Old orogen – young topography: lithological contrasts controlling erosion and relief formation in the Bohemian Massif

Abstract. In several low mountain ranges throughout Europe, high-grade metamorphic and granitic rocks of the Variscan orogen are exposed – even though the topography of this Paleozoic mountain range was largely leveled during the Permian and later covered by sediments. The Bohemian Massif is one of these low mountain ranges and consists of high-grade metamorphic and magmatic rocks that dip southward below the weakly consolidated Neogene sediments of the Alpine Molasse Basin. Morphologically, the Bohemian Massif is characterized by rolling hills and extensive low-relief surfaces above 500 m, which contrast with deeply incised canyons characterized by steep and morphologically active valley flanks. These morphological features and the occurrence of marine sediments several hundred meters above sea level are a clear indication of relief rejuvenation due to significant surface uplift during the last few million years.

To constrain landscape change and its rate, we used the concentration of cosmogenic ¹⁰Be in river sands to determine 20 catchment-wide erosion rates and correlated these with topographic metrics characterizing both the hillslopes and the drainage systems. Erosion rates range from 22 to 51 m per million years, which is generally low compared to tectonically active mountain ranges such as the Alps. Low erosion rates in the Bohemian Massif seem to contradict the steep topography observed close to the receiving streams (i.e., the Danube River and the Vltava River), which have morphological characteristics of alpine landscapes. Erosion rate is correlated with catchment-wide topographic metrics. Highest erosion rates occur in catchments featuring high channel steepness and a large area fraction with significant geophysical relief. Catchments with abundant deeply incised canyons erode about twice as fast as those characterized primarily by low-relief surfaces. Separating the catchments in four elevation quartiles, we found that the degree of correlation between erosion rate and landscape metrics decreases from the lowest to the highest elevation quarter of the catchments. We interpret this as an increasing decoupling of erosion rate and topographic features with distance to the sample location.

We interpret the measured erosion rates and related topographic patterns as the landscape response to slow and large-scale uplift in concert with strong variations in bedrock erodibility between rocks of the Bohemian Massif and the Neogene Molasse basin. We propose that lithology is ultimately responsible for the topographic difference between the mountainous Bohemian Massif and the low-relief Molasse zone despite a common uplift history during the last few million years. As erosion progresses basement rocks with their high resistance to erosion are exposed. The repeated emergence of such bedrock barriers reduces the erosion rate during topographic adjustment and governs the evolution of low-relief surfaces at different elevation levels. The resulting stepped landscape requires neither spatial nor temporal changes in uplift rate but can form by erodibility contrasts under uniform uplift conditions.