THE CENOZOIC CRUSTAL STRESS STATE OF MONGOLIA ACCORDING TO GEOLOGICAL AND STRUCTURAL DATA (REVIEW)

Abstract

This article gives a chronological review of the main published research results concerning the Cenozoic crustal stress-strain state in Mongolia and adjacent territories. The studies commenced in the southern Baikal rift zone in the 1970s and were extended further southwards to cover mobile regions neighbouring the Siberian platform. Geological, structural and morphostructural data were collected and analysed to define the crustal stress types and spatial characteristics. The authors have consolidated their reconstructions of the crustal stress-strain state of Mongolia, which were based on tectonic fracturing data and displacements along fractures in fault zones active in the Cenozoic. We consolidated a database of reconstructed stress tensors, which now contains more than 750+ solutions. The Late Cenozoic stress field was mapped. The map shows domains differing in types of the paleostress state of the crust. The reconstructions were compared to our calculations of the present-day crustal stress state, which were based on earthquake focal mechanisms, and to calculations by other authors. At the Late Cenozoic and current stages, the maximum horizontal compression axis (S Hmax ) has varying orientations, from submeridional (Western Mongolia) to NE and ENE (Eastern Mongolia). The role of compression increases from the northern domains, where the reconstructions show shear and transtension, to the southern domains with prevailing transpression and compression. Regular changes occur in the stress state and rupture parageneses along the largest latitudinal faults, North Khangai and Dolinoozersky; such changes are related to left-lateral strike-slip faulting. We analysed the sequence of the occurrence of stress fields differing in types and spatial characteristics, and revealed the main regularities in the evolution of the crustal stress-strain state in time. In the Cenozoic history of crust deformation in Mongolia, we can distinguish several episodes that differ in the dominant impacts of various tectonic force sources or combinations of such impacts. At the beginning of the Cenozoic, tectonic structures developed mainly under the influence of the interaction of East Asia and the Pacific Plate, which was manifested in the southeastern domains of the study area. The long-term SE-trending asthenospheric flow caused crustal stretching, which initiated the formation of tectonic structures comprising the Baikal rift system. Starting from the Pliocene, crustal stretching took place in combination with NNE compression caused by the India–Eurasia convergence. As a result, shearing occurred along the large faults. At this background, the Khangai and Khentei uplifts (including crust extension zones at their crests) are large structures that developed due to the dynamic effect of local mantle anomalies.