Late Pleistocene-Holocene denudation rates in the Bavarian Forest from in-situ produced 10Be cosmogenic nuclides

The Variscan basement units of the Bohemian Massif and other Variscan domains in Central Europe are thought to be less affected by Cenozoic tectonics. However, analyses using geomorphic indices and river profiles reveal spatial variations in the evolved topography in the Bavarian Forest, the southwestern segment of the Bohemian Massif. The geomorphic analyses suggest disequilibrium along drainage divides and in river profiles. It has been suggested that the topography in the southern parts of the Bohemian Massif has rejuvenated (Zebari et al., 2024). However, interpretations aimed at extracting tectonic signals from geomorphic analysis are inherently relative, and it is sometimes challenging to separate tectonic signals from climate signals or differential erodibility. To further quantify landscape dynamics in the Bavarian Forest, we estimated long-term watershed-averaged denudation rates from in-situ produced ¹⁰Be cosmogenic nuclide analysis of sand samples collected from outlets of 15 representative watersheds in the Bavarian Forest.

Our initial results indicate that watershed-averaged denudation rates in the Bavarian Forest range from 21.1 ± 2.4 to 40.5 ± 4.8 mm/kyr. These denudation rates represent a time period equivalent to the removal of approximately 60 cm of rocks, corresponding to about 15 kyr for the fastest-eroding watershed and about 28.5 kyr for the slowest one. There are spatial variations in the denudation rates, with the watersheds clustering into several distinct subregions. Overall, watersheds in the southeast, within or around the Ilz catchment, and those in the high-elevation areas of the Hinterer Bayerischer Wald, have relatively higher denudation rates than those in the central segment of the Vorderer Bayerischer Wald. Furthermore, notable differences are also found across the drainage divide between the Regen and Danube rivers in the adjacent watersheds, and these differences are consistent with the analysis of drainage divide dynamics using the χ (Chi) integral. Additionally, the calculated denudation rates also correlate with the topographic metrics of watershed and river profiles.

The same climatic conditions and minimal contrast in rock erodibility are expected for the adjacent watersheds in the Bavarian Forest; therefore, these denudation rates may represent a brief time window within a longer span of tectonic processes that shaped the relief there. Broad surface uplift of the Northern Alpine Foreland Basin and beyond, since prior to 6 Ma, and associated drainage network reorganization may have also affected the southern regions of the Bohemian Massif, resulting in the rejuvenation of topography in the Bavarian Forest, notably within its southeastern part. Thus, it is expected that the driving forces for the broad-scale recent surface uplift event(s), which affected the Bavarian Forest, also reactivated the major bounding faults.

References:

Zebari, M., Friedrich, A. M., Ludat, A. L., Kahle, B., Rieger, S. M., & Kübler, S. (2024). The role of late Cenozoic intraplate tectonic in shaping the topography of the Bavarian Forest, southwestern Bohemian Massif, Germany. Geologica Bavarica, 130: 35–55.