Crustal Evolution of Oceanic arc and Mechanism of Continent Formation
The Institute for Research on Earth Evolution (IFREE: Yoshio Fukao, Director-General), the Japan Agency for Marine-Science and Technology (JAMSTEC: Mr. Yasuhiro Kato, President) conducted seismic study to clarify crustal structure at Mariana arc-backarc region using controlled sources (air gun shooting) and ocean-bottom seismographs (OBSs), which resulted in the clarification of an aspect of the evolution that oceanic crust evolving into continental crust.
The crustal evolutional process clarified by this study enable application to the Izu-Ogasawara oceanic island arc with the same process as this region would be useful for understanding Japanese geological backgrounds, which could be a basis for extending continental shelf territory.
This result would be published on the American science journal "Geology" on February 27 (US time)
There are continental crust (*1) with relatively light rocks and oceanic crust (*2) with relatively denser rocks. On the other hand, there are the crustal blocks called island arc developed by subduction of the oceanic crust and the crust is neither continent nor oceanic.
An island arc in Izu-Ogasawara or Mariana region is called intra-oceanic island arc, and it is considered to be created by subduction of an oceanic crust into another oceanic crust approximately 50 million years ago. It is clarified that the northern Izu-Ogasawara island arc is composed of continental materials (mainly granite and andesite) with light elements (Suyehiro et al., 1996). Since this area is surrounded by oceanic crust, it was assumed that no continental crust around there before the island arc was formed. The mechanism of the crustal evolution to the continent had been still unknown yet.
3. Method of the Crustal Structure Study
This study conducted at 1000 km south off Ogasawara islands in 2003, as a part of collaborative study (Multi-Scale Seismic Imaging of the Mariana Subduction Factory, Japanese representative: Kiyoshi Suyehiro, JAMSTEC) in corporation with the Earthquake Research Institute of the University of Tokyo, University of Hawaii, Stanford University and Washington University. OBSs (Pic.2) were deployed at 106 points by our Research Vessel KAIYO (Pic.1), acoustic signals generated on the sea surface by large-capacity air guns (Pic.3) are recorded on OBSs through crust and mantle.(fig.1) The structures of crust and mantle are indicated based on wave velocity distribution resulted from this OBS data analysis.
1. Previously, it is only known that the oceanic island arc includes crustal materials with light elements like continental crust. Now we clarified three important characteristics from this velocity distribution. At West Mariana Ridge and Mariana island arc, (1) we identified three layers of upper crust, middle crust and lower crust; (2) there is an enlarged middle crust (green area with velocity of 6.0~6.9 km/s) which compositions are mainly of granitic or andesitic continental materials; (3) heterogeneity in the upper mantle with slow velocities was detected (pale blue area with velocity of 7.6~7.7 km/s). (Fig.2)
2.Continental materials in the middle crust are considered to be produced by differentiation of uplifting mantle-derived basaltic magmas based on the crustal differentiation model of the petrologic study (*3).
3.The volume of the lower curst obtained from this study data is significantly smaller comparing the one calculated by the crustal differentiation model using observed volumes of the upper and middle crust. This indicates that a part of the crust with denser materials is transformed from the crust to the mantle. (Fig.3).
In this way, we understood the evolutional process of the arc crust with "crustal change" to the continental crust.
5. Significance and Future Development
It would be the one of the geographical backgrounds for deciding continental shelf territory to find out spatially-extent or continuity of the continental crust produced by "crustal change". JAMSTEC has been conducting the seismic study entitled "the crustal structure exploration for continental shelf territorial delimitation" using our research vessels from 2004 in corporation with the Japan Coast Guard, based on "the Basic Policy for Continental Shelf Delimitation (August, 2004)" at "the Commission in the Concerned Government Ministries on the Continental Shelf Exploration and the Marine Resources". From now on, we will extend out study area to entire Izu-Ogasawara region.