nt km - km Origin of magnetic dipole anomaly of Shikoku Yukari KIDO Kantaro FUJIOKA Shiki MACHIDA Hiroshi SATO Takahumi KUROKI Hidetoshi SHIBUYA Masum

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1 ntkm-km Origin of magnetic dipole anomaly of Shikoku Yukari KIDO Kantaro FUJIOKA Shiki MACHIDA Hiroshi SATO Takahumi KUROKI Hidetoshi SHIBUYA Masumi SAKAGUCHI Azusa TONOTANI and Hideo ISHIZUKA All of the dipole anomalies observed in the Kurosegawa tectonic zone can be interpreted in terms of structural model, in which the anomalous sources of magnetization are located in a mid-crustal layer and probably comprise intrusive rocks. These intrusions are probably serpentinite diapirs associated with the dehydration of subducted slabs of the Philippine Sea Plate. Using a fine-scale magnetic anomaly data based on high density air-borne surveys, we have identified several dipole anomalies aligned east-west direction exactly on the Chichibu South zone to Kurosegawa tectonic zone, which constitutes a part of an accretionary prism in the southwest Japan. We have been investigating four sites along the tectonic line and applied a magnetic inversion to one of the dipole anomalies found here, and figured out the magnetic source as a spheroidal shape standing in parallel to the tectonic line and slightly inclined southward. Keywords : magnetic dipole anomaly, serpentine, diapir, Kurosegawa tectonic zone IFREE Institute for Frontier Research on Earth Evolution, Japan Marine Science and Technology Center (JAMSTEC) Deep Sea Research Department, JAMSTEC Ocean Floor Geotectonics Division, Ocean Research Institute, Univ. of Tokyo Dept. Earth Sciences, Kumamoto Univ. Science, Earth History and Environmental Science, Kochi Univ.

Kido et al., ntkm km Kido et al., Maruyama et al.

120 130 140 (b) D A B C (c) 35 34 33 1 2 3 4 5 6 7 8 9 [nt] 100 21 0-17 -31-43 -56-70 -86-109 132 133 134

Color indexes (same as Figure ) display that biolet; ultramafic, yellow through right green; sedimentary

2 Kido et al., ntkm km Kido et al., Maruyama et al., ; (a) Median Tectonic Line 40 Kurosegawa Tectonic Zone (KTZ) Nankai Trough 30 Philippine Sea Plate (b) D A B C (c) [nt] (a) (b) ABCD (c) - Figure (a) and (b): Geological maps of whole Shikoku. Color indexes (same as Figure ) display that biolet; ultramafic, yellow through right green; sedimentary rocks, red; plutons, blue; volcanic rocks, and circles are area of research sites. Figure (c): Geomagnetic anomaly map of SW Japan. Magnetic dipole position is displayed by box.

3 Table Position, amplitude of Shikoku magnetic dipole anomaly Characteristics of ten dipole anomalies. Name description, location of latitude and longitude, profile wave length and diameter of dipole anomaly are presented. Kido et al., - nt c ab ntkm ntkm Murakami & Yoshikura, ; Yoshikura et al.,

4 C-- -C-.km C3 C2 C1 C4 - Fig. - Chouja, Ino, Engyoji, Kochi Pref. D2 D1 D3 - Fig. - Yaechi, Tokushima Pref, the eastern part of Shikoku. A1 A2 A- A A B-, -, -, -, - -; B---- km EWN Fig. Geological map with sampling site. Fig. - Yahatahama, the westem part of shikoku. Sh Sh Sh Sh Am B1 B2 B3 - Fig. - Godan Highland~Takashinano, Ehime Pref. - Fig. - Legend of Chichibu zone.

5 - Fig. - Takashinano site. - Fig. - Takashinano serpentine body. - Fig. - Serpentine Road in Yusuhara. - Fig. - Co-authors in front of Takashinano site. - Fig. - Chouja block.

-C.km Phyllitic Actinolite schistmagnetite

km Rodingite Rodingite - cm --- - - SI - m - -

- Ouyochi, Tokushima Pref. Pelitic rock - Fig.

- Aitani quarry, in Engyoji site (Toyo Denka

6 -C.km Phyllitic Actinolite schistmagnetite Actinolite -C.km -C-.km Rodingite Rodingite - cm SI - m - - Quartz Vein Rodingite - Fig. - Ouyochi, Tokushima Pref. Pelitic rock - Fig. - Handy susceptibility meter - Fig. - Aitani quarry, in Engyoji site (Toyo Denka Kogyo Co., Ltd.) - SI Fig. - At north site of Aitani quarry. Attached numbers are susceptibility value (SI unit).

7 C-C D-- -D -D- D- D - SI mm - - SI.. magnetic intensity Actinolite. magnetic intensity D ntkm Lowrie & Fuller Test,,,,,,,,,,,,,,mTstep step:,,,,, Weight: 28.27g W Up N S N 1.0 Jo=1.1 E Dn Demag Level NRM 5, 10, 15, 20, 25, 30 Div=2.0e-1 mt stepstep:mt Fig. Vector component diagrams (Zijderveld plots) of sample (serpentine).

8 Weight Atmosphere Applied field Rate Smoothing mg ln air 377T 11 C/min 5 point C C C C C C Fig. Thermo-magnetic susceptibility profile. Left: stepwise TRM measurement ( C C C C C). Right: Max rate of TRM is C. mt C C C C C C C km Fryer et al., ;

9 Ao Ax Ao Ax AZo AZx - Fig. Microphotographs of serpentine samples;, -,. Left: open nicol, right: crossed nicol.

10 .... Kido et al., Fryer, P., and G. J. Fryer, in Seamounts, Islands, and Atolls, B. Keating, P. Fryer, R. Batiza and G. W. Boehlert eds., AGU Monogr.,, -,. Fryer, P., J. A. Pearce, L. B. Stokking, et al.,, Proc. ODP, Sci. Results, : College Station, TX (Ocean Drilling Program). GSJ (Geological Survey of Japan), Geological Map of Japan, :,, CD-ROM Version, rd edition,. GSJ (Geological Survey of Japan) and Coordinating CCOP, Magnetic Anomaly Map of East Asia, :,, CD-ROM Version,. Kido, Y., K. Fujioka, S. Machida, and H. Sato, A serpentine diapir as a source of magnetic dipole anomalies along a tectonic line in the southwest Japan, Abstract on Geological Society of Japan,. Maruyama, S, The Kurosegawa serpentinite melange zone and its role in the evolution of southwest Japan. Geol. Soc. China,, -,,. Maruyama, S, S. Banno, T. Matsuda, and T. Nakajima, Kurosegawa zone and its bearing on the development of the Japanese Island. Tectonophys.,, -,. Murakami, H., and S. Yoshikura, Geomagnetic features by use of aeromagnetic data in Shikoku, Southwest Japan, Earth Monthly,, -, (in Japanese). Yoshikura, S., Igneous and high-grade metamorphic rocks in the Kurosegawa Tectonic Zone and its tectonic significance, J. Geosci. Osaka City Univ.,, -,. Yoshikura, S., S. Hada, and Y. Isozaki, Pre-Jurassic Evolution of Eastern Asia, in Kurosegawa Terrane, IGCP Project,, -,.

地質調査研究報告/Bulletin of the Geological Survey of Japan

地質調査研究報告/Bulletin of the Geological Survey of Japan Takayuki Uchino and Makoto Kawamura (2010) Glaucophane found from meta-basalt in the Nedamo Terrane, Northeast Japan, and its geologic significance. Bull. Geol. Surv. Japan, vol. 61 (11/12), p. 445-452,