10:15 〜 10:30
[SCG52-06] Subduction of the Philippine Sea plate constrained by middle to late Miocene trench-proximal magmatism in southwest Japan
キーワード：中新世、西南日本、火成活動、フィリピン海プレート、四国海盆
The separation of southwest Japan and northeast Japan from the Eurasian continent during the Miocene led to the establishment of the Japanese archipelago in its current position. Paleomagnetic studies indicate that southwest Japan underwent a rapid clockwise rotation in the final stage of this separation. Recent detailed paleomagnetic investigations suggest that this rotation was completed around 16 Ma [1]. However, uncertainties remain regarding the plate configuration before and after the rotation.
The clockwise rotation of the Southwest Japan Arc coincided with widespread trench-proximal magmatism, which had been regarded as occurring “almost simultaneously” with the rotation. The geochemistry and spatiotemporal distribution of igneous rocks provide crucial insights into the tectonic environment of magma generation. This presentation re-evaluates the spatiotemporal distribution of trench-proximal magmatic activity in Southwest Japan, focusing on recent zircon U-Pb dating results. The aim is to constrain the mode of subduction of the Philippine Sea Plate beneath southwest Japan during the Miocene.
The Miocene magmatism in trench-proximal regions of southwest Japan can be categorized into three major groups: felsic igneous rocks in the Outer Zone, the Setouchi Volcanic Rocks, and trench-proximal basaltic magmatism in the marginal zone. Recent zircon U-Pb dating suggests that all these magmatic activities occurred after the completion of rotation. The felsic igneous rocks in the Outer Zone have been dated to 14.5 ± 1.0 Ma [2]. The Setouchi Volcanic Rocks, mostly active between 15 and 14 Ma [3][4][5] exhibit a uniform age and lithology along the arc extension from the Kii Peninsula to western Kyushu.
If we accept the conventional petrogenetic interpretations that (1) the felsic igneous rocks in the Outer Zone were formed by the melting of accretionary complex or lower crust due to the subduction of the young hence hot Shikoku Basin [6] and (2) the high-Mg andesites characteristic of the Setouchi Volcanic Rocks resulted from reactions between slab-derived melts and mantle peridotite [7], then by ~15 Ma, the Shikoku Basin slab must have already reached depths beneath to the Setouchi–Amakusa region, in the area from the Kii Peninsula to western Kyushu. This implies that the Kyushu-Palau Ridge was located to the west of Kyushu around 15 Ma [8].
Furthermore, tholeiitic basalts intruding into Cape Shionomisaki and Cape Muroto have been traditionally attributed to marginal tholeiites of the terminal spreading phase of the Shikoku Basin [9]. The age of the Muroto gabbroic body, dated at 15.6 Ma [10], immediately after the end of rotation, strongly suggests that the Kii Peninsula–Kyushu region was directly facing the Shikoku Basin at that time.
Alkaline basalts found in trench-proximal regions such as Tanegashima, Cape Ashizuri, Shingu, and Bankawa exhibit geochemical characteristics similar to ocean island basalts, with ages ranging from 15.2 to 7.5 Ma [11][12]. Since the distribution of the Setouchi Volcanic Rocks indicates that the Shikoku Basin slab was already present beneath this region at that time, the sporadic occurrence of deep mantle-derived alkaline basalt activity in this region may be related to some form of slab break-off or similar structure.
[1] Hoshi et al. (2015) EPS,67: 92. [2] Shinjoe et al. (2021) Geol. Mag.158, 47. [3] Hoshi et al. (2000) JMPS, 95, 203. [4] Shinjoe & Orihashi (2017), J. Geol. Soc. Jpn, 123, 423.[5] Shinjoe et al. (2024), Isl. Arc, 33 : 12506. [6] Shinjoe (1997) Chem. Geol.,134, 237. [7] Tatsumi & Hanyu (2003) G3, 9: 1081. [8] Tatsumi et al. (2020) Sci. Rep., 10: 15005. [9] Kimura et al. (2005) GSA Bull. 117, 969. [10] Shinjoe et al. (2023) 130th Annual Meeting of Geol. Soc. Jpn, T5-O-6. [11] Shinjoe et al. (2010) Geochem. J., 44, 257. [12] Seike (2016) Bull. Saitama Museum of Natural History, 10, 67.
The clockwise rotation of the Southwest Japan Arc coincided with widespread trench-proximal magmatism, which had been regarded as occurring “almost simultaneously” with the rotation. The geochemistry and spatiotemporal distribution of igneous rocks provide crucial insights into the tectonic environment of magma generation. This presentation re-evaluates the spatiotemporal distribution of trench-proximal magmatic activity in Southwest Japan, focusing on recent zircon U-Pb dating results. The aim is to constrain the mode of subduction of the Philippine Sea Plate beneath southwest Japan during the Miocene.
The Miocene magmatism in trench-proximal regions of southwest Japan can be categorized into three major groups: felsic igneous rocks in the Outer Zone, the Setouchi Volcanic Rocks, and trench-proximal basaltic magmatism in the marginal zone. Recent zircon U-Pb dating suggests that all these magmatic activities occurred after the completion of rotation. The felsic igneous rocks in the Outer Zone have been dated to 14.5 ± 1.0 Ma [2]. The Setouchi Volcanic Rocks, mostly active between 15 and 14 Ma [3][4][5] exhibit a uniform age and lithology along the arc extension from the Kii Peninsula to western Kyushu.
If we accept the conventional petrogenetic interpretations that (1) the felsic igneous rocks in the Outer Zone were formed by the melting of accretionary complex or lower crust due to the subduction of the young hence hot Shikoku Basin [6] and (2) the high-Mg andesites characteristic of the Setouchi Volcanic Rocks resulted from reactions between slab-derived melts and mantle peridotite [7], then by ~15 Ma, the Shikoku Basin slab must have already reached depths beneath to the Setouchi–Amakusa region, in the area from the Kii Peninsula to western Kyushu. This implies that the Kyushu-Palau Ridge was located to the west of Kyushu around 15 Ma [8].
Furthermore, tholeiitic basalts intruding into Cape Shionomisaki and Cape Muroto have been traditionally attributed to marginal tholeiites of the terminal spreading phase of the Shikoku Basin [9]. The age of the Muroto gabbroic body, dated at 15.6 Ma [10], immediately after the end of rotation, strongly suggests that the Kii Peninsula–Kyushu region was directly facing the Shikoku Basin at that time.
Alkaline basalts found in trench-proximal regions such as Tanegashima, Cape Ashizuri, Shingu, and Bankawa exhibit geochemical characteristics similar to ocean island basalts, with ages ranging from 15.2 to 7.5 Ma [11][12]. Since the distribution of the Setouchi Volcanic Rocks indicates that the Shikoku Basin slab was already present beneath this region at that time, the sporadic occurrence of deep mantle-derived alkaline basalt activity in this region may be related to some form of slab break-off or similar structure.
[1] Hoshi et al. (2015) EPS,67: 92. [2] Shinjoe et al. (2021) Geol. Mag.158, 47. [3] Hoshi et al. (2000) JMPS, 95, 203. [4] Shinjoe & Orihashi (2017), J. Geol. Soc. Jpn, 123, 423.[5] Shinjoe et al. (2024), Isl. Arc, 33 : 12506. [6] Shinjoe (1997) Chem. Geol.,134, 237. [7] Tatsumi & Hanyu (2003) G3, 9: 1081. [8] Tatsumi et al. (2020) Sci. Rep., 10: 15005. [9] Kimura et al. (2005) GSA Bull. 117, 969. [10] Shinjoe et al. (2023) 130th Annual Meeting of Geol. Soc. Jpn, T5-O-6. [11] Shinjoe et al. (2010) Geochem. J., 44, 257. [12] Seike (2016) Bull. Saitama Museum of Natural History, 10, 67.