A Study Of Deep Electrical Structure Characteristics And Geothermal Genesis Mechanism In The Hetao Basin And Northern Margin Of Ordos Block

The Hetao Basin and the northern margin of Ordos block locate in the western part of the North China Craton.The Ordos block is one of the oldest and most stable Craton in Asia and has undergone a series of major tectonic events: Suture of North China Craton and Mongolian Block in the Permian/Triassic Period;Collision between North China Craton and Yangtze Craton in the Triassic Period;Subduction of the Paleo-Pacific plate begining at the Cretaceous.In order to better see the crust-mantle structure and tectonic characteristics of the lithosphere,understand the control of deep lithospheric processes on the formation of Hetao Basin,and the influence on the distribution of geothermal resources in the study area.The paper views the broadband(139sites)and long-period(14sites)magnetotelluric data in the Sinoprobe-01-02 as the research subjects.The crust-mantle electrical structure of the region is obtained,and the dependence between the electrical structure and the deep thermal state is further analyzed,in order to study the geothermal mechanism of the region and discriminate the electrical properties from different tectonic models.The perspective provides the powerful constraints and evidence.The raw time-series data collected in the field is processed through a series of standard processes,and finally high-quality tensor impedance data is obtained.Based on this,2D and 3D inversion studies were conducted.The final electrical structure model reflects that the mantle upwelling occurred in the Early Cretaceous even earlier,which is concluded that the asthenosphere containing high-conductivity sulfides upwelled along the weak suture zone,causing the lithosphere destruction and thinning,and the local area of the crust is melted,and the existence of deep faults provides important channels for the invasion and ejection of magma.The subsidence of the Hetao Basin,and the uplifting of the northern margin of Ordos block and Yinshan orogenic belt are closely related to the upwelling of the asthenosphere under the regional extensional tectonic environment.The adjustment of gravity balance caused by the extension of the crust in the Hetao Basin and the passive uplifting of the lithospheric mantle,forming the thinner "hot" lithosphere and higher temperature mantle heat flow.Deep heat can continuously warm the Cenozoic phreatic water in Hetao Basin by conduction,and become a stable heat source of sedimentary low-temperature geothermal field.The heat transfer of the mantle heat flow and the thermal convection of the deep fault constitute the heat source mechanism of the low-temperature geothermal field in the Hetao Basin.The geothermal mechanism of the study area is discussed based on the electrical structure model,and the Hashin-Shtrikman boundary condition is used to constrain the upper mantle geothermal structure,and the feasibility of this research idea is demonstrated by the actual verification analysis of the high-conductivity anomaly.Verification results: the Baotou geothermal well has a depth of 2604.34 m,and the main water intake section is 1577.69-2581.83 m,and the bottom of hole temperature is 70.8?,and the effluent water temperature is 64?.