INVERSION AND INTERPRETATION OF MAGNETOTELLURIC DATA ALONG THE BEIJING-BAIGEZHUANG PROFILE: A CONTINUOUS VARIATION MODEL IN RESISTIVITY

Amethod of continuous resiotivity variation is used for the inversion of the ma-gnetotelluric sounding data along the Beijing-Baigezhuang profile in the northern part of North China, and the latest result is obtained about the electrical structure of the crust and the upper mantle. To test the reliability of the acceptable models the Backus-Gilbert appraisal method is applied. In the electrical structure obtained the shallow low resistivity zones correspond to Cenozoic sediments. The high conductivity zones occur at the middle crust in the studied region, and they are thicker beneath depressions than those beneath uplifts. The depth of high conductivity zones in the upper mantle varies laterally and significantly. For example, the depth of resistivity contour of one ohm-meter is more than70km beneath the Cangxian uplift, about 60km beneath the central Hebei depression, and about 50 krn beneath the Huanghua depression. The resistivity value of the high conductivity zone in the upper mantle tends to decrease from west to east.The resistivity-depth curves within the crust are in good agreement with the velocity-depth curves for three principal tectonic units: the central Hebei depression, the Cangxian uplift and the Huanghua depression. The high conductivity zones in the upper mantle coincide with inverted velocity layers obtained by teleseismic data. The difference in electrical resistivity between uplift and depression reflects a lateral in-homogenety in the deep structure, and also has close relation Jo the temperature distribution in the depth. These results are interesting for further investigation of the tectonic movement and the evolution of the basins and the earthquake background of the North China plain.