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

Abstract

At subduction zones, aqueous fluids released from the subducting slab play an important role in island arc volcanism, triggering partial melting and accretion magmatism. We used the Network‐magnetotelluric (MT) method to obtain the deep (>100 km), large‐scale electrical resistivity structure beneath Kyushu, Japan, with the aim of identifying regions of deep fluids and magma that feeds subduction zone volcanoes in the region. Network‐MT observations, in which dipoles of about 10–30 km in length were used to measure the electric potential difference, were performed over all of Kyushu, in the Southwest Japan Arc, where the Philippine Sea Plate (PSP) is subducting beneath the Eurasian Plate at a high angle. Using the Network‐MT data, we obtained the large‐scale electrical resistivity structure along four profiles across each of four Quaternary active volcanoes at the volcanic front of the Kyushu subduction zone. The resistivity models have two features in common: a conductor beneath each volcano, whose base extends to the backarc side, and a resistor along the hinge line of the subducting PSP in the forearc. The conductor indicates regions of fluid released from the slab or partial melting of the mantle, representing a magma source for subduction zone volcanoes in this region.

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

“…Little is known about the cause of the separation of the active volcanic regions in Kyushu and about the relationship between magmatism (and/or volcanism) and electrical resistivity structures in the mantle wedge beneath Kyushu. To explore these questions in our previous studies, we performed two‐dimensional (2‐D) and three‐dimensional (3‐D) inversion analyses to obtain electrical resistivity structures beneath all of Kyushu, using Network‐Magnetotelluric (Network‐MT) data obtained at 14 periods between 480 and 40,960 s; these data contain geoelectromagnetic information at lithospheric scales [ Hata et al ., , ]. Two major findings of previous studies are described below.…”

Three‐dimensional electromagnetic imaging of upwelling fluids in the Kyushu subduction zone, Japan

“…The Two features that were detected by the 2-D analysis of Hata et al [2012] are also apparent in the final 3-D resistivity model of the present study. First, conductive blocks exist beneath volcanoes (excluding Yufu volcano, whose conductive block extends to both sides with depth) on the VF, the bases of which extend to greater depths in the back arc (Figure 7).…”

“…In our previous study, we performed two sensitivity tests for four profiles (I, J, K, and L) through 2-D forward modeling to examine whether the above two features are well resolved [Hata et al, 2012]. In the sensitivity …”

“…For details on network-MT data processing in the 2-D case, see Hata et al [2012]. Details on the different constraint conditions for the subducting slab in the 2-D modeling are as follows: no prior knowledge ( Figure A1a), a discontinuity between the slab and the overlying mantle ( Figure A1b), two discontinuities between the slab and the overlying/underlying mantle ( Figure A1c), a resistive slab (unfixed value of 1000 Ωm) ( Figure A1d), a discontinuity between a resistive slab (unfixed value of 1000 Ωm) and the overlying mantle ( Figure A1e), and two discontinuities between a resistive slab (unfixed value of 1000 Ωm) and the overlying/underlying mantle ( Figure A1f ).…”

“…After this pioneering work, several network-MT observations were conducted in Japan, including on Kyushu Island [Uyeshima et al, 2002]. Hata et al [2012] used long-period network-MT data, which contained resistivity information from the crust to upper mantle, to produce 2-D electrical resistivity models along four profiles across the active Quaternary volcanoes of Kyushu Island. The 2-D models show upwelling magma sources (aqueous fluids and/or partially molten mantle) produced during the subduction of the PSP.…”