Geomorphic imprints of lithospheric flexure in central Australia

Abstract

The Finke River in central Australia is counted among the world's oldest drainage systems; this raises the prospect of investigating how (tectonic and sub-lithospheric) processes have shaped the landscape in the long term. The Finke's upper reaches display an enigmatic set of intertwined active and relict bedrock gorges that suggest a complex history of incision, aggradation and re-incision. We measured cosmogenic 10Be and 26Al in fluvial gravels preserved in the gorges, and we applied a Markov chain-Monte Carlo-based inversion model to test two limiting-case hypotheses about the timing of gravel deposition and exhumation. Our results suggest that the nuclide memory contained within the gravels was essentially erased during sediment storage exceeding several million years. Previous studies attribute central Australia's geomorphic development to intensified post-Miocene aridity in tune with the perception of a quiescent tectonic regime. However, the close correlation between drainage patterns and gravity anomalies leads us to propose, instead, that cyclical in-plane loading in the presence of extreme lithospheric density anomalies caused to-and-fro tilting of the upper Finke. We show that relief variations of order 100 m accompany mild in-plane loading of ∼1–2 x 1012 N m−1, and we speculate that loading variations are likely the result of tractions imposed at the base of the plate rather than at plate boundaries. Acting together, these tectonic and sub-lithospheric processes triggered the phases of incision and aggradation that created the Finke's unusual intertwined bedrock gorges over a multi-million-year timescale.