DEFORMATION REGIMES AND LOW-RANK PALEOTECTONIC STRESSES OF THE WESTERN TIAN SHAN IN THE MESOZOIC – CENOZOIC

Abstract

There is a description of the results of studies of deformation regimes and reconstruction of stress fields of certain time intervals of the Mesozoic – Cenozoic history of the western part of the Tien Shan based on the geological and structural data and earthquake focal mechanisms. The relevance of research is determined by the existing difficulties in the age reference of paleostress fields, which is associated with the retrospective nature of the initial data and the features of geological evolution, as well as the existence of controversial, unresolved issues regarding the development of the western periclinal part of the Tien Shan in the Mesozoic – Cenozoic. The aim of the research is to reconstruct the rank components of paleostresses in the earth’s crust in the western part of the Tien Shan at separate stages of the Mesozoic – Cenozoic development. The time intervals for which the stress field diagrams were obtained represent cyclically manifested phases of the first-rank rhythms, which are characterized by alternation of different deformation modes. The method for reconstructing paleostresses was based on the concept of the formation of superimposed systems of cracks of various generations, which were sequentially manifested at various stages of the development of deformation processes. Analysis of the data showed that with increasing age of rocks the number of different fracture systems actually increases – the earlier systems are superimposed by the later. At the same time, the characteristic elements in Quaternary rocks manifest themselves in different ways in the older rocks. This allows them to be used for the reconstruction of the Late Cenozoic stress field based on mass measurements of fracturing in rocks of earlier Mesozoic – Cenozoic ages. Based on the step-by-step identification of the heterochronous cracks system and determination of a pair of maxima of the fracture density with asymmetric scattering, region-based diagrams of the first-rank stress fields were obtained for different stages of manifestation of Mesozoic – Cenozoic deformation regimes: 1 – from the Permian-Triassic to the Middle Jurassic; 2 – Middle-Late Jurassic; 3 – Late Jurassic – Early Cretaceous; 4 – Late Cretaceous – Paleocene – Eocene; 5 – Oligocene – Quaternary. The results obtained indicate the alternation of compression and extension phases, first-rank for the region, and give an idea of the main trends in its geodynamic evolution. They can be used in the development of a tectodynamic model of the region.