Lateral variations in mechanical properties of the Romanian external Carpathians: Inferences of flexure and gravity modelling

We present the results of two-dimensional flexure and gravity modelling of the subsidence of the Romanian Carpathian foreland based on twenty profiles through the Southern and Eastern Carpathians. The narrow spacing of the grid of profiles allows us to investigate the lateral tectonic variations transverse to the belt, but also along strike. The topographic elevation of the Romanian Carpathian mountain belt is modest and the minimum in the Bouguer gravity anomaly, characteristic for flexural control of subsidence, is located relatively far towards the foreland in respect to the mountain belt. As the contribution of topographic loading to the evolution of the Romanian Carpathian foreland system is small, a subduction (underplating)-dominated tectonic regime controlled the nappes emplacement and basin shortening in the external flysch and molasse basins during the Late Tertiary. The modelling results support the existence of important variations in effective elastic thicknesses (T_e) and plate boundary forces. High flexural bending stresses in the western part of the Southern Carpathians can explain low T_e values, whereas the increase of T_e to the east can be explained by changes in rheological properties of the Moesian Platform. Field observations indicate that variations in deflection along strike are probably also related to basement unregularities, stress field rotations and strike-slip movement along lateral ramps during the Tertiary. In the Eastern Carpathians the model results indicate a N-S trend of lateral back stepping of the subduction system. A distinct back step in the vicinity of the seismically active Vrancea area supports the involvement of the E-W crustal scale Trotuş fault, which forms the transition from the East European Platform to the Scythian and Moesian Platforms. Development of a possible N-S migration of plate boundary activity or slab detachment and successive intersection with the Trotuş fault can explain the large Pliocene subsidence in the Focşani depression and relative uplift of the East European Platform north of the mentioned fault.