09:00 〜 09:15
[SCG55-01] 海嶺沈み込みに起因する前弧海盆の局所的な変形構造〜新第三系宮崎層群〜
キーワード：前弧海盆、宮崎層群、古応力解析、九州ーパラオ海嶺、海山沈み込み
The Neogene Miyazaki Group comprises forearc basin sediments that is characterized by significant spatial variations in consolidation despite the minor differences in depositional ages. The Miyazaki Group is divided into three lithofacies from south to north (i.e., the Aoshima, Miyazaki, and Tsuma facies) corresponding to the variation, consolidated in the south and less in the north. The spatial variation in physical properties (e.g., porosity and maximum effective stress) of sediments in the Miyazaki Group showed that the Aoshima Facies had locally uplifted ~2000 m larger than the northern parts (Yoshimoto+, 2023).
To examine the uplift and deformation history of the Aoshima Facies, we performed the paleo-stress inversion based on the meso-scale fault-slip data in the Miyazaki Group. The Aoshima Facies is mainly cut by N–S, NE–SW, and NW–SE oriented normal faults. The paleo-stress states were calculated from 166 fault-slip data using the Hough transform Inverse Method.
Two independent major paleo-stress states were identified in the Aoshima Facies. The first one (stress α) was observed everywhere in the Aoshima Facies, which has a subvertical σ1-axis and NW–SE oriented σ3-axis with a stress ratio at ~0.4. Similar arc-perpendicular extensions in the latest Miocene to Pliocene in the Kyushu forearc were reported from the Miyazaki and Tsuma Facies (Yamaji, 2003).
On the other hand, the second stress state (stress β) was dominant around the boundary between the Aoshima and Miyazaki facies. It is subvertical to WNW–ESE trending σ1-axis and NNE–SSW oriented σ3-axis. The girdle distribution of the σ1-axis is probably due to the high-stress ratio (~0.9; σ1 ≈ σ2) because the vertical σ1 and WNW–ESE trending σ2 can be switched easily. The switched maximum horizontal stress corresponds to the arc-perpendicular compression.
The causes of the arc-perpendicular extension (stress α) in the Kyushu forearc have been considered to be the back-arc rifting along the Okinawa Trough and the rollback of the subducting slab in southern Kyushu. Apart from the regional extension, we identified the localized arc-perpendicular compression (stress β) which is possibly caused by the seamount subduction along the Kyushu–Palau Ridge, being subducted beneath the Miyazaki Group since the early Pliocene. The subduction and localized underplating of seamounts apparently caused the localized uplift of the Aoshima Facies and horizontal compression around the boundary of the Aoshima and Miyazaki facies.
[Reference]
Yamaji, A., 2003. Slab rollback suggested by latest Miocene to Pliocene forearc stress and migration of volcanic front in southern Kyushu, northern Ryukyu Arc. Tectonophysics, 364, 9-24. https://doi .org /10.1016/S0040-1951(03)00047-7
Yoshimoto, T., Chiyonobu, S., Omori, Y., Zhang, F., Yamamoto, Y., 2023. Spatial variations in the consolidation of sediments in the Neogene Miyazaki forearc basin, Southwest Japan. Tectonophysics, 862, 229922. https://doi.org/10.1016/j.tecto.2023.229922
To examine the uplift and deformation history of the Aoshima Facies, we performed the paleo-stress inversion based on the meso-scale fault-slip data in the Miyazaki Group. The Aoshima Facies is mainly cut by N–S, NE–SW, and NW–SE oriented normal faults. The paleo-stress states were calculated from 166 fault-slip data using the Hough transform Inverse Method.
Two independent major paleo-stress states were identified in the Aoshima Facies. The first one (stress α) was observed everywhere in the Aoshima Facies, which has a subvertical σ1-axis and NW–SE oriented σ3-axis with a stress ratio at ~0.4. Similar arc-perpendicular extensions in the latest Miocene to Pliocene in the Kyushu forearc were reported from the Miyazaki and Tsuma Facies (Yamaji, 2003).
On the other hand, the second stress state (stress β) was dominant around the boundary between the Aoshima and Miyazaki facies. It is subvertical to WNW–ESE trending σ1-axis and NNE–SSW oriented σ3-axis. The girdle distribution of the σ1-axis is probably due to the high-stress ratio (~0.9; σ1 ≈ σ2) because the vertical σ1 and WNW–ESE trending σ2 can be switched easily. The switched maximum horizontal stress corresponds to the arc-perpendicular compression.
The causes of the arc-perpendicular extension (stress α) in the Kyushu forearc have been considered to be the back-arc rifting along the Okinawa Trough and the rollback of the subducting slab in southern Kyushu. Apart from the regional extension, we identified the localized arc-perpendicular compression (stress β) which is possibly caused by the seamount subduction along the Kyushu–Palau Ridge, being subducted beneath the Miyazaki Group since the early Pliocene. The subduction and localized underplating of seamounts apparently caused the localized uplift of the Aoshima Facies and horizontal compression around the boundary of the Aoshima and Miyazaki facies.
[Reference]
Yamaji, A., 2003. Slab rollback suggested by latest Miocene to Pliocene forearc stress and migration of volcanic front in southern Kyushu, northern Ryukyu Arc. Tectonophysics, 364, 9-24. https://doi .org /10.1016/S0040-1951(03)00047-7
Yoshimoto, T., Chiyonobu, S., Omori, Y., Zhang, F., Yamamoto, Y., 2023. Spatial variations in the consolidation of sediments in the Neogene Miyazaki forearc basin, Southwest Japan. Tectonophysics, 862, 229922. https://doi.org/10.1016/j.tecto.2023.229922