Restoration of Exotic Terranes Along the Median Tectonic Line, Japanese Islands: Overview

This paper reviews recent progress on the geotectonic evolution of exotic Paleozoic terranes in Southwest Japan, namely the Paleo-Ryoke and Kurosegawa terranes. The Paleo-Ryoke Terrane is composed mainly of Permian granitic rocks with hornfels, mid-Cretaceous high-grade metamorphic rocks associated with granitic rocks, and Upper Cretaceous sedimentary cover. They form nappe structures on the Sambagawa metamorphic rocks. The Permian granitic rocks are correlative with granitic clasts in Permian conglomerates in the South Kitakami Terrane, whereas the mid-Cretaceous rocks are correlative with those in the Abukuma Terrane. This correlation suggests that the elements of Northeast Japan to the northeast of the Tanakura Tectonic Line were connected in between the paired metamorphic belt along the Median Tectonic Line, Southwest Japan. The Kurosegawa Terrane is composed of various Paleozoic rocks with serpentinite and occurs as disrupted bodies bounded by faults in the middle part of the Jurassic Chichibu Terrane accretionary complex. It is correlated with the South Kitakami Terrane in Northeast Japan. The constituents of both terranes are considered to have been originally distributed more closely and overlay the Jurassic accretionary terrane as nappes. The current sporadic occurrence of these terranes can possibly be attributed to the difference in erosion level and later stage depression or transtension along strike-slip faults. The constituents of both exotic terranes, especially the Ordovician granite in the Kurosegawa-South Kitakami Terrane and the Permian granite in the Paleo-Ryoke Terrane provide a significant key to reconstructing these exotic terranes by correlating them with Paleozoic granitoids in the eastern Asia continent.