Main fault zones controlling the late Alpine structure in the area east of Sofia (Srednogorie Zone, Western Bulgaria)

The northwestern narrow tip of the Upper Cretaceous Panagyurishte sedimentary strip is cropping out along the eastern-north-eastern margin of the Neogene Sofia graben. In this area the sedimentary strip is split in two parts separated by an uplifted pre-Upper Cretaceous basement block. Both parts show syncline geometries with cores built of Upper Cretaceous rocks. The synclines are striking NW-SE and are extremely asymmetrical with vertical to overturned southwestern limbs. Within the overturned limbs meso-scale structural features such as imbrications, folds and small-displacement faults indicate strong flattening of the rock volume under generally top-to-the-N shear. These features were observed along the Kamenitsa-Rakovishki and Negushevo fault zones. Due to the relatively poor outcrop conditions the precise dips of these fault zones are still poorly known and there are no data about the fault cores. Thus, the general north-vergent character of the both fault zones is assumed on the basis of the semi-penetrative small-scale structures within their immediate footwalls and field relations. Our new data suggest that the northward translations along the Negushevo fault zone resulted in formation of approx. 30 long and 2 km wide basement uplift.