CAPTURING THE ROLE OF CLIMATE IS A PREREQUISITE TO DISENTANGLING TECTONIC AND LITHOLOGIC CONTROLS ON LANDSCAPE EVOLUTION

The role of climate in shaping the topography of tectonically active landscapes relative to tectonics and lithology is hotly debated. In large part this debate persists because clear signatures of climate’s influence have proven elusive. Here, we show that orographic rainfall gradients strongly affect fluvial erosional efficiency and thus topography across >1500 km of the northern-central Andes, and this effect is captured by a simple metric proportional to unit stream power that combines channel steepness and mean-annual rainfall, k_snQ. Importantly, determining channel steepness depends on knowledge of the reference concavity index, which describes the expected relationship between channel gradient and discharge assuming uniform climate, rock uplift rate, and rock erodibility. By analyzing river profiles across diverse climatic, tectonic, and morphologic regimes we show that once spatial variations in rainfall are accounted for the common assumption of a reference concavity near 0.5 is well supported. Also, accounting for rainfall patterns increases the sensitivity of channel steepness to lithologic and tectonic controls on topography, enhancing predictions of erosion and rock uplift rates and allows us to quantify climatic, tectonic, and lithologic effects on topography.

In parallel, we compare results using the standard channel steepness metric, k_sn, which intrinsically assumes that erosional efficiency and thus climate are spatially uniform. Using k_sn where rainfall is spatially variable undermines efforts to distinguish the effect of climate from those of tectonics and lithology. Further, failure to account for rainfall gradients can bias reference concavity estimates, compromising the utility of channel steepness values. Together, these effects can completely obscure the apparent influence of climate, even in settings with strong climate gradients that impart clear signatures of their influence in the topography. This can also ultimately lead to false impressions about rock uplift patterns and relative influences of other environmental variables. Therefore, we conclude that accounting for climate, particularly in mountain landscapes where orographic rainfall gradients are common, is an important prerequisite to understanding primary controls on landscape evolution.