Geophysical constraints on the deep structure beneath the Xiong'an New Area geothermal field, North China

The region known as the Xiong'an New Area serves as a testing and validation research laboratory for the development and utilization of geothermal energy in China. To better understand the lithospheric structure and associated geothermal features in the region, a comprehensive approach was employed, combining deep seismic reflection and an overlapping magnetotelluric sounding (MT) profile analysis. The analysis reveals arched reflections, coinciding with a relatively high-resistivity anomaly, demonstrating concentric patterns that indicate the presence of lithospheric gneiss domes beneath the Rongcheng and Niutuozhen structural highs. Additionally, the seismic image reveals a marked transparent zone characterised by low-resistivity beneath the Niubei slope. These dome-like features are interpreted as plutons and/or intrusive bodies that were formed by upwelling of the asthenosphere during the early Paleogene era. These relatively deep-rooted plutons are highly likely to be the primary heat sources in this geothermal region. Deep-seated faults beneath the Niubei slope act as channels, allowing the flow of fluids and the transfer of heat from the source to shallower levels. Isotope data supports that the notion that near-surface water flows through a shallow network of fractures, ultimately reaching the geothermal reservoir. The interpreted geometry suggests that the gneiss domes likely resulted from a RayleighTaylor instability process. This process involves diapiric flow induced by density inversion, leading to an upwelling of the asthenosphere. These dynamics, superimposed onto a melted layered structure, generate the distinctive arcuate seismic reflectors found at the core of these domes. Surface geology analysis indicates that the initiation and final emplacement of these domes most likely occurred during the late Mesozoic period.