Tectono-thermal regimes and lithosphere behaviour in the External Dacides in the Upper Triassic and Jurassic Tethyan opening (Romanian Carpathians)

Traces of Mesozoic basic magmatism occur in the Romanian Carpathians in two different tectono-stratigraphic positions: (1) associated with the opening of the Tethys, and (2) associated with intracontinental basins. The latter were generated within three sedimentary basins, which are now inverted, constituting the External Dacides: the Black Flysch, Ceahlau (Eastern Carpathians) and Severin (South Carpathians) basins. Assuming a normal Jurassic lithosphere and a pure shear opening mechanism, stretching factors of 1.25 and 1.7 have been calculated for the Black Flysch and Ceahlau basins, respectively. These values were in turn used to determine the theoretical values of the syn-rift sediment thickness, and compared with those obtained by backstripping techniques. The occurrence of basic magmatism in the Black Flysch basin associated to such a low stretching factor, suggests that melt generation was possible due to 'wet' composition of the mantle peridotite. The thermal state was different in the Ceahlau and Severin basins, the lithosphere being affected by updoming processes prior to rifting. Upper Triassic and Jurassic thermal states of the European Lithosphere are related to the coeval opening mechanism of the Tethys. There was a transition from a simple shear between Austro-Alpine (= Apuseni Mountains) and the South Carpathians, to a pure shear mechanism (accompanied by strike-slip movements) between Austro-Alpine and the northern part of the Eastern Carpathians. Related to this change in the position of the master detachment fault, an astenospheric uprise developed within the European plate. It was largest in the Severin zone, medium in the Ceahlau zone and became smaller toward the Black Flysch zone. The Dacidian Swell developed as a result of this astenospheric rise. Starting with the Lower and Middle Jurassic, the Dacidian Swell became subject of rifting.