| Haiyuan arc tectonic belt is located at the junction of the Qilianshan collisional orogenic belt,the Alax block and the Erdos block in the northeastern margin of the Tibetan Plateau.It is the frontier of the northeastwards extension and the latest component of the Tibetan Plateau,with strong tectonic deformation and seismic activity during the Late Cenozoic.Haiyuan arc tectonic belt and its adjacent areas are the best places to study the reomote effect of the uplift of the Tibetan Plateau and the block contact relationship.The study of deep structure beneath this area is helpful to reveal the interaction between the Tibetan Plateau and the Alax block and the Erdos block and to explore the environment for strong earthquakes in the interior of the continent.In this paper,one long-period magnetotelluric sounding profile(LMT)and two broadband magnetotelluric sounding profiles(BBMT)across Haiyuan arc tectonic belt in the northeastern margin of the Tibetan Plateau are used to modelling the crust and uppermost mantle electrical structure in this area.Huining-Barunbieli profile is nearly N-S trending,located at the junction of the eastern Qilian tectonic belt and the Alax block in the northeastern margin of the Tibetan Plateau.The others are Jingyuan-Yanchi profile and Huining-Qingcheng profile,located at the junction of the eastern Qilian tectonic belt and the Erdos block in the northeastern margin of the Tibetan Plateau.Through the MT data processing flow,high-quality impedance tensor data are obtained,and the LMT and the corresponding BBMT data are merged.The information of 2D skewness and strike direction is extracted by using GB decomposition and phase tensor analysis technology,and be analyzed in detail.2D inversion were conducted,respectivly.The inversion results of different data modes were evaluated in terms of data RMS and structural characteristics.3D inversion were also conducted to further constrain the reliability of the 2D inversion model.Integrating the regional geological and geophysical data in the study areas,electrical structure is analyzed in detail,and a series of new results is achieved.(1)A high-resistivity wedge-shaped structure toward S or SSW is found in the crust beneath Hexi corridor belt.The result of the Huining-Barunbieli profile shows that from the south to the north,the crust and uppermost mantle electrical structure model can be divided into: the eastern part of Qilianshan high-low-high resistivity stratoid electrical structure,Hexi Corridor low resistivity belt electrical structure and the southern margin of the Alxa block high-low-high resistivity layered structure.A high-resistivity wedge-shaped structure toward S or SSW is found in the crust beneath Hexi corridor belt.Integrating the analysis of regional tectonic evolution,it is inferred that the wedge-shaped high-resistivity blocks may be the reflection of island chain paleo-uplift or underwater paleo-uplift in the early Paleozoic oceanic basin.This wedge-shaped structure may indicate the dynamic process of wedging of Alax block into the Qilian collision orogenic belt in the northeastern margin of the Tibetan Plateau.(2)Revealing the deep contact relationship between the northeastern margin of the Tibetan Plateau and the southwestern margin of Erdos block.The results of Jingyuan-Yanchi profile and Huining-Qingcheng profiles show that from southwest to east,the crust and uppermost mantle electrical model can be divided into the eastern part of Qilianshan high-low-high resistivity stratoid electrical structure and the southwestern margin of Erdos wedge-shaped high-resistivity structure,and the latter can be subdivided into the Liupanshan high-resistivity wedge-shaped structure and the southwestern margin of Erdos high resistivity block.The Liupanshan high-resistivity wedge-shaped structure correspond to Haiyuan arc tectonic blet in surface.The Liupanshan high-resistivity wedge-shaped structure has a tendency to wedging westward or southwestward,and the eastern Qilian tectonic belt and the Haiyuan arc tectonic belt located on the top of crust thrust toward east or northeast to form the thrust-nappe belt in western margin of the Erdos block.(3)Modeling the 3D curst and uppermost mantle electrical structure beneath Haiyuan arc tectonic belt and its adjacent areas.In order to further reveal the spatial distribution characteristics of the curst and uppermost mantle wedge-shaped high resistivity block beneath the junction of the northeastern margin of the Tibetan Plateau,the Alax block and Erdos block,By collecting the published research results of 14 MT profiles in this area,a 3D electrical structure model of the crust and uppermost mantle beneath Haiyuan arc tectonic belt and its adjacent areas is constructed by using GOCAD,which include: 3D structural model of the main faults,3D crust and uppermost mantle electrical structure model of the blocks,high resistivity wedge-shaped structure model of the junction belt and the crustal electrical structure model of the strong earthquake focal region.3D structural model of the main faults: Haiyuan-Liupanshan fault dips to the southwest,and disappears in the low resistivity layer distributed in lower crust beneath the eastern Qilian tectonic belt,which belongs to the intracrustal fault.Haiyuan segment has a steep dip,and Liupanshan segment has a gentle dip,and the dip increasing southward.Tianjingshan fault has a steep dip in shallow curst and gentle shape in middle crust.From southeast to northwest,the dip angle gradually becomes steeper,and the deep part extends to the southwest to the low-resistivity layer distributed in the north side of the Haiyuan fault,forming the slip surface of the overlying thrusting nappe structure.Longshoushan-Chahan-Qingtongxia-Guyuan fault is generally steep in shallow and gentle in deep,and the deep part extends to the southwest to Hexi corridor belt in a near-horizontal form,forming the main slip surface of the Haiyuan arc tectonic belt extending from the southwest to the northeast.3D electrical structural characteristics of crust and uppermost mantle of blocks: the eastern Qilian tectonic belt shows a high-low-high resistivity stratoid electrical structure.The eastern part of Hexi corridor belt show a low-high resistivity layered electrical structure near the side of the eastern Qilian tectonic belt,and a high resistivity wedge-shaped structure near the side of Alax and Erdos block.The southern margin of the Alax block shows a high-low-high resistivity layered curst and uppermost mantle electrical structure and the southwestern margin of Erdos has a high resistivity wedge-shaped curst and uppermost mantle electrical structure.High-resistivity wedge-shaped structure model beneath the junction belt: Based on the 3D electrical structure model of the crust and mantle,systematic and comprehensive analysis and research have been carried out.It is found that the wedge-shaped high resistivity structure exists widely beneath the junction belt between the northeastern margin of the Tibetan Plateau and the North China Plate.The wedge-shaped structure may indicate that the North China Plate has not blocked the NE-trending expansion of the Tibetan Plateau passively,but wedged into the northeastern margin of the Tibetan Plateau.We infer that the wedge-shaped high resistivity structure in the crust and mantle may be one of the most basic types of electrical structure beneath collisional orogenic belt.The crustal electrical structure model of the strong earthquake focal region: On the basis of systematic analysis and comprehensive study of the spatial relationship between the 3D electrical structure model and the distribution of earthquake focal in the northeastern margin of the Tibetan Plateau,the crustal electrical structure model of the strong destructive earthquake focal region in the northeastern margin of the Tibetan Plateau is preliminarily established,the crustal electrical structure consisting of a number of irregularly shaped,unconnected “fragmented” extremely high resistivity blocks in mid-upper crust and stratoid low resistivity layer in mid-low crust is one of the optimal crustal structures for strong and destructive earthquakes occurrence in the northeast margin of Tibet plateau. |
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