11:45 〜 12:00
[SCG52-15] Geophysical characteristics of Nankaido Megamullions in the Shikoku Basin: tectonic implications for backarc spreading initiation
キーワード：背弧拡大、四国海盆、海洋コアコンプレックス、重力異常、地磁気
Why and how backarc spreading initiates and ceases have been essential questions in plate tectonics. Understanding the process of backarc spreading is necessary to understand the origin of the Philippine Sea, which is largely composed of backarc basins. The backarc spreading of Shikoku and Parece Vela Basin in the Philippine Sea is considered to have started around 29–26 Ma and terminated around 15 Ma.
Oceanic core complexes (OCCs), or megamullions, are domal structures with corrugations on their surface seen on spreading seafloors. They develop when the melt supply from the spreading center is diminished (~50%). At the OCCs, the rocks from the lower crust and the mantle are exhumed along large detachment faults to the seafloor. Therefore, investigating the OCCs helps comprehend the evolution of the backarc basins. Indeed, several OCCs along the Parece Vela and Shikoku remnant backarc spreading center have been intensively studied (Godzilla Megamullion, Mado Megamullion) and provide information on the termination of the Shikoku and Parece Vela backarc basin. However, the information on the backarc spreading initiation has been lacking.
The cruise YK22-18S of R/V Yokosuka aimed to conduct a geophysical survey and sample the rocks over the several OCCs in the westernmost part of the Shikoku Basin. We tentatively named these OCCs “Nankaido Megamullions”, which also include Mado Megamullion in the Shikoku Basin remnant backarc spreading center. Most of the OCCs of the Nankaido Megamullions are located near the Kyushu-Palau Ridge, including the Sui-Shin Megamullion, which was investigated during YK21-06S. The Kyushu-Palau Ridge is a remnant half of the proto-Izu-Bonin arc, so the OCCs near the Kyushu-Palau Ridge are expected to provide information on 22–20 Ma, the period of backarc spreading initiation.
From the multibeam bathymetry data and total field magnetic data obtained during YK21-06S and YK22-18S, we present a bathymetric map, Mantle Bouguer Anomaly (MBA) map, and a magnetic anomaly map. Judging from the domal structures with a relative height of ~1000 m, the corrugations parallel to the spreading direction, and the ~140 mGal positive MBA, we identified three OCCs among the Nankaido Megamullions. Furthermore, we calculated the magnetic anomaly from the total magnetic field data and obtained a geomagnetic stripe pattern. We also present the deskewed geomagnetic anomaly pattern and determine the magnetic chrons precisely.
Oceanic core complexes (OCCs), or megamullions, are domal structures with corrugations on their surface seen on spreading seafloors. They develop when the melt supply from the spreading center is diminished (~50%). At the OCCs, the rocks from the lower crust and the mantle are exhumed along large detachment faults to the seafloor. Therefore, investigating the OCCs helps comprehend the evolution of the backarc basins. Indeed, several OCCs along the Parece Vela and Shikoku remnant backarc spreading center have been intensively studied (Godzilla Megamullion, Mado Megamullion) and provide information on the termination of the Shikoku and Parece Vela backarc basin. However, the information on the backarc spreading initiation has been lacking.
The cruise YK22-18S of R/V Yokosuka aimed to conduct a geophysical survey and sample the rocks over the several OCCs in the westernmost part of the Shikoku Basin. We tentatively named these OCCs “Nankaido Megamullions”, which also include Mado Megamullion in the Shikoku Basin remnant backarc spreading center. Most of the OCCs of the Nankaido Megamullions are located near the Kyushu-Palau Ridge, including the Sui-Shin Megamullion, which was investigated during YK21-06S. The Kyushu-Palau Ridge is a remnant half of the proto-Izu-Bonin arc, so the OCCs near the Kyushu-Palau Ridge are expected to provide information on 22–20 Ma, the period of backarc spreading initiation.
From the multibeam bathymetry data and total field magnetic data obtained during YK21-06S and YK22-18S, we present a bathymetric map, Mantle Bouguer Anomaly (MBA) map, and a magnetic anomaly map. Judging from the domal structures with a relative height of ~1000 m, the corrugations parallel to the spreading direction, and the ~140 mGal positive MBA, we identified three OCCs among the Nankaido Megamullions. Furthermore, we calculated the magnetic anomaly from the total magnetic field data and obtained a geomagnetic stripe pattern. We also present the deskewed geomagnetic anomaly pattern and determine the magnetic chrons precisely.