3‐D electrical resistivity structure based on geomagnetic transfer functions exploring the features of arc magmatism beneath Kyushu, Southwest Japan Arc

Abstract

Our 3‐D electrical resistivity model clearly detects particular subsurface features for magmatism associated with subduction of the Philippine Sea Plate (PSP) in three regions: a southern and a northern volcanic region, and a nonvolcanic region on the island of Kyushu. We apply 3‐D inversion analyses for geomagnetic transfer function data of a short‐period band, in combination with results of a previous 3‐D model that was determined by using Network‐Magnetotelluric response function data of a longer‐period band as an initial model in the present inversion to improve resolution at shallow depths; specifically, a two‐stage inversion is used instead of a joint inversion. In contrast to the previous model, the presented model clearly reveals a conductive block on the back‐arc side of Kirishima volcano at shallow depths of ~50 km; the block is associated with hydrothermal fluids and hydrothermal alteration zones related to the formation of epithermal gold deposits. A second feature revealed by the model is another conductive block regarded as upwelling fluids, extending from the upper surface of the PSP in the mantle under Kirishima volcano in the southern volcanic region. Third, a resistive crustal layer, which confines the conductive block in the mantle, is distributed beneath the nonvolcanic region. Fourth, our model reveals a significant resistive block, which extends below the continental Moho at the fore‐arc side of the volcanic front and extends into the nonvolcanic region in central Kyushu.

Electrical Investigation of Natural Lawsonite and Application to Subduction Contexts

“…Geotherms (gray lines) and average P‐T paths of the top of the slab in old/cold subduction zones and young/warm subduction zones are from Penniston‐Dorland et al (). Previous studies of lawsonite stability field suggest that lawsonite is stable on most P‐T path of old crusts (such as Japan; Hata et al, ), and breakdown occurs in young crusts such as Cascadia (McGary et al, ) at depths where conductive anomalies are observed (2.5–4 GPa).…”

Constraints on fluids in subduction zones from electromagnetic data

“…The Kyushu-Palau Ridge was formed as part of the backarc system and features anomalously thick oceanic crust (Nishizawa et al, 2007). The projection of this ridge in the direction of plate convergence is roughly in the region of high mantle conductivity, as discussed in Hata et al (2017).…”

“…Similarly, in the Kyushu subduction zone, which has featured 3D electromagnetic imaging using both network MT and geomagnetic transfer function approaches (Hata et al, 2015(Hata et al, , 2017, there is significant variability in the mantle wedge conductivity, with only one region, beneath Kirishima volcano, associated with a significant melt-related conductor (Fig. 7).…”

Three‐Dimensional Electrical Resistivity Distribution Beneath the Beppu–Shimabara Graben With a Focus on Aso Caldera, Southwest Japan Subduction Zone

“…Two additional features were identified from the electrical resistivity distribution beneath Aso caldera based on three-dimensional (3-D) resistivity models (Hata et al, 2016): a magma pathway imaged as a series of northward-dipping conductive anomalies from depths of~10 km to the Aso volcano edifice and earthquake clusters within resistive blocks rather than conductive blocks (i.e., the inferred magma pathway). In addition, previous 3-D resistivity models at coarser scales (>10 km), based on network-MT and geomagnetic depth sounding data, have suggested that the crust beneath the graben is a discontinuous high-conductivity belt, in which resistivity values are higher around Aso caldera than at comparable depths around other volcanoes (Handa, 2005;Hata et al, 2015Hata et al, , 2017. On the other hand, we need a 3-D resistivity model/distribution for the crustal structure beneath the whole of the graben at finer scale (<several kilometers) to further verify the relation between the resistivity distribution and the volcanic range and seismicity in and around the graben.…”