An improved apparent polar wander path for southwest Japan: post-Cretaceous multiphase rotations with respect to the Asian continent

To construct the Mesozoic apparent polar wander path (APWP) for the inner arc of the southwestern Japanese islands (referred to as southwest Japan) and compare it to that of East Asia, a 110 Ma paleomagnetic pole for southwest Japan was determined. Mudstone and sandstone samples were collected from 16 sites for paleomagnetic analysis in the Lower Cretaceous Inakura Formation of the Inakura area in the central part of southwest Japan. A high-temperature magnetization component, with unblocking temperatures of 670-695 degrees C, was isolated from 12 sites of red mudstone. Of these, 11 sites revealed a primary remanent magnetization during the Early Cretaceous. The primary directions combined with the previously reported ones provide a new mean direction (D = 79.7 degrees, I = 47.4 degrees, alpha(95) = 6.5 degrees, N = 17), and a corresponding paleomagnetic pole that is representative of southwest Japan (24.6 degrees N, 203.1 degrees E, A(95) = 6.8 degrees). The Early Cretaceous paleomagnetic pole, together with the Late Cretaceous and Cenozoic poles, constitute a new APWP for southwest Japan. The new APWP illustrates a standstill polar position during 110-70 Ma, suggesting tectonic quiescence of this region. This standstill was followed by two large tracks during the Cenozoic. We interpret these tracks as clockwise tectonic rotations of southwest Japan that occurred twice during the Cenozoic. The earlier tectonic rotation occurred for a tectonic unit positioned below northeast China, the Liaodong and Korean Peninsulas, and southwest Japan (East Tan-Lu Block) during the Paleogene. The later rotation took place only under southwest Japan during the Neogene. Cenozoic multiphase rifting activity in the eastern margin of the Asian continent was responsible for the tectonic rotations that are observed from the paleomagnetic studies. Intermittent rifting may constitute a series of phenomena due to asthenospheric convection, induced by the growth of the Eurasian mega-continent in the Mesozoic.