PALEOGENE RETROARC FLEXURAL BASIN BENEATH THE NEOGENE PANNONIAN BASIN - A GEODYNAMIC MODEL

The Neogene (Middle Miocene-Recent) Pannonian Basin is superimposed on an earlier Paleogene (Middle Eocene-Early Miocene) basin complex as a result of back-arc extension. Although the stratigraphy of the latter is very well known due to raw material exploration, its formation and structural evolution is much less understood. A transtensional origin was generally proposed for the Paleogene Basin of Hungary, assuming an origin analogous to that of the overlying Neogene Pannonian Basin. We suggest an alternative geodynamic scenario for the development of the Paleogene basin complex, as we think that the observations are better understood in terms of a retroarc foredeep basin model. According to this, a flexural basin developed south of the backthrust inner West Carpathian units. The thrust load resulted in a generally deep, underfilled ''flysch'' basin during Middle Eocene-Early Oligocene times. In the Late Oligocene-Early Miocene predominantly shallow marine and continental deposition occurred indicating the gradual cessation of thrusting in the adjacent thrust-fold belt and the beginning of ''molasse'' sedimentation. Throughout the whole Paleogene, a shift can be observed of the bathyal, dominantly siliciclastic (in the axis of the foredeep) and neritic, partly carbonate (in the foreland flank of the foredeep) facies belts. This migration of the Middle Eocene-Early Miocene depocenters of the HPB (Hungarian Paleogene Basin) to the east-northeast in present-day coordinates is the result of two processes: (1) the advancement of backthrusting to the southeast; and (2) along-strike shift of thrusting to the northeast. While a thin-skinned thrust front can be reconstructed for the Late Eocene (Buda Anticline), the Diosjeno-Hurbanovo line is interpreted as the result of a major thick-skinned thrusting event at the base of the NP24 zone (approximately 31 Ma). The subsidence pattern, the distinct depositional environments and the structural observations can be more easily integrated into a flexural basin model than into a wrench basin model. Based on the flexural basin model of this work the early Tertiary Western Carpathians formed a compressional arc. While this arc probably developed above a B-subduction zone, during the Late Eocene with a corresponding trench (Magura), a forearc basin (Central Carpathian or Podhale) and a retroarc flexural basin (Hungarian Paleogene), from the Early Oligocene to the Early Miocene it was characterized by A-subduction with a corresponding peripheral foredeep (Dukla and Silesian) and a retroarc flexural basin (Hungarian Paleogene).