The degree of study of geophysical methods in the territory of Mongolia is extremely uneven and still insufficient. This fully applies to gravimetric studies. The information given in this article will partially fill this gap. The quantitative data obtained by means of gravimetry on the structure of the Cenozoic basins will supplement the existing knowledge of the features of the development of the intermontane depressions of the south of Eastern Siberia and Mongolia. The Tolskaya Cenozoic depression is confined to the Khentei trough, which forms the central part of the Khentei arch of the Khangai-Khentei fold system of Mongolia. It has a sublatitudinal orientation (across the strike of the rocks of the pre-Cainozoic basement) and the planned dimensions of 30–35×4–10 km. Based on the results of an area-based gravity survey of medium scale performed by the authors on the territory of the depression, a map of gravity anomalies in the reduction of the Bug River was compiled. The work was carried out using scintrex autograv CG-5 gravimeters using the standard method. The heights of the observation points were determined by the Trimble 5700 GPS devices. The correction for the relief was introduced within a radius of 200 km. Analysis of the gravitational field made it possible to clarify the structure of the upper part of the earth's crust, the thickness of loose deposits and the morphology of the surface of the basement of the basin. Based on the interpretation of gravimetric data, geological and geophysical sections were constructed, intersecting the depression in various directions, and the isopachite scheme of sedimentary deposits. A quantitative interpretation of the gravitational anomalies was carried out with the help of a 3-D program for the semi-automated inversion of gravity anomalies when the bodies are approximated by horizontal prisms with vertical sections that have the form of polygons. The foundation of the hollow has a block structure. It is complicated by two basins: western and eastern (Selbinskaya), divided by a transverse uplift of the enclosing strata. Judging by the results of the interpretation of the anomalies of gravity, the thickness of sedimentary deposits in the western basin is 150 m, in the Selbinskaya can reach 400 m. The relative youth of the Tolsky basin allows us to understand that the seismic potential of this region can be quite high. According to geological and geophysical data, the South-Tolsky and Selbinsky regional faults control the position of the sub-latitudinal tectonic structure of the Tolsky basin. The development of these structures is associated with these faults, which are also characteristic of modern seismotectonic activity. Therefore, the area of intersection of the Selbin and Tolsky fault systems, to which, according to gravimetric data, is confined, the maximum thickness of the sedimentary deposits of the basin can be a powerful seismogenerating structure.

Turutanov Evgeny Khrisanfovich, Doctor of Sciences (Geology and Mineralogy), Chief Researcher, Head, Laboratory Complex Geophysics, Institute of the Earth's Crust SB RAS, 128, Lermontov st., Irkutsk, 664033, Russian Federation, tel.: 8(3952)42-87-92, e-mail: tur@crust.irk.ru 

Buyantogtokh Bazarin, Postgraduate, Institute of the Earth's Crust SB RAS, Research Scientist, Center for Astronomy and Geophysics of MAS, PO-788, Ulaanbaatar-210613, Mongolia, tel.: 976-11-458024, e-mail: bazaryn_b@yahoo.com 

Tengis Badma, Postgraduate, Institute of the Earth's Crust SB RAS, Research Scientist, Center for Astronomy and Geophysics of MAS, P.O.-788, Ulaanbaatar-210613, Mongolia, tel.: 976-11-458024

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