17:15 〜 18:30
[SCG45-P13] 背弧リフティングステージにおける火山活動とテクトニクスの関係：南部沖縄トラフの３火山の解析から
キーワード：重磁力異常、背弧海盆、沖縄トラフ
Back-arc basins are semi-closed seas developed under extensional stress field along subduction zones. Though the origin of the extensional field is still debating, the evolution generally follows the common process; arc or continental crust thinning, development of normal faults (=rifting), breaking off the pre-existed crust, and finally continuous oceanic crust formation. The initial rifting stage is sometimes accompanied by sporadic volcanism, but the link between arc/back-arc volcanism and rifting has not been well-understood. The Okinawa Trough was a back-arc basin at the rifting stage, where the Philippine Sea Plate subducts beneath the Yangtze Plate. The previous studies on the back-arc rifting mainly dealt with rifting in oceanic island arcs. Since the southern Okinawa Trough (OT) is one of few places where we can observe the back-arc rifting on the continental crust, it provides us a valuable opportunity to understand the details of the continental rifting process and its relationship with volcanism.
In our research, geophysical data of the Yonaguni Knolls, the Irabu Knolls, and the Tarama Knolls - three volcanic areas near the rifting axis locating in the southern Okinawa Trough are analyzed. These three volcanic areas are located in different tectonic settings. Through the bathymetry, gravity anomaly, magnetic anomaly, and other compiled data, I tried to elucidate the character of these knolls and the adjacent areas and then to discuss the relationship with their tectonic background.
The axial rift valley generally extends the E-W direction and segmented. The rift valley is 18 km-wide and is accompanied by a high Bouguer anomaly in the eastern Yonaguni area. Both the widths and the Bouguer anomalies of the rift valleys surrounding the Irabu and Tarama Knolls decrease eastward. These observations suggest that the rifting stage is more progressed in the western end of the Okinawa Trough. High magnetic anomalies are observed at minor ridge and knolls within the rift valley and along the rift walls, indicating magma injection. Three volcanic areas show different geophysical features, reflecting their tectonic settings.
The Yonaguni Knolls, located at the western end of the Okinawa Trough, forms an en echelon arrangement of the E-W trending knoll chain (row), likely controlled by the rift faults.
Irabu Knolls locates at the intersection of two differently trending segments of the Okinawa Trough. It may form many faults that enhance the magma intrusion.
Tarama Knolls is located on the arc side flat seafloor. Two types of volcanic eruption are presumed here; fissure-eruption with higher magnetic anomaly and explosive eruption with a large volume of pumice causing low gravity anomaly and no magnetic anomaly.
As the result of the forward modeling of magnetic anomalies, I estimated the magnetization of Yonaguni, Tarama, and Irabu Knolls as 14, 10, and 2 A/m, respectively. This result is consistent with previously collected rock samples.
In our research, geophysical data of the Yonaguni Knolls, the Irabu Knolls, and the Tarama Knolls - three volcanic areas near the rifting axis locating in the southern Okinawa Trough are analyzed. These three volcanic areas are located in different tectonic settings. Through the bathymetry, gravity anomaly, magnetic anomaly, and other compiled data, I tried to elucidate the character of these knolls and the adjacent areas and then to discuss the relationship with their tectonic background.
The axial rift valley generally extends the E-W direction and segmented. The rift valley is 18 km-wide and is accompanied by a high Bouguer anomaly in the eastern Yonaguni area. Both the widths and the Bouguer anomalies of the rift valleys surrounding the Irabu and Tarama Knolls decrease eastward. These observations suggest that the rifting stage is more progressed in the western end of the Okinawa Trough. High magnetic anomalies are observed at minor ridge and knolls within the rift valley and along the rift walls, indicating magma injection. Three volcanic areas show different geophysical features, reflecting their tectonic settings.
The Yonaguni Knolls, located at the western end of the Okinawa Trough, forms an en echelon arrangement of the E-W trending knoll chain (row), likely controlled by the rift faults.
Irabu Knolls locates at the intersection of two differently trending segments of the Okinawa Trough. It may form many faults that enhance the magma intrusion.
Tarama Knolls is located on the arc side flat seafloor. Two types of volcanic eruption are presumed here; fissure-eruption with higher magnetic anomaly and explosive eruption with a large volume of pumice causing low gravity anomaly and no magnetic anomaly.
As the result of the forward modeling of magnetic anomalies, I estimated the magnetization of Yonaguni, Tarama, and Irabu Knolls as 14, 10, and 2 A/m, respectively. This result is consistent with previously collected rock samples.