The post‐Palaeozoic uplift history of south‐eastern Australia

Abstract

Vertical movements within mountainous terrain may be indicative of either active tectonism or of passive isostatic rebound of the crust to the erosion of the highlands. During the active orogenic phase, the landscape evolution is controlled largely by the tectonic process but once this ceases or is reduced in intensity, the erosional unloading and concomitant rebound becomes the dominant landscape‐forming process. It is this latter phase that is examined here. It is argued that the southeastern highlands are a residual of the Palaeozoic Lachlan Fold Belt, rather than having been rejuvenated in Tertiary time. It is concluded that the erosional rebound model explains many of the recent geomorphological observations that attest to little landscape evolution since the early Tertiary. The model adopted is of a mountain range that is initially in local isostatic equilibrium but which responds regionally to the erosional unloading. The crust or lithosphere is modelled as a viscoelastic layer so that rebound is not instantaneous. The rate of erosion at any time is assumed to be proportional to elevation above sealevel with a time constant of the order 10⁸ years. For a given present‐day topography the elevations, erosion, rebound, stress and gravity can be computed throughout time as a function of model parameters. The time by which the rebound phase became the dominant process is 200–250 Ma ago and the elevations at that time were about 75% greater than present values. An erosion time constant of 200 Ma produces average Tertiary erosion rates of a few metres/million years, rates that are consistent with geomorphological observations in several areas of south‐eastern Australia.