Tertiary tectonic evolution of the external East Carpathians (Romania)

Abstract

Paleostress calculation and analysis of mesoscopic structures are integrated with depth interpreted geological profiles based on seismic studies and well correlation to derive a Tertiary tectonic model for the East Carpathians. Following Early Miocene and older orogenic phases, the first tectonic event that affected the studied area is characterised by a WSW–ENE-oriented shortening of Middle Miocene (Late Burdigalian) in age. Resulting deformations induced ENE-ward thrusting of Tarcau and Marginal units, as well as the internal part of the Subcarpathian nappe. A second shortening event with an E–W to WSW–ENE contraction direction took place in the Late Miocene (Sarmatian), characterised by further foreland thrusting of the Subcarpathian nappe and out-of-sequence deformation in the Tarcau and Marginal Folds nappes. Along strike, differences in deformation mechanisms are controlled by the friction coefficients along the main detachment layers, by the lateral variations in the wedge thickness and by the involvement in the northern part of the thrusting system of the thick, competent East European platform. Tear faulting occurred in both tectonic events, the main resulting structure being the triangle zone developed south of the Trotus valley. The Latest Miocene (Latest Sarmatian)–Early Pliocene is characterised by a strike–slip stress field with NNE–SSW compression and WNW–ESE tension axis, left-lateral faults being dominant. The last deformation which affected the studied area is characterised by NNW–SSE shortening during the Pliocene, major deformations taking place mainly in the SW-most bending zone.