Geodynamic Conditions of Evolution of the Tunka Branch
in the Baikal Rift System

A. V. Parfeevets and V. A. San’kov

Institute of the Earth’s Crust, Siberian Division, Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033 Russia

e-mail: aparf@crust.irk.ru

Received June 14, 2005

Abstract—The Cenozoic paleostress state of the Earth’s crust at the southwestern flank of the Baikal Rift Sys-
tem (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 com-
parison of the obtained reconstructions, including the dated stress fields, with the Pleistocene–Holocene defor-
mations 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 tran-
stensional regime from the Oligocene to the late Miocene or early Pliocene. At the early–late Pliocene bound-
ary, 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 pos-
sibility of forming pull-apart basin was existent at the late stages.

DOI: 10.1134/S0016852106050050

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