Could geophysical modeling solve some of the Transylvanian Basin (Romania) structural problems?

The Transylvanian Basin is composed of crystalline basement assigned either to the Internal Dacides/Median Dacides or to the Tisia/Dacia terrain, of Mesozoic deposits (belonging to the Vardar Ocean ?), sporadically of Paleogene deposits, of Upper Miocene deposits (7000-9000 m) formed in a fast subsidence regime and structurally of two pre- Miocene depressions (Puini Basin at west and Tarnave Basin at east) separated by a crystalline basement crest (Pogaceaua) with a maximum apex at 3500m. The Miocene subsidence depocenter with major development in the two basins preserving Jurassic (locally), Cretaceous and Paleogene deposits coincides with the presence of a supposed ophiolitic layer in the basement (seismic data), although the drillings revealed island-arc volcanics exclusively, similar to those outcropping in the Apuseni Mts. The thermal flux data (partially registered on the geothermal map of the Geological Institute of Romania, 1986) suggest a negative regime on areas with maximum Miocene subsidence. Maximum magnetic anomalies of (+250nT) and minimum gravimetric anomalies of (-64mgal) overlap the areas with increased subsidence. The interpretative variants which explain the starting of the Miocene subsidence summarize the generation of a back-arc basin (similar to the Tisian Basin of Pannonia) as a consequence of the Peninic slab subduction, as back-arc extension associated with an upwelling asthenospheric mantle. The major contradictory element of this hypothesis remains the deciphering of the Badenian-Pannonian continuous convergence which invalidates the back-arc extension hypothesis. Another element of the structural analyses on the Transylvanian Basin outlines the Moho and Conrad surfaces, both exhibiting important concave shape, overlapping the maximal subsidence zones (convex shape) of Puini and Tarnave depressions. The seismic and drilling investigation supplied the detailed geometry of the Mesozoic, Paleogene and Miocene sedimentary deposits, of the island-arc and presumed ophiolitic volcanics and of the contact surfaces with the crystalline basement respectively. A geophysical data modeling based on the tridimensional geometry of all stratigraphic compounds of the Transylvanian Basin, on the registered magnetic and gravimetric data and on the statistically processed petro-physical data, was also performed. The starting point concerns two types of structures, corresponding to two possible hypothetical models on the basement: 1. the development of a complex continental crust known as Tisia/Dacia with convex configuration at the Moho and Conrad surfaces level, beneath a concave basin filling (less explained); 2. the development of two trapped fragments of oceanic crust (Puini Basin and Tarnavelor Basin) designed after the collision of the Apuseni Mts. and the East Carpathians continental crusts, under which the mantle lies, rapidly cooled at their contact. The cooling mantle triggered the subsidence during Badenian. The modeling demonstrates that the second variant is the unique solution for the presence of several oceanic crust fragments trapped between collision continental crusts, rapidly cooled during post collision – a process that allowed the mantle compression and the basin subsidence.