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Additional file 1: Sounding curves for all MT sites.

Additional file 2: Sounding curves of SSQ invariant impedances. Colors indicate the west–east zoning of site locations corresponding to the site colors in Figure 1b.

Additional file 4: Inverted 3-D resistivity model showing the areas in which inverted resistivities were replaced in the sensitivity tests. Yellow and red polygons show

Additional file 4: Figure S4. Photo image of collected basaltic rock samples during the dive 6K#1331 near the Yokoniwa hydrothermal field of the Central Indian Ridge. Photographic data are cited from Data and Sample Research System for Whole Cruise Information â DARWINâ manage by JAMSTEC ( http://www.godac.jamstec.go.jp/darwin/ ).

Additional file 1: Figure S1. Analysis of observed magnetic anomaly along the 6K#1171 track assuming a magnetization direction parallel to the geocentric axial-dipole field; an inclination of − 43° and a declination of 0°. From top to bottom: observed (red line) and synthetic (blue line) magnetic anomaly intensity of x, y, and z components, magnetization, phase, vehicle altitude, and seafloor/vehicle geometry. Black and gray circles are those with high coherency of >0.5 and from 0.3–0.5 on both the x and z components, respectively. The profile was divided into four sections depending on the survey direction, which is essential for two-dimensional analysis of magnetic anomalies. WL, window length.

Additional file 4: Figure S1. Backfield DC demagnetization curves of the studied lava samples; 6K#569R4-2, 6K#569R4-3, 6K#562R1, and 6K#569R4-1. The measurements were taken in limited field range to extract only remanent coercivity (Hcr). The 6K#569R4-3 sample display both fresh (solid line, top) and altered (broken line, bottom) specimens.

Additional file 2: Figure S2. Analysis of observed magnetic anomaly along the 6K#1332 track assuming a magnetization direction parallel to the geocentric axial-dipole field; an inclination of − 43° and a declination of 0°. From top to bottom: observed (red line) and synthetic (blue line) magnetic anomaly intensity of x, y, and z components, magnetization, phase, vehicle altitude, and seafloor/vehicle geometry. Black and gray circles are those with high coherency of >0.5 and from 0.3–0.5 on both the x and z components, respectively. The profile was divided into three sections depending on the survey direction, which is essential for two-dimensional analysis of magnetic anomalies. WL, window length.

Additional file 5: Table S1. Major element composition for lava samples collected along the dive 6K#918 near the Kairei hydrothermal field. The data were cited from Kumagai et al. (2008).

Additional file 3: Figure S3. Photograph of collected basaltic rock samples during the dive 6K#918 near the Kaire hydrothermal field of the Central Indian Ridge. Photographic data are cited from Data and Sample Research System for Whole Cruise Information â DARWINâ manage by JAMSTEC ( http://www.godac.jamstec.go.jp/darwin/ ).