Beyond threshold hillslopes: Channel adjustment to base-level fall in tectonically active mountain ranges

Abstract

Numerous empirical and model-based studies argue that, in general, hillslopes and river channels increase their gradients to accommodate high rates of base-level fall. To date, however, few data sets show the dynamic range of both these relationships needed to test theoretical models of hillslope evolution and river incision. Here, we utilize concentrations of 10 Be in quartz extracted from river sand on the eastern margin of the Tibetan Plateau to explore relationships among short-term (10 2 -10 5 a) erosion rate, hillslope gradient, and channel steepness. Our data illustrate nonlinear behavior and a threshold in the relationship between erosion rate and mean hillslope gradient, confi rming the generalization that hillslopes around the world are limited by slope stability and cease to provide a metric for erosion at high rates (>~0.2 mm/a). The relationship between channel steepness index and erosion rate is also nonlinear, but channels continue to steepen beyond the point where threshold hillslopes emerge up to at least 0.6 mm/a, demonstrating that channel steepness is a more reliable topographic metric than mean hillslope gradient for erosion rate and that channels ultimately drive landscape adjustment to increasing rates of base-level fall in tectonically active settings.

Geochronology of the Baye Mn oxide deposit, southern Yunnan Plateau: Implications for the late Miocene to Pleistocene paleoclimatic conditions and topographic evolution

“…330 m. Myr -1 for the Dadu, Yalong, and Yangtze River gorges since about 10 Ma (Ouimet et al, 2010). The incision rates for the Nanpie River are also comparable to the results calculated from 10 Be exposure and 14 C dating of the Quaternary fluvial terraces on the eastern Tibetan Plateau ranging from 200-800 m. Myr -1 (Ouimet et al, 2009;Godard et al, 2010;Hetzel, 2013). The river incision rates inferred from the Pleistocene to Holocene terraces on the eastern margin of the Tibetan Plateau range from 200-500 m. Myr -1 at low gradient channel rivers to 1000-3000 m. Myr -1 at high gradient channel rivers (Kirby et al, 2003), with the former being similar to the incision rate of the Nanpie River.…”

Tectonic controls of Holocene erosion in a glaciated orogen

“…High rock uplift rates in many of these ranges led to the buildup of topography and in some cases high relief, steep river channels and hillslopes, and commensurate high erosion rates (Willett, 1999). Because of the covariation between climate, topography, and rock uplift, erosion rates in fluvially dominated orogens have been shown to correlate with climatic and topographic metrics such as precipitation rate, relief, hillslope angle, and channel steepness via linear, non-linear, and threshold relationships (Ahnert, 1970;Montgomery and Brandon, 2002;Ouimet et al, 2009;DiBiase et al, 2010). The development of rugged mountain belts led to an increase in cooler, higher-elevation landscapes, which created the necessary conditions for alpine glaciers to form in the late Cenozoic.…”

Tectonic controls of Holocene erosion in a glaciated orogen

“…6B). This limit has been observed in many other landscapes around the world (DiBiase et al, 2010;Ouimet et al, 2009;Montgomery and Brandon, 2002;Binnie et al, 2007). Basin-averaged hillslope angle values tend to reach a maximum around 34°, as also shown by Montgomery (2001) using 100 km 2 grids across the Olympic range.…”

“…We emphasize that this technique provides a robust, geometric construct for understanding the importance of spatial changes in channel relief without demanding an understanding of all parameters within a specific incision law (fluvial or glacial). Unlike hillslope angle calculations, channel steepness values may be able to record changes in erosion/rock uplift rates in regions where hillslopes have reached a threshold (Ouimet et al, 2009). 125…”

“…High rock uplift rates in many of these ranges led to the buildup of topography and in some cases high relief, steep river channels and hillslopes, and commensurate high erosion rates (Willett, 1999). Because of the covariation between these characteristics, erosion rates in fluvially dominated orogens have been shown to correlate with climatic and topographic metrics such as precipitation rate, 25 relief, hillslope angle, and channel steepness via linear, non-linear and threshold relationships (Ahnert, 1970;Montgomery and Brandon, 2002;Ouimet et al, 2009;DiBiase et al, 2010). In the Late Cenozoic, alpine glaciers formed, and then advanced and receded many times due to climate oscillations.…”