2002 Fiscal Year Final Research Report Summary
Spreading process of the Mariana Trough
| Project/Area Number |
12440116
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
固体地球物理学
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| Research Institution | Kobe University |
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Principal Investigator |
SEAMA Nobukazu Kobe University, Research Center for Inland Seas, Associate Professor, 内海域機能教育研究センター, 助教授 (30270862)
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| Co-Investigator(Kenkyū-buntansha) |
ISEZAKI Nobuhiro Chiba University, Department of Earth Sciences, Professor, 理学部, 教授 (60107943)
HYODO Masayuki Kobe University, Research Center for Inland Seas, Professor, 内海域機能教育研究センター, 教授 (60183919)
YAMAZAKI Toshitsugu AIST, Institute for Marine Resources and Environment, Group Leader, 海洋資源環境研究部門, 研究グループ長
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| Project Period (FY) |
2000 – 2002
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| Keywords | Mariana Trough / Back-arc Basin / Sea-floor Spreading / Ocean Bottom Electro-Magnetometers |
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| Research Abstract |
The Mariana Trough is a typical intra-oceanic back-arc basin that shows a variety of features. We have compiled extensive surface geophysical data (bathymetry, magnetic and gravity) of the Mariana Trough, which were collected during several Japanese cruises over the last few years. Topographic features and the mantle Bouguer anomaly (MBA) reveal distinct differences between regions. The north of 22N shows extremely low MBA indicating an incipient rifting. The central region between 21N and 14N shows characteristic slow-spreading features; existence of median valley neovolcanic zone and "Bull's eyes" MBA along the axes. The south of 14N shows an EPR type axial relief morphology and lower MBA than that in the central. Tectonic evolution can be inferred from the direction of spreading fabric, the trace of non-transform offsets, and by crustal ages identified by vector geomagnetic anomaly field. The crustal ages indicate that the seafloor spreading started at 4Ma, 6Ma and 3Ma at the northern end, the central, and the southern end, respectively. The spreading rates are generally slow (~20 mm/yr) in the northern and central areas, while they are fast (~35 mm/yr) in the south. Spreading axis directions have gradually changed by 15 degrees clockwise with age in the north of 17.5N, while they occurred at 3.5-3.0Ma from N35W to N5W just south of 17.5N. On the other hand, they are relatively constant in the southern end.
We have developed a new type of ocean bottom electro-magnetometer (OBEM), which is small and easy to handle with Filloux-type electrodes. Preliminary results of an OBEM observation for one year in the Mariana region suggest that very conductive zone (about 70km in depth) below the Marianas island arc and low conductivity zone with thickness of a few ten km beneath the Mariana Trough.
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