On the stability of isostatically compensated mountain belts

Abstract

The formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence. The evolution of potential energy stored in an orogen is dependent on (1) the density structure, (2) the thermal evolution, and (3) the way convergent deformation is partitioned between crust and mantle lithosphere. It is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension, or collapse, may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened (e.g., England, 1987). In this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the “Brace‐Goetze” model for the rheology of the lithosphere. The reference lithosphere, defined to be in potential energy and isostatic balance with the mid‐ocean ridges, changes with different thermal parameters of the lithosphere. The instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts (measured relative to the reference state) with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range. It is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates (greater than a few times 10−16 s−1) and leading to significant finite extension providing the Moho temperatures exceed about 650–700°C, a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust. Consequently, processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse. The calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening (i.e., fl ≤ fc/2, where fl and fc are the lithospheric and crustal thickening factors).