IS CERNA-JIU FAULT (SOUTH CARPATHIANS, ROMANIA) BEING REACTIVATED? BEHAVIOUR PATTERNS RESEMBLING THOSE ENCOUNTERED AT THE WESTERN TERMINATION OF THE NORTH ANATOLIAN FAULT

Two basement nappe systems (designated as “Lower Danubian” and “Upper Danubian” respectively) occupy the lowermost structural position within the South Carpathians Alpine nappe pile (Iancu et al., 2005). Each of those two Danubian units includes a Neoproterozoic metamorphic rocks basement, overlain by a Mesozoic (Liassic to Late Cretaceous) sedimentary cover. In the SW extremity of the South Carpathians, a major control on the geometry of the contact between the two Danubian nappe systems has been exerted by the Tertiary age Cerna-Jiu strike-slip major fault, for which a dextral average displacement of 35 km was estimated (Berza and Drăgănescu, 1988).

Along the Cerna-Jiu fault, the South Carpathians orogen has been completing a clockwise rotation around the Moesian Platform western edge. The last stage of this process has been accompanied (e.g., Linzer et al., 1998) by extensional deformation resulting in the opening, during the Badenian (ca. 15 Ma ago), of several sedimentary basins: Bahna, Orşova, Donji Milanovac, Mehadia, Bozovici, Liubcova. Those pull-apart and transtensional basins became disconnected from each other after the Badenian, then sedimentation progressively ceased – last, during the Early Pannonian (ca. 8 Ma ago), in Mehadia basin.

The more recent evolution of the region is less well-constrained. It could only be conjectured (Leever, 2007) that significant differential uplift operated after the Early Pannonian, given that the maximum present-day elevation at which Badenian sediments occur in the Liubcova, Orşova and Donji Milanovac basins is less than 500 m, while in the nearby Mehadia basin it exceeds 700 m. Differential uplift has been invoked also by ter Borgh (2013), as a possible cause for the obvious disequilibrium which nowadays exists between the catchments of the two main rivers of this region - the Danube and the Jiu: the Danube has only short tributaries which, nonetheless, exert stream piracy at the expense of the Jiu drainage area. The entire present-day catchment of Jiu is developed on deposits belonging to the former Dacian Basin, where continental-fluviatile sedimentation was definitely established (Jipa and Olariu, 2009) no earlier than the Dacian to Romanian transition (~4 Ma). Hence the obviously unstable drainage divide between the Danube and the Jiu catchments – and, implicitly, the present-day Danube course, the Iron Gates gorge included – has to be younger. It is worth mentioning, in addition, that the Danube terraces seem to indicate accelerated uplift rates in the Quaternary (the resulting incision being in excess of 100 m - Leever, 2007). It is hence reasonable to assume that some significant tectonic activity has concerned the considered region since ~4 Ma. Present-day seismic activity, together with discharges of thermal water accompanied by large concentrations of He could be signatures of that tectonic activity that initiated no earlier than ~4 Ma ago.