| Continental collision is one of the important manifestations of the movement of Earth’s tectonic plates and plays an important role in the evolution of modern continents.The Tibetan Plateau is the largest plateau in the world and one of the youngest plateaus in the world.Since the Cenozoic,the continental collision between India and Asia has formed the majestic surface features of the Tibetan Plateau.Its unique geological features make it a golden key to study the tectonic evolution of continental convergence and intracontinental subduction.However,there are still controversies about a series of scientific issues such as the uplift mode of the Tibetqn Plateau and the location of lithospheric subduction between India and Asia.Due to its high altitude and harsh environment,field exploration is difficult to carry out,so many regions still lack magnetotelluric profiles that reflect their deep and fine structures.Under the support of the Second Tibetan Plateau Scientific Expedition and Research Program-"Fine Lithospheric Structure and Superficial Response in Key Areas" and the National Natural Science Foundation of China(41590863),a 1400 km long magnetotelluric(MT)profile starting from the Tethys Himalayan terrane in the south and ending in the Tarim Basin in the north across the entire Tibetan Plateau was carried out within 88 ° E-89 ° E.This MT profile comprised a total of 261 MT stations,including 61 long-period MT stations.This study not only supplements the magnetotelluric data reflecting the fine structure of the Hohxil terrane,but also provides electrical evidence for studying the structural characteristics of the deep subduction front of the Indian and Asian plate.Two dimensional and three-dimensional inversion were performed on the data,and the inversion results have a certain degree of similarity.Due to the excessive redundancy of structures in two-dimensional inversion,the results of three-dimensional inversion were ultimately chosen as the electrical structure model for the deep part of the study area.The inversion results show that different terrains on the Tibetan Plateau have different electrical structural characteristics.Among them,the crust of the Tethys-Himalayan terrane as a whole exhibits high resistance,while the crust of the Lhasa terrane contains high conductors that tilt northward.The high conductors in the crust of the North Qiangtang terrane tilt southward,and the high conductors below the Jinsha River suture zone have a tendency to extend deeper.The high conductor of the Hoh Xil terrane tilts northward and penetrates into the southern part of the Kunlun orogenic belt.There is a very obvious southward subducting high conductor beneath the Altyn Tagh fault.Based on the analysis of other geological and geophysical data,this paper has gained the following understanding of the convergence behavior of the Indian and Asian plates beneath the Tibetan Plateau:(1)The position of the subduction front of the Indian crust in the study area may be at 31°N,where crustal duplexing occurred in the Lhasa crust.The three-dimensional electrical results in the southern plateau indicate the presence of a high conductor tilted northward at a position of approximately 31°N.Based on previous research,this paper speculates that it represents the position of the subduction front of the Indian lower crust,and that the Lhasa terrane at the front has undergone crustal duplexing,resulting in crustal thickening.Therefore,the large-scale high conductivity anomaly in the lower crust of the Lhasa terrane may be due to intense thrust friction in the Lhasa crust,resulting in partial melting.The high conductivity anomaly is the electrical response of the subduction front of the Indian crust.Based on the different empirical relationships between resistivity and melt fraction,it is estimated that the lowest melt fraction range for the lower crustal conductor is 6.2% ～12.7%,indicating that decoupling occurred during the subduction process between the Indian plate crust and the lithospheric mantle,and the lithospheric mantle may continue to subduct northward.Comparing the electrical characteristics of the Indian crustal subduction front and the oceanic-continental subduction front,it is found that the formation of high conductors in the oceanic-continental convergence front is more related to "water",which is significantly different from the formation of high conductors in the continental convergence front.In addition,crustal duplexing may occur at the front edge of land-land convergence,resulting in crustal thickening,which did not occur at the front edge of ocean land convergence.(2)The front area of the northward expansion of the Tibetan Plateau may be located in the Kumkol Basin,while the uplift of the Altyn Tagh Mountains may be related to the decoupled northward thrust of the upper crust.The inversion results of the Kunlun orogenic belt and the Altyn Tagh fault zone in the northern part of the Tibetan Plateau show that the crustal flow from within the plateau is hindered by the high resistance of the Qimantag Mountain crust and does not continue to penetrate northward,only penetrating below the Kumkol Basin.Although both the Qaidam Basin and the Kumkol Basin are located in the front edge area of the northward expansion of the plateau,the crust exhibits different electrical characteristics,indicating that the latest front edge area of the northward expansion of the plateau may be located below the Kumkol Basin.The Altyn Tagh Fault Zone is a deep and large fault at the scale of cutting the lithosphere,and the inversion results have identified two branch faults in the north.There are traces of the Asian plate subducting southward along the Altyn Tagh fault zone.The high conductivity inside the crust beneath Altyn Tagh fault may be caused by tectonic heat generated by the subduction of the Asian crust,which may be a material response to the subduction front of the Asian crust.Different models and empirical relationships have calculated that the melt fraction of high conductors in the middle and lower crust below the Altyn Tagh Mountains ranges from 3.4% to 8.1%,which may indicate a decoupling between the lower and upper crust.Affected by the far-field effect of the northward push of the Indian Plate and the southward subduction of the Asian lithospheric mantle,the upper crust beneath the Altyn Tagh Mountains undergoes a northward thrust along the thrust fault,causing strong surface deformation and resulting in the uplift of the Altyn Tagh Mountains.(3)The convergence of the Indian and Asian lithosphere on the northern Tibetan Plateau has reactivated the Jinsha River suture zone.The inversion results indicate that the Qiangtang and Hohxil terranes may be located at the front edge of the convergence of the Indian and Asian lithosphere.The partially melted high conductor extending to a depth of 100 km below the Jinsha River suture zone indicates that the suture zone has been reactivated since the Cenozoic.The convergence of the Indian and Asian plate lithosphere beneath the northern Tibetan Plateau leads to convection in the mantle,and the reactivated Jinsha River suture zone becomes a pathway for mantle thermal material upwelling,resulting in large-scale partial melting of the middle and lower crust.The large-scale high conductivity represents the material response of the convergence of the Indian and Asian plate lithosphere beneath the northern Tibetan Plateau.This result confirms that the reactivation of ancient suture zones is a direct manifestation of the convergence of the Cenozoic lithosphere.The melt fraction of the lower crust in the Jinsha River suture zone is 6.2%～12.7%,and this strength difference is enough to cause a decrease of one order of magnitude in the strength and viscosity of the rocks.Therefore,there may be crustal flow below the northern Tibetan Plateau,and the upwelling asthenosphere magma along the tectonic weak area can provide replenishment for crustal material flowing eastward.(4)Bangong Nujiang Ocean and Jinsha River Ocean experienced bidirectional subduction,respectively.Electrical results indicate the presence of high-conductivity anomalies associated with both northward and southward subduction along the Bangong-Nujiang suture zone and the Jinsha suture zone.Combining previous research,this study suggests that these anomalies represent traces of bidirectional subduction of the Bangong-Nujiang Ocean and the Jinsha River Ocean.There are significant differences in the electrical characteristics between the Bangong Nujiang suture zone and the Jinsha River suture zone.There are high conductors extending deep beneath the Jinsha River suture zone,which may reflect the continuous northward movement of the Indian Plate since the Cenozoic. |
