Assessment of the influence of modern crustal movements and the recently activated Precambrian structural plan on the relief of the Lake Ladoga region (the southeastern Baltic Shield)
This paper describes the influence of modern crustal movements and the recently activated Precambrian structural plan on the relief of the Lake Ladoga region. It presents the results of comprehensive studies, including processing of the regional geological and geomorphological data by the modern methods, as the major novelty of our work. The solutions of earthquake focal mechanisms suggest the current subhorizontal NW compression in the study area. Based on the computer simulation by the Roxar software, we have identified areas wherein new fractures are most likely to occur, determined the dominant directions of such fractures, and revealed the areas of intense vertical movements in the given stress state. The input database included a digital model of the relief and the spatial patterns of ancient faults represented by large-size inhomogeneities influencing the stress field. Strain values were estimated from the horizontal displacements recorded by the International GPS Service for Geodynamics (IGS) and the GPS networks in the Republic of Karelia and the southeastern regions of Finland. Using the LESSA software, we have estimated the relief orientation characteristics: the density of lineaments, and elongation lines, which are indicative of the changes in the dominant directions of the strike of the lineaments (‘hatches') in the study area. By interpreting the satellite images and the topographic maps (scale 1:20000), we reveal a number of geological structures, such as granite-gneiss domes and large-size faults, which are directly reflected in the relief. The study results give grounds to establish an indirect relationship between the relief and the modern field of deformation: the areas with high strain values correspond to the areas with steep slopes. The computer simulation data show a NE-trending linear zone with the increased amplitudes of vertical movements. This zone occupies the region along the NW shore Lake Ladoga. In the block divisibility scheme based on the relief analysis, this region is comprised of blocks that are small in area: the Pearson correlation coefficient between the density of the block boundaries and the vertical displacements amounts to 65 %. In addition, this zone is distinguished by an increased permeability of the crust, which was estimated from the helium survey data. Most of the seismic events occurred in the areas characterized by either high or medium probability of new fracturing in case of NW compression. This gives an argument in support of the conclusion that compression takes place in the Ladoga Lake region at the present stage. According to estimations by the LESSA software, the maximum densities of the linear relief elements correlate with the spatial patterns of the Precambrian faults. A number of faults and granite-gneiss domes are directly manifested in the relief, reflecting the inherited nature of the development of the latter.