We calculate variations in the thickness of the rigid uppermost layer of the lithosphere of South Africa, including its 2.6 to 3.6 billion-year-old embedded Kaapvaal craton. Results show that this layer, known as the competent lithosphere, beneath the craton attained its present thickness of up to 120km only after 2.0 billion years ago. Prior to that the thickness of the competent lithosphere of the craton may have been up to 60% less because its upper mantle was mechanically decoupled from its upper crust. The observed variance in the depth to the base of the competent lithosphere between the Archean Kaapvaal Craton and the surrounding Mesoproterozoic terrain are interpreted in terms of distinct chemical differences. These findings have implications for understanding the ageing of continents and Archean tectonics.