Deep Electric Structures Beneath The Northeastern Margin Of The Tibetan Plateau And Its Tectonic Implications

The northeastern margin of the Tibetan Plateau and adjacent areas are a focused topic of current geosciences in the world. This thesis presents the newly accomplished study on the deep structure beneath this region based on magnetotelluric (MT) sounding data.The study area lies in the range 99o-110oE, 33o-40oN, involves several major tectonic units. Its southwest is the Bayankala block, and Qaidam block. The Alashan block lies in the northwest. And it abuts the Ordos block in northeast. The work of this thesis is based on 13 profiles, some of which were recently completed in the field. These profiles pass through thr Qaidam block, Haiyuan boundary between the Qilian and Ordos blocks, Gulang boundary between the Qilian and Alshan blocks, and Yingchuan boundary between the Alashan and Ordos blocks. MT data of high quality at totally 1156 sites on these profiles are available for this study.In data processing, the remote reference and Robust methods are used to improve data quality. All measurements are regularized. The objects of analysis include many parameters of sites, such as orientations of principal electric axes, skewness, magnetic induction vectors, apparent resistivity, and impedance phases. Impedance vector decomposition and static correction are performed to those site data which have local distortion and static shift, respectively. The 2D inversions by NLGG, OCCAM, RRI and REBOCC methods are compared. Finally the NLGG method is used to make 2D inversions for all profiles, including joint inversions of apparent resistivity and impedance phase data by TE and TM polarity modes. From these inversions, 2D electric structure images of the 13 MT profiles are obtained. In conjunction with geological and geophysical data of the study area, the low-resistivity distributions at depth is determined. And their relation with deep structures of each tectonic units is analyzed. The conclusions of this thesis are summarized below.(1)The electric structure of crust and upper mantle beneath the Bayankala, Qaidam, Qilian, Alashan and Ordos blocks is characterized by layered low-resistivity (LR) materials of tensΩm. The LR layer below the Bayankala block is at about 40km depth. The burial depth of the LR layer below the Qaidam block is larger, some 60km, with thick and intact overlying high-resistivity (HR) strata. The depth of the LR layer of the Qilian block is between 30~40km. And those of the Alashan and Ordos blocks are relatively shallow, 20~30km deep.(2)Below the Haiyuan, Gulang and Yingchuan boundary zones, deep electric structures are complicated, where HR and LR bodies are mixed each other, thus no large-scale layered LR material exists. In the Haiyuan boundary zone, the LR body of shallow crust is wide in north and narrow in south, or say spreading out toward northwest and converging to southeast, like a deep in southwest and shallow in northwest listric surface. In the Yingchuan boundary zone, HR and LR bodies exist alternatively, and the LR body of upper crust is shallow in west and deep in east. And the whole Gulang boundary zone exhibits a HR body tilting toward southwest.(3)The contact zones between blocks or between a block and a boundary zone correspond to several major faults, which are the Maqing, Maxianshan, west Haiyuan, Qingtongxia-Guyuan and Yellow River faults, respectively. On either side of these faults, electric structures are apparently different, or resistivity changes rapidly across these faults.(4)The west Haiyuan-Haiyuan-Liupanshan fault and Qingtongxia-Guyuan fault are distinct electric boundaries which are of segmentation. Along these faults the localities of LR bodies or their vicinities are prone to medium-and large-sized earthquakes. Southwest to the west Haiyuan-Haiyuan-Liupanshan fault exists continuous layered LR material in crust, while in its northeast side there is no such LR layer in crust.(5)The deep electric structure pictures show that the sources of the 1920 Haiyuan M8.5 and 1927 Gulang M8 shocks are located in the boundaries where electric property changes remarkably. The deep electric structure is consistent with the long-extending steep strike-slip of the Haiyuan earthquake source, and northeastward overthrusting of the Gulang earthquake source, respectively.(6)In comparison with tectonics of the study area, stable or weakly deformed blocks have layered resistivity structures. And those highly active and deformed blocks, and tectonic boundary zones are characterized by complex electric structures at depth.(7)The deep electric structures below the study area revealed by MT sounding show that several blocks and boundary zones in the northeastern margin of the Tibetan Plateau have extremely complicated crustal structures. They are associated with special tectonic and dynamic settings of this region, and deserve further studies.