Non-structural origins of asymmetric topography in semi-arid environments

Geoscientists have long noted that landscapes in some regions have pole-facing slopes that are steeper than equator-facing slopes. In some cases, the asymmetry has a simple structural cause, such as slopes that form parallel to bedrock strata, but in others, the absence of obvious structural controls and the consistent pole-equator orientation of the asymmetry suggest that different microclimates on opposing slopes may be the cause. Compelling as the microclimatic correlation may be, it has not been demonstrated how microclimatic effects can influence long-term landscape evolution sufficiently to generate asymmetric topography. Two conflicting hypotheses that depend on microclimates have been proposed. In one hypothesis, a microclimate-induced contrast in the efficiency of erosion processes, such as channel incision, creates a difference in erosion rates on opposing slopes that drives drainage divide migration until the slope asymmetry compensates for the difference in erosional efficiency. In the other, more popular hypothesis, the asymmetric erosional efficiency is not a sufficient condition. Instead, faster sediment aggradation at the foot of more efficiently eroding slopes forces axial streams to undercut the opposing slopes, eventually creating an asymmetry in steepness. We seek to determine whether undercutting is a necessary mechanism, and focus our efforts on understanding the mechanisms responsible for the topographic asymmetry at Gabilan Mesa, CA, a landscape with a high degree of asymmetry and a simple underlying lithology. We investigate the long-term topographic consequences of these mechanisms with a landscape evolution model. In the first set of model experiments, we explore the effects of aspect-dependent differences in the efficiency of soil creep, the magnitude of a channel incision threshold, and runoff production. The second model experiment includes undercutting of slopes in response to lateral base level migration. We examined which mechanism most accurately matches the observed topography of Gabilan Mesa, in terms of both the degree of slope asymmetry and other characteristics, including the degree of valley incision, mean gradient, and relief. Both the erosional efficiency model and the undercutting model are capable of producing landscapes with the same degree of asymmetry as Gabilan Mesa while also reproducing the other topographic characteristics. When paired with field evidence that rills and gullies are more abundant on equator-facing slopes, we believe that a discrepancy in either channel incision thresholds or runoff production may be enough to cause the observed topographic asymmetry at Gabilan Mesa without the aid of undercutting.