Topographic relief driven by variations in surface rock density

Abstract

Earth’s surface topography is generated by tectonically induced variations in crustal thickness combined with erosion and, to a lesser degree, by vertical stresses caused by convection in the underlying mantle. Rock hardness and resistance to erosion are also commonly thought to influence topography because hard rocks, such as granites and basalts, usually form topographic highs in the landscape. Here we use analytical and numerical models to simulate the erosion-induced isostatic rebound of rocks. We find that the isostatic rebound that accompanies erosion causes denser rocks to occupy higher elevations in the landscape, thereby creating topographic relief that is proportional to surface rock density differences rather than rock hardness. We quantify this effect, taking into account the flexural strength of the continental lithosphere. We show that in a steady-state erosional setting, density-dependent isostatic rebound can cause the densest rocks to be exhumed at double the rate of surrounding, less-dense rocks and has a stronger effect than typical rock hardness variations. The results imply that denser rock formations should erode faster and therefore be characterized by younger thermochronological ages. Thermochronological data sets from the Kinabalu granite in Borneo and the Shakhdara–Alichur gneiss domes in Pamir confirm this counter-intuitive result. Our findings imply that lateral variations in surface rock density have significant control on the shaping of the large-scale features of Earth’s surface. The high elevation in Earth’s topography of hard rocks, such as granites and basalts, was thought to be caused by their inherent resistance to erosion. Numerical modelling now demonstrates, counterintuitively, that erosion-induced isostatic rebound of rocks, which is density dependent, causes granites and basalts to occupy high elevations because they are more dense than surrounding rocks.