Upper and lower crustal deformation and residual topography in a continental back-arc: Inferences from the Pannonian-Transylvanian Basins

The topography and subsidence history of sedimentary basins are commonly related to crustal and lithospheric thinning linked to isostasy, also influenced by flexure and dynamic topography. The static component of the topography relative to a reference level can be calculated by the assumption that a lithospheric column consisting of crustal layers and a lithospheric mantle lid float within the asthenosphere. Here, we discuss the observed and calculated residual topography of the Pannonian Basin, i.e. the difference between the actual and calculated isostatic topography. The residual topography calculation is based on new geophysical constraints on the sedimentary, upper and lower crustal and lithospheric thicknesses based on reflection seismic and new receiver function analysis. The crustal thickness decreases from 40 to 45 km beneath the Eastern Alps to 22 km in the eastern Great Hungarian Plain that is floored by less than 60 km thick lithosphere affected by Miocene extension. The sedimentary thickness reaches more than 6 km in the deepest depocenters. The crust is much less attenuated in the Transdanubian Range (28 km), Apuseni Mountains or Transylvanian Basin (32.5–35 km). The interpreted lower crustal thickness reaches maximum 15–20 km in the Eastern Alps, 12–15 km in the Apuseni Mountains, 10 km in the Transdanubian Range and thins to 5 km in the Great Hungarian Plain. The new four-layer lithospheric model shows a much lower residual topography value than previously suggested for the Pannonian Basin. Only the Transdanubian Range is affected by maximum 300 m of positive residual topography, whereas the Transylvanian Basin shows 300–400 m of negative residual topographic values. The former is interpreted to be affected by small-scale asthenospheric upwelling effects, likely also contributing to the Miocene-Pliocene volcanic activity of the area. While the currently uplifting Transylvanian Basin undergoes tectonic re-adjustment linked to the gradual Vrancea slab break-off. Our results demonstrate the important effects of the thick sedimentary succession and the different crustal thinning values on the observed and predicted topographic variations in extensional sedimentary basins.