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.