From plateaus to mountain peaks: identifying climatic and tectonic controls on peak elevation and plateau longevity

Many of the Earth's highest mountain peaks are located at the dissected fringe of large orogenic plateaus such as the Tibetan Plateau or the Altiplano. The striking spatial coexistence of exceptionally high peaks with rivers that incise the edge of the plateau led Wager to propose the co-evolution of valleys and mountain peaks more than a hundred years ago: focused erosion in valleys triggers the rise of mountain peaks due to erosional unloading and isostatically driven uplift. In addition to this interaction between localized erosion and ridgeline uplift, precipitation gradients due to orography introduce additional complexity. Amplified by rising ridgelines, the plateau slope forms a strong orographic barrier with wet conditions at the windward and dry conditions towards the plateau center. This in turn affects the spatial pattern of erosion and isostatically driven uplift.

We propose that the co-evolution of topography and precipitation (a) controls the spatial distribution and maximum height of mountain peaks that prominently tower above the plateau elevation and (b) limit the longevity of orogenic plateaus.  In this study, we compare the spatial distribution of mountain peaks along the Tibetan Plateau with results of a numerical model. The model considers orographic precipitation based on the advection and diffusion of moisture and its reaction on topographic barriers, fluvial erosion based on the stream power law, and flexural isostasy including viscous relaxation to account for erosional unloading and isostatic compensation. Our findings reveal that climatic factors (i.e. how far precipitation extends over the ridgeline), tectonic conditions (i.e. the pace and spatial pattern of plateau uplift) and lithospheric parameters (i.e. length-scale of lithospheric flexure) represent principal controls of the coupled precipitation­­-topography system. Only a few parameter combinations lead the evolution of peaks exceeding 8 km while maintaining the longevity of the plateau in the rain shadow of the ridgeline. Our experiments show that rapid plateau uplift is required, so that the main precipitation falls on the southern slope of the plateau even in the early phase of topography evolution. The longevity of the plateau requires the formation of a drainage divide in the rain shadow immediately behind the ridgeline of the highest mountains. Whether a drainage divide forms and where its position is depends on the ratio of the length scales for lithospheric flexure and orographic precipitation. Without the emergence of such a drainage divide, the plateau is rapidly dissected by river systems, without the formation of mountain peaks exceeding 8 km.