Escarpment evolution on high‐elevation rifted margins: Insights derived from a surface processes model that combines diffusion, advection, and reaction

Abstract

Experiments with a surface processes model of large‐scale (1–1000 km) long‐term (1–100 m.y.) erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent. The model describes changes in topographic form as a result of simultaneous short‐ and long‐range mass transport representing hillslope (diffusive) processes and fluvial transport (advection), respectively. Fluvial entrainment is modeled as a first‐order kinetic reaction which reflects the credibility of the substrate, and therefore the fluvial system is not necessarily carrying at capacity. One‐dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are (1) antecedent topography/drainage; (2) the timescale (or equivalently a length scale) in the fluvial entrainment reaction; (3) the flexural response of the lithosphere to denudation; and (4) the relative efficiencies of the short‐ and long‐range transport processes. When rainfall and substrate lithology are uniform, a significant amount of discharge draining over the escarpment top causes it to degrade. Only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions. An additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment. This corresponds to a substrate that is hard to detach by fluvial erosion, and therefore to fluvial erosion that is not transport limited. Continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide. It is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology. Low flexural rigidities promote steep and slowly retreating escarpments. For given rainfall and substrate conditions, the morphology of a retreating model escarpment is determined by the ratio of the short‐range diffusive and long‐range advective transport efficiencies. A low ratio (which is interpreted to correspond to a relatively arid climate and weathering‐limited conditions) promotes steep, sharp‐topped escarpments with straight main slopes, and escarpment retreat occurs over a wide range of height scales. A high ratio (interpreted to correspond to a more humid, temperate climate) produces a convex upper slope, and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient. Lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock. However, resistant caprocks are not an essential requirement for model scarps to retreat. We conclude that the inferred controls and model behavior are both consistent with the present‐day morphology of rifted continental margins and with modern conceptual models of landscape evolution.

Cenozoic river profile development in the Upper Lachlan catchment (SE Australia) as a test of quantitative fluvial incision models

“…RMS misfit contours are subparallel and have a positive slope in the K t À L f plot, demonstrating that the mean amount of incision along the river profiles is controlled by the ratio between these two parameters [van der Beek and Braun, 1998]. For any ratio of K t /L f , the distribution of incision along the profile is controlled by L f : high L f values tend to concentrate fluvial incision downstream, whereas it spreads upstream for lower L f values [Kooi and Beaumont, 1994;Braun, 1998, 1999]. This is demonstrated by the misfit structure plot, which shows positive s values for L f > $20 km (and constant K t /L f ) and negative s for smaller L f .…”

“…Initial model runs with this model are conducted to test the original formulation of Beaumont et al [1992] and Kooi and Beaumont [1994], that is, a linear dependence of carrying capacity on drainage area and slope (m t = n t = 1 in equation (4)) and a constant river width (here taken to be 50 m, an approximate average width for the rivers we are studying). This description limits the number of free parameters to two: the transport parameter K t and the length scale for fluvial incision L f .…”

“…On the other hand, for large L f (L f ) dx), Q s ( Q eq and the model tends toward a detachment-limited stream power model. In the original formulation of this model [Beaumont et al, 1992;Kooi and Beaumont, 1994], W was included implicitly only because rivers were assumed to have unit width. In order to study the possible influence of varying river width downstream, however, we have chosen to explicitly include W. Also, whereas Beaumont et al [1992] and Kooi and Beaumont [1994] implicitly assumed that Q eq is proportional to linear stream power (m t = n t = 1), we do not restrict ourselves to this case.…”

“…[11] Beaumont et al [1992] and Kooi and Beaumont [1994] derived a fluvial incision algorithm that takes the shielding effect of sediments into account. They describe bedrock incision as a first-order kinetic reaction in which downstream sediment flux variations are inversely proportional to a characteristic length scale L f and directly proportional to the degree of disequilibrium (the ''undercapacity'') in the fluvial sediment flux:…”

South Atlantic continental margins of Africa: A comparison of the tectonic vs climate interplay on the evolution of equatorial west Africa and SW Africa margins

“…Topography of SW Africa margin is dominated by the Great Escarpment (1000 -1500 m altitude) that represents the inherited rift-shoulder uplift (Summerfield, 1991;Kooi and Beaumomt, 1994). Numerical models of coupled tectonic and surface processes (Gilchrist et al, 1994) indicate that 1) denudation mostly affects the coastal, west-facing, narrow catchment; 2) delayed denudation affects the much wider interior catchment once the rift shoulder is breached; 3) the bulk of denudation occurred before the Tertiary.…”