Effects of a low‐viscosity lower crust on topography and gravity at convergent mountain belts during gravitational instability of mantle lithosphere

Abstract

We examine the gravitational stability of a stratified structure consisting of a low-density crust through which viscosity decreases exponentially, over a denser mantle lithosphere of constant viscosity, which in turn overlies an inviscid, slightly less dense layer like the asthenosphere. The most important aspect of the viscosity structure is the contrast in viscosity at the Moho. Surface uplift above a mantle downwelling is greatest when the viscosity contrast at the Moho is negligible. The surface is depressed when the viscosity of the lowermost crust is much greater than mantle viscosity. For a wide range of viscosity structures, calculated gravity anomalies and in particular admittances (ratios of the Fourier transform of gravity to that of topography) produced by Rayleigh-Taylor instability differ markedly from those observed across convergent mountain belts. In particular, whereas values of calculated admittance are negative, or very small, observed admittances are consistently positive. Thus, if downwelling of mantle lithosphere directly beneath mountain ranges plays an important role in the construction of mountain belts, it cannot be the main process that induces crustal thickening.

Crustal deformation induced by mantle dynamics: insights from models of gravitational lithosphere removal

“…These results are compatible with earlier studies that investigate the dynamics of weak crust above foundering mantle lithosphere (e.g. Neil & Houseman 1999;Pysklywec & Shahnas 2003;Pysklywec & Beaumont 2004;Elkins-Tanton 2007;Molnar & Houseman 2013;Wang et al 2014;Molnar & Houseman 2015;Wang et al 2015).…”

“…As a result, the effective viscosity of the crust varies with temperature, strain rate, and possibly pressure. Studies that consider a Newtonian crust with a viscosity that decreases with increasing depth (temperature) show that topographic uplift decreases as the viscosity variation in the crust increases (Elkins-Tanton 2007;Molnar & Houseman 2015). Only the lowermost crust is able to flow, which limits the amount of thickening and uplift.…”

“…Only the lowermost crust is able to flow, which limits the amount of thickening and uplift. The viscosity ratio at the crust-mantle boundary is important, with the greater thickening and surface uplift where the crust and mantle viscosities are similar (Molnar & Houseman 2015). This is likely because the crust experiences greater shearing by the underlying mantle.…”

“…A number of studies have used linear stability analysis to assess crustal deformation during gravitational removal, assuming either a single-layer viscous crust (e.g. Neil & Houseman 1999;Molnar & Houseman 2013) or one with an exponential decrease in viscosity with depth (Molnar & Houseman 2015). These studies demonstrate if the crust is relatively strong (>13-30 times more viscous than mantle), lithosphere removal is accompanied by surface subsidence.…”

Gravitational instability of mantle lithosphere and core complexes

“…In a continuation of recent work [ Molnar and Houseman , , ], I examine deformation of the upper crust during growth of Rayleigh‐Taylor instability of the underlying mantle lithosphere. Depending on the viscosity structure of the crust and mantle lithosphere, the entire crust might be pulled down or the thickening lower crust might lead to a rise and divergence of the overlying upper crust (Figure ).…”