Earth and Planetary Science Letters, 62 (1983) 349-359 349 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [4] Paleomagnetic evidence for the clockwise rotation of Southwest Japan Yo-ichiro Otofuji l and Takaaki Matsuda 2 I Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Kyoto 606 (Japan) 2 Department of Geology, Himeji Institute of Technology, Himeji 671 -22 (Japan) Received August 3, 1982 Revised version received November 8, 1982 Over 500 oriented samples of felsic rocks of Cretaceous to Middle Miocene age were collected along the Go River in the central part of Southwest Japan, in an attempt to detect the process of tectonic rotation of Southwest Japan from the paleomagnetic view point. Thermal demagnetization was successful in isolating characteristic directions from the remanent magnetization of samples. Reliability of the paleomagnetic direction is ascertained through the agreement of directions from different kinds of rocks as well as the presence of both normal and reversed polarities. The paleomagnetic results establish that Southwest Japan began to rotate clockwise through 58 _+ 14 ° later than 28 Ma and ceased its motion by about 12 Ma. Southwest Japan has undergone no detectable north-south translation since 28 Ma. These results imply that southwest Japan was rotated about the pivot around 34°N, 129°E between 28 Ma and 12 Ma in association with the opening of the Japan Sea. 1. Introduction Paleomagnetic studies of Cretaceous rocks of Southwest Japan [1,2] have established that the directions of Cretaceous sedimentary and volcanic rocks have shown clockwise deflection from the expected paleofield direction. This anomalous di- rection is explained by a clockwise rotation of Southwest Japan associated with the opening of the Japan sea [3,4]. Similar easterly declination data are reported from Paleogene rocks [5]. These facts leave little doubt that the clockwise rotation mainly occurred during the latter part of the Ter- tiary. The aim of the present work is to measure the process of clockwise rotation of Southwest Japan by paleomagnetism in an attempt to assess the timing and the amount of the rotation. The paleomagnetic results shed some light on the origin of the Japan Sea. Felsic igneous rocks of Late Cretaceous to Miocene age are common over large areas of the 0012-821x/83/$03.00 © 1983 Elsevier Science Publishers B.V. Inner Zone of Southwest Japan. These rocks are ideal for studying the tectonic rotation of South- west Japan, because these rocks of Cretaceous and Paleogene age have easterly characteristic direc- tions of Southwest Japan [1,5]. These igneous rocks are extensively exposed along the G~ River and the neighbouring region in the central part of Southwest Japan, where this study was carried out (Fig. 1). This area has been geologically mapped in detail, and its stratigraphic relations have been well studied [6,7]: The igneous rocks of the area consist of a sequence of pyroclastic flow deposits and lava flows, which are intruded by plutonic rocks and dykes. Dacitic to rhyolitic compositions predominate between the Cretaceous and the Paleogene. During the Miocene, the composition changes to intermediate. Recently chronological studies by Matsuda [8] using fission-track methods have greatly clarified the history of igneous activ- ity in this area which was originally worked out by Kawano and Ueda [9] using the K-Ar dating