Topography of mountain belts controlled by rheology and surface processes

The most pronounced topographic features on Earth are large collisional mountain belts such as the European Alps, Himalayas, Andes, and Southern Alps of New Zealand. While it is well known that these mountain belts formed by collision of tectonic plates, it is not well understood what controls their variability and how climate and erosion affect the topography and size of these systems. Does erosion and thereby climate limit mountain belt height and size, or are mountain belt height and size limited by the internal strength of the colliding plates?

To solve these questions, we use new state-of-the-art numerical models that simulate the interaction between tectonics and surface processes during growth and decay of mountain belts over a time span of several 10s of million years and develop new theory. The new theory captures the main parameters controlling the interaction between surface processes and tectonics in a single quantity that we call the Beaumont number after Christopher Beaumont, who is one of the pioneers in quantitative understanding of plate tectonics. We show that the three main factors that control mountain belt size and topography are the velocity of the colliding plates, their crustal strength, and the efficiency of surface processes. 

In our article we demonstrate that mountain belts can be of three types: Type 1 actively widening mountain belts such as the Himalayas with a maximum height that is controlled by crustal strength, Type 2 mountain belts such as in Taiwan that have a constant size and an elevation controlled by crustal strength, and Type 3 small mountain belts such as the southern Alps of New Zealand with a constant width and an elevation that is controlled by the erosion rate. Each of these mountain belt Types can be characterized and understood by its unique Beaumont number that can be derived from simple observations, such as plate convergence velocity, mountain height, and widening rate. The classification can in turn be used to determine the relative contribution of each of the three controlling factors for each mountain belt, thus finally answering the question of what controls mountain belt elevation on Earth. The results and classification of these natural examples suggests that erosion-limited mountain belts are rather the exception than the norm so that the height of most collisional mountain belts is limited by the strength of their colliding plates and not climate.

We also investigated how mountain belt topography decays once they stop growing and find that the removal of topography is mostly controlled by the erosional power. Our results highlight that survival of topography for several 10s to 100s of million years is likely quite common given the erosional power that we observe in many mountain belts on Earth.

The results presented in our article provide for the first time a unified framework for the controls of collisional mountainbelts subject to the interaction between surface processes and tectonics.