Resistivity structure of high-angle subduction zone in the southern Kyushu district, southwestern Japan

Abstract

Magnetotelluric observations were carried out in the southern Kyushu district of southwestern Japan to investigate the characteristics of the electrical resistivity structure of a high-angle subduction zone. We constructed a 2-D resistivity model parallel to the subducting plate motion by using the inversion technique with the Akaike Bayesian Information Criterion (ABIC) smoothness constraint. The general features of the obtained resistivity structure are as follows: (1) a conductive block (below 1 ·m) is found beneath the volcanic zone and is widespread bilaterally below 40 km depth, (2) a resistive block (about 1000 ·m) distributes from 10 to 25 km depth in the forearc region and (3) a conductor (1 ∼ 30 ·m) is embedded beneath the resistive block, which may correspond to the negative Bouguer gravity anomaly observed in this region. We propose the following for the high-angle subduction zone: A serpentinized block is generated in the lower crust of the forearc region and a partial melting and hydrothermal fluid are well developed beneath the volcanic front.

Fluid upwelling beneath arc volcanoes above the subducting Philippine Sea Plate: Evidence from regional electrical resistivity structure

“…In this study, the existence of a conductor (<40 Ωm) beneath each volcano, which is a remarkable feature of our best fit resistivity models, is consistent with Ichiki et al 's [2000] resistivity model by the ULF‐MT data of 0.1–3,000 s across the Kirishima volcanoes. In their model, a conductive block (0.1–30.0 Ωm) is also found beneath the volcanoes.…”

“… Ichiki et al [2000] proposed a two‐dimensional (2D) resistivity model inferred from a magnetotelluric (MT) survey in the ultra‐low‐frequency band (ULF‐MT) along a N60°W–S60°E profile across the Kirishima volcanoes, which almost coincides with our Profile I. The authors reported a remarkable feature: a symmetrical conductive block of about 1 Ωm in both the forearc and backarc beneath the volcanoes to below a depth of 40 km.…”

Influence of dehydration on the electrical conductivity of epidote and implications for high‐conductivity anomalies in subduction zones

“…On the other hand, it has been suggested that the high conductive accessory minerals such as oxides (magnetite), phlogopite, and sulfides (or saline fluids) could account for the high conductivity anomalies in subduction zones by the conductivity model of two phase medium consisting of a resistive matrix and those conductive materials [ Reynard et al , ; Li et al , ; Manthilake et al , ; Sakuma and Ichiki , ]. These results are somewhat efficient for the interpretation of some regional highly conductive anomalies, but not consistent with the widespread interpretation of highly conductive zones in terms of partial melting or the accumulation of free fluids [ Ichiki et al , ; Soyer and Unsworth , ; Worzewski et al , ; Evans et al , ; McGary et al , ; Pommier , ]. Since the thermal structure of a subduction zone determines the depth at which the hydrous minerals become dehydrated in the subducting slab, therefore, if regardless of the complexity and diversity of thermal and petrological structure in different subduction zones, a single high conductivity mechanism observed from experimental conductivity study on a single hydrous mineral can hardly explain the widespread highly conductive anomalies in subduction zones.…”

Two‐dimensional electrical section beneath the eastern margin of Japan Sea

“…For instance, Kamiya and Kobayashi [2000] identified a serpentinite body beneath the Kanto district, Japan, as a region of high Poisson's ratio, and related its aseismicity to the ductile nature of serpentinite. Based on a land MT transect in the southern Kyushu district, Japan, Ichiki et al [2000] also found a high conductivity anomaly beneath the forearc region of the high‐angle subduction zone, and interpreted it as a serpentinized block. It is also worth noting that Ichiki et al [2006] recently reported presence of water even in the mantle transition zone probably transported by the Pacific plate which is stagnant beneath the northeastern China, the back‐arc region of northeast Japan.…”