The geothermal field genesis mechanism in the central uplift of the Jizhong depression: A study based on magnetotelluric imaging

Abstract

The North China Craton has undergone destruction due to westward subduction of the Paleo-Pacific Plate, causing significant thinning of its eastern lithosphere. The thinner lithosphere facilitates easier heat conduction to the shallow crust. The Jizhong Depression is located at the center of the cratonic destruction area, featuring numerous large uplift-type geothermal fields. To investigate the geothermal genesis mechanism of uplift-type geothermal fields in the Jizhong Depression, this study employs 237 magnetotelluric data. It constructs, for the first time, a three-dimensional electrical structure model of the Jizhong Depression and then combines regional data to analyze the geothermal system from the perspectives of caprock, heat source, water channel, and heat reservoir. The shallow part of the Central Uplift Zone exhibits low electrical resistivity, surmising the presence of a sand-mud caprock. In the mid-lower crust, a large-scale low-resistivity body C2 is identified, speculated to be a shear zone formed by the Taihang Mountain Piedmont Fault rather than a magma chamber. The upper mantle contains a low-resistivity body C4, which may be partially melting due to the upwelling of upper mantle material along the tectonically weak zones of the lithosphere since the Meso–Cenozoic. Based on calculations, the water content and melt fraction of the low-resistivity body C4 are estimated as 0.5∼3 wt% and 2∼12 %, respectively. The result indicates that the Central Uplift Zone has a significant burial depth for the sand-mud caprock, and the thicker caprock plays an influential insulating role. The Gaoyang and Niudong–Hexiwu faults are major deep-seated faults that transfer deep heat to the shallow crust through thermal convection. Geothermal energy primarily originates from the mantle, where mantle heat flow is transferred to the shallow layers through thermal conduction and convection without an additional magma chamber for heating.