How erosional efficiency modulates landscape response to drainage reorganization

This blog post is written by Helbert García-Delgado (also here) on his recent paper in Geomorphology. Helbert is a researcher at Syrakus University. If he is not working on longterm landscape evolution, he also documents landslides and natural hazards in the Colombian Andes. Thanks for having me on board and sharing your post here.

The shift from longitudinal to transverse drainage systems is a pivotal area of investigation within fluvial geomorphology because it sheds light on how we interpret landscape adjustments. Typically, it is theorized that in the advanced stages of mountain-building processes, longitudinal rivers are gradually replaced by transverse ones due to the expansion of their headwater basins. To test this hypothesis, we assembled a multidisciplinary team of geoscientists from Colombia, Germany, Chile, and the United States. We gathered new empirical data, calculated drainage divide metrics and conducted traditional river profile analysis from the Andes Mountains to envision the rates at which landscapes adjust.

Our observations unveiled an intriguing phenomenon: a delay in the transition from longitudinal to transverse drainage systems due to what we term “out-of-sequence topographic rejuvenation.” This delay is attributed to the balancing effect it has on erosion rates between the flanks of the mountain range (the orogen flanks) and the elevated central plateau (the axial plateau). This unique landscape configuration may have arisen as a result of significant plate tectonic reorganization during the Late Cenozoic in the Northern Andes. If this hypothesis holds true, it underscores the importance of considering changes in the Earth’s dynamic processes to gain a comprehensive understanding of how drainage systems evolve over geological time.

In addition to these primary findings, our research uncovered another intriguing aspect: the influence of lithological (rock composition) and structural (rock deformation) variations on the rates of headward expansion of drainage systems and the migration of erosional features (knickpoints). In regions where dissimilar rock types with varying degrees of deformation (horizontal versus folded structures) were exposed, erosion rates were notably reduced, often by a factor of two to four. Furthermore, the creation of waterfalls on flat-topped landforms acted as an obstacle, impeding the upstream progression of erosional processes caused by discrete captures.

In summary, our interpretations provide critical insights into the hypothesis that the efficiency of erosion, governed by the interplay of contrasting lithologies and structural characteristics, plays a pivotal role in maintaining flat-topped landscapes in a transient state. This conclusion could be applicable to other regions where ancient flat-topped landscapes remain prominent as elevated features, even though tectonic activity has waned millions of years ago.

References

García-Delgado, H., Schwanghart, W., Hoke, G.K., Guerrero, B., Velandia, F., 2023: How erosional efficiency modulates landscape response to drainage reorganization: new empirical evidence from the Andes. Geomorphology, 440, 108893. [DOI: 10.1016/j.geomorph.2023.108893] [free download until Oct 25, 2023]