Imaging the Partially Melt Rock System of Gonghe Basin, Qinghai, China, with Magnetotellurics in Three Dimensions

A three-dimensional (3-D) electrical resistivity model of Gonghe Basin in northeastern Tibetan plateau, unveils a complex subsurface filled with zones of partial melt, sedimentary mantles, cold plutons, and regional faults. Between the years of 2018 to 2022, 1350 broadband magnetotelluric stations were collected in an array around Gonghe Basin from which a 3-D electrical resistivity model was created with a resolvable depth of 50 km. Our resistivity model reveals a significant feature of step discontinuities between high and low resistivity blocks at different depths in the upper and middle crust. Our 3D resistivity model also shows a large higher resistivity ( $30-200\varOmega.m$ ) body at depths of 3–10 km located beneath the hot springs around the Basin. We explain that this anomaly, which is smaller than the normal resistivity range of intact granite, is mainly influenced by the high temperature environment. The most electrical resistivity robust features are two huge conductive bodies ( $1-10\varOmega.m$ ) that underlie the northeast and southern margin of Gonghe Basin below 10 km depth. The preferred electrical resistivity model suggests that the source reservoir is a narrow northeast-southwest elongated body which like a goldfish 10∼30 km below the middle basin and a vertical conductive channel in the depth could reach to 7 km below dalianhai country. Below the southern part of the basin, there is a larger high conductor that does not merge into one more diffusive and broad anomaly for a range of 1 0-40 km. Combined with the analysis of s-wave low-velocity, we believe that the most important heat source could be a zone of 5-10% partial melt in the crust.