On the link between the formation of the Pannonian basin and the extensional collapse of the Dinarides

Classical models of evolution in the Alps-Carpathians-Dinaridic domain assume that the formation of the Pannonian back-arc basin is related to the rapid roll-back of European slab and the invasion of the Tisza-Dacia and Alcapa upper plate blocks into the so-called Carpathians embayment starting at ~20Ma. The general mechanism assumes a gradual evolution, an initial mechanical phase of extensional detachments being recognized near the transition between the Alps and the Pannonian basin, which was subsequently followed by upper crustal normal faulting and a thermal phase during the Middle-late Miocene, observed in the central part of the Pannonian basin. Hence, an always standing contradiction existed between the limited amounts of extension recorded in the crust and the large scale astenospheric upraise which took place beneath the basin centre. This apparent contradiction can be mechanically resolved by the existence of large scale extensional structures in other areas neighboring the Pannonian basin, such as the Dinarides. These structures can be large scale extensional detachments, unroofing the footwalls over the orogenic structures of the Dinarides and collapsing the hanging-walls in the area of the Pannonian basin. Our study demonstrates the existence of three such major extensional detachments in the central Serbian part of the Dinarides, at the transition towards the Pannonian basin. These Miocene detachments were mapped and dated in the areas of the Cer, Bukulja and Fruska Gora Mountains and in all cases follow a major weakness zone, inherited from the Cretaceous-Paleogene stage of mountain building, i.e the contact between the Dinaridic upper plate (Tisza-Dacia) and lower plate (Adria) along the Alpine Tethys (Sava) subduction zone. The footwall of these detachments exhume Jadar (Adria) basement and its Triassic-Jurassic cover (including obducted ophiolitic zones), altogether metamorphosed during previous phases of Cretaceous and/or Eocene crustal shortening. Detachment zones seem to be developed mainly in the Late Cretaceous-Eocene flysch of the Sava zone, which can be found metamorphosed in their footwall and non-metamorphosed in their immediate hanging-wall. The regional hanging-wall of these detachments is in all cases the Pannonian basin with its observed upper crustal extensional structures. In Cer and Fuska Gora Mountains these accommodate a Middle-late Miocene normal faulting, while the Bukulja detachment accommodate the formation of the lower Miocene Morava basin (or “peri-Pannonian” depression). By correlating these observations with other recent research in areas of Southern Serbia and Bosnia-Croatia, an overall image of large scale extensional collapse along detachment zones is observed along the entire central and internal Dinarides during the Miocene. Therefore, a full mechanical explanation can be provided for the Pannonian basin extension by incorporating this Dinaridic collapse. Two directions of extension were observed by field kinematic mapping, and initial Early-Middle Miocene top-N was followed by subsequent Middle-late Miocene top-E direction of extensional movement. While the second direction of movement is compatible with the invasion of the Tisza-Dacia block into the Carpathians embayment, the first might alternatively suggest the existence of a phase of Dinaridic extension driven by the roll-back of an Adriatic slab, prior to its detachment somewhere after the late Miocene.