Abstract
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 °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°, I = 47.4°, α95 = 6.5°, N = 17), and a corresponding paleomagnetic pole that is representative of southwest Japan (24.6° N, 203.1° E, A95 = 6.8°). 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.
Highlights
Apparent polar wander paths (APWPs) for cratonic blocks are useful in understanding the paleogeographic evolution of a region over geological time scales
Paleomagnetism Initial Natural remanent magnetization (NRM) intensities ranged between 6.1 × 10−2 and 5.8 × 10−3 Am−1
We suggest that the clockwise rotation observed in the Korean Peninsula, which corresponds to the first clockwise rotation for southwest Japan, occurred in association with intra-cratonic rifting that resulted in the formation of extensional basins, such as the Bohai Bay Basin, Yilan–Yitong Graben, and Songliao Basin (Fig. 1a)
Summary
Apparent polar wander paths (APWPs) for cratonic blocks are useful in understanding the paleogeographic evolution of a region over geological time scales. The APWPs for the eastern Asia blocks, constructed from the Paleozoic to the present, provide the age of amalgamation and associated closure of the ocean between these blocks (e.g., Enkin et al 1992; Wu et al 2017; Huang et al 2018a). The Cenozoic segment of the APWP for southwest Japan suggests the opening of the Japan Sea in the Miocene (Otofuji and Matsuda 1987; Kodama and Takeda 2002). The pre-Miocene poles for this region are shifted toward lower latitudes with respect to the coeval poles for the eastern Asia blocks (Otofuji 1996), indicating that southwest Japan drifted from the continent due to the opening of the Japan Sea during the Miocene. Southwest Japan drifted in a clockwise rotation of ~ 40° about a pivot located at the western end of this region (Ishikawa 1997; Uno 2002; Hoshi 2018)
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