ABSTRACT
According to the results of magnetotelluric sounding the geoelectrical structure of crust and upper mantle was studied. The variation of resistivity at depth 30 km seems irregular and proved that the geoelectrical structure of upper crust is complex, it consists of blocks of various size and different resistivity. Generally, most of low resistivity areas correspond to high geothermal areas. The geoelectrical structure at depth of 90 km is simpler, some low resistivity anomalies are distributed against high resistivity background, of them the Songliao, the eastern part of North China Platform and western part of Yangtze Platform are connected to each other, formed an immense northeast–southwest trending low resistivity zone. Daxingganling–Taihanshan gravity gradient zone, Fen–Wei graben, Longmenshan–Panxi tectonic zone are located at the western fringe of the low resistivity zone; Yilan–Yitong fracture zone and TanLu fracture zone are situated on the eastern fringe, it was inferred that the low resistivity zone was related to the formation of tectonic zones abovementioned. Low resistivity zones at 90 km depth are distributed at the thin down regions of lithosphere, where partial melting mantle material may exist. The low resistivity is taken as background at depth of 150 km, on which scattered blocks with high resistivity are distributed, the axial line of high resistivity blocks formed some “tectonic line” and cut apart Chinese mainland into a few geotectonic units. The low resistivity layer in crust at shallow depth exists in most active tectonic areas. The focuses of shallow-focus earthquakes are generally located over low resistivity layer in crust. The depth of low resistivity layer in upper mantle varies greatly, from shallowest 50–60 km to deepest more than 200 km, the average depth in eastern regions is about 100 km, and that in western is nearly 120 km. A great northeast–southwest trending upwarping zone of low resistivity layer in upper mantle was discovered, from Songliao of Northeast China through east fringe of North China Platform to southwest along Fen–Wei graben, in the final to the west fringe of Yangtze Platform. Upwarping region of low resistivity layer in upper mantle mostly corresponds with high heat flow region and strong earthquake region.
