Tectonics and sedimentation during convergence of the ALCAPA and Tisza-Dacia continental blocks: The Pienide nappe emplacement and its foredeep (N. Romania)

The juxtaposition of the ALCAPA and Tisza-Dacia continental blocks, although one of the key issues in the evolution of the Carpathians, is not well known in terms of associated effects on the sedimentary systems during frontal foreland development. Most of the Contact between ALCAPA and Tisza-Dacia being covered by post-tectonic deposits, these effects can best be observed in northern Romania. Sedimentological data on facies, palaeocurrents and modal composition of sandstones combined with micropalaeontological data and 2D well-calibrated seismic lines constrain the tectonic history of the contact zone between ALCAPA and Tisza-Dacia. Pervasive deposition of sand-dominated siliciclastics beginning in late Early Oligocene (Late Rupelian) times is interpreted to reflect the onset of convergence between ALCAPA and Tisza-Dacia in the study area. The depocentre of coarse siliciclastic material migrates southward, finally forming a southeastward-thinning clastic wedge in the Transylvanian Basin. This Burdigalian-age clastic wedge is interpreted as fill of a flexural foreland basin that formed in response to the coeval thrusting of parts of ALCAPA (Pienides) over Tisza-Dacia. A shift from an E-W to SE-NW striking basin axis during Oligocene times towards a WSW-ENE oriented basin axis during Burdigalian times is interpreted as a result of clockwise rotation of Tisza-Dacia during basin formation.