Earth Planets Space, Vol. 59 (No. 7), pp. 897-909, 2007
Japan Coast Guard Academy, 5-1, Wakaba-cho, Kure-shi, Hiroshima, Japan
(Received September 20, 2006; Revised February 27, 2007; Accepted April 4, 2007; Online published July 20, 2007)
Magnetizations of the seamounts in the Izu-Ogasawara arc are calculated using correlation analyses of magnetic anomalies and topographic data. The calculated results of the seamounts in the Sitito-Iozima ridge (present volcanic front) show normal magnetizations with a mean value of 5.10 ± 1.38 A/m. The results also show that the majority of the seamounts in the Nisi-Sitito ridge, which are Pliocene and Middle Miocene in origin, are magnetized in a normal magnetic field direction with a mean value of 2.74 ± 1.07 A/m. Seamounts in the Kyushu-Palau ridge, which are Oligocene in origin, also show predominantly normal polarities, with a mean of 2.67 ± 0.71 A/m. For such a polarity bias to be explained by induced magnetization components (IM), including viscous remanent magnetization (VRM), the intensity of IM should be comparable to the mean of 2.74 A/m. However, no significant differences in the standard deviations of magnetization intensities are recognized between the seamounts in the Nisi-Sitito Ridge and those in the Sitito-Iozima Ridge, contrary to expectations if the normal polarity bias is IM in origin. Three-dimensional (3-D) multi-block model analyses are also applied to 25 seamounts having magnetization directions considerably different from those of the present magnetic field to estimate the normally and reversely magnetized volumes of the seamounts. The results show that 88% (22/25) of the seamounts have relatively greater volumes of normal magnetization compared to reverse magnetization. The IM component estimated from the 3-D multi-block model is a maximum of 0.66 A/m, which is too small to explain the observed normal polarity bias. A possible alternative explanation for the observed normal polarity bias may be enhanced volcanic activity during normal magnetic periods, although this is difficult to justify theoretically at the present time.
Key words: Arc volcanism, magnetic anomaly, polarity bias, seamounts, paleomagnetism, Izu-Ogasawara arc, Kyushu-Palau ridge.