related to ridge subduction in Southwest Japan and the East Asian continental margin from the Mesozoic to the Paleogene

Abstract In the inner zone of Southwest Japan, the northern part of the Median Tectonic Line (MTL), granites and their volcanic equivalents were formed in large quantities during the late Cretaceous to early Tertiary. Migration of magmatic activity related to ridge subduction is proposed to explain the widely distributed magmatism and its tectonics. For this purpose, the ages of granitic rocks were carefully inspected — according to the criteria of the dating methods and procedures concerned — and compiled with the sampling localities. They clearly show a trend in rock ages along the MTL, in agreement with the general direction of strikes in Southwest Japan. From the trend of the ages and the cooling histories of the bodies, it was concluded that the magmatism migrated eastward along the MTL with a rate of 30 km/m.y. This migration concept is extended to the Mesozoic igneous rocks of the East Asian continental margin. There, two magmatic belts - ranging in age from Mesozoic to Paleogene - are found to become younger northeastward along the margins, which is ascribed to the subduction of the Farallon-Izanagi and Kula-Pacific ridges.

Possible manifestations of slab window magmatisms in Cretaceous southwest Japan

Abstract The slab window system related to ridge subduction is studied to explain the conspicuous activity and the along-arc migration of the magmatisms in the eastern margin of Eurasian continent in the late Mesozoic. The trend of the granite ages is scrutinized with systems approach and analyzed by the statistical method in Cretaceous southwest Japan, a reference field of the Eurasian eastern margin. The magmatic distribution of simultaneous activity was determined to be the V-shape of the slab window, excluding dating errors from the located age data. The slab window magmatism has the most dominant zone of the acidic rocks at or near the centerline of the window (the spreading center of two plates), and also has the subdominant zone of the acidic to intermediate rocks on either side of the window. As the slab window opens due to the plate motions, the upwelling current is adiabatically induced in the asthenosphere to fill the window gap. The partial melt is generated in the upwelling flux and transports heat to the lower crust to make greater granitic magma chambers by heat conduction to, and assimilation with, the crustal matter. The younger and hotter slabs outside the window also play a role in partial slab melting and/or dehydration to the asthenosphere and generate the subdominant granodioritic and adakitic and/or high-magnesian andesitic magmatisms as the pre- and post-activities of the main granite genesis. The slab window formed by the subducted ridge between the Kula and Pacific plates was the strong heat source of the active magmatisms and migrated at the rate of 2.8 cm/year from southwest to northeast about 8000 km along the continental margin. The growth of the continent associated with the active acidic to intermediate magmatisms including the adakitic and/or high-magnesian andesitic activities is modeled by the slab window system.