17:15 〜 18:30
[SCG45-P08] Origin of Philippine Sea basins -Tectonic setting leading to subduction initiation of Izu-Bonin-Mariana arc-
キーワード：沈み込み開始、伊豆小笠原マリアナ弧、フィリピン海プレート
Robust tectonic reconstruction of the evolving Philippine Sea Plate for the period immediately before and after subduction initiation at ~52 Ma to form the Izu-Bonin-Mariana (IBM) arc is prerequisite to understand cause of subduction initiation (SI) and test competing hypotheses for SI such as spontaneous or induced nucleation. Understanding of nature and origin of overriding and subducting plates is especially important because plate density is a key parameter controlling SI based on numerical modeling (e.g., Leng and Gurnis 2015). There is increasing evidence that multiple geological events related to changing stress fields took place in and around Philippine Sea plate about the time of SI ~52 Ma (Ishizuka et al., 2011). For our understanding of the early IBM arc system to increase, it is important to understand the pattern and tempo of these geological events, particularly the duration and extent of seafloor spreading in the proto arc associated with SI, and its temporal relationship with spreading in the West Philippine Basin (WPB) and structural development of the Daito Ridge Group.
Recent cruises (KS-17-15 and YK19-07S cruises) in the Philippine Sea basins investigated origin and age of formation of ocean basins in and around the Daito Ridge group. The data are still preliminary, but some fresh basalt samples returned well-defined 40Ar/39Ar ages older than the age of onset of subduction along the IBM arc at c. 52 Ma. Combined with interpretation of seafloor magnetic anomaly data, this strongly implies that some part of the Philippine Sea basins such as the West Philippine Basin existed prior to the IBM arc, and this basin could be a part of overriding plate at subduction initiation.
Temporal variation of geochemical characteristics of basalts in the Philippine Sea basins appears to indicate that major compositional variation occurred in sub-Philippine Sea mantle subsequent to subduction initiation. This might imply that sinking of large volume of Pacific Plate along the entire IBM arc margin triggered counterflow of asthenospheric mantle and reorganization of sub-Philippine Sea mantle.
The Kita-Daito Basin separates the Amami Plateau and the Daito Ridge. No rock sampling has been reported from this basin to constrain its age and origin of this basin. The Vp model of the Kita-Daito Basin suggests the presence of a 4 – 6 km thick crust, similar to the backarc basin oceanic crust in the Shikoku and Parece Vela Basins, as revealed by Nishizawa et al. (2011, 2013). These features seem to imply that rifting and seafloor spreading occurred between the Amami Plateau and Daito Ridge to form the Kita-Daito Basin. Dredge sampling and Shinkai 6500 submersible survey observed and recovered samples from the crustal sections exposed in the deepest part of the basin. Collected samples are mostly composed of volcanic rocks, porphyries and trace amount of sedimentary rock and conglomerate. The volcanic rocks are basalt to andesite clasts. These samples show geochemical characteristics implying influence of material released from subducting slab. This implies that these rocks are not part of ocean crust composed of MORB. These volcanic rocks gave ages of middle Eocene, which indicates that these volcanic rocks formed after the period of subduction initiation of Izu-Bonin-Mariana arc. These observations seem to imply that the Kita-Daito Basin did not exist at the subduction initiation, and formed by rifting in middle Eocene accompanied by basaltic to andesitic volcanism.
Recent cruises (KS-17-15 and YK19-07S cruises) in the Philippine Sea basins investigated origin and age of formation of ocean basins in and around the Daito Ridge group. The data are still preliminary, but some fresh basalt samples returned well-defined 40Ar/39Ar ages older than the age of onset of subduction along the IBM arc at c. 52 Ma. Combined with interpretation of seafloor magnetic anomaly data, this strongly implies that some part of the Philippine Sea basins such as the West Philippine Basin existed prior to the IBM arc, and this basin could be a part of overriding plate at subduction initiation.
Temporal variation of geochemical characteristics of basalts in the Philippine Sea basins appears to indicate that major compositional variation occurred in sub-Philippine Sea mantle subsequent to subduction initiation. This might imply that sinking of large volume of Pacific Plate along the entire IBM arc margin triggered counterflow of asthenospheric mantle and reorganization of sub-Philippine Sea mantle.
The Kita-Daito Basin separates the Amami Plateau and the Daito Ridge. No rock sampling has been reported from this basin to constrain its age and origin of this basin. The Vp model of the Kita-Daito Basin suggests the presence of a 4 – 6 km thick crust, similar to the backarc basin oceanic crust in the Shikoku and Parece Vela Basins, as revealed by Nishizawa et al. (2011, 2013). These features seem to imply that rifting and seafloor spreading occurred between the Amami Plateau and Daito Ridge to form the Kita-Daito Basin. Dredge sampling and Shinkai 6500 submersible survey observed and recovered samples from the crustal sections exposed in the deepest part of the basin. Collected samples are mostly composed of volcanic rocks, porphyries and trace amount of sedimentary rock and conglomerate. The volcanic rocks are basalt to andesite clasts. These samples show geochemical characteristics implying influence of material released from subducting slab. This implies that these rocks are not part of ocean crust composed of MORB. These volcanic rocks gave ages of middle Eocene, which indicates that these volcanic rocks formed after the period of subduction initiation of Izu-Bonin-Mariana arc. These observations seem to imply that the Kita-Daito Basin did not exist at the subduction initiation, and formed by rifting in middle Eocene accompanied by basaltic to andesitic volcanism.