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The subduction of the Philippine Sea (PHS) plate along the Nankai Trough in in southwest Japan is a relatively recent process compared with subduction along the Japan Trench in northeast Japan. However, the tectonic evolution of the PHS plate along the Nankai Trough is still controversial and not fully understood. There are several competing hypotheses based on different estimates for the time variations of convergence rate and plate age. Our study employs numerical modelling of subduction in order to evaluate the slab evolution for the last 15 Myr and aims to evaluate each tectonic scenario against the present-day slab geometry along a profile passing through the Shikoku and Chugoku regions. The modelling strategy involves a parameter study where subduction initiation and various subduction parameters are analyzed in terms of subduction geometry evolution. Two-dimensional visco-elasto-plastic numerical simulations of spontaneous bending subduction predict that convergence rate and plate age variations play an important role in the evolution of subduction geometry. Modeling results after 15 Myr of evolution reveal that the tectonic model based on a high convergence rate between ~ 15 Ma and ~ 3 Ma produces a slab geometry that agrees well with the observed present-day slab shape specific for the Shikoku and Chugoku regions.
The subduction of the Philippine Sea (PHS) plate along the Nankai Trough in in southwest Japan is a relatively recent process compared with subduction along the Japan Trench in northeast Japan. However, the tectonic evolution of the PHS plate along the Nankai Trough is still controversial and not fully understood. There are several competing hypotheses based on different estimates for the time variations of convergence rate and plate age. Our study employs numerical modelling of subduction in order to evaluate the slab evolution for the last 15 Myr and aims to evaluate each tectonic scenario against the present-day slab geometry along a profile passing through the Shikoku and Chugoku regions. The modelling strategy involves a parameter study where subduction initiation and various subduction parameters are analyzed in terms of subduction geometry evolution. Two-dimensional visco-elasto-plastic numerical simulations of spontaneous bending subduction predict that convergence rate and plate age variations play an important role in the evolution of subduction geometry. Modeling results after 15 Myr of evolution reveal that the tectonic model based on a high convergence rate between ~ 15 Ma and ~ 3 Ma produces a slab geometry that agrees well with the observed present-day slab shape specific for the Shikoku and Chugoku regions.
The opening of the Japan Sea led to the separation of southwest Japan from the Eurasian continent. Subsequent to this event, a diverse range of igneous activities occurred in southwest Japan. On the back‐arc side of the region, igneous activity commenced at approximately 22 Ma and persisted for an extended period. In the trench‐proximal region of southwest Japan, magmatism initiated around 15.6 Ma, immediately following the cessation of the Japan Sea opening, in correlation with the subduction of the Philippine Sea plate beneath southwest Japan. The Amakusa Islands in western Kyushu host felsic to intermediate igneous rocks with Miocene radiometric ages. There has been a debate regarding the attribution of the igneous rocks in Amakusa Island among the Miocene igneous rocks in southwest Japan. To address this issue, we conducted zircon U–Pb dating and analyzed the major‐ and trace‐element compositions of felsic igneous rocks in the Amakusa Islands to elucidate their characteristics. The obtained U–Pb ages range from 14.5 to 14.8 Ma, suggesting contemporaneity between magmatism in the Amakusa Islands and the Setouchi Volcanic Rocks in the trench‐proximal region of southwest Japan. The major and trace element compositions of the felsic igneous rocks exhibit similarities to the dacites of the Setouchi Volcanic Rocks. These findings support previous suggestions that the magmatism in the Amakusa Islands can be correlated with the Setouchi Volcanic Rocks, based on the discovery of a high‐Mg andesite dike and paleo‐stress analysis utilizing the direction of dikes and sills. Therefore, the Setouchi Volcanic Belt is proposed to extend further west than the previously identified Ohno volcanic rocks in eastern Kyushu. The subduction of the Shikoku Basin of the Philippine Sea plate toward western Kyushu supports the hypothesis that the Kyushu‐Palau Ridge was positioned west of Kyushu at ~15 Ma.
The lower Miocene strata in the San in region were presumed to be a graben-fill deposit based on their distribution and the extensional setting. However, previous research has presented little evidence for grabens; only a few syndepositional faults have been found, and the regional stress has been called into question. To collect more basic data that can improve our understanding of the origin of the Miocene basins, we present geological data for the Tajima Mihonoura area, eastern San in region, Southwest Japan, including the structure of the base of the lower Miocene Yoka Formation, the zircon U-Pb age of the formation, and the paleostress during its deposition. There is talus or basal breccia at the contact between the basement and the Yoka Formation, and no faults were found along the contact. These observations indicate that the Yoka Formation unconformably overlies the basement. Although the thickness of the lower Miocene strata was used to estimate the shape of the grabens in the San in region, we show that the thickness in the study area was dependent on the paleogeography during the early Miocene. Zircon from a felsic pumice tuff from the Yoka Formation was dated, yielding a weighted mean 238 U-206 Pb age of 19.6±0.15 Ma (2σ) from the grains with concordant Miocene ages. This mean age is consistent with other chronological constraints on the formation obtained in the Tango Peninsula and the Tajima-Myokensan area, which form its eastern and southern margins, respectively. Paleostress inversion from the orientation of dikes related to the Yoka Formation suggests that the study area was subject to NE-SW extension with a low stress ratio. This stress probably represents the regional stress in the eastern San in region, given the similarity of paleostresses reported recently from other areas of Hokutan.
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