LATERAL VARIATIONS IN MECHANICAL-PROPERTIES OF THE ROMANIAN EXTERNAL CARPATHIANS - INFERENCES OF FLEXURE AND GRAVITY MODELING

We present the results of two-dimensional flexure and gravity modellin g of the subsidence of the Romanian Carpathian foreland based on twent y profiles through the Southern and Eastern Carpathians. The narrow sp acing of the grid of profiles allows us to investigate the lateral tec tonic variations transverse to the belt, but also along strike. The to pographic elevation of the Romanian Carpathian mountain belt is modest and the minimum in the Bouguer gravity anomaly, characteristic for fl exural control of subsidence, is located relatively far towards the fo reland in respect to the mountain belt. As the contribution of topogra phic loading to the evolution of the Romanian Carpathian foreland syst em is small, a subduction (underplating)-dominated tectonic regime con trolled the nappes emplacement and basin shortening in the external fl ysch and molasse basins during the Late Tertiary. The modelling result s support the existence of important variations in effective elastic t hicknesses (T-e) and plate boundary forces. High flexural bending stre sses 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 obse rvations indicate that variations in deflection along strike are proba bly also related to basement unregularities, stress field rotations an d 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 v icinity of the seismically active Vrancea area supports the involvemen t of the E-W crustal scale Trotus fault, which forms the transition fr om the East European Platform to the Scythian and Moesian Platforms. D evelopment of a possible NS migration of plate boundary activity or sl ab detachment and successive intersection with the Trotus fault can ex plain the large Pliocene subsidence in the Focsani depression and rela tive uplift of the East European Platform north of the mentioned fault .