Late Alpine deformations, Neotectonic evolution and Active tectonics of the southern border of Central Balkan Mountain: a new contribution

Abstract

The published till now data and interpretations show the dominant role of the active faults in the structures of Karlovo and Sheynovo grabens and along the southern slope of the Central Balkan Mountain. The new investigations along the southern border of the Balkan Mountain included studies of faulting and fracturing at micro and mezzo-levels. A quantity of striae on slickensides and shear joints were measured. Thin sections have been prepared from five samples collected from outcrops of the surface of thrusting of the big granite over-thrust. The microscope analyses have shown that the process of thrusting was in low-temperature conditions. The secondary deformations at micro-level are reflections of the next tectonic impacts. Geophysical information has been also analysed (data from the seismic profiling, as well as reinterpretations of the gravity field). The tectonic stress fields from the Early-Alpine time till now have been reconstructed. For the first time it has been proved the existence of deformations from the Atean phase (Late Miocene) along the southern slope of the Balkan Mountains. This stress field is characterised by sub-horizontal and NE—SW directed axis s3 and axis s1 varying from subvertical to NW—SE direction. The youngest tectonic stress field has dominant N—S extension. The main conclusion of the study is that it was not possible to find proves for the existence of significant fault extended along the southern slope of the Balkan Mountains. A combination of fault structures is present, some of them being old surfaces of over-thrusting. No traits of recent active tectonic were found, and the seismicity is very low. It can be assumed that the recent movements are facilitated by listric-type faults, and this is related to the change of the bedding angle of the sediments filling the adjacent young basins.