Carpathian Foreland Fold and Thrust Belt and Its Relation to Pannonian and Other Basins

ABSTRACT The foreland fold and thrust belt of the Carpathians can be traced from Austria through the western and eastern Carpathians to the south Carpathian bend in Romania where most of its structural units plunge beneath younger Pliocene-Pleistocene cover. Folds in the youngest rocks at the south Carpathian bend continue westward, until all surface expression disappears before reaching the Danube River. The fold and thrust belt is flanked by elements of the European, Russian, and Moesian cratonal areas which are overlain by a variable width and thickness of foredeep deposits that are partly involved within the external fold and thrust belt. Internal to the fold and thrust belt are older parts of the Carpathian orogene formed mainly on continental crust. These older structures have significant differences in evolution between the western and eastern Carpathians. The foreland fold and thrust belt consists predominantly of flysch in its inner parts and molasse in its outer parts. These sedimentary rocks are cut into a series of thrust sheets that have moved relatively toward the craton. Oldest flysch units are Late Jurassic and the youngest units are Miocene. Some of the flysch units of the inner part of the belt may have been underlain by oceanic crust, whereas the younger, more external molasse units were underlain by continental crust. Timing of thrusting is only well-constrained locally, but suggests that deformation in the western Carpathians developed during Oligocene to Miocene time. In the eastern Carpathians, structural activity continued into the Pliocene, and deep earthquake activity at Vrancea suggests subduction. Deformation is still locally active. A volcanic arc of Miocene to Holocene age lies internal to the fold and thrust belt. Magmatic activity appears to have migrated externally and eastward in time. Contemporaneous with thrusting was the development of Neogene basins in the intra-Carpathian region. These basins exhibit a two-phase subsidence history (except for the Pannonian and Transylvanian basins)—fast initial subsidence, followed by a period of slower linear subsidence. The Pannonian basin shows only a reasonably fast linear subsidence. Structural and thermal models indicate the basins were formed by about 100% stretching. The convex east loop of the Carpathian foreland fold and thrust belt is a result of the collision of continental fragments against a recess in the combined European-Moesian plate. In this region, subduction or downbending of the subducted slab results in an extensional stress field, suggesting that thrusting and apparent compression along the mountain belt are only thin-skinned, superficial effects due to the inability to subduct light upper crustal material and detachment of the crust from the underlying lithosphere. The dominant forces seem to be those acting on the downgoing slab, which produce shortening along the plate boundary, and plate geometry that inhibits rigid plate convergence.