[S-GL23] Geologic structure and tectonic history of East Asia and Japanese Islands

[SGL23-01] Factors controlling tectonics in the East Asia since the Cretaceous: Collision of the Indian subcontinent and subduction of Izanagi-Pacific plate

*Toru Takeshita¹ (1.Adviser, Land Infrastructure Division, Pacific Consultants Co., Ltd.)

Keywords:Tectonics in the East Asia since the Cretaceous, Collision of the Indian subcontinent, Retreat of the Pacific plate subduction interface, NE-SW trending sinistral fault and E-W extension, Kinematics of the Median Tectonic Line in the Paleogene, Izanagi-Pacific ridge subduction

Recently, it has become clear that the Izanagi-Pacific ridge subducted beneath the East Asia continent at c. 60-50 Ma (earliest Tertiary) based on the studies of mantle tomography (e.g., Seton et al., 2015). Also, thanks to the increasing degree of the quality and quantity of radiometric ages, the timing of the initial India-Asia collision has been dated as 53-47 Ma (Early Eocene, e.g., Tong et al., 2024). It could be intuitively understood that the formation of the Himalaya and Tibetan plateau was caused by the collision of the Indian subcontinent. In addition, on the formation of rift zones in the East Asia and marginal seas at its eastern and southern borders, Tapponnier et al. (1982) interpreted that sinistral displacement along the Red River fault led to the extrusion of the Indochina Continent, which resulted in the formation of the South Chaina Sea based on analogue modelling. Similarly, they inferred that rift basins in the Shanxi Province and Baikal Rift could have been formed by an E-W extension caused by the activation of the NE-SW trending sinistral faults. Jiao, Tapponnier et al. (2023) have further conducted numerical DEM modelling for tectonics in the East Asia, and refined their model. However, it should be noted that the French school led by Tapponnier postulated that the boundaries at the east and south sides are a free boundary, and never incorporated the interaction between the East Asian continent and subducting plate. On the other hand, Schellart et al. (2019) conducted analogue modelling, where the Indian subcontinent extrudes into the Asian Continent at a constant velocity, whereas the subduction interface (i.e. trench) retreats at a constant velocity, varying the ratio of the two velocities. As a result, the folded belts in the Himalaya, rift zones and amount of E-W extension in the East Asian Continent could be nicely reproduced by the velocity of trench retreat at one third of the one of extrusion. However, not only the modeled main NE-SW trending sinistral faults are slightly offset from the real ones, but also the first-class sinistral faults such as the Tan-Lu fault cannot be reproduced by the model. Furthermore, the E-W shortening which for example has occurred in the Japanese Islands since 3 Ma, can be never reproduced in the model, which could be caused by the advancement of the Japan trench towards the west (Takahashi, 2017). Nevertheless, the analogue modelling results by Schellart et al. (2019) could explain the first-order main tectonic features in the East Asia very well.
Although the correlation of tectonics in the Japanese Islands and the East Asia continents is of great interest for us, the researches on this topic have been little advanced. Recently, Kubota et al. (2020) have clarified that there were two major movements around the Median Tectonic Line (MTL) in southwest Japan in the Paleogene period: One is a large-scale normal faulting caused by extension in the N-S direction (present geographic orientation before the opening of the Japan Sea) in the Paleocene (c. 59 Ma), and the other is a sinistral transpression caused by the N-S shortening (ibid.) in the Early Eocene (c. 47-46 Ma). Kubota et al. (2020) inferred that the extension in the N-S direction at c. 59 Ma was caused by the buoyant ridge subduction, while the timing of the sinistral transpression at c. 47-46 Ma could be correlated with both initiation of the collision of the Indian Subcontinent as mentioned above and subduction along the Mariana trench (48±10 Ma, Wu et al., 2016). However, it has been not known at all if the sinistral transpression along the MTL are genetically related to these geological events.