Critical form and feedbacks in mountain-belt dynamics: Role of rheology as a tectonic governor

Abstract

[1] The rheology that governs deformation within a convergent orogen also controls its topographic form, or in other words, the covaration of its height (or thickness) and its width. Under conditions pertaining to small orogens and for rheologies ranging from linear viscous to Coulomb plastic, we show that this topographic form is insensitive to the distributions of fluxes into and out of the wedge and is thus a fundamental property of the system. It can therefore be thought of as a “critical” topographic form, directly analogous to the well-studied case of a Coulomb plastic rheology, which predicts a constant critical taper angle, independent of its size. The tendency of the system to evolve toward a critical topographic form can be regarded as a “tectonic governor” that strongly damps the response of the orogen to changes in climate or tectonic forcing. Scaling relationships can be derived for the variation of orogen width, height, and exhumation rate as a function of accretionary flux and precipitation rate. This study explores how sensitive these scaling relationships are to the assumed rheology. It is found that the scaling relationships vary by less than a factor of 2 across the range of typical geological rheologies, from Coulomb friction, to power law viscous, to linear viscous. These scaling relationships provide a first-order representation of the behavior of convergent wedges and are relatively insensitive to the underlying assumptions.