Quantifying Erosional vs. Tectonic Controls on Divide Asymmetry

Mountain ranges commonly exhibit asymmetric topography, with main drainage divides offset from the range center. These asymmetries are often attributed to differential rock uplift, but divide positions also respond to bedrock erodibility contrasts and base-level differences between opposing flanks. How these controls interact to determine divide position and whether base-level differences can rival tectonic forcing remain poorly constrained. Resolving these questions is fundamental to interpreting topographic asymmetry in active orogens and extracting reliable tectonic information from landscape morphology. Building on previous work, we present an analytical framework based on the stream power model that quantifies how uplift, erodibility, and base-level elevation jointly control divide positions. We derive dimensionless divide asymmetry numbers that quantify the tectonic-to-erosional forcing ratio controlling divide position, where values <1, ≈1, and >1 indicate tectonic dominance, comparable forcing, and erosional dominance, respectively.

Numerical landscape evolution experiments test our analytical predictions, demonstrating that base-level differences can influence divide position as strongly as differential uplift. The experiments test our analytical framework across a wide range of boundary conditions, confirming that the dimensionless parameters successfully capture divide behavior under diverse tectonic and erosional settings. Applying this framework to the Sub-Himalayan Mohand Range, we find that the observed divide position can be reproduced only when base-level differences between the Himalayan hinterland and Indo-Gangetic foreland are explicitly incorporated. Application to three additional Sub-Himalayan anticlines reveals dramatic variations in divide position across structurally similar fault-related folds, with divide asymmetry numbers ranging from 0.15 to 2.0. These variations correspond to base-level differences of 0-150 m between opposing flanks, demonstrating that base-level offsets of this magnitude control divide asymmetry more strongly than differential uplift in these actively deforming structures.

Our results demonstrate that base-level configuration can exert comparable or even stronger influence than differential uplift on divide position in certain settings. While the potential importance of base-level differences has been recognized, our dimensionless framework provides the first quantitative approach for systematically partitioning these competing controls. This advance enables more robust interpretation of divide asymmetry in active orogens, particularly in settings where topographic gradients naturally generate base-level contrasts between opposing drainage networks. The framework offers a valuable complement to existing approaches for extracting tectonic signals from mountain range morphology.