13:45 〜 15:15
[SCG52-P08] Revisiting Mesozoic fracture zones in the northwestern Pacific Ocean
キーワード：断裂帯、磁気異常縞模様、北西太平洋
We will present topographic expression of several major fracture zones in the northwestern Pacific Ocean. Nakanishi (1993) described the topographic expression of these Mesozoic fracture zones in the northwestern Pacific Ocean by bathymetric and seismic reflection profiles. The Kashima Fracture Zone (KFZ) is located along the western margin of the northwestern Pacific Ocean south of the Japan Trench. The northern end of this fracture zone seems to have been subducted at the Japan Trench near Daiichi Kashima Seamount. The Nosappu Fracture Zone (NFZ) extends more than 1,000 km from the northwestern edge of the Shatsky Rise to the Kuril Trench. The South Shatsky Fracture Zone (SFZ) situated south of the Shatsky Rise is approximately 600 km long and obliquely cuts the Hawaiian lineation set.
Multibeam bathymetric data collected by only a few cruises were used in the previous work. Since then, multibeam data were collected by many research vessels in the northwestern Pacific Ocean. We compiled these data from the fracture zones to better define their physiography and structural pattern. The multibeam data incorporated in this study were obtained from the databases of several institutions (e.g., Japan Agency for Marine-Earth Science and Technology, National Oceanic and Atmospheric Administration/National Geophysical Data Center). We processed the multibeam data using the MB-System (Caress and Chayes, 1996) and GMT (Wessel et al., 2019) to produce a bathymetric grid.
The data set reveals remarkable topographic features of the fracture zones. The location of the KFZ north of 27°N determined by this study approximately corresponds to that by Nakanishi et al. (1989). The KFZ of this portion is characterized by a complex trough morphology. We could not determine the location of the fracture zone south of 27°N only by the bathymetric data because Cretaceous mid-plate volcanism obscured the topography of the KFZ.
The location of the NFZ north of 38°N approximately corresponds to that by Nakanishi et al. (1989). The NFZ of this portion manifests various topographic features such as a ridge, trough, and scarp. The NFZ between 36°N and 38°N is characterized by a complex morphology. En echelon knolls across the NFZ are observed there as described in Ogawa et al. (2003). Topographic features associated to the fracture zone are rarely visible south of 36°N. Magnetic anomaly lineations about 140 Ma are identified here. These imply that the NFZ commenced forming around 140 Ma; although, Nakanishi et al. (1989) suggested that the NFZ was born around 145 Ma just after the reorganization of the plate boundaries arising from the emergence of the Shatsky Rise.
The traces of propagating rifts are observed along the SFZ. This observation supports that the SFZ originated from apseudofault due to rift propagation of the Pacific-Farallon Ridge proposed by the previous works.
Multibeam bathymetric data collected by only a few cruises were used in the previous work. Since then, multibeam data were collected by many research vessels in the northwestern Pacific Ocean. We compiled these data from the fracture zones to better define their physiography and structural pattern. The multibeam data incorporated in this study were obtained from the databases of several institutions (e.g., Japan Agency for Marine-Earth Science and Technology, National Oceanic and Atmospheric Administration/National Geophysical Data Center). We processed the multibeam data using the MB-System (Caress and Chayes, 1996) and GMT (Wessel et al., 2019) to produce a bathymetric grid.
The data set reveals remarkable topographic features of the fracture zones. The location of the KFZ north of 27°N determined by this study approximately corresponds to that by Nakanishi et al. (1989). The KFZ of this portion is characterized by a complex trough morphology. We could not determine the location of the fracture zone south of 27°N only by the bathymetric data because Cretaceous mid-plate volcanism obscured the topography of the KFZ.
The location of the NFZ north of 38°N approximately corresponds to that by Nakanishi et al. (1989). The NFZ of this portion manifests various topographic features such as a ridge, trough, and scarp. The NFZ between 36°N and 38°N is characterized by a complex morphology. En echelon knolls across the NFZ are observed there as described in Ogawa et al. (2003). Topographic features associated to the fracture zone are rarely visible south of 36°N. Magnetic anomaly lineations about 140 Ma are identified here. These imply that the NFZ commenced forming around 140 Ma; although, Nakanishi et al. (1989) suggested that the NFZ was born around 145 Ma just after the reorganization of the plate boundaries arising from the emergence of the Shatsky Rise.
The traces of propagating rifts are observed along the SFZ. This observation supports that the SFZ originated from apseudofault due to rift propagation of the Pacific-Farallon Ridge proposed by the previous works.