We studied the Nivyanin fault segment from the Devene fault system in NW Bulgaria, using shallow electrical resistivity profiling. The measured profile was located on the Iskar floodplain near Chomakovtsi village and crossed a lineament that connects the northernmost exposures of Upper Cretaceous limestone and Miocene conglomerate on elevated surfaces in which the Iskar valley was incised. The results suggest that the studied fault segment was a strike-slip fault that was active after the deposition during the Miocene. The fault zone is broad over 100 m and consists of sub-vertical fractures and breccia. Vertical fault displacement did not affect the Quaternary fluvial deposit.

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.

The Devene fault system, a major strike-slip structure at the boundary between the Balkan Range and the Moesian Platform in NW Bulgaria, remains a subject of debate regarding its Quaternary activity. This study investigates the shallow expression of the fault at two representative sites, Tri Kladentsi and Beli Breg, using high-resolution electrical resistivity profiling to differentiate tectonic deformation from climatically driven landscape evolution. At Tri Kladentsi, resistivity profiles confirm a steeply dipping structural boundary within the Miocene bedrock, juxtaposing limestone against sands. The overlying 25 m thick loess cover, however, remains sub-horizontal and undisturbed. Likewise, at Beli Breg, the complex architecture of stacked channel sequences and tributary deposits at the Ogosta River confluence reveals no identifiable fault displacement. Our results suggest a high degree of morphological mimicry, where asymmetric river valleys produced by selective erosion and differential loess accumulation superficially converge with tectonic signatures. The long-term left-lateral slip rate is estimated at 0.14–0.19 mm/yr based on a 20 km Miocene offset. Nevertheless, the lack of modern surface rupture indicates a deceleration of fault slip rate and a transition to a buried fault top during the Quaternary. These findings necessitate a re-evaluation of regional seismic hazard assessments, because the absence of continuous surface traces physically constrains the maximum earthquake potential.

The Devene fault system, a major strike–slip structure at the boundary between the Balkan Range and the Moesian Platform in NW Bulgaria, remains a subject of debate regarding its Quaternary activity. This study investigates the shallow expression of the fault at two representative sites, Tri Kladentsi and Beli Breg, using high-resolution electrical resistivity profiling to differentiate tectonic deformation from climatically driven landscape evolution. At Tri Kladentsi, resistivity profiles confirm a steeply dipping structural boundary within the Miocene bedrock, juxtaposing limestone against sands. The overlying 25 m thick loess cover, however, remains sub-horizontal and undisturbed. Likewise, at Beli Breg, the complex architecture of stacked channel sequences and tributary deposits at the Ogosta River confluence reveals no identifiable fault displacement. Our results suggest a high degree of morphological mimicry, where asymmetric river valleys produced by selective erosion and differential loess accumulation superficially converge with tectonic signatures. The long-term left-lateral slip rate is estimated at 0.14–0.19 mm/yr based on a 20 km Miocene offset. Nevertheless, the lack of modern surface rupture indicates a deceleration of the fault slip rate and a transition to a buried fault top during the Quaternary. These findings necessitate a re-evaluation of regional seismic hazard assessments, because the absence of continuous surface traces physically constrains the maximum earthquake potential.