Wearing down, wearing back and gorge extension in the long-term denudation of a highland mass: Quantitative evidence from the Shoalhaven catchment, southeast Australia

The long standing issue of the dominance of scarp retreat versus summit lowering in the denudation of a highland mass is investigated with supporting evidence provided by Tertiary basalts throughout the Shoalhaven catchment in southeast Australia. Both of these forms of denudation are found to be insignificant compared to the role of fluvial gorge extension over the last 30 m.y. Headward advancement of the Shoalhaven Gorge has been occurring at approximately 15 times the rate of major escarpment retreat, 250 times the average rate of summit lowering, and over 500 times the rate of interfluve consumption. These estimates show that the headward erosion of gorges is the most important process denuding the highlands in the Shoalhaven region and possibly elsewhere in the highlands of eastern Australia. Over the long term, the highlands in this region will become considerably more dissected well before they decrease substantially in height or are narrowed. This conclusion has important implications for models predicting isostatic rebound from assumed character and rates of denudation.