15:30 〜 15:45
[SCG55-07] OBS/MCS探査が明らかにした北海道下に沈み込む太平洋プレートの構造変化とその解釈
★招待講演
キーワード：沈み込む太平洋プレート、千島海溝アウターライズ域、海山、構造探査
The Pacific Plate subducting to the Japan and Kuril trenches is topographically characterized by bending-related faults, fracture zones, large seamounts, and therefore various heterogeneity seem to exist in the incoming plate. Previous seismic studies in these regions concluded that plate-bending faults at the trench outer slope is impactful for incoming plate hydration based on seismic velocity reduction. We have also analyzed an OBS seismic survey data newly collected in the Kuril Trench axial region to understand a state of the incoming plate development. Our previous model by first arrival and wide-angle PmP traveltime inversion proposed a low Vp (~7.5 km/s) mantle, expecting serpentinization of 40–50 %, quite larger than in the Japan and Kuril trenches (15–25 % serpentinization, Fujie et al., 2018), but similar to mantle Vp beneath the seamount (Kodaira et al., 2002; Nishizawa et al., 2009). Furthermore, a systematic difference in the estimated oceanic Moho depth was found between OBS and multi-channel seismic (MCS) data, meaning our model has the 8–10 km thick crustal-Vp (< 7 km/s) layer. We consider that this systematic difference can be explained by a presence of a zone with gradual increase of Vp beneath the MCS Moho; identified wide-angle phases seem to be not reflection but refraction in the corresponding zone. In this presentation, we quantitatively investigate the state of the low Vp anomaly around the Moho by an additional analysis based on traveltime data of the MCS profile in addition to OBS record sections.
The survey was conducted by R/Vs Kairei and Yokosuka of JAMSTEC in 2019. A 208 km-length survey line KT209 runs from the land slope to the trench outer slope across bending-related faults and a seamount (call as the KT209 seamount). 80 of OBSs were deployed every ~2 km along the line. An airgun array with a total volume of 7,800 cu.in. was used as a controlled source and shoot every 200 m. Multi-channel seismic (MCS) reflection data was also collected by towing a 5.5 km-length multichannel hydrophone streamer cable. Observed first arrival traveltimes in OBS record sections and two-way-time of near-vertical oceanic Moho reflection were used for modeling P-wave velocity (Vp) structure by a 2-D traveltime tomographic inversion.
The revised Vp model is featured by a deep low-Vp anomaly in the incoming plate; the anomaly is less than 7 km/s and still reaches 10 kilometers depth from the seafloor same as the previous model. Because this anomaly expands beneath the oceanic Moho depth defined by the MCS profile, we interpret that the anomaly causes not in but beneath the crust. This anomaly is also significant and thickest beneath the KT209 seamount and continuously appears to at least 30 km landward from the trench. We understand above all features are close to crustal thickening beneath the Erimo and the Daiichi-Kashima Seamounts imaged from wide-angle data (Nishizawa et al., 2009). In addition, traveltimes of wide-angle PmP are projected along the bottom of low Vp anomaly in the revised model. We therefore interpret that low Vp anomaly beneath the oceanic crustal basement, that is the Moho, would be underplated magma material and wide-angle PmP seem to be reflections at the bottom of underplate layer. The age of the Erimo and Ryofu Seamounts near the KT209 are dated at ~100 Ma which are at least 20 Ma younger than surrounding seafloor, and produced by Cretaceous off-ridge volcanism (Takigami et al., 1989). Although an origin of the KT209 seamount is unknown, the fact of large seamounts distribution limitedly in the southmost Kuril Trench implies that the KT209 seamounts would belong to these Cretaceous volcanoes. Tens of small seamounts can be also recognized on the bathymetric topography in the northwest Pacific. We suggest that the Pacific Plate in this region subducts with a strong spatial heterogeneity.
The survey was conducted by R/Vs Kairei and Yokosuka of JAMSTEC in 2019. A 208 km-length survey line KT209 runs from the land slope to the trench outer slope across bending-related faults and a seamount (call as the KT209 seamount). 80 of OBSs were deployed every ~2 km along the line. An airgun array with a total volume of 7,800 cu.in. was used as a controlled source and shoot every 200 m. Multi-channel seismic (MCS) reflection data was also collected by towing a 5.5 km-length multichannel hydrophone streamer cable. Observed first arrival traveltimes in OBS record sections and two-way-time of near-vertical oceanic Moho reflection were used for modeling P-wave velocity (Vp) structure by a 2-D traveltime tomographic inversion.
The revised Vp model is featured by a deep low-Vp anomaly in the incoming plate; the anomaly is less than 7 km/s and still reaches 10 kilometers depth from the seafloor same as the previous model. Because this anomaly expands beneath the oceanic Moho depth defined by the MCS profile, we interpret that the anomaly causes not in but beneath the crust. This anomaly is also significant and thickest beneath the KT209 seamount and continuously appears to at least 30 km landward from the trench. We understand above all features are close to crustal thickening beneath the Erimo and the Daiichi-Kashima Seamounts imaged from wide-angle data (Nishizawa et al., 2009). In addition, traveltimes of wide-angle PmP are projected along the bottom of low Vp anomaly in the revised model. We therefore interpret that low Vp anomaly beneath the oceanic crustal basement, that is the Moho, would be underplated magma material and wide-angle PmP seem to be reflections at the bottom of underplate layer. The age of the Erimo and Ryofu Seamounts near the KT209 are dated at ~100 Ma which are at least 20 Ma younger than surrounding seafloor, and produced by Cretaceous off-ridge volcanism (Takigami et al., 1989). Although an origin of the KT209 seamount is unknown, the fact of large seamounts distribution limitedly in the southmost Kuril Trench implies that the KT209 seamounts would belong to these Cretaceous volcanoes. Tens of small seamounts can be also recognized on the bathymetric topography in the northwest Pacific. We suggest that the Pacific Plate in this region subducts with a strong spatial heterogeneity.