Crustal structure of the Ulaanbaatar region, Mongolia according to gravimetric data

Establishing relationshipsbetween the regional seismicity andthe depth geometry and compositions of large-size density inhomogeneities of the upper crust is one of the ways to determine the parameters of mean long-term seismicity. Theoriginof geological bodies and mechanisms of penetration of intrusive bodiesinto the upper crust aredetectable from the shapes and sizes of such bodies.Knowledge on the shapes and sizes of large intrusive bodies is important for prospecting and exploration of mineral resources. Based on the medium-scale areal gravimetric survey data, a regional map of Bouguer gravity anomalies was constructed for theUlaanbaatar region, Central Mongolia.Interpreted gravimetric data from the solutions of the inverse problem of gravimetry based on the body-shape selection were used to developthe 3D models showing the largest crustal density inhomogeneities of the study area – the Tolskaya (Ulaanbaatar) basin, Nalaiha depression and Bogdoulin granite massif. It is confirmed that the basement of these depressionshas a block structure. The basement of the Tolskaya basinincludes two depressions, the western and eastern ones.In the western depression, the sediment thickness is 150 m. It amounts to 400 m in the eastern depression at the intersection of the Selbin and Tolskaya deep fault systems. The information on the shapes and thicknesses of the sediment bodiesisused in the seismic studies aimed at obtaining a more precise assessment of seismic hazard of the Ulaanbaatar city and the region. In the Nalaiha depression, the sediments are 800 m thick.The morphology of the basement and the thickness of sediments in this depression can be used to update the mining prospects of the Nalaiha coal deposit. The maximum thickness of granitoids in the Bogdoulin massif is 9.0 km. The area of its projection to the ground surface is twice as big as the area of the outcrops. Our research results can be useful for assessing the metallogenic specialization of granitoids, which indicators in relation to some minerals are related to the specific features of the Bogdoulinsky massif. Such indicators have beendiscovered by the geological, geophysical and geochemical methods. Our study shows that the geophysical methods (in particular, gravimetry) can considerably expand the knowledge of the morphology of geological objects at depth and facilitate reaching a new level in structural analysis, which is critical for developingnew ideas to clarify the geological history and tectonic conditions of the formation of the Mongolia-Siberian region.

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