15:30 〜 15:45
[SCG67-06] プチスポットの配列と海洋プレートの変形構造
キーワード：プチスポット, アルカリ玄武岩, 太平洋プレート, アウターライズ, リソスフェア
Tiny submarine volcanoes, known as petit-spots, occur in regions of plate flexure prior to subduction and seamount-loading (Hirano et al., 2006, 2013). The surface morphology and distribution of petit-spot monogenetic volcanoes are influenced by the stress field in the lithosphere. The magmas produced by the accumulation of melt originating from asthenosphere just below the site of plate-flexure, are able to rapidly ascend to the surface from the base of the lithosphere (Hirano et al., 2006; 2011; Valentine & Hirano, 2011). As monogenetic petit-spot volcanoes are too small to be detected by satellite altimetry, their study requires a research vessel equipped for shipboard acoustic multibeam surveys. Although previously surveyed areas are limited to the regions off the Japan Trench, the eruption ages of petit-spot volcanoes in this region represent monogenetic eruptions over a period of >9 Myr over a large area, and the eruptions are related to the outer rise bathymetry. Such tiny volcanoes are ubiquitous in regions of plate flexure worldwide, and have been recently reported from the Tonga Trench (Hirano et al., 2008), the Basin and Range province (Valentine & Hirano, 2010), south of Greenland (Uenzelmann-Neben et al., 2012), the Chile Trench (Hirano et al., 2013), an accretionary complex in Costa Rica (Buchs et al., 2013), and submarine French Polynesia (Hirano et al., submitted).The spatial distribution of submarine petit-spot volcanoes remains poorly constrained because shipboard bathymetry has not covered the entire northwestern Pacific Plate. If petit-spot volcanoes occur only in regions of plate flexure, then tiny submarine volcanoes should appear homogeneously on the submarine surface of outer rises. However, areas devoid of volcanoes and lava have been found surrounding areas of petit-spot volcanoes (i.e., sites A?C in Hirano et al., 2006), indicating that the local characteristics of the lithosphere, in addition to plate flexure, control the occurrence of petit-spot volcanoes. Here we report that the distribution of petit-spot volcanoes is controlled by the tectonic structure of the seafloor. We conducted submersible dives along the linearly distributed petit-spot knolls by JAMSTEC Shinkai6500 in April 2014. Areas with tectonic fabrics appear on the subducting Pacific Plate off the Japan Trench, including a ridge-perpendicular fabric zone (RPPFZ), ridge-parallel abyssal hills (RPRAH), and subducting 'horst and graven' structures (HAGS) (Nakanishi et al., 2011). At Site C, which is a trench-oceanward slope offshore from Soma City, Fukushima Prefecture, the trend of the Japan Trench changes from N?S in the north to NE?SW in the south, where two areas of trench-parallel HAGSs are intersecting in a complex manner (Nakanishi et al., 2011). The distribution of young volcanic cones of more than 80 petit-spots, reported from Site C by Hirano et al. (2008), seems to be controlled by the fabrics of RPPFZ and RPRAH, the trends of which are continuous with the HAGS to the north and south along the trench-oceanward slope, respectively. Although the RPPFZ is not recognized as a fracture zone, its trend is sub-parallel to that of the neighboring Nosappu and Kashima fracture zones (Nakanishi, 1993). As the RPPFZ and RPRAH that control the petit-spot distribution are clearly original structures of the lithosphere (in contrast to HAGS), the occurrence of petit-spot eruptions is possibly related to lithospheric structures.