Miocene clockwise rotation of southwest Japan and formation of curvature of the Median Tectonic Line: Paleomagnetic implications

The origin of the curvature of the Median Tectonic Line (MTL) in southwest Japan is still controversial. In order to investigate its formation, we car ried out a paleomagnetic study in the Nohi area, north of the curved part o f the MTL in the eastern part of southwest Japan. More than 420 paleomagnet ic samples were collected at 35 sites from Cretaceous to Paleogene welded t uffs in the Kasagatake Rhyolites, Oamamiyama Group, and Nohi Rhyolite. Char acteristic directions with high unblocking temperature component above 560 degrees C were isolated from 32 sites. Paleomagnetic directions of the Kasa gatake Rhyolites and Oamamiyama Group (55-66 Ma) have a clockwise deflected declination (D = 51.7 degrees, I = 52.6 degrees, alpha(95) = 8.6 degrees) which is consistent with the characteristic Paleogene paleomagnetic directi on of the central part of southwest Japan. A similarly deflected declinatio n is also observed in the peripheral part of the Nohi Rhyolite region, far away from the Atera fault system. Paleomagnetic data indicate that the Nohi area experienced a clockwise rotation through more than 45 degrees with re spect to the Asian continent as a part of southwest Japan but that the amou nt of rotation of the Nohi area is 23 degrees smaller than that of the cent ral part of southwest Japan. A new model is developed to explain the tecton ic rotation. At about 15 Ma during the latest stage of the clockwise rotati on of southwest Japan, the eastern end moved southward by more than 60 km a long a left-lateral strike-slip fault which comprised the present north-sou th segment of the MTL and the Akaishi Tectonic Line. We conclude that the c urvature of the MTL is a composite of the 15 Ma strike-slip fault system an d a small bend of the MTL associated with the small differential rotation o f the Nohi area with respect to the central part of southwest Japan.