Neotectonic and modern stresses of South Sakhalin

Tectonophysical studies are conducted in South Sakhalin for identification of temporal–spatial changes in the geodynamic settings of the formation of the local structures. Analysis of the field data reveals 11 local stress state (LSS) in the large newest megastructures, which were formed on geological basement of different ages. The parameters of the tectonic stresses are significantly distinct in each LSS, especially the orientations (up to reindexation) of the compression and extension axes in different fault wings. Tectonic stresses of two ages and constant latitudinal and horizontal compression axis are reconstructed. The earlier cofold shear stress field with a horizontal and longitudinal extension axis is post-Miocene and the later stress field of the reversed fault with a vertical extension axis is orogenic. The LSSs reconstructed for the first time by the displacement vectors on slickensides, along with the data on the earthquake mechanisms, substantiate the reindexation of the horizontal extension axis with the vertical intermediate axis of major normal stresses at the postfold orogenic stage of evolution of the territory. These results are in agreement with previous data on the transformation of the dextral to reverse thrust displacements along the longitudinal fault systems. The young stress field is more confidently interpreted in the activation fault zones, which limit the orogenic blocks, whereas the traces of cofold deformations without younger orogenic stress fields better remain inside the blocks which are composed of older and strongly dislocated Mesozoic rocks.