Quantifying the growth and decay of topography in collisional orogens

It is widely recognized that mountain belt topography is generated by crustal thickening and lowered by river incision, linking climate and tectonics. However, it remains enigmatic whether surface processes or lithospheric strength control mountain belt height, width, and longevity, reconciling high erosion rates observed for instance in Taiwan and New Zealand and low erosion rates in the Tibetan and Andean plateaus, as well as long-term survival of mountain belts for several 100s of million years as observed in the Urals and Appalachians. Here we use a tight coupling between a landscape evolution model (FastScape) and a thermo-mechanically coupled mantle-scale tectonic model (Fantom) to investigate mountain belt growth and decay. Using several end-member models and introducing the new non-dimensional Beaumont number, Bm, we quantify how surface processes and tectonics control mountain growth and define three end-member types of growing orogens: Type 1, non-steady state, strength controlled; Type 2, flux steady state, strength controlled; and Type 3, flux steady state, erosion controlled. Orogenic decay is determined by erosional efficiency and can be subdivided into two phases with variable isostatic rebound characteristics and associated timescales: First short-wavelength relief is removed within a few Myr, followed by removal of long-wavelength topography and effectively local isostatic rebound. Comparing model and scaling results to natural orogens explains why different orogens on Earth are rheology or erosion- (climate)-limited, and why long-term survival of topography seems to be the norm rather than the exception.