Controls on post-mid-Cretaceous landscape evolution in the southeastern highlands of Australia: Insights from numerical surface process models

The tectonic and geomorphic evolution of the southeastern Australian highlands has been a subject of controversy among geophysicists, geologists, and geomorphologists for several decades. We employ a numerical surface process model that includes long-range fluvial transport, hillslope diffusion, and landsliding, in order to quantitatively assess the tectonic, lithological, and structural controls on the landscape evolution and denudation history of SE Australia. We constrain fluvial erosion parameters for our model by fitting river profiles. Rockfall and landsliding are the most important processes controlling the concentration of erosion in river valleys. We therefore model hillslope evolution using a very low diffusion coefficient and a threshold slope for landsliding. The "initial" (prebreakup) topography of the highlands provides a fundamental control on their subsequent evolution, together with the style and rate of fluvial incision. Secondary controls are exerted by lateral variations in lithology and local tectonic uplift along faults. Our modeling results indicate that (1) the observed highlands morphology requires that the drainage divide was established at its present location prior to opening of the Tasman Sea; whether the divide is inherited from Paleozoic orogeny or resulted from Mesozoic syn-rift uplift cannot be established, however; (2) in contrast to traditional interpretations, escarpment retreat does not appear to be the fundamental process eroding the highlands; and (3) the observed barbed river drainage may be imprinted as a result of lithological variation; it does not necessarily evolve through capture. The driving mechanism that is most consistent with the inferred uplift pattern of the southeastern highlands appears to be magmatic underplating.