17:15 〜 18:45
[SCG48-P32] 琉球弧北部トカラ列島東方海域の音響層序及び海底地質構造
キーワード：マルチチャンネル反射法地震探査、琉球弧、トカラ列島
Tokara Islands are located between Yakushima Island and Oshima Island in the northern Ryukyu arc, and mainly consist of the Quaternary volcanic arc. The geological history on these area plays an important role in constraining tectonic system of ongoing backarc rifting, structural development between the northern Ryukyu arc and Kyusyu, and paleoceanographic changes related to the Kuroshio Current. Here we present seismic stratigraphy and structural characteristics to discuss the tectonic evolution east of Tokara Islands. The detail seismic and sampling data provide evidence of uplift and subsidence from Pliocene to present in the forearc region.
We conducted multichannel seismic reflection surveys in FY2020–2022 in the vicinity of Tokara Islands for marine geological mapping at the scale of 1:200,000. Seismic stratigraphy was defined after correlation of more than fifty seismic profiles obtained along the arc and perpendicular to the arc. The ages of each acoustic unit were determined by outcrop sedimentary rocks obtained by dredge and grab sampling. In this study, four major acoustic units (Units TY1, TY2, TY3, and TY4 in ascending order) were identified based on widely distributed unconformities. Unit TY1 is the acoustic basement distributed on the surface of forearc ridge. Units TY2, TY3, and TY4 are characterized by clearly stratified deposits filling the basin to the east of the forearc ridge, indicating two subsidence events. The truncation observed along the Unit TY2 surface suggests uplift occurred before the sedimentation of Unit TY3. The thickness of Unit TY4, the uppermost strata, in the western basin of Tokara Valley increases southwestward, showing ongoing significant subsidence in that region. The geological structures associated with deformations in Units TY2–TY4 strata are mainly characterized by NNE-SSW trending normal faults. In addition, WNW-ESE trending normal faults with strike-slip components are also observed along small (tens-kilometer scale) ridges west of the forearc ridge. The detrital U-Pb dating of sandstones showed that Unit TY1 consists of the Cretaceous accretionary complexes (unpublished data), and biostratigraphic analysis showed that sedimentary rocks belonging to Units TY2 and TY3 correspond to the Pliocene and Early Pleistocene, respectively (Arimoto & Utsunomiya, 2023; Arimoto & Tanaka, submitted). Our results suggest that the northern Ryukyu arc preserves sediments correlated to the Shimajiri Group, which was formed under the early stage of back-arc rifting in the Pliocene followed by uplift. Successively, the Early Pleistocene sedimentary unit was formed under remarkable subsidence in the southwestern part of the basin, as siliciclastic hemipelagic facies in contrast to the contemporaneous reef complex of the Ryukyu Group. In this presentation, we will discuss geological history based on detailed structures observed in each acoustic unit.
We conducted multichannel seismic reflection surveys in FY2020–2022 in the vicinity of Tokara Islands for marine geological mapping at the scale of 1:200,000. Seismic stratigraphy was defined after correlation of more than fifty seismic profiles obtained along the arc and perpendicular to the arc. The ages of each acoustic unit were determined by outcrop sedimentary rocks obtained by dredge and grab sampling. In this study, four major acoustic units (Units TY1, TY2, TY3, and TY4 in ascending order) were identified based on widely distributed unconformities. Unit TY1 is the acoustic basement distributed on the surface of forearc ridge. Units TY2, TY3, and TY4 are characterized by clearly stratified deposits filling the basin to the east of the forearc ridge, indicating two subsidence events. The truncation observed along the Unit TY2 surface suggests uplift occurred before the sedimentation of Unit TY3. The thickness of Unit TY4, the uppermost strata, in the western basin of Tokara Valley increases southwestward, showing ongoing significant subsidence in that region. The geological structures associated with deformations in Units TY2–TY4 strata are mainly characterized by NNE-SSW trending normal faults. In addition, WNW-ESE trending normal faults with strike-slip components are also observed along small (tens-kilometer scale) ridges west of the forearc ridge. The detrital U-Pb dating of sandstones showed that Unit TY1 consists of the Cretaceous accretionary complexes (unpublished data), and biostratigraphic analysis showed that sedimentary rocks belonging to Units TY2 and TY3 correspond to the Pliocene and Early Pleistocene, respectively (Arimoto & Utsunomiya, 2023; Arimoto & Tanaka, submitted). Our results suggest that the northern Ryukyu arc preserves sediments correlated to the Shimajiri Group, which was formed under the early stage of back-arc rifting in the Pliocene followed by uplift. Successively, the Early Pleistocene sedimentary unit was formed under remarkable subsidence in the southwestern part of the basin, as siliciclastic hemipelagic facies in contrast to the contemporaneous reef complex of the Ryukyu Group. In this presentation, we will discuss geological history based on detailed structures observed in each acoustic unit.