Deep Electric Structure Beneath The Yingjiang-longling Area In Southwestern Yunnan And Its Implications For Kinetics

The Yingjiang-Longling area, located in western Yunnan, is one focused research regions in currentgeosciences. This thesis presents the newly accomplished study on the deep structure beneath thisregion based on magnetotelluric (MT) sounding data.The research region spans97.5°-99°E and24°-25.5°N, respectively, including the Tengchong blockand the adjacent north Baoshan block. MT data have been collected on three profiles (ZM01、ZM02,and ZM03) in this region, which have the roughly same azimuth. These sounding profiles cross thegeologic structures in southwestern Yunnan. The content of this thesis focuses on the ZM02soundingprofile, with roughly NW32°azimuth,130km length and21sounding sites. It stretches over a series ofNE-trending faults in the research area, such as the Dayingjiang fault, Longling-Ruili fault andWanding fault.Using the advanced remote reference observation means in data collection and the Robustprocessing technology, the quality of observational data has been improved. In the process ofqualitative analysis and interpretation to the data, this work employs the GB impendence tensordecomposition to correct local distortion effects embraced in observational data. Then it obtained theelectric structure dimensional character beneath the ZM02profile by analyzing1D skew,2D skew andreal induction vector parameters. Through the statistics of electrical structure angles by the multi-siteand multi-frequency patterns, the electric structural axis azimuths of different areas beneath the ZM02profile are delineated, meanwhile the electric region angles for two-dimensional inversion aredetermined. The apparent resistivity and phase curves which are rotated to the best reasonable regionalelectric orientation are qualitatively analyzed on integral and individual scales to roughly describe theelectrical property of deep structure beneath the profile. In the course of using NLCG inversionalgorithm to calculate the2D electrical model, several inversion parameters, such as the inversionmode, regularization factor, starting model and inversion mesh are discussed in detail to obtain a highdegree of confidence of the two-dimensional deep electrical structure model. Based on the electricalstructure of the ZM02profile, combined with the2D electrical structure model of the ZM03profile andZM01partial profile, the overall understanding of deep electrical structure beneath theYingjiang-Longling area in western Yunnan province is attained. And then, through comprehensiveanalysis of the geological, geophysical and other information of the study area and adjacent areas, thisthesis studies the deep context, mechanisms and dynamics of major earthquakes in this region. Themain research results and perspectives are as follows:(1)The deep electrical structure of Yingjiang-Longling area has a clear “sandwich” layeringstructurefeature in the vertical direction. The average thickness of the high resistive upper crust is10km.A notable high-conductivity layer (HCL) with several ms is present at depths10-20km in the crust(middle crust). While the base of the lower crust and upper mantle are uniform layers of relatively highresistivity. The top interfaces of the high-conductivity layers in the middle and lower crust are laterally heterogeneous to some extent, which up lifts to a depth of5km below the Dayingjiang fault, becominggradually deeper toward east. These top interfaces are cut by the Longling-Ruili fault, forming theboundary between the Tengchong micso-plate and Baoshan micro-place.(2) The formation of the HCL in thecrust of the Yingjiang-Longling area is probably associatedwith partial melting of media containing saline water, which is presumably the southwestwardextension of weak crustal flow from the eastern margin of the Tibetan plateau. In the study area, thesoutheast flow extruded from the Tibetan plateau turns to southwest, and the weak, intensivelydeformed HCL in the lower crust is decoupled with that of the upper crust. Under the forces from theplate boundary and surrounding blocks, these HCL materials move independently toward southwest,accommodating many NE-trending left-lateral strike-slip faults in this region. The back-arc extensioncaused by eastward subduction of the Indian plate produces material lack east the India-MyanmarMountains. Thus the part of previously flowing southward deflects to southwest under gravity, whichconstitutes the primary driving force in the study area.(3) Earthquakes concentrate along the Dayingjiang fault and Longling-Ruili fault, which havedistinct features at depth. The top of the HCL below the Dayingjiang fault is shallow, which mayexplain smaller focal depths around this fault. Meanwhile the high-resistivity bodies on its either sideare too thin to generate large interaction, so the earthquakes there are of small magnitudes. In contrast,the Longling-Ruili fault as the boundary between micro-plates is large and extends downward deep, thethick high-resistivity bodies on its either side interact strongly to provide stress built up for generationof major earthquakes. The1976M7.3and M7.4earthquakes at this fault are evidence to support thisspeculation. Nevertheless, the earthquakes in the Yingjiang-Longling area share same generatingmechanism, i.e. during clockwise rotation of the blocks, the interaction among brittle crust blocks at theintersection between the NE trending and nearly NS striking structures leads to high stressaccumulation, eventually causing occurrences of major shocks in this region.