09:30 〜 09:45
[SCG55-14] The reconstruction of the plate boundary between the Pacific-Izanagi plates during mid-Cretaceous time
キーワード：太平洋プレート、アビサルヒル、磁気異常縞模様、太平洋ーイザナギ海嶺
The Pacific Plate is the largest existing oceanic plate. The plate was born in Early Jurassic, about 190 Ma (e.g., Nakanishi et al., 1992). The detailed tectonic history of the plate during mid-Cretaceous time, especially around the Cretaceous Normal Superchron, remain unclear. The Pacific Plate was surrounded by Izanagi, Farallon, and Phoenix plates during the period. The magnetic anomaly lineations of the Pacific Plate from Late Jurassic to Early Cretaceous were identified by several works (e.g., Larson and Chase 1972; Sager et al., 1988; Nakanishi et al., 1989; 1992; 2015).
The Pacific-Phoenix ridge reorganized around 120 Ma, the beginning of the Cretaceous Normal Superchron (e.g., Nakanishi et al, 1992; Larson, 1997). Several studies suggested that the reorganization was related with the formation of the largest oceanic plateau, Ontong Java Nui (e.g., Taylor, 2006). Magnetic anomaly lineations south of the Kuril Trench indicate that the reorganization of the Pacific-Izanagi ridge occurred after chron M3 (126 Ma, Nakanishi et al., 1989). There is no detailed information of the reorganization of the Pacific-Farallon ridge.
We are proceeding to examine magnetic anomaly lineations and inherited seafloor spreading fabric as abyssal hills to reveal the tectonic history of the Pacific Plate from Late Jurassic to Early Cretaceous using the latest geomagnetic and multibeam bathymetric data. Here, we present clues about the reconstruction of the plate boundary between the Pacific-Izanagi plates during mid-Cretaceous time. Our study area is bounded by the Kuril Trench to the north, Emperor Seamount Chain to the east, Shatsky Rise to the south, and Nosappu Fracture Zone to the west. The seafloor in the study area was formed at the Pacific-Izanagi ridge in this period. The Hokkaido Trough is situated in the eastern part of the study area and one of the remarkable topographic features in the study area.
Geomagnetic and multibeam bathymetric data incorporated in this study were derived from several different sources. We conducted geomagnetic and bathymetric surveys during several cruises by R/Vs Hakuho-maru, Kairei, Mirai, Yokosuka, Falkor, and Thomas G. Thompsonin the study area. Additional data were obtained from the domestic and international institutions and projects.
We calculated magnetic anomalies using the Comprehensive Model 4 (Sabaka et al., 2004) and International Geomagnetic Reference Field 14th. The geomagnetic polarity timescale used in this study is that proposed by Ogg (2020). A bathymetric grid data set was produced at a 100 m spacing after recalculation of and manual editing of multibeam bathymetric data.
Our bathymetric map exposes complex chaotic topographic relief west of the Hokkaido Trough. The topographic relief has the similar size of abyssal hill fabric formed seafloor spreading. Some relief has the strike with an NE-SW strike, which is different from the strike of magnetic anomaly lineations in the southern part of the study area. Similar topographic relief is observed in the pseudo-fault zone associated with a ridge propagation (e.g., Hey et al., 1986). The magnetic anomaly lineations from M6 to M3 south of the area of the chaotic topographic relief shorten with age. We could not identify any younger magnetic anomaly lineations with the same strike of the magnetic anomaly lineation M3 in the area of the chaotic topographic relief. We thus conclude that the chaotic topographic relief was formed by propagation of the Pacific-Izanagi ridge that began around M6 (ca. 128 Ma).
The Pacific-Phoenix ridge reorganized around 120 Ma, the beginning of the Cretaceous Normal Superchron (e.g., Nakanishi et al, 1992; Larson, 1997). Several studies suggested that the reorganization was related with the formation of the largest oceanic plateau, Ontong Java Nui (e.g., Taylor, 2006). Magnetic anomaly lineations south of the Kuril Trench indicate that the reorganization of the Pacific-Izanagi ridge occurred after chron M3 (126 Ma, Nakanishi et al., 1989). There is no detailed information of the reorganization of the Pacific-Farallon ridge.
We are proceeding to examine magnetic anomaly lineations and inherited seafloor spreading fabric as abyssal hills to reveal the tectonic history of the Pacific Plate from Late Jurassic to Early Cretaceous using the latest geomagnetic and multibeam bathymetric data. Here, we present clues about the reconstruction of the plate boundary between the Pacific-Izanagi plates during mid-Cretaceous time. Our study area is bounded by the Kuril Trench to the north, Emperor Seamount Chain to the east, Shatsky Rise to the south, and Nosappu Fracture Zone to the west. The seafloor in the study area was formed at the Pacific-Izanagi ridge in this period. The Hokkaido Trough is situated in the eastern part of the study area and one of the remarkable topographic features in the study area.
Geomagnetic and multibeam bathymetric data incorporated in this study were derived from several different sources. We conducted geomagnetic and bathymetric surveys during several cruises by R/Vs Hakuho-maru, Kairei, Mirai, Yokosuka, Falkor, and Thomas G. Thompsonin the study area. Additional data were obtained from the domestic and international institutions and projects.
We calculated magnetic anomalies using the Comprehensive Model 4 (Sabaka et al., 2004) and International Geomagnetic Reference Field 14th. The geomagnetic polarity timescale used in this study is that proposed by Ogg (2020). A bathymetric grid data set was produced at a 100 m spacing after recalculation of and manual editing of multibeam bathymetric data.
Our bathymetric map exposes complex chaotic topographic relief west of the Hokkaido Trough. The topographic relief has the similar size of abyssal hill fabric formed seafloor spreading. Some relief has the strike with an NE-SW strike, which is different from the strike of magnetic anomaly lineations in the southern part of the study area. Similar topographic relief is observed in the pseudo-fault zone associated with a ridge propagation (e.g., Hey et al., 1986). The magnetic anomaly lineations from M6 to M3 south of the area of the chaotic topographic relief shorten with age. We could not identify any younger magnetic anomaly lineations with the same strike of the magnetic anomaly lineation M3 in the area of the chaotic topographic relief. We thus conclude that the chaotic topographic relief was formed by propagation of the Pacific-Izanagi ridge that began around M6 (ca. 128 Ma).