17:15 〜 18:30
[SCG45-P04] Magnetic and gravity constraints on crustal structure of the Nosappu Fracture Zone, Northwestern Pacific
キーワード：断裂帯、地磁気異常、海域地球物理観測
The structure and lithology of fracture zones (FZ) are essential for understanding long-term tectonic evolution of the seafloor as well as Earth water and material cycles. Widely obtained marine geophysical dataset such as magnetic anomalies can provide strong constraint on crustal structures, in which several historical information such as magnetic field variations, crustal accretion processes, and temporal change of lithology should be reserved. Here,we present analyses of magnetic dataset obtained along and across the Nosappu Fracture Zone (NFZ). The NFZ is located on the old Pacific lithosphere just before subducting into the Kuril trench. It extends over 1000 km NNE-SSW direction from around 33°N,154°E; the Shatsky Rise, to around 42° N,148°E; the Kuril Trench. The horizontal distance of offset is about 350 km based on the magnetic isochron M5r, which is 6 Myr or more converted to age (Nakanishi, 1993). The purpose of this study is to understand the geophysical properties of the magnetic, as well as gravity and topography, in the NFZ for providing constraints on the crustal structure.
Multibeam bathymetry, total and vector magnetic fields, and gravity data were acquired along and across the NFZ during two research cruises; the KR06-03 with R/V Kairei and YK14-09 with R/V Yokosuka. The magnetic anomaly was calculated by subtracting the International Geomagnetic Reference Field (IGRF12; e.g., Thébault et al., 2015) from each magnetic field data point. The amplitude of the magnetic anomaly ranged from -500 to +600 nT for the east side of the NFZ, and from -200 to +400 nT for the west side. The clear magnetic stripes appeared in the magnetic anomaly map and were divided into the east and west sequences, which corresponds to each formation age of the crust in contact with the NFZ. The crustal magnetization structure was estimated from the calculated magnetic anomaly based on the well-established inversion methods (e.g., Macdonald et al., 1980; Parker & Huestis, 1974). The inversion calculation was carried out with a focus on the area where dense data were obtained, assuming a 1.0 km thick magnetized source layer without vertical variation of magnetization. No annihilator was added. For estimating paleo-latitude when the oceanic crust was formed, we examined various magnetization directions to determine the optimal one by comparing them with the results of the topography and gravity analysis. To determine the FZ area semi-objectively, slope angle was calculated from the multibeam bathymetric data using the Generic Mapping Tools (GMT; Wessel & Smith, 1991). Slope azimuth was also calculated to detect the strike of abyssal hills using GMT. We determined the east and west boundaries of the FZ area between normal oceanic crust by using the maximum value of the slope angle. The strike of abyssal hills around the NFZ was also detected by using the results of slope azimuth. In addition, mantle bouguer anomaly (MBA) was calculated from the satellite-derived free-air gravity anomaly (Sandwell et al., 2014) by subtracting the predicted gravity for the gridded multibeam bathymetry data and a constant thickness crust using the method of Parker (1973).
The low intensity of the crustal magnetization was observed along the FZ area. Two magnetic boundaries parallel to the NFZ were identified by the vector magnetic anomaly and the crustal magnetization. Assuming the magnetization inclination was -30°, the magnetic boundaries almost coincided with the east and west boundaries of the FZ area based on the bathymetric results. The region where low MBA value appeared also corresponded to the FZ area. Inside the FZ area, several local areas with high magnetization intensity were observed at the areas with local low value of the MBA. In this presentation, we aim to discuss geophysical insights to the formation background of crustal structure at regional scales.
Multibeam bathymetry, total and vector magnetic fields, and gravity data were acquired along and across the NFZ during two research cruises; the KR06-03 with R/V Kairei and YK14-09 with R/V Yokosuka. The magnetic anomaly was calculated by subtracting the International Geomagnetic Reference Field (IGRF12; e.g., Thébault et al., 2015) from each magnetic field data point. The amplitude of the magnetic anomaly ranged from -500 to +600 nT for the east side of the NFZ, and from -200 to +400 nT for the west side. The clear magnetic stripes appeared in the magnetic anomaly map and were divided into the east and west sequences, which corresponds to each formation age of the crust in contact with the NFZ. The crustal magnetization structure was estimated from the calculated magnetic anomaly based on the well-established inversion methods (e.g., Macdonald et al., 1980; Parker & Huestis, 1974). The inversion calculation was carried out with a focus on the area where dense data were obtained, assuming a 1.0 km thick magnetized source layer without vertical variation of magnetization. No annihilator was added. For estimating paleo-latitude when the oceanic crust was formed, we examined various magnetization directions to determine the optimal one by comparing them with the results of the topography and gravity analysis. To determine the FZ area semi-objectively, slope angle was calculated from the multibeam bathymetric data using the Generic Mapping Tools (GMT; Wessel & Smith, 1991). Slope azimuth was also calculated to detect the strike of abyssal hills using GMT. We determined the east and west boundaries of the FZ area between normal oceanic crust by using the maximum value of the slope angle. The strike of abyssal hills around the NFZ was also detected by using the results of slope azimuth. In addition, mantle bouguer anomaly (MBA) was calculated from the satellite-derived free-air gravity anomaly (Sandwell et al., 2014) by subtracting the predicted gravity for the gridded multibeam bathymetry data and a constant thickness crust using the method of Parker (1973).
The low intensity of the crustal magnetization was observed along the FZ area. Two magnetic boundaries parallel to the NFZ were identified by the vector magnetic anomaly and the crustal magnetization. Assuming the magnetization inclination was -30°, the magnetic boundaries almost coincided with the east and west boundaries of the FZ area based on the bathymetric results. The region where low MBA value appeared also corresponded to the FZ area. Inside the FZ area, several local areas with high magnetization intensity were observed at the areas with local low value of the MBA. In this presentation, we aim to discuss geophysical insights to the formation background of crustal structure at regional scales.