The Electrical Resistivity Structure And Its Tectonic Implications Of The Ailaoshan-Red River Tectonic Belt And Adjacent Areas

The Ailaoshan-Red River tectonic belt is located at the junction of South China block and Simao-Indosinian block on the southeast margin of Tibet Plateau.From the Paleozoic to the Mesozoic,it experienced the opening,subduction and closure of the Ailaoshan Paleo-Tethys Ocean,followed by the collision and orogeny of the Indosinian block and the South China block.Since the Cenozoic,it was affected by the long-range effect of the collision between the Indian plate and the Eurasian plate,and has undergone strong tectonic deformation and seismic activity as an important regulatory response area on the SE Tibet Plateau,thus becoming an important window for studying the lateral expansion of the Tibet Plateau and the dynamic process of the interaction between adjacent blocks.However,due to the insufficient research on deep structure,the contact relationship of adjacent blocks,structural deformation mechanism and crust-mantle dynamics are still unclear.In this paper,broadband magnetotelluric(BMT)and long-period magnetotelluric(LMT)were carried out to establish the crustal and upper mantle electrical resistivity structure.Based on the regional geological and geophysical data,the key scientific issues such as the contact relationship between Indochina block and South China block,the deep dynamic process of Ailaoshan-Red river tectonic belt and its adjacent areas,and the characteristics of seismogenic environment are discussed.The main research contents and understandings are as follows:(1)Establishing the electrical resistivity structure of Ailaoshan-Red River tectonic belt and adjacent areas.A total of 240 BMT sites and 81 LMT sites were collected along 8 crisscrossed profiles,covering an area of about 150000 km~2.The MT data are inversed by using three schemes,including 3D inversion with galvanic distortion,conventional profile2D inversion,Mod EM 3D inversion.3D inversion with galvanic distortion was tested and carried out.The theoretical models show that in the case of galvanic distortion,the results from conventional Mod EM 3D inversion may deviate from the true model.Therefore,MT dataset in the study area are also inversed by the method of 3D inversion with galvanic distortion.A comprehensive comparison of the models from conventional Mod EM 3D inversion and 3D inversion with galvanic distortion shows that the latter is more credible.Furthermore,sensitivity tests were carried out to analysis the effective detection depth of and the robustness of the resistivity model.(2)The characteristics of electrical resistivity structure and interpretation.Combined with other geological and geophysical data,the characteristics and tectonic implications of the resistivity model are discussed.The deep structure of Simao block has obvious stratification and zoning characteristics.There is a wide range of conductor in the lower crust and lithospheric mantle,which provides a deep material source for the tertiary magmatic activities in Ning’er and Tongguan volcanic areas.However,the asthenosphere mantle as a whole is characterized by medium high resistivity,indicating that the asthenosphere mantle activity of Simao block is not obvious.There is a high conductor extending to the asthenosphere mantle in the Southeast Yunnan area of the South China block,which is well corresponding to the magmatic activity from the mantle in this area.However,the low resistivity bodies in the crust of Pingbian and Maguan volcanic areas are obvious,indicating that there is no magma chamber in the crust.It is a direct volcano with magma chamber located in the mantle,which is mainly controlled by faults.There is a west-dipping resistor in the lithospheric mantle of the Ailaoshan-Red river tectonic belt.Combined with regional geological and seismic tomography,it is interpreted as the residual subducted slabs of South China block that subducted towards Indosinian block after the closure of the Ailaoshan Paleo-Tethys Ocean in the Early Triassic.The resistors in the middle and upper crust are the corresponding relic of napping to the Indosinian block.It appears as a typical crocodile mouth structure remnant in the plate convergence zone,and the contact relationship model between the Indosinian block and the South China block was established.According to the Amojiang-Lixianjiang fault corresponding to the thrust nappe front of the resistors in the middle and upper crust and its electrical interface can extend to the residual slab of the deep lithospheric mantle,the specific location of the Paleo-Tethys suture should be in the area of the Amojiang-Lixianjiang fault.(3)Studying on distribution of materials flow and dynamic mechanismCombined the resistivity model with the previous experimental results of deep rock conductivity,the partial melting and water content beneath the study areas are estimated.The results show that the lower crust and lithospheric mantle at the southern end of the Xiaojiang fault has a high melting percentage and water content,and has the conditions for weak material plastic flow,but it has not spread across the Red River fault to Simao block or to Southeast Yunnan area.The asthenosphere mantle in Southeast Yunnan area has high water content,which indicating the asthenosphere upwelling.Comprehensive analysis shows that there is a southward weak material flow channel in the lower crust and lithospheric mantle at the southern end of the Xiaojiang fault.However,it is blocked by the residual subducted slabs of South China block and traces of magmatic activity on the southwest side of the Red River fault.There is an upwelling of thermal material from the depth of asthenosphere mantle near Wenshan-Malipo fault,which may be related to the upwelling branch of Hainan mantle plume in the southeast.The upwelling of asthenosphere mantle material in Southeast Yunnan areas interacts with the southward lower crust and lithosphere mantle material flow of the Xiaojiang fault zone,resulting in complex material processes in the Ailaoshan-Red river tectonic belt and adjacent areas.(4)Discussing the seismogenic environment and mechanismBy comparing the distribution of seismicity,active faults and the characteristics of electrical structure,the seismogenic environment characteristics of Tonghai and Wenshan earthquake areas are discussed.The earthquakes in Wenshan area are mainly distributed around the fluids-related conductors in the middle and upper crust.The most obvious feature is that there is a conductor extending to the asthenosphere mantle.Considering that the current tectonic deformation in this area is not strong,it is considered that the seismic activity in Wenshan seismic area is mainly related to the deep fluids activity and migration,and the fluids can be derived from the upwelling activity of the asthenosphere mantle material.The seismicity in Tonghai earthquake areas can be divided into two parts.The first is the clamping area between Qujiang fault and Shiping fault.Due to the differential movement of the fragmented resistors of the middle and upper crust in the process of southward migration under the compression of the Sichuan-Yunnan block and the traction of the plastic flow,the stress accumulation occurs in the resistors and the transition zone with the underlying conductors.Second is the southeast end of Qujiang-Shiping fault system,because the strike slip movement in the southeast direction of the fault and the compression formed by the passive blocking of the lithospheric resistors of the South China block,earthquakes were conceived and occurred.