Geodynamic conditions of evolution of the Tunka Branch in the Baikal Rift System

Abstract

The Cenozoic paleostress state of the Earth’s crust at the southwestern flank of the Baikal Rift System (Tunka system of basins) is reconstructed. With allowance for known facts about the geologic history of the Tunka system of basins, the evolution of the stress field and its formation conditions are established by comparison of the obtained reconstructions, including the dated stress fields, with the Pleistocene-Holocene deformations in active fault zones and the present-day stress state (seismotectonic deformations calculated from the focal mechanisms of earthquakes). The opening of basins in the NW-SE direction was proceeding in the transtensional regime from the Oligocene to the late Miocene or early Pliocene. At the early-late Pliocene boundary, this process was followed by the transpressional regime with compression in the NW direction. In the late Pliocene, the situation at the southwestern flank changed drastically. Since that time, deformation has occurred in the transpressional regime and the compression axis has been oriented in the NE direction. The alternative models of the evolution of the Tunka system of basins—oblique extension, the transform fracture zone, or a pull-apart system—are considered. Both models are combined in the framework of the suggested stress-field reconstruction. The oblique extension (transtension) was related to the early stages of evolution, whereas a possibility of forming pull-apart basin was existent at the late stages.