Dados da Submissão

Título

River captures govern the style, patterns and rates of escarpment evolution

Texto do resumo

Introduction
Escarpments exist in all climates and tectonic settings. Despite marked differences between examples, all escarpments share a common evolutionary trajectory: they tend to retreat backwards due to the relief asymmetry between the steep escarpment and the gentle plateau above. Geoscientists conceptualised over a century ago that escarpments retreat in a parallel manner, maintaining their shape and slope angle. This concept implies escarpments retreat with spatially uniform rates, resulting in a linear map-view geometry. However, in detail, the geometry of most, if not all, escarpments is indented, with alternating spurs and embayments. The degree of escarpment sinuosity has been argued to be linked to the escarpment age, implying that indentation evolves with escarpment retreat. Parallel escarpment retreat is at odds with an evolving spur-embayment escarpment geometry.

Objective
We employ landscape evolution modelling to explore how systematic captures of plateau rivers by escarpment-draining rivers affect the style and rates of escarpment retreat.

Methods
We conducted numerical experiments using the Landlab modelling toolkit to simulate drainage capture dynamics in a retreating escarpment context. We implemented a well-established modelling approach known to induce systematic drainage capture events. This approach involved imposing an instantaneous perturbation in the rock uplift field oriented perpendicular to the main flow direction of the pre-existing drainage network. This generated a large plateau bounded by an escarpment spanning the entire boundary of the model domain. We quantified the time evolution of this escarpment landscape, focusing on the morphological and erosional changes following river capture events.

Results
Simulations show that the steep escarpment retreats due to relief asymmetry with the gentle plateau, resulting in multiple captures of plateau rivers. Each capture event drives rapid local erosion, resulting in a sharp increase in river steepness termed 'knickpoint' that propagates upstream through former plateau rivers. As these knickpoints propagate, they create indentations (i.e., embayments), warping the initially linear escarpment geometry. The escarpment's sinuosity increases by growing embayments at the incising river valleys and remaining protruding spurs in between. The fast local river incision also moves the drainage divide into the plateau. When the knickpoints against the escarpment side reach the sub-catchment's headwaters, they breach through and cut off original escarpment sections. This is a "dissection of the escarpment from the back" that slices frontal scarp sections into separate residual landforms known as 'mesas' or 'buttes'. Finally, a new escarpment position (and drainage divide) is established inward of the plateau, leaving dissected residual landforms in front, which decay over time, independent of the new escarpment.

Conclusion
Our simulations suggest a capture-driven rapid and stepwise mechanistic dissection process of escarpments, contrasting with the prevailing assumption of general uniform, slow escarpment retreat. Escarpment-draining rivers can capture plateau rivers and slice off whole escarpment sections, partly dissecting these into residual landforms. Current escarpment positions thus could be rapid outcomes of drainage capture events on the plateau.

Palavras Chave

landscape evolution; numerical modelling; escarpment; geomorphology