Escarpment Retreat- Intuitions, Mechanisms, and Rates

Escarpments are preserved rift shoulders formed during continental rifting, such as the Red-Sea escarpment or those created during the breakup of Gondwana. Morphologically, escarpments function as highly asymmetrical water divides, separating steep, sea-draining rivers from low-gradient rivers draining the interior plateau. The evolution of an escarpment landscape is characterized by horizontal retreat driven by differential erosion rates across the water divide, even in tectonically inactive settings.

The horizontal migration of escarpments at rifted margins challenges conventional intuitions based primarily on two key observations:

• Observed denudation and rock exhumation rates, typically below 50 m/Ma over geological scales, with escarpment basins exhibiting higher rates than plateau basins;
• The “kinked” profile morphology of escarpment-draining rivers, where morphological knickzones are universally identifiable.

These observations are often interpreted in terms of temporal or spatial variations in uplift rates, attributed to tectonic rejuvenation, mantle dynamics-driven uplift, or the influence of precipitation and lithology.

In this study, I present insights derived from numerical model simulations of escarpment systems, demonstrating that the escarpment retreat can be conceptualized as a process of topographic advection. In a topographic advection system at steady state, rock erosion rates balance the combined effects of vertical rock uplift and the product of the directional topographic slope and horizontal rock advection velocity, relative to the topographic system’s boundaries (i.e. base level or water divide). In such settings, river morphologies adjust to the horizontal advection velocity by modifying their steepness to align with the erosion rate, maintaining a consistent relationship between steepness and erosion dynamics. The origin of the advection velocity may arise from tectonic activity or laterally moving boundaries, or any mechanism that generates horizontal rock velocities relative to the boundaries.

In an escarpment system, the advection velocity arises from the steady migration of the water divide, which expands the escarpment area while shrinking the plateau area, and create rock velocity relative to the divide. Although the observed erosion rates in tectonically inactive escarpments appear “slow”, they reflect the product of the topographic slope and a “fast” divide migration rate. The divide migration rate is typically one or two magnitude higher than the measured erosion rate, as the topographic slope is naturally less than one. This relationship applies to both escarpment basins and plateau basins. Morphologically, plateau basins exhibit lower gradients than escarpment basins, resulting in lower erosion rates on the plateau side, as the divide migration velocity remains constant. In escarpment-draining rivers, steep upper reaches originate from the major water divide, maintaining distinct morphologies consistent with the “top-down” dynamics of constant divide migration under constant base levels.