| This thesis presents the newly accomplished study on the deep structure of the Qiangtang terrane and adjacent areas based on magnetotelluric (MT) sounding data, provides some new understandings of the geotectonics and dynamics about this region.The study area in this thesis locates in the central and north Tibet Plateau. The work of this thesis is based on five profiles, modern more advanced processing, analysis, and modelling techniques were used to obtain the final modle of these five profiles. Many parameters of every site were analysed, such as skewness, regional strike, phase ellipse and skin depth, and so on. The final 2-D resistivity models for the five profiles were obtained by NLCG method. In conjunction with geological and geophysical data of this study area, the stratigraphic distridution, the distribution of the high-conductivity layer and related dynamic process is analyzed. The main conclusions of this thesis are summarized below.(1) From shallow to deep, the study area can be roughly divided into three layers, the high-resistivity layer, the high-conductivity layer and the relative high-resistivity layer. Moreover, there are two high-conductivity layers in the crust of southern Qiangtang terrane, confirmed that the double high-conductivity layer in the crust of southern Qiangtang terrane in the western and central Tibet Plateau were exist.(2) There is an apparent N-S dichotomy in the electrical structure of the southern and northern Qiangtang terrane. In the southern Qiangtang terrane, the high-conductivity layer dips southwards from the near surface in the central Qiangtang uplift; in the northern Qiangtang terrane, the high-conductivity layer has a hook-like shape. It extends upwards to the near surface of the central Qiangtang uplift in the south and the Jinsha River suture (JRS) in the north and downward to the lower crust or even the upper mantle in the central part of the Qiangtang terrane. In addition, the high-conductivity layer beneath the southern and northern Qiangtang terrane both extends upward to the near surface in the Qiangtang uplift and forms an antiformal structure with high conductivity. According to the special shape of the high-conductivity layer beneath the Qiangtang terrane, we suspect that this high-conductivity layer is related to the palaotectonic track of the southward subduction of the melange during the Triassic Period. According to our inversion models, a large part of the crystalline basement of the Qiangtang terrane has been tectonically removed and replaced by melange that was underthrust southwards from the JRS. The water-rich melange in the crust of Qiangtang terrane is prone to partial melting at high-temperatures, and provide a reasonable explanation for the pronounced N-S change in geophysical features below the approximate surface trace of the BNS.(3) The deep electric structures of the Qiangtang terrane revealed by MT sounding show that the crustal flows beneath the west and central Qiangtang terrane was occurred beneath the central Qiangtang terrane, not along the JRS. And the up welling of mantle-derived magma in the central Qiangtang terrane may paly an important role in keeping the balance between the changeless of altitude of Qiangtang terrane and the eastward crustal flow.(4) The largest high-conductivity body was below the Gaize-Siling Co thrust system (GST) on the surface, about 30 to 40 km south of the BNS. The antiformal structure of high conductivity beneath the GST is related to the palaotectonic track of the subduction of the crust of meso-Tethys. In addition, the dislocation of the electrical structure in the shallow and deep depth beneath the BNS indicated that stronge thrust-nappe structure was occurred on the both side of BNS during the Cenozoic.(5) An obvious low-density anomaly data throughout the entire crust beneath the Shuanghu rift in central Qiangtang terrane suggests that this north-south trending rift is a large-scale fault. And it may be formed by convective thinning of the lithosphere root. |
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