Erosion-isostatic rebound models for uplift: an application to south-eastern Australia

Abstract

A mountain's history includes two distinctive phases, one of active tectonism and construction followed by one of erosion and passive isostatic rebound. In the first phase uplift is driven by tectonic mechanisms while in the second phase base levels of the terrain are regionally uplifted. It is this latter phase that is modelled here. The starting model is a mountain range, initially in isostatic equilibrium, on a viscoelastic plate defined by the effective flexural rigidity D and relaxation time τv. The rate of erosion at any time t is assumed to be proportional to the elevation at that time, with an erosional time constant τe. For a given present-day topography the uplift, erosion, gravity, and stress can be computed through time as functions of D,τv, τe, and the time t0 at which the erosional-rebound mechanism became the dominant landscaping process. The model has been applied to the highlands of south-eastern Australia which we assume to be an erosional residue of the Palaeozoic Lachlan Fold Belt. Observations of rivers cutting through Cainozoic basalts and other geomorphological indicators of uplift can then be interpreted in terms of the isostatic rebound and there is no need to invoke active tectonic uplift mechanisms. The model parameters that fit the observations are τe = 150–250 Myr, Dτv ∼ 2.5 × 1024 N m Myr and t0∼ 180–200 Myr. The predicted Late Palaeozoic fold-belt topography is about 75 per cent greater than the present-day values. Topographic depressions are predicted to have occurred along the margins of the formerly elevated areas and the model is relevant to understanding the evolution of basins flanking the highlands. Within the highland area erosion generally exceeds uplift but in some regions the two are approximately equal, in keeping with recent geomorphological observations. The flexural stresses associated with the erosion initially increase with time and maximum values in south-eastern Australia are predicted to occur in Late Cainozoic time, with the stress state near the surface being mainly tensional.