Three-dimensional audio-frequency magnetotelluric imaging of Akebasitao granitic intrusions in Western Junggar, NW China

• We map the 3D electrical resistivity of the Akebasitao pluton in Western Junggar, China.
• An AMT array consisting of 463 sites is inverted by using ModEM.
• The Akebasitao pluton is imaged as an asymmetrical mushroom-like resistor.
• The Darbut Fault is imaged as a sub-vertical narrow conductive zone.
• An extension regime at the pluton's forming age in Western Junggar is suggested.

An audio-frequency magnetotelluric (AMT) array was deployed here to map the resistivity structure of the Akebasitao intrusions and the surrounding area in Western Junggar, China. High quality AMT data were acquired at 463 sites that covered the whole Akebasitao intrusions. The latest 3D electromagnetic inversion system, ModEM, was employed to invert the AMT dataset. A high resolution resistivity model was recovered by using a nonlinear conjugate-gradient scheme. The Akebasitao intrusions, the most pronounced resistive features in the preferred model, are clearly imaged from shallow depths to more than 10 km, with some conductive zones and spots in and surrounding it. The shape of the Akebasitao pluton is asymmetrical mushroom-like, suggesting an extension stress environment at its forming age (the Late Carboniferous to the Early Permian) in Western Junggar. Our explanation is consistent with the subducting spreading ridge tectonic model for Western Junggar in the Late Carboniferous, with a paleo-Asia ocean plate subducting northwestward beneath Karamay arc, as former studies proposed. The upwelling channel of the Akebasitao pluton seems to be located at its center. The Darbut Fault has been clearly imaged as a subvertical narrow conductive zone extending from the surface to 3–5 km or deeper. The most prominent conductors are two elongate zones in depths deeper than 500 m. Other conductive zones can also be identified surrounding the boundaries of the Akebasitao pluton, which can be interpreted as pyrometasomatic metamorphism relating to magmatic activities. Because the ophiolitic rocks outcrop as some lens along these two boundaries, and as previous studies showed that the serpentine metamorphosed from peridotite with well-connected magnetite possessing high conductivity, the conductors may therefore also represent their sources in depth.