Abstract:
On the basis of a structural-geomorphological analysis of the deformation of the benchmark initial denudation surface area, the supporting foothill Gelasien and early Pleistocene inclined levels, the spectrum of river terraces, the changes in the thickness of Neogene and Quaternary lithologycally stratigraphic and lithologycally facial vertical sections and the stratification of ancient and young alluvial-diluvial-proluvial mantle trains, the network of contrasting tectonic deformations which underly the pattern of block-mosaic morphological structure of the central segment of the Sofia seismic-tectonic zone has been identified. In morphotectonic aspect, Sofia seismic and tectonic area is within the scope of the high west block foot of the KraishteSrednogorie morphotectonic zone. In the current plastic of the Earth crust, the foot is demonstrated as an integrated and deep, peneplanationed, oro-structural pedestal. In its morphotectonic style, a complex block-mosaic structure has been diagnosed, which is determined by the presence of several major submeridionally-oriented fault structures: Kolosh, Tran-Kosharevo, Pernik and Zadbalkan. These major fault structures outline the spatial parameters of a system of sub-parallelly alternating oro-structural chains with graben depressions and valleys between the mountains. The northern oro-structural frame of the seismic and tectonic zone is outlined by the massive segment of the West Balkanide horst-block morphostructure. The central place in the internal structure of the studied area is occupied by the linearly withdrawn from the southeast to the northwest Ruy-Plana swell-shaped horst-imaged ascent, with relatively stable mode of slow and moderate positive neo-tectonic deformations. The southern fence-frame of the central segment of the Sofia seismic and tectonic zone is formed by the asymmetric Verila, Kolosh and Zemenska Mountains’ orostructural morpho block. In the spaces between the central swell-shaped ascent and the southern and northern fence-frame of the studied area are – unilaterally imposed – the Radomir and Sofia graben valleys. The main geotectonic process, which determines the general regularities in the spatial location of the earthquakes, is the dominant regional subductional extensional and transforming mode after Early Paleogene stage of the development of the research area. In this background, within the frames of the researched area, the basic seismogenerating morphostructures appear to be the unilaterally set and asymmetric graben morphostructures: the ones of Radomir, Sofia and Pernik. The basic energy of the impulse tectonic processes is generated by the appearance of tensions of extension and active subequatorial and sub-parallel abruptions, and as a result of it – a mechanism of step-like differentiation of their hollow foundations occur most often. Quaternary geodynamic localities fixed by contrast listric fault deformations are formed. A map of the spatial values of Quaternary tectonic processes and linear abruptions along the Early Quaternary and especially Holocene listric faults – the ones of Kalishtе, Kolosh, Izvor, Chervena Mogila, Radomir, North Vitosha, Iskar, Bezden-Gorna Malina and Rudartsi-Divotino fault-flexure zone, etc. has been drawn up. The values of deformations along those faults range between 40 and 100-120 m. The highest values of deformations are found along the length of the fault planes of Kolosh, Izvor, Kalishte, Bezden-Gorna Malina, North Lozen and North Vitosha, as well as the Iskar submeridional fault. On the basis of the morphotectonic model, a direct spatial and dynamic relation between the seismic potential on one hand, and the Quaternary extensional mechanism of formation of the block-mosaic structure in the central segment of the Sofia seismic zone on the other, has been found.