Japan's continental shelf survey has revealed that the Ogasawara Plateau is colliding with the Izu–Ogasawara Arc and accreting to it (Ohara et al., 2015). The Japan Arc is considered to have developed through the collision and accretion of bathymetric highs (e.g., Taira, 2001); here, an ongoing continental growth process is observed. The collision between the Ogasawara Plateau and the Izu–Ogasawara forearc causes shoaling of the axis of the Izu–Ogasawara Trench. The depth of the axis at the collision point is 3450 m, 〜6000 m shallower than the deepest point within the Izu–Ogasawara Trench. This collision process caused numerous lineaments interpreted as faults in the western part of the Ogasawara Plateau, and tectonic emplacement of the Hahajima Seamount (HS) at the forearc just in front of the collision point. The Commission on the Limits of the Continental Shelf (CLCS) recognized the accretion of the Ogasawara Plateau to the Izu–Ogasawara Arc, and concluded in the recommendation to Japan that the western part of the Ogasawara Plateau is considered to be a “submarine elevation” in the sense of Article 76, paragraph 6 of the United Nations Convention on the Law of the Sea (UNCLOS). In other words, the western part of the Ogasawara Plateau is considered to be a natural prolongation of Japan's landmass on the Izu–Ogasawara Arc, including Chichi-jima and Haha-jima Islands. (Yasuhiko OHARA, Yukihiro KATO and Akira NISHIMURA)
The Commission on the Limits of the Continental Shelf (CLCS) established pursuant to the United Nations Convention on the Law of the Sea issued its recommendations on Japan's extended continental shelf in April 2012, confirming Japan's rights over the vast areas within the Philippine Sea and Pacific Plates. Japan submitted information on the limits of its continental shelf beyond the EEZ to the CLCS in November 2008, which was the result of a 25-year continental shelf survey project begun in1983, involving all of Japan's agencies related to geosciences (Hydrographic and Oceanographic Department of Japan Coast Guard, Japan Agency for Marine-Earth Science and Technology, Japan Oil, Gas and Metals National Corporation, and Geological Survey of Japan) . The huge volume of geological and geophysical data obtained by the project gives the scientists an unprecedented opportunity to study the geology and tectonics of the Philippine Sea and Pacific Plates. In this contribution, we review the results of these studies, summarizing the following seven main themes: (1) Extensive and high-density mapping of the Philippine Sea, establishing a precise evolutionary history of the plate; (2) Discovery of oceanic core complexes within the Philippine Sea Plate; (3) Dense bottom sampling of the Philippine Sea Plate, establishing a precise tectono-magmatic history of the plate; (4) Understanding the crustal structure of ridges and basins within the Philippine Sea Plate; (5) Understanding the collision tectonics of the Ogasawara Plateau; (6) Understanding the crustal structure of seamounts and basins within the Pacific Plate; (7) Discovery of hydrothermal field within the Okinawa Trough and Izu-Ogasawara Arc.
Geomorphic information is a fundamental reference for the physical processes of land and sea formation and is a baseline in space and time for describing characteristics of the Earth's surface. In contrast to a long history of topographic mapping of land, technological advances in detailed seafloor mapping and regional compilation of larger scale maps and charts have only been achieved with the swath bathymetric technique, or multi-narrow beam echo sounders, within the last several decades. Japan's national Extended Continental Shelf (ECS) Survey, complying with UNCLOS, or United Nations Convention of the Law of the Sea, has produced an almost full-coverage, high-resolution bathymetric database of Japanese waters, as well as related geological and geophysical information. Details are discussed on the primary importance of high-resolution bathymetric data supported by geoscientific evidence and proofs, as exemplified in Japanese ECS Submission, in practicing UNCLOS.
Marine magnetic anomalies are used to date the seafloor, characterize the oceanic crust, and reconstruct the evolutionary process of ocean basins. The Japanese continental shelf survey has collected high-quality, dense magnetic data from around Japan, which have increased our understanding of the tectonic history of the area. The Philippine Sea consists of volcanic arcs, paleo-arcs, active, and inactive backarc basins and fragments of older arc and oceanic crusts. Clear magnetic lineation patterns in the basins reveal the seafloor spreading process of the backarc region. Magnetic anomalies associated with paleo-arcs provide us with information on the paleo-latitude and/or rotation of the Philippine Sea Plate, as well as magmatism in the transitional phase from arc volcanism to backarc volcanism. Magnetic anomaly data are compiled for the Philippine Sea north of 15°N collected during the continental shelf survey, and the tectonic interpretation is summarized on the basis of a new magnetic anomaly map reduced to the pole.
A three-dimensional crustal density structure model of the western Pacific area is constructed using detailed marine gravity data, as well as 19 lines of seismic crustal structure data collected by the Hydrographic and Oceanographic Department, Japan Coast Guard during Japanese continental shelf surveys. The model consists of five layers: seawater, sediments, upper crust, lower crust and mantle, with densities of 1.03, 2.3, 2.4, 3.0 and 3.3 g/cm3, respectively. The boundaries between the seawater and the sediments and between the sediments and the upper crust are determined using existing bathymetric and sediment thickness data. The differences between the observed free air anomalies and the gravity anomalies in the initial five-layer model are divided into three components by their wavelengths: the density inhomogeneity in the mantle, variations in depths of the Moho and the top of the lower crust. The depths of the top of the lower crust and the Moho are obtained with an inversion calculation from their anomaly contributions. According to the results, most large seamounts are associated with Mohos of about 15 km or greater in depth. However, some have almost no Moho lows below them, and others are even located over Moho highs. Fujibakama and Kaede Escarpments and their southeastward extensions are clearly visible in the free air gravity anomaly map, and steps in the top surface of lower crust layer of a few hundred to several hundred meters in height are obtained as results of the model calculation.
Characterizating the igneous basement and reconstructing the volcanotectonic history of the Philippine Sea are the major targets of surveys for delineating Japan's continental shelves operated by Ministry of Economy, Technology and Industry. Sampling using R/V Hakurei-maru No. 2 equipped with Deep-sea Boring Machine System (BMS) and arm dredges covers a wide area of the Philippine Sea. The results are summarized of 40Ar/39Ar dating and chemical and isotopic analyses on igneous rock samples from the Philippine Sea, particularly from the West Philippine Basin, and its surrounding area, and Kinan Seamount Chain in the Shikoku Basin. Both regions are characterized by Ocean Island Basalt (OIB)-like magmatism which formed oceanic plateaus (e.g., Urdaneta Plateau, Oki-Daito Rise) and seamounts (e.g., seamounts in Minami Daito Basin, Kinan Seamount Chain). However, the geochemical characteristics of OIB-like basalts and pattern of spatial and temporal variations of this type of magmatism indicate that sources and processes involved in magmatism in these two areas are clearly distinct.
Japan Coast Guard conducted seismic reflection experiments with multi-channel streamer cable and refraction experiments with ocean bottom seismographs from 2004 to 2008, as part of continental shelf surveys, to clarify the crustal structures of various submarine topographic features around Japan. These provide useful information on their formation process and tectonics. The total number of survey lines approaches 100 and their total length is over 50,000 km. The total number of ocean bottom seismographs deployed for the experiments is over 5,200. Seven sea areas are established in the current research: 1) Minami-Tori Shima area, 2) Ogasawara Plateau area, 3) Japan Trench area, 4) Shikoku Basin and Parece Vela Basin area, 5) Kyushu–Palau Ridge area, 6) Daito Ridges area and 7) West Philippine Basin area. Characteristic velocity structures are shown in each area, as described below: 1)Characteristic topographic features around Minami-Tori Shima are seamounts formed by intraplate igneous activities on an oceanic crust. The largest seamount in this area, Takuyo-Daigo Seamount, has several intrusion cores and thickened crust. The Northwest Pacific Basin displays a typical oceanic crust with large velocity anisotropy in the uppermost mantle. 2)Ogasawara Plateau is located at the edge of the Pacific Plate, where it collides with the adjacent Philippine Sea Plate. The crustal thickness of the plateau is estimated to be over 20 km, which might make subduction below the trench difficult. 3)Erimo Seamount and Daiichi-Kashima Seamount are subducting below the North American Plate. Sections of plate boundary with the seamounts are obtained. 4)The Shikoku Basin and Parece Vela Basin are back-arc basins, formed by rifting of the proto Izu–Ogasawara Arc. The crustal thickness of these basins is ～5 km, which is thinner than that of the typical oceanic crust produced at a mid-ocean ridge (～7–8 km). 5)The Kyushu–Palau Ridge is a remnant arc of the proto Izu–Ogasawara intra-oceanic island arc. Although crustal models of the Kyushu–Palau Ridge vary along the ridge, the crusts are significantly thicker than those of oceanic basins and show similar characteristics to the Izu–Ogasawara arc crust. 6)The Daito Ridges are composed of the Amami Plateau, Daito Ridge and Oki-Daito Ridge. These bathymetric highs have a thicker crust at 15–25 km and also have same paleo-island arc features. 7)The West Philippine Basin is believed to spread from the CBF Rift. The crustal thickness of the basin is approximately 4–6 km, which is similar to that of the Shikoku Basin and Parece Vela Basin. However, the higher velocities of the lower crust and uppermost mantle are meaningfully different from the neighboring backarc basins.
Japan Agency for Marine-Earth Science and Technology carried out seismic surveys using ocean bottom seismographs (OBSs) and a multi-channel reflection survey system from 2004 to understand the structural characteristics and the continuity of the Izu–Ogasawara Arc crust. The Izu–Ogasawara Arc developed from the oceanic crust and produced andesitic middle crusts. The velocity is similar to that identified in the continental crust, and the initial continental crust might have been produced during development of the arc crust. To investigate the process of the Izu–Ogasawara Arc crust, many 2-D velocity structures are compared using unified specifications of data acquisition and analysis, and structural commonalities and differences are evaluated. The specification was confirmed previously through simulation studies using the structure obtained. These arc crustal structures have common characteristics, which are an upper crust with a Vp of 4.5–6.0 km/s, a middle crust with a Vp of 6.0–6.5 km/s, and a lower crust with a Vp of 6.5–7.5 km/s. The lower crust is composed of two layers; the upper part has a Vp of 6.5–6.8 km/s and the lower part has a Vp of 6.8–7.5 km/s. The uppermost mantle has a Vp of less than 8.0 km/s. Development of the arc crust results in crustal thickening accompanied by rifting. Back arc opening after rifting plays the role of crustal thinning. The Shikoku Basin, which is the older backarc basin, has a relatively thin crust with a thickness of approximately 10 km, and the eastern part has a high velocity lower crust with a Vp of over 7 km/s. In addition, the upper crust of the eastern part of the Shikoku Basin has some intrusive materials and strike slip faults with few vertical displacements. Such a high-velocity lower crust is not distributed in the Parece Vela basin. The Ogasawara Ridge has different characteristics from the above arc crust, which are a crustal thickness of approximately 20 km but a complicated structure including a narrow and thin crust in the N–S direction. Here, we introduce the structural characteristics of the entire Izu–Ogasawara Arc crusts based on unified seismic surveys and data analysis methods.
Some small basins have developed in the northern Philippine Sea along the northern and southern sides of the Daito Ridge (DR): the Kita-Daito Basin (KDB), Amami-Sankaku Basin (ASB) and Minami-Daito Basin (MDB). From the interpretation of multichannel reflection seismic profiles crossing these basins, their noticeable differences of geological character can be recognized. There are several E–W trending normal faults in the main part of the KDB which suggest that this basin was formed by seafloor spreading. A seismic section crossing the Oki-Daito Ridge (ODR) and DR presents a reflector, possibly indicating subduction of the slab containing the ODR. The subduction probably caused back-arc spreading of the KDB, which finally ceased upon the collision of both ridges. Based on the trace of characteristic reflectors in seismic sections and correlations with several drill holes, sediments are divided into Units I through V, ranging from Lower Eocene to Holocene. The Choju Seamounts occupy the middle part of the MDB, dividing the basin into the eastern main basin and the western small subbasin (Minami-Daito Nishi Sub Basin; MDNSB). The seismic character of Unit II (≒ Middle to Upper Eocene) is noticeably different among the ASB, the main basin of MDB, and the MDNSB, indicating different sources of sediments. The thickness of Unit II in the ASB increases towards the Kyushu-Palau Ridge (KPR), indicating that sediments were supplied by the KPR. Unit II in the main basin of MDB has no direct relationship with the KPR, and should have had another source, such as the DR or the ODR. The thickness of Unit II in the MDNSB increases towards the DR, suggesting an equivalent origin of the carbonate layer at the top of the DR.
Interpretation of seismic reflection surveys shot by Japan Oil, Gas and Metals National Corporation (JOGMEC), Japan National Oil Corporation (JNOC) and Geological Survey of Japan in the Philippine Sea controlled by Ocean Drilling Program (ODP) as well as Deep Sea Drilling Project (DSDP) cores and ocean bottom samplings reveal the basin framework of the Izu–Ogasawara Arc. Seven seismic horizons from the acoustic basement to the seabed are interpreted, and isochron maps of six seismic units from Eocene to Quaternary are mapped to study the history of basin evolution. The Izu–Ogasawara Forearc Basin is located east of the volcanic front, is about 50 km in width and 2 to 3.5 sec. in Two Way Time (TWT) thickness. It extends more than 1000 km from east of the Ioto to off the Boso Peninsula, and subducts along the Sagami Trough. Paleogene sediments develop only in the forearc side. The Paleogene basin is bounded by NNE–SSW and NNW–SSE faults, has a zigzag pattern, elongates throughout the Izu–Ogasawara Forearc, and consists of echelon horsts and half grabens. Horsts are composed of volcanic rocks of Eocene to Oligocene age, and grabens are filled with turbidite sediments of the same age. Divergent seismic reflection patterns indicate that the turbidite sediments were deposited synchronously with the tilting and subsiding of the half grabens during the syn-rift period of the basin that was initiated by the rifting of the Proto-Izu-Ogasawara Arc. The latest Oligocene to Early Miocene sediments are mainly composed of calcareous nannofossil rich marls, which were deposited both on the Paleogene forearc basin as post-rift sediments, and in the backarc basin that was initiated when the Shikoku Basin was opened by the rifting of the Proto Izu–Ogasawara Arc. Well-stratified and flat seismic reflection indicates that the marls were deposited in a calm environment when volcanic activity ceased. The Izu–Ogasawara Arc re-started volcanic activity in the Middle to Late Miocene at the Nishi-Shichito Ridge in the backarc side. Sediments of this period are thick around the Nishi-Shichito Ridge. Seismic reflection is strong, discontinuous, and wavy near volcanoes, and weak, continuous, and parallel away from the arc. The former facies is mainly composed of coarse volcaniclastics, and the latter of finer clastics. In the Pliocene and Quaternary, the depocenter shifted from west to east as the volcanic front shifted.