Magnetic properties of pillow basalt from the Kinan seamount chain, the Shikoku Basin.

Furuta, Toshio; Tonouchi, Shyoji; Nakada, Masayuki

doi:10.5636/jgg.32.567

Cited by 12 publications

(8 citation statements)

References 10 publications

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“…The 15 km isodepth curve of the Philippine Sea Plate in the study area almost coincides with the 300°C isothermal line (Figure 4b) [ Higashikata et al , 2001]. It is possible to explain the observed reduction in magnetization as the temperature effect if a main magnetic carrier of the oceanic basalt is titanomagnetites, which has Curie temperatures between ∼150° and 250°C [ Furuta et al , 1980; Furuta , 1993]. However, the thermal condition based on heat flow observations shows no significant differences between AS and KP (Figure 4b) although they are a pair of back‐arc basin flanks.…”

Section: Discussionmentioning

confidence: 80%

Regional variation of magnetization of oceanic crust subducting beneath the Nankai Trough

Kido

Fujiwara

2004

Geochem Geophys Geosyst

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[1] We present the characteristics of marine magnetic anomalies in the Nankai Trough and the northern Shikoku Basin. The marine magnetic anomalies are clearly visible over the Shikoku Basin but gradually disappear across the Nankai Trough. The expected reason for the decrease in the marine magnetic anomaly amplitudes across the trough is the increase in depth of the magnetic source: the subducting oceanic crust. Forward and inverse magnetic modeling results, which take into account the slab depth distribution, show variations in magnetization that cannot be explained solely by increasing depth of the slab. We identify five zones along the trough axis on the basis of the pattern of the magnetic amplitude profile: two ridges, a seamount chain, and two basement zones. The magnetic inversion analysis indicates cyclic intensity variations along the ridge zones. The largest contrast in magnetization along the N-S lineation before the deformation front occurs off Ashizuri (AS). Low-temperature chemical oxidation effects are most likely the main cause of the reduction in magnetization. There are considerable differences in the original carrier of magnetic intensity in the AS and Kii Peninsula (KP) zones, and the original carrier in AS is characterized by much higher original magnetic intensities. This indicates several possibilities: the oceanic crust originated from asymmetric spreading; there was magma injection during the late stage volcanism; or the AS zone was affected by an anomalous heat source. It would be concluded from the current data that the basement in AS originally had a much stronger magnetic carrier and suffered stronger low-temperature oxidation effects than the eastern part of the Shikoku Basin. The regional differences in intensity strongly suggest a variety of magnetic minerals and phase changes associated with temperature and mechanical dislocation.

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Section: Discussionmentioning

confidence: 80%

Regional variation of magnetization of oceanic crust subducting beneath the Nankai Trough

Kido

Fujiwara

2004

Geochem Geophys Geosyst

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“…The three basic approaches to sampling have been dredging (e.g., Merrill and Burns, 1972;Furuta et al, 1980;Gee et al, 1988), drilling (Marshall, 1978;Rice et al, 1980;, and studying subaerially exposed sections on oceanic islands (Gee et al, 1989). In general, these studies have shown that seamount magnetic properties not only differ from those found in "typical" seafloor basalts but are highly variable both within and among seamounts.…”

Section: Introductionmentioning

confidence: 99%

Magnetic Petrology of Basalts from Ninetyeast Ridge

Smith¹,

Gee²,

Klootwijk³

1991

Proceedings of the Ocean Drilling Program

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Given the importance of the inversion of seamount magnetic anomalies, particularly to the motion of the Pacific plate, it is important to gain a better understanding of the nature of the magnetic source of these features. Although different in detail, Ninetyeast Ridge is composed of submarine and subaerial igneous rocks that are similar to those found at many seamounts, making it a suitable proxy. We report here on the magnetic petrology of a collection of samples from Ninetyeast Ridge in the Indian Ocean. Our purpose is to determine the relationship between primary petrology, subsequent alteration, and magnetic properties of the recovered rocks. Such information will eventually lead to a more complete understanding of the magnetization of seamounts and presumably improvements in the accuracy of anomaly inversions. Three basement sites were drilled on Ninetyeast Ridge, with recovery of subaerial basalt flows at the first two (Sites 756 and 757) and submarine massive and pillow flows at the final one (Site 758). The three sites were distinctly different. Site 756 was dominated by ilmenite. What titanomagnetite was present had undergone deuteric alteration and secondary hematite was present in many samples. The magnetization was moderate and stable although it yielded a paleolatitude somewhat lower than expected. Site 757 was highly oxidized, presumably while above sea level. It was dominated by primary titanomagnetite, which was deuterically altered. Secondary hematite was common. Magnetization was relatively weak but quite stable. The paleolatitude for all but the lowermost flows was approximately 40° lower than expected. Site 758 was also dominated by primary titanomagnetite. There was relatively little oxidation with most primary titanomagnetite showing no evidence of high-temperature alteration. No secondary hematite was in evidence. This site had the highest magnetization of the three (although somewhat low relative to other seamounts) but was relatively unstable with significant viscous remanence in many samples. Paleolatitude was close to the expected value. It is not possible, at present, to confidently associate these rocks with specific locations in a seamount structure. A possible and highly speculative model would place rocks similar to Site 757 near the top of the edifice, Site 756 lower down but still erupted above sea level, and Site 758 underlying these units, erupted while the seamount was still below sea level.

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“…In spite of numerous studies on magnetization of seamounts from magnetic anomalies (e.g., Sager and Pringle, 1988;, the magnetic structure of seamounts is still unclear. Previous paleomagnetic studies on dredged and drilled samples from seamounts and islands show that the magnetic properties of seamount rocks are highly variable (Furuta et al, 1980;Kono, 1980;Gee et al, 1988Gee et al, , 1989. Gee et al (1989) suggest that induced magnetization significantly contributes to seamount magnetization.…”

Section: Introductionmentioning

confidence: 99%

“…On the other hand, Sager and Pringle (1988) suggest that the magnitude of seamount magnetization derived from the seamount magnetic anomaly is almost the same as that obtained from dredged samples and that the induced magnetization of seamounts is negligible. Pillow basalts dredged from seamounts show high Q ratios (Furuta et al, 1980;Gee et al, 1988).…”

Section: Introductionmentioning

confidence: 99%

Magnetic Structures of Seamounts in the Western Pacific Ocean Deduced from Leg 144 Downhole Magnetometer Logs

Ito¹,

Nogi²

1995

Proceedings of the Ocean Drilling Program, 144 Scientific Results

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Downhole magnetometer logs obtained during Ocean Drilling Program Leg 144 from Holes 873A (Wodejebato Guyot), 878A (MIT Guyot), and 879A (Takuyo-Daisan Guyot) in the western Pacific Ocean were used to constrain the nature and origin of the igneous sections of the seamounts. Magnetic boundaries, apparent inclination, polarity of magnetization (normal or reverse), and the hemispheric origin (Northern or Southern) was determined by inspecting the variations and the amplitudes of the horizontal and the vertical magnetic field.Magnetization of the basalts between 175 and 200 mbsf in Hole 873A (Wodejebato Guyot) was acquired during a reversed polarity chron in the Southern Hemisphere; the apparent inclination is about 21.6°. Magnetization below about 200 mbsf is most likely dominated by induced or viscous magnetization. The apparent inclination above 200 mbsf is in good agreement with that derived from paleomagnetic studies of the core sample. Basalts from Hole 878A in MIT Guyot below 860 mbsf show that magnetization originated during a reversed polarity chron in the Southern Hemisphere. The apparent inclination below 860 mbsf is 21.8°. Magnetization of basalts above 860 mbsf possibly was acquired during a normal chron in the Southern Hemisphere. Magnetization of the basalts and volcaniclastic breccias in Hole 879A (Takuyo-Daisan Guyot) indicate an origin in the Southern Hemisphere during a normal polarity chron. The apparent inclination (-20.6°) is almost the same as that of the core-sample measurements.Many magnetic boundaries were obtained from the logs of Holes 873A (Wodejebato Guyot) and 878A (MIT Guyot). These boundaries were not caused by changes in magnetic polarity but by intensity variations within the same polarity.In this study, we suggest that induced magnetization in the upper basalt section is not significant in Wodejebato (Site 873), MIT (Site 878), and Takuyo-Daisan (Site 879) guyots.

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Magnetic properties of pillow basalt from the Kinan seamount chain, the Shikoku Basin.

Cited by 12 publications

References 10 publications

Regional variation of magnetization of oceanic crust subducting beneath the Nankai Trough

Regional variation of magnetization of oceanic crust subducting beneath the Nankai Trough

Magnetic Petrology of Basalts from Ninetyeast Ridge

Magnetic Structures of Seamounts in the Western Pacific Ocean Deduced from Leg 144 Downhole Magnetometer Logs

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