Geoscience Reference
In-Depth Information
NOAA/NGDC (GEODAS;
http://www.ngdc.noaa.gov/mgg/gdas/gd_sys.html
).
The geomagnetic data obtained in KR08-10 cruise by R/V
KAIREI
was personally
obtained from the shipboard scientists of the cruises.
We calculated magnetic anomalies using the 11th Generation International
Geomagnetic Reference Field (International Association of Geomagnetism and
Aeronomy, Working Group V-MOD 2010). The geomagnetic reversal timescale used
in this study is that proposed by Gradstein et al. (
2004
). Fig.
3
shows magnetic
anomaly profiles in the study area.
The magnetic data were analyzed in the usual fashion for magnetic lineation
studies: anomalies were plotted as wiggles perpendicular to ship tracks using GMT
and MAGBAT and then correlated between tracks by eye. The analysis of geomagnetic
data began with the previous magnetic lineation map (Nakanishi et al.
1989
) and
focused on tracing the lineations. As with any magnetic lineation identification study,
the picks were based on a combination of anomaly shape and spacing, extension from
better picks, and constraints imposed by geometry and orientation. To help identify
individual anomalies, profiles were compared with a synthetic magnetic model based
on the Mesozoic geomagnetic reversal timescale of Gradstein et al. (
2004
).
3
Results
3.1
Overview of the Bathymetric Features
The bathymetric map (Fig.
4
) abundantly exposes topography of the seafloor of the
Pacific plate. Bathymetric data extend across the trench slope and outer swell
to the undeformed oceanic plate. Data coverage spreads more than 200 km from the
trench axes and is wider than previous works (e.g., Kobayashi et al.
1998
; Sasaki
2003
). The wide coverage makes more detailed description of topographic struc-
tures of seafloor possible than in previous works.
The crest of the outer swell, Hokkaido Rise, a gently uplifted topographic feature
parallel to the trench axis, along the Kuril Trench, has a depth of 5,100 m, which is
nearly 1,000 m shallower than the northwestern Pacific basin. The distance between
the crest of the outer swell and trench axis is approximately 70 km in the Kuril
Trench. The outer swell of the Japan Trench is slightly less clear compared to that of
the Kuril Trench. Its crest is deeper than 5,200 m and situated about 80 km east of the
Japan Trench axis. The outer swell is distinctly identified north of 37°N in the Japan
Trench, north of the Joban Seamounts, but is obscure south of 37°N, near the Joban
Seamounts. The outer swell along the Izu-Ogasawara Trench is narrower than those
along Kuril and Japan trenches. Kashima Fracture Zone (Nakanishi
1993
; Nakanishi
et al.
1989
) is situated just east of the outer swell. Kashima Fracture Zone prevents
the eastern side of the fracture zone from uplifting to make an outer swell.
The remarkable topographic structures on the deep-sea floor, except for deep-sea
trenches and outer swells, are Kashima and Nosappu fracture zones, Joban Seamounts
and subducting seamounts, Erimo, Daiichi-Kashima, and Mogi. The Kashima
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