Is the electrical conductivity of the northwestern Pacific upper mantle normal?

Abstract

[1] The Normal Oceanic Mantle project, based on ocean bottom geophysical observations, has been underway since 2010 to investigate the physical state of the oceanic lithosphere and asthenosphere. We have conducted electromagnetic surveys on old (130 Ma) seafloor in the northwestern Pacific region, where no active tectonic processes have been identified, in order to image the electrical conductivity structure beneath the region. So far, data have been collected at four sites through a pilot survey conducted from June 2010 to August 2012. A one-dimensional electrical conductivity model was obtained by preliminary analysis of the data by using the magnetotelluric method. The model shows that the resistive (<0.01 S m 21) lithospheric mantle is as thick as 80 km, and that the asthenospheric mantle below has a conductivity of 0.03 S m 21. The resistive layer is slightly thicker than that beneath the Philippine Sea but significantly thinner than that beneath the area off the Bonin Trench in the Pacific Ocean. There is a greater difference in age between the survey area and the Philippine Sea (0-60 Ma) than between the survey area and the area off the Bonin Trench (140)(141)(142)(143)(144)(145)(146)(147)(148)(149)(150)(151)(152)(153)(154)(155). This comparison suggests that the relation between age and lithospheric thickness is not as simple as that predicted by the concept of lithospheric cooling. It also suggests that the lithosphere beneath the area off the Bonin Trench in the Pacific Ocean is abnormally thick if the mantle beneath the survey area in this study is ''normal,'' as expected from the plate cooling model.

Electrical conductivity of old oceanic mantle in the northwestern Pacific I: 1-D profiles suggesting differences in thermal structure not predictable from a plate cooling model

“…The positions given in italicized characters are the representative positions for each site. Water depths at each site were retrieved from the up-to-date compilation of available MBES data, and the values for some sites are therefore slightly different from those previously reported by Baba et al (2010Baba et al ( , 2013a where r is the observed position and T is the period. The responses were estimated for each OBEM site using a bounded influence algorithm (Chave and Thomson 2004).…”

“…This phenomenon is smaller for Area B, which is more distant from the coastlines; therefore, the coast effect is likely a possible cause. In fact, our previous study showed that splitting in the responses for Area A is partly explained by the topography, including coastlines (Baba et al 2013a). However, for Area A, the splitting tends to be slightly more significant at the western sites, NM01, NM02, and NM04 ( Fig.…”

“…Recently, we carried out the normal oceanic mantle (NOMan) project (http://www.eri.u-tokyo.ac.jp/ yesman/), which aimed to investigate the state of the old Pacific mantle via marine seismic and electromagnetic (EM) observations. Baba et al (2013a) analyzed magnetotelluric (MT) data obtained by the pilot survey and generated a preliminary one-dimensional (1-D) electrical conductivity profile for the upper mantle, which shows that the resistive lithospheric mantle is as thick as ~80 km. One important finding was that the resistive layer in this area of the lithosphere is unexpectedly different from that detected in the area off the Bonin Trench (Baba et al 2010), although the lithospheric ages differ little between the two areas.…”

“…First, we introduce the datasets and 1-D conductivity profiles for the upper mantle, which are newly obtained for two areas: the northwest and the southeast of the Shatsky Rise. Compared with the dataset used by Baba et al (2013a), (1) the number of observation sites was increased and the area covered is wider, (2) the data were collected from an additional area to examine the generality of structural features, and (3) the period range to be analyzed was extended to include both shorter and longer periods and thus yields more constraints for shallower and deeper structures. We discuss the features of the LAS beneath the northwestern Pacific by comparing the 1-D profiles from the NOMan project with those obtained with similar methods from the other two areas in the northwestern Pacific where data were acquired in past experiments (Baba et al 2010(Baba et al , 2013b.…”

“…We originally intended to compare the two areas to examine the generality of the "normal" mantle state. Observations were initiated at five sites (NM01-NM05) in Area A as a pilot survey (Baba et al 2013a), after which iterative deployments and retrievals of instruments in the two areas were performed each year through the main observation phase until 2015. A total of seven cruises were carried out for this experiment using the R/V KAIREI of the Japan Agency for Marine Science and Technology (JAMSTEC) and the W/V KAIYU of Offshore Operation Co., Ltd.…”