The Research On The Lithospheric Conductivity Structure Of Ordos Block And Its Surrounding Regions

Ordos block is located in the western part of the North China Craton, it is considered as a relatively stable block. However, the faults and grabens around Ordos block develop intensive tectonic and seismic activities. Because of the special geographic location, Geodynamics problems in Ordos block and its surrounding regions have attracted the attention of many earth scientists.This paper selected seven MT profiles form the MT time series data which were acquired from 2010 to 2014 by the projects named “the research on the conductivity of the lithosphere of the Central Orogenic in China” and “the experiment research of MT standard network observation in the area of Qinghai-Tibetan Plateau and the North-China”, they cover Ordos block and its surrounding regions with a total of 159 broadband MT. High-quality impedance tensor data were acquired through data processing, based on the analysis of dimensionality features and tectonic trends, 2-D and 3-D modelings with nonlinear conjugate inversion were carried out to get a reliable electrical model of the research area。The final result shows that electrical structure beneath the north Ordos block can be separated into three layers as low resistor sediment- high resistor upper crust- low resistor lower crust from shallow to deep. Lower crustal beneath north of the Ordos block to Yinshan orogenic belt is rich in conductive carriers as 3Fe+ and Na+, resulting in low resistivity; moreover, because of lower crust beneath the southern region of Yijinhuoluoqi and Etuokeqi is rich in fluid, lower crust of the region shows high conductivity. Statistics of the directions of geoelectrical principal axes reflects that the upper mantle beneath center of Ordos block is a complete and solid block, but the crust exist some degree of deformation, a deformation zone may exist between the lower crust and upper mantle. The western thrust belt, Tianhuan Depressions, Yishan slope and the south of Jinxi fault-fold belt located in the Ordos block still remain core part of the Archean craton and thickened lithosphere now.Combined with Electrical structure model, we can infer the high heat flow、high Vp/Vs ratio and low resistivity anomaly in the crust and mantle of Ordos block and its surrounding regions may be connected with the hot upwelling and partial melting in the lower crust and upper mantle. Moreover, in the upwelling process, the mantle material carry high conductive and high Vp/Vs sulfide, which improve the conductivity and Vp/Vs ratio of crust and mantle. The reconstruction degree to the Midwest of North China Craton lithosphere is different. Large-scale mantle upwelling beneath Hetao Rift Basin, Yinchuan Rift Basin and Yinshan orogenic belt caused severe destruction and lithosphere thinning, the destruction degree of Weihe Rift Basin and its surrounding regions relatively weak, while Shanxi Rift Basin and its surrounding regions may only remain two channels for the hot upwelling of asthenospheric materials, due to relatively small scale of them, Shanxi Rift Basin lithospheric thinning phenomenon does not appear severely.