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

Abstract

E stablish ing relationship s between the regional seismicity and the depth geometry and composition s of large -size density inhomogeneities of the upper crust is one of the ways to determine the p arameters of mean long-term seismicity . The origin of geological bodies and mechanism s of penetration of intrusive bodies in to the upper crust are detectable from the shape s and size s of such bodies. K nowledge o n the shape s and size s of large intrusive bodies is important for prospecting and exploration of mineral resources. Based on t he medium - scale areal gravimetric survey data, a regional map of Bouguer gravity anomalies was constructed for the Ulaanbaatar region , Central Mongolia. I nterpret ed gravimetric data from the sol utions of the inverse problem of gravimetry based on the body - shape selection were used to develop the 3D models showing the largest crustal density inhomogeneities of the study area – the Tolskaya ( Ula a n ba at a r) basin , Nalaiha depression and Bogdoulin granite massif. It is confirmed that the basement of these depression s has a block structure. The basement of the Tolskaya basin includes two depressions , the western and eastern ones . In the western depression, t he sediment thickness is 150 m . It amounts to 400 m i n the eastern depression at the intersection of the Selbin and Tolsk aya deep fault systems. The i nformation on the shape s and thicknesses of the sediment bodies is used in the seismic studies aimed at obtaining a more precise assessment of seismic hazard of the Ul a an ba at a r city and the region . In the Nalai ha depression, the sediments are 8 00 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 . T he 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 o f the Bogdoulinsky massif . Such indicators have been discovered by the geological, geophysical and geochemical methods . Our study shows that t he geophysical methods (in particular, gravimetr y ) can considerably expand the knowledge of the morphology of geological objects at depth and facilitate reaching a new level in structural analysis, wh ich is critical for develop ing new ideas to clarify the geological history and tectonic conditions of the formation of the Mongolia-Siberian region.