17:15 〜 18:45
[SCG52-P05] 沈み込む帯へのインプット直前の太平洋プレート構成岩石
キーワード：ホットスポット、プチスポット、太平洋プレート、海山、アルカリ玄武岩
The western Pacific plate just prior to subduction into the Kuril-Japan-Izu-Ogasawara-Mariana trench systems is largely modified due to intraplate volcanic activities since the generation of a typical oceanic plate observed as a model at present mid-ocean ridges and ophiolites. Several kinds of volcanic edifice are observed on the western Pacific plate as some protrusions, which plausibly perform some erratic matters during the facility of subduction inputs. For example, subducting seamounts and petit-spot volcanic fields plausibly control the hypocentral locations of earthquakes along the plate boundary (e.g. Mochizuki et al., 2008; Singh et al., 2011; Fujie et al., 2020; Akizawa et al., 2022). Otherwise, the subduction inputs of such volcanic edifices also control geochemistry of island arc magma (e.g. Gamble et al., 1997; Nishizawa et al., 2017). This study summerizes the eruption ages and geological features of volcanic edifice on the incoming Pacific plate.
Most seamounts and oceanic plateaus occurred within the western Pacific plate, which are all older than 70 Ma as the West Pacific Seamount Province (WPSP) where the seamount distributions and eruption ages cannot be explained by the simple hotspot model. Those were caused by secondary plumelets such as those below the present French Polynesian region (Koppers et al., 2003). Some overprinting lavas on WPSP, however, are newly recognized, which are created during the Eocene based on Ar–Ar dates from Ogasawara Plateau, Uyeda Ridge, and Minamitorishima (Marcus) Island (Hirano et al, 2021). We have called this discovery “belated seamount rejuvenations”, implying a Tertiary hotspot track based on a Pacific plate reconstruction model. Petit-spot volcanoes of the western Pacific Plate were recognized as some volcanic fields composed of ten to hundred monogenetic eruptions along the zone of concave flexure of the outer rise prior to plate subduction. Submersible dive observations and dredge samplings near the axis of the Japan Trench observed petit-spot volcanic edifices truncated by normal faults on the oceanward slope of the trench, where the petit-spot volcanoes consist of a volcanic cone built upon the petit-spot lava sheets and the basement chert layers (Hirano et al., 2006). Likewise, the shipboard geophysical survey observed that volcanic sills beneath petit-spot edifices (Fujiwara et al., 2008). The petit-spot volcanic field was recognized as an area of thin apparent sediment layers in the seismic reflection survey, implying the significant disturbance of sediment layer on the incoming plate to the trench due to the petit-spot volcanic activities (Akizawa et al., 2022). Once subducted, the disturbed pelagic clays on the subducting slab associated with petit-spot volcanic fields are potentially a factor controlling the hypocentral locations of 2011 Tohoku Earthquake (Fujie et al., 2020).
Most seamounts and oceanic plateaus occurred within the western Pacific plate, which are all older than 70 Ma as the West Pacific Seamount Province (WPSP) where the seamount distributions and eruption ages cannot be explained by the simple hotspot model. Those were caused by secondary plumelets such as those below the present French Polynesian region (Koppers et al., 2003). Some overprinting lavas on WPSP, however, are newly recognized, which are created during the Eocene based on Ar–Ar dates from Ogasawara Plateau, Uyeda Ridge, and Minamitorishima (Marcus) Island (Hirano et al, 2021). We have called this discovery “belated seamount rejuvenations”, implying a Tertiary hotspot track based on a Pacific plate reconstruction model. Petit-spot volcanoes of the western Pacific Plate were recognized as some volcanic fields composed of ten to hundred monogenetic eruptions along the zone of concave flexure of the outer rise prior to plate subduction. Submersible dive observations and dredge samplings near the axis of the Japan Trench observed petit-spot volcanic edifices truncated by normal faults on the oceanward slope of the trench, where the petit-spot volcanoes consist of a volcanic cone built upon the petit-spot lava sheets and the basement chert layers (Hirano et al., 2006). Likewise, the shipboard geophysical survey observed that volcanic sills beneath petit-spot edifices (Fujiwara et al., 2008). The petit-spot volcanic field was recognized as an area of thin apparent sediment layers in the seismic reflection survey, implying the significant disturbance of sediment layer on the incoming plate to the trench due to the petit-spot volcanic activities (Akizawa et al., 2022). Once subducted, the disturbed pelagic clays on the subducting slab associated with petit-spot volcanic fields are potentially a factor controlling the hypocentral locations of 2011 Tohoku Earthquake (Fujie et al., 2020).