Old orogen - young topography: erodibility contrast as superior control of relief rejuvenation in the Bohemian Massif?

Although the topography of the Variscan orogen was largely leveled in the Permian, outcrops of Variscan rocks occur in the form of several low mountain ranges forming tectonic windows framed by Neogene sediments. The Bohemian Massif is one of these low mountain ranges and consists of high-grade metamorphic rocks and magmatic intrusions that dip towards the south under the weakly consolidated Neogene sediments of the Molasse Basin. Timing and rates of Neogene uplift of the region are largely unconstrained, but the occurrence of marine sediments several hundred meters above sea level is a clear indication of significant surface uplift during the last few million years. Morphologically, the Bohemian massif is characterized by rolling hills and extended planation surfaces above 500 m that are contrasted by deeply incised gorges with steep and morphological active valley flanks. The central ridge of the Bohemian Massif forms a continental divide with the Vlatava and the Danube draining the northern and southern part of the mountain range. To constrain the pattern of landscape change and its rates, we computed topographic metrics and determined catchment-wide erosion rates from the concentration of cosmogenic ¹⁰Be in river sands.

The morphometric analysis indicates an out-of-equilibrium landscape. River length profiles feature knickpoints abundantly at elevations of about 500 m separating steep channel segments at lower elevations from less steep channel segments at higher elevations. Hypsometric maxima near knickpoint elevations along with high and low values in geophysical relief down- and upstream of major knickpoints testify of a bimodal landscape. The continental divide shows a distinct asymmetry, which is expressed by across-divide gradients in channel steepness.  The higher average channel steepness within the southerly Danube catchment predicts the northward migration of the Danube-Vltava drainage divide. Erosion rates of 20 to 50 m per million years in the 20 catchments studied are very low compared to the Alps and appear to contradict the steep topography close to the receiving streams. The lowest erosion rates occur in catchments with a large proportion of low relief areas at medium altitudes. The highest erosion rates occur in elongated catchments of Danube tributaries whereby these basins also have a large proportion of low gradient topography.

Based on our results we suggest that the Bohemian Massif was affected by low but long-lasting uplift without significant gradients between the Bohemian Massif and the nearby Molasse Basin. In our model, the occurrence of contrasting bedrock properties between Neogene sediments of the Molasse Basin and the crystalline basement represents the superior control on the topographic evolution of the entire region. As the river incision progresses, there is a transition from easily erodible sediments to the much less erodible crystalline rocks below, which abruptly reduces the ability of a river to incise. Consequently, relief forms and channel gradients increase until the erosion rate can balance out the uplift rate. We suggest that the Bohemian Massif is currently at such a transient state, which is expressed by landscape bimodality, where the two contrasting landscape types are separated by upstream migrating knickpoints.