The Variation of the stress-and-strain state of the Vuoksi Fault Zone area (Vyborg district)

The stress-and-strain state of the rock massif and the kinematic type of faults of the Vuoksi fault zone (Karelian Isthmus) for the latest stage of tectonic history were determined by field tectonophysical methods. The main information was obtained from geological stress/strain indicators, which were mostly represented by measurements of minor faults and slickensides with kinematic information (the direction of relative displacement) on the fault plane. Near the Vuoksi fault zone, systems of faults with an orientation close to the strike of this regional fault structure are observed. Most widespread are strike-slip faults, whereas faults with normal or reverse displacement make up less than a quarter of the total number of faults. The STRESSgeol software based on the method of cataclastic analysis was used to restore stress-and-strain state. The variable orientation of the principal stress axes was determined in the studied area. The two main directions of maximum compression are NE-SW (to ENE-WSW) and NW-SE (to WNW-ESE). The axes of the maximum deviatory extension are subhorizontal and are directed in the NNE-SSW and NW (to WNW-ESE) direction. The intermediate axis often occupies a sub-vertical position. The horizontal shear stress state type prevails for the entire zone, and lateral shear displacements are also characteristic of this fault zone. This stress state type of the studied part of the Baltic Shield is significantly different from the stress state type of orogenic belts, where, along with horizontal shear stress state type, the situation of horizontal compression stress state type dominates. The obtained variability of the directions of maximum compression in the studied area can be preliminarily correlated with the previously identified zone of “unstable tectonic stresses” of the East European Platform.