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August 25, 2015

Deep Sea Drilling at the Amami Sankaku Basin Revealed
Wide Distribution of Fore-arc Basalts across the Izu-Bonin-Mariana Arc
~Record of spontaneous subduction initiated 52 million years ago~

As reported on March 24, 2014, IODP Expedition 351 carried out scientific drilling in the Amami Sankaku Basin, about 100km west of the Kyushu-Palau Ridge, a remnant part of the Izu-Bonin-Mariana (IBM) arc (Figure 1). From the Japan Agency for Marine-Earth Science and Technology (JAMSTEC: Asahiko Taira, President), Dr. Osamu Ishizua and Dr. Morihisa Hamada participated in this expedition with international scientists.

Based on analyses of igneous basement rocks and overlying sediments sampled from the Amami Sankaku Basin, the research team found that the uppermost basement rocks are not mid-ocean ridge basalts (MORBs), but are very similar to fore-arc basalts (FABs) commonly found in the adjacent IBM fore-arc region erupted around 52 million years ago, when subduction of the Pacific plate beneath the Philippine Sea plate was initiated. These FABs are confirmed to be distributing not only in the IBM fore-arc region, but also in the Amami Sankaku Basin, which is at the rear-arc side of the IBM arc.

This unexpected widespread distribution of FABs suggests that the subduction zone was under extensional environment across the arc, and that the igneous basement of the IBM arc was formed over a much wider area during the subduction inception in a mode consistent with “spontaneous” subduction. These results will provide significant insights into the process of subduction initiation in this area, as well as into subsequent evolution of the IBM arc.

These new findings were published online in Nature Geoscience on August 25th, 2015 (JST).

Title: A record of spontaneous subduction initiation in the Izu-Bonin-Mariana arc

Authors: Richard J. Arculus1, Osamu Ishizuka2,3, Kara A. Bogus4, Michael Gurnis5, Rosemary Hickey-Vargas6, Mohammed H. Aljahdali7, Alexandre N. Bandini-Maeder8, Andrew P. Barth9, Philipp A. Brandl1,10, Laureen Drab11, Rodrigo do Monte Guerra12, Morihisa Hamada13, Fuqing Jiang14, Kyoko Kanayama15,a, Sev Kender16,17, Yuki Kusano15,b, He Li18, Lorne C. Loudin19, Marco Maffione20, Kathleen M. Marsaglia21, Anders McCarthy22, Sebastién Meffre23, Antony Morris24, Martin Neuhaus25, Ivan P. Savov26, Clara Sena27, Frank J. Tepley III28, Cees van der Land29, Gene M. Yogodzinski30, Zhaohui Zhang31
1Research School of Earth Sciences, Australian National University, 2Geological Survey of Japan/AIST, 3Research and Development Center for Ocean Drilling Science, Japan Agency for Marine-Earth Science and Technology, 4International Ocean Discovery Program, Texas A&M University, 5Division of Geological and Planetary Sciences, California Institute of Technology, 6Earth and Environment Department, Florida International University, 7Department of Earth, Ocean and Atmospheric Science, Florida State University, 8Center for Energy Geoscience, School of Earth and Environment, University of Western Australia, 9Department of Earth Sciences, Indiana University-Purdue University, 10Geozentrum Nordbayern, Friedrich-Alexander-Universität Erlangen-Nürnberg, 11Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia University, 12Technological Institute of Micropaleontology, University of Vale do Rio dos Sinos, 93022, Brazil. 13Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, 14Institute of Oceanology, Chinese Academy of Sciences, 15College of Science and Engineering, Kanazawa University, 16School of Geography, University of Nottingham, University Park, 17British Geological Survey, 18Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 19Department of Earth Sciences, University of New Hampshire, 56 College Road, 214 James Hall, Durham, NH 03824, USA. 20Institute of Earth Sciences, Utrecht University, Budapestlaan 17, Utrecht 3584, The Netherlands. 21Department of Geological Sciences, California State University, 22Institute of Earth Sciences, University of Lausanne, 23School of Earth Sciences, University of Tasmania, 24School of Geography, Earth and Environmental Sciences, Plymouth University, 25Institut für Geophysik und Extraterrestrische Physik, Technische Universität Braunschweig, 26School of Earth and Environment, University of Leeds, 27Geosciences Department/CESAM, Universidade de Aveiro, 28College of Earth, Ocean and Atmospheric Sciences, Oregon State University, 29School of Civil Engineering and Geosciences, University of Newcastle, 30Department of Earth & Ocean Sciences, University of South Carolina, 31Earth Sciences, Nanjing University

Current affiliation: San'in Kaigan Global Geopark Promotion Office, Tottori Prefectural Government
Current affiliation: Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology

Figure 1: Location of the Amami Sankaku Basin, the Kyushu-Palau Ridge and the drilling site U1438

Figure 2: Scientific drillings at the IBM arc system and genesis of oceanic crusts
In addition to the Amami Sankaku Basin (IBM-1), the back-arc (IBM-3) was drilled in IODP Expedition 350 and the fore-arc (IBM-2) in IODP Expedition 352. To sample middle crusts, a future drilling operation is planned at the IBM-4.

Figure 3: Graphic lithologic summary, biostratigraphic- and paleomagnetic-based age-depth plots for the drilling site U1438
Based on nanofossil dating, the age of the lowest part of Unit IV (the lowest part of sedimentary units at 1,461m below the seafloor) was estimated to be at 51Ma - to 64Ma, while that of Unit 1 (igneous basement rocks) is probably of the same age or older.
Abbreviations are: mbsf = metres below sea floor; Cl =clay, Si = silt, Vfs-fs = very fine sand–fine sand; Ms-vcs = medium sand–very coarse sand,
Gr = gravel; Pleist.=Pleistocene, Plio.=Pliocene, Paleoc.=Paleocene.

Figure 4: Comparative geochemical plots of mid-ocean ridge and subduction-related basalts Oceanic crusts in the Amami Sankaku Basin are indicated by a red sphere, while those in the IBM FABs are indicated by blue and green boxes.
Collected samples in the Amami Sankaku Basin contained lower incompatible elements (for example, lower zirconium concentration at given yittrium concentration) compared with MORBs, showing a remarkable similiarity to FABs from the IBM arc.

Figure 5: Comparison of the models on evolution of the IBM arc before drilling (left) and after drilling (right)
This drilling operation found FABs which was formed during the inception of the IBM subduction (52-48 Ma) from the Amami Sankaku Basin, back-arc side of the IBM arc. The Amami Sankaku Basin and Kyushu-Palau Ridge were separated from the growing IBM arc (48-25 Ma) as a result of opening of the Shikoku Basin and the Parece Vela Basin (25-15 Ma).


(For this study)
Morihisa Hamada, Mantle and Continental Crust Drilling Research Group, Research and Development (R&D) Center for Ocean Drilling Science (ODS)
(For press release)
Hiroyasu Matsui, Manager, Press Division, Public Relations Department
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