Deep Electrical Resistivity Structure And Geological Significance Of The Eastern Segment Of The Qilian Orogenic Belt At The Northeastern Margin Of The Qinghai-tibet Plateau

The eastern segment of Qilian orogenic belt is located in the northeastern margin of the Qinghai-Tibet Plateau.It is the intersection zone of multiple tectonic units and shows the geomorphic characteristics of the basin now.The region is in the forefront of the outward expansion of the plateau,which is the key area to study the geodynamic process of crustal thickening and material flow.Along the northwest striking Maxianshan fault,it can be divided into the Linxia block on the southwest side and the Longzhong block on the northeast side.It has experienced a number of tectonic evolution events,which formed a complex tectonic pattern and led to frequent seismic events,including the Qilian ocean closure and the far-field affect of the collision between the India and Eurasia plates.There were lots of previous studies on the northeastern margin of the Qinghai-Tibet Plateau based on geophysical,geochemical,petrological and chronological methods.They were mainly focused on the evolution process of the North Qilian fold belt and the contact relationship between tectonic units.The understanding of the basement properties of the Linxia block and Longzhong block is still unclear.This paper studies the deep electrical structure of Linxia terrane and Longzhong terrane in the Eastern Qilian orogenic belt,which is of great significance to clarify the differences between the two terranes and infer the regional evolution process.This paper uses long-period and broadband magnetotelluric data along the two northeast profiles.54 broadband stations and 10 long-period stations were recorded.The broadband station spacing is about 5 km,and the long-period station spacing is about 20 km,passing through three tectonic units including the Western Qinling block,the Linxia block and the Longzhong block.Through fine data processing and analysis,this paper gained the dimensionality and electrical principal axis of the subsurface structure.Two-dimensional inversion and three-dimensional inversion were carried out based on nonlinear conjugate gradient algorithm to obtain the resistivity model of two profiles at a depth of 60 km.The results show that:(1)the Linxia block on the south side of the Eastern Qilian orogenic belt is characterized by layered electrical properties.The low resistivity structure on the shallow surface corresponds to the sediment of the basin.There is a medium high resistivity structure on the upper crust.The middle and lower crust is a relatively continuous high conductivity layer with a resistivity of about 10 ?·m;(2)the shallow layer of the Longzhong block in the west of the Eastern Qilian orogenic belt is a 3 km thick low resistivity sediment.The crustal structure is characterized by a integrated high resistivity structure.(3)the Maxianshan fault between the two blocks is a deep electrical gradient zone,which can extend downward to the lower crust or even Moho.Based on the characteristics of the resistivity model and the regional geological background,this paper reveals the basement properties and the differences between the Linxia and Longzhong blocks,and discusses the evolution process of the study area:(1)the crustal structure of the Linxia block represents the structure characteristic of Japanese type island arc,and the high conductivity layer is the crustal material with deformation,cracks and saline fluids;(2)the crustal structure of the Longzhong block represents the structure characteristic of Mariana type island arc.Its high resistivity and complete crustal structure is the characteristics of basalt formed in oceanic environment;(3)the Maxianshan fault may represent the position of the eastern suture of the Qilian ocean closure,and in addition to the Japanese island arc(now the Linxia block)on one side of the Qilian terrane,ocean-ocean subduction took place in the ocean domain during subduction,resulting in the formation of the Mariana island arc(now the Longzhong block),followed by ocean closure,two types of island arc collided.(4)During the Qilian ocean closure,the compressive stress led to the deformation of the lower crust of the Linxia block.The long-field stress of Cenozoic collision was absorbed in the middle and lower crust in the form of plastic deformation,while the rigid upper crust was conducive to the storage and release of energy,which provided good conditions for the occurrence of earthquakes.