The upper mantle structure and Cenozoic volcanism in Central Mongolia

Abstract

The study of formation mechanisms and origin of large structures in the continental lithosphere is one of the fundamental directions of recent tectonic and geodynamic investigations. Cenozoic structure-forming processes are reflected clearly in the continental lithosphere of Central Asia. It is accepted that the tectonic evolution of its lithosphere was determined by different factors: external stress fields at boundaries of large interacting lithospheric plates and internal stress fields produced by processes in the sublithospheric mantle. This work is dedicated to defining the role of these factors in the formation of structures in Central Mongolia based on petrological‐geochemical and seismotomographic materials combined with geostructural and morphotectonic data. Central Mongolia demonstrates features related to Cenozoic intracontinental deformation processes. Their influence is reflected in the neotectonic structure of the region, which includes positive morphostructures of the Gobi Altai, Khangai Range (Upland), intervenient Lake Valley (Fig. 1), and contrasting structures around Lake Khubsugul. In addition, the Cenozoic stage was marked by the formation of basaltic fields, which spatially coincide with fields of Late Mesozoic volcanics in the southern part of this region. The volcanics are largely represented by subalkali olivine basalts and alkali basaltoids. The volcanic complexes of different ages are discriminated largely based on their geomorphologic position, K‐Ar dating, paleomagnetic data, and geological relationships with paleontologically well-substantiated sedimentary sequences [2‐5]. Most Cenozoic basalts fields of different sizes and shapes are localized in the Khangai Upland and its spurs mainly east of 100 ° E. Volcanic bodies form usually small watershed plateaus, fill fault-line Cenozoic depressions, and make up elements of river valleys (high and low terraces). The northern Khangai Upland comprises three areas (Fig. 1) with volcanics of different stages: Late Pliocene (Taryat, Orkhon, and Khanuingol), Middle‐Late Pleistocene (Taryat and Orkhon), and Holocene (Taryat) [2]. The study of volcanic edifices in these fields shows that they represent volcanoes of the central type and occur in marginal parts of Cenozoic depressions. The Upper Pliocene basalt section includes five to seven flows 2‐70 m thick; the Middle‐Upper Pleistocene section, two to three flows 5‐15 m thick; and the Holocene section, only one flow (8‐10 m thick) or monogenic scoria cones. Volcanics are dominated by light to dark