188宝金博页面版

FULL PAPER Open AccessWide-angle refraction experiments in the DaitoRidges region at the northwestern end of thePhilippine Sea plateAzusa Nishizawa * , Kentaro Kaneda, Yasutaka Katagiri and Mitsuhiro OikawaAbstractThree large bathymetric highs (from north to south: the Amami Plateau, the Daito Ridge, and the Oki-Daito Ridge)originating from paleo-island arcs characterize the northwestern end of the Philippine Sea plate. We obtained 10seismic refraction and multi-channel seismic reflection profiles across and along these bathymetric highs and obtainedP wave velocity (Vp) models of the crust and the uppermost mantle. Although there are large variations in the crustalstructure throughout this region, these bathymetric highs usually have a middle crust with Vp of 6.3 to 6.8 km/s, alower crust with Vp of 6.8 to 7.2 km/s, a Pn velocity of 7.6 to 7.8 km/s, and a total crustal thickness of 15 to 25 km. Thesefeatures are similar to those of the Izu-Ogasawara (Bonin)-Mariana island arc and the Kyushu-Palau Ridge, whichare immature paleo-island arcs. However, the crust at the southwestern part of the Oki-Daito Ridge contains arelatively thin middle crust and a smaller total crustal thickness compared with other ridges in this region. Inaddition, we identified a deep reflector beneath the ridge, with these properties indicating a different origin,such as intraplate volcanism.Keywords: Daito Ridges; P wave velocity structure; Paleo-island arcBackgroundThe Philippine Sea plate is composed of three inactivemarginal basins, the Shikoku Basin, the Parece Vela Basin,and the West Philippine Basin, as well as many bathymetrichighs (Figure 1). The evolutionary histories of these basinshave been mostly elucidated based on geophysical and geo-logical data (e.g., Kobayashi et al. 1995; Okino et al. 1999;Deschamps and Lallemand 2002). The northwestern end ofthe Philippine Sea plate is characterized by several oldbathymetric highs, known as the Daito Ridges. From northto south, these comprise the Amami Plateau, the DaitoRidge, and the Oki-Daito Ridge, with models for theirevolution having been presented in various studies (e.g.,Tokuyama et al. 1986; Tokuyama 2007; Ishizuka et al.2013). The latitude at which these bathymetric highsformed was investigated by Ueda (2004) using topographicand magnetic anomaly data. Paleolatitudes of 15.1°±4.6°Nfor the Amami Plateau, 20.1°±8.2°N for the Daito Ridge,and 4.4°±11°S for the Oki-Daito Ridge were obtained,showing that these bathymetric highs initially formedsouth of their present positions and have since migratednorthwards.The topographic highs of the Amami Plateau consist ofthree east-west-oriented ridges that extend for approxi-mately 300 km, with the middle ridge being both thelargest and the shallowest in depth (less than 2,000 m). Thenorthwestern end of the plateau is currently subductingbeneath the Ryukyu (Nansei-Shoto) Trench (Nishizawaet al. 2009). Igneous rocks obtained from the plateausuggest that its pedestal is Cretaceous in age, and theridge portion situated on the pedestal formed in theMiddle Eocene (Tokuyama et al. 1986; Ishizuka and Yuasa2007). Hickey-Vargas (2005) described basalt and tonalitefrom the Amami Plateau that have geochemical character-istics of intra-oceanic island arc rocks and which formedat 70 to 118 Ma during the Cretaceous. Based on thesedata, Tokuyama (2007) concluded that the Amami Plateauwas an active island arc before the Eocene.The Daito Ridge is situated south of the Amami Plateau;its western end approaches the Ryukyu Trench, and itseastern end connects to the Kyushu-Palau Ridge. The* Correspondence: azusa@mail1.accsnet.ne.jpHydrographic and Oceanographic Department, Japan Coast Guard, 2-5-18Aomi, Koto-ku, Tokyo 135-0064, Japan© 2014 Nishizawa et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative CommonsAttribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproductionin any medium, provided the original work is properly credited.Nishizawa et al. Earth, Planets and Space 2014, 66:25http://www.earth-planets-space.com/content/66/1/25