Resistivity Characteristics At The Intersection Of The Ordos And Yangtze Blocks And Its Dynamic Significance

The Qinling orogenic belt located on the northeast margin of the Qinghai-Tibet Plateau is connected to the Yangtze block to the south and the North China block to the north and is the transition zone between the Paleo Asian Ocean tectonic domain and Tethys tectonic domain.This paper uses a north-south magnetotelluric sounding profile which starts at Sichuan basin of Yangtze block,passes through the Qinling orogenic belt and Ordos block,and reaches the Hetao graben.The contact relationship between the Ordos block in the north and the Yangtze block in the south along Qinling orogenic belt,and the northeast escape of material of the Qinghai-Tibet Plateau,can be discussed.The magnetotelluric profile studied in this paper is about 1200 km long,and 61 broadband electromagnetic sounding points are arranged on the profile.Dimensionality analysis and strike analysis were performed on the impedance tensor decomposition and phase tensor analysis.The four periods of 0.01 ? 0.1 s,1 ? 10 s,10 ? 100 s and 100 ?1000 s was decomposed.Combined with the geological structure data of the study area,it was determined that the tectonic strike angle is NE80 °.Based on the results of Swift decomposition and phase tensor decomposition,we got the Pseudosection of the Swift skew of impedance tensor and the MT phase tensor analysis at different periods.The results shows that along the MT profile,most of the tectonic areas in the study area with two-dimensional structure.Based on the obtained structural strike angle,the MT soundings was rotated,and resistivity model was obtained by two-dimensional inversion.The resistivity model shows that there are low-resistance bodies with a thickness of4-10 km in the upper crust in the Ordos block,and high-resistance bodies with a resistivity value of 300-1000 ?m in the lower crust,which generally show a “Layered”distribution.The electrical structure of the crust in the Qinling orogenic belt is more complicated,and the difference between the north and south sides is obvious;the south side is dominated by large and thick high-resistance distribution,and the north side shows the characteristics of high-low resistance interphase distribution.The Yangtze block is dominated by high-resistance bodies,with larger and deeper distributions,and shallow parts with low-resistance bodies.Large-scale “bottleneck” low-resistivity anomaly found in the northern Qinling orogenic belt may be upwelling channels for mantle material in the opposite subduction edge of the North China and South China blocks,or channel of middle and lower crustal material to escape eastward along the structurally weak area in the Qinling orogenic belt.A south-dipping low-resistivity anomaly was found at the boundary between the Qinling orogenic belt and the Dabashan arc zone,which is interpreted as an electrical reflection of the thrust fault at the northern edge of the Dabashan arc zone.In the resistivity model,the high-resistiviy anomalies in the Yangtze block slopes northward to the Qinling orogenic belt.But the high-resistivity anomaly in the northern Ordos block slopes southward to the Qinling orogenic belt.The high-resistivity anomalies can usually only be considered as a reflection of stable lithosphere,it is inferred that the two groups of high-resistivity anomalies subducted beneath the Qinling orogenic belt are indicating the opposite subduction of the North China and Yangtze blocks beneath the Qinling.And it is the basis of electrical structure.The Ordos block has large differences in electrical properties between its southern and northern parts.Large-scale low-resistivity anomalies with resistivity values between 3 and100 ?m are distributed in the middle and lower crust in the north,which is explained by joint action of the upwelling of the lithosphere mantle and the inclusion of salt-containing fluid after partial melting.