| The Sichuan-Yunnan block(SYB),located in the southeast margin of the Tibet Plateau,is an important component of the southeast expansion of the Tibet Plateau since the late Cenozoic,and a key site to study the uplift model and deformation process of the Plateau.The Xiaojinhe-Qinghe tectonic belt(Lijiang-Xiaojinhe fault in the North Branch,Jinhe-Qinghe fault and Chenghai fault in the South Branch)cuts the SYB obliquely.Previous studies suggest that the tectonic belt has shielding and absorbing effect on the southeast extension of the Tibet Plateau,but its deep driving mechanism and dynamic process are not clear.At the same time,there are frequent earthquakes in the study area,and its seismogenic background remains to be combed.In order to further understand the scientific problem which is the deep-shallow tectonic coupling relationship and seismogenic background of the Xiaojinhe-Qinghe tectonic belt(XQTB),two broad-frequency magnetotelluric(MT)profiles and one broad-frequency and long-period magnetotelluric(MT+LMT)profile were deployed and completed.Three-dimensional curst-mantle electrical structure model was established.combined with seismic,gravity and magnetic data,we deployed the comprehensive interpretation,including surface deformation data analysis;In addition,geology investigations were completed,including three tectonic points and one remote sensing site.First,through the precise processing and analysis of magnetotelluric data,the qualitative understanding of deep structure in the study area is obtained,and the inversion scheme is defined.Based on the fine two-dimensional and three-dimensional inversion,the reliable crust-mantle electrical structure model of central SYB is obtained.Then,combined with the existing research data,the comprehensive geological and geophysical analysis of the electrical structure model of each section is carried out,the shallow stratigraphic and tectonic units are further divided,and the deep extension of XQTB is speculated.At the same time,combined with the previous research results of geophysics,the genetic mechanism and tectonism of the large crust-mantle geological body with high resistivity(LCMG)in the middle of SYB are discussed.Secondly,according to the characteristics of deep electrical structure model and the demands for deep-shallow dynamic relationship analysis,we carried out the surface geological investigation and research,the shallow structural deformation pattern of XQTB is basically clarified.It is considered that the unbalanced crustal movement in the middle of SYB is not only reflected in the segmented difference of surface tectonic properties,but also in the block difference of deep electrical structure,which seems to indicate that there is some coupling relationship between deep structure and shallow structure.On this basis,the tectonic activity time and reginal kinematic characteristics of the study area are further combed,the dynamic process is discussed,and the deep-shallow tectonic coupling relationship of XQTB has been constructed in three stages since the late Cenozoic.At last,based on the analysis of the distribution of historical earthquake hypocenters,combined with the tectonic characteristics and regional kinematic characteristics,the seismogenic background of the study area are further summarized.In general,the following innovative achievements have been achieved in this paper:(1)The electric structure of crust-mantle in the middle of SYB is characterized by block.The deep parts of Muli-Panzhihua(north-south)and Yongsheng-Mianning(east-west)show high resistivity structure(LCMG)and develop a small amount of high conductive layers,which is surrounded by a large set of high conductive layers.These high conductive layers are significant anisotropy,not only in the spatial distribution,but also in the amplitude of the resistivity,which play an important role in adjusting the southeast expansion of the Tibetan Plateau.(2)The LCMG in the middle of SYB.The LCMG in the middle of SYB shows the geophysical characteristics of three highs(high resistivity,high velocity,high density),which may have the same origin.It is speculated that it is the result of the paleo mantle plume tectonism in the Late Permian,with strong petrological properties,and may be a stable and rigid geological body,which formed the deep dynamic background of passive blocking to the southeast extension of the Tibet Plateau in the late Cenozoic.(3)The shallow structure of XQTB is characterized by segmentation.The northeast section of XQTB: northeast section of Lijiang-Xiaojinhe fault(LXF)and Jinhe-Qinghe fault(JQF)are weak active or inactive thrust at present.The scale and deformation degree of JQF are significantly larger than that of northeast section of LXF.The southwest section of XQTB: southwest section of LXF is a sinistral strike slip fault with thrust component,Chenghai fault is a normal and sinistral strike slip fault.(4)Since the late Cenozoic,the XQTB has been divided into three stages of deep-shallow tectonic coupling and its adjusting to the southeast expansion of the Tibet Plateau:Before the middle Miocene,under the tectonism of the crust-mantle high conductive layers,the crust medium of the Tibet Plateau expanded eastward,and the upper crust was blocked by the rigid LCMG,which led to the uplift of the upper crust medium,and formed the XQTB in form of thrust as the main property,which adjusted the eastward expansion of the Plateau as an important regulating component;In the middle and late Miocene,the Xianshuihe fault splited the eastern margin of the Tibet Plateau and began the sinistral strike slip.With the continuous southeast extension of the eastern margin of the Plateau,the thrust of the XQTB was further strengthened,which resulted in the emplacement of the northeast section of the tectonic belt and further adjusted the southeast extension of the Plateau;Since the Pliocene,with the tectonism of multiple stages,the LCMG may have been splited,showing less rigidity than the early stage,especially in the northwest;The Xiaojiang fault begun with sinistral strike slip,and the sliding component of the Xianshuihe fault more passed on to a series of North-South faults such as the Xiaojiang Fault to adjust the tectonic deformation of the eastern SYB,northeast section of LXF and JQF stopped large-scale deformation and may no longer participate in adjusting as an important coordinate component;At the same time,the Red river fault began with dextral strike slip.Influenced by the tail tension effect of Red river fault,the southwestern section of LXF and Chenghai fault began with sinistral strike slip and continue to adjust the southeast expansion of the Plateau.(5)The seismogenic background of the central SYB.The Holocene active structure is the basic structural background of earthquake,and the corner zone with multiple faults is one of the backgrounds of frequent occurrence of large earthquakes.The high and low resistivity steep zone is the best combination of electrical structure in the study area.The eastward extension of the Tibet Plateau is the basic dynamic condition for the occurrence of large earthquakes in the central SYB.In the future,the southwest section of XQTB may be in danger of large earthquake disaster. |
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