Geomorphic and cosmogenic nuclide constraints on escarpment evolution in an intraplate setting, Darling Escarpment, Western Australia

Abstract

The ~900km long Darling Scarp in Western Australia is one of the most prominent linear topographic features on Earth. Despite the presence of over-steepened reaches in all westerly flowing streams crossing the scarp, and significant seismic activity within 100km of the scarp, there is no historical seismicity and no reported evidence for Quaternary tectonic displacements on the underlying Darling Fault. Consequently, it is unclear whether the scarp is a rapidly evolving landform responding to recent tectonic and/or climatic forcing or a more slowly evolving landform. In order to quantify late Quaternary rates of erosion and scarp relief processes, we obtained measurements of the cosmic-ray produced nuclide beryllium-10 (10Be) from outcropping bedrock surfaces along the scarp summit and face, in valley floors, and at stream knickpoints. Erosion rates of bedrock outcrops along the scarp summit surface range from 0·5 to 4·0mMyr-1. These are in the same range as erosion rates of 2·1 to 3·6mMyr-1 on the scarp face and similar to river incision rates of 2·6 to 11·0mMyr-1 from valley floor bedrock straths, indicating that the Darling Scarp is a slowly eroding 'steady state' landform, without any significant contemporary relief production over the last several 100kyr and possibly several million years. Knickpoint retreat rates determined from 10Be concentrations at the bases of two knickpoints on small streams incised into the scarp are 36 and 46mMyr-1. If these erosion rates were sustained over longer timescales, then associated knickpoints may have initiated in the mid-Tertiary to early Neogene, consistent with early-mid Tertiary marginal uplift. Ongoing maintenance of stream disequilibrium longitudinal profiles is consistent with slow, regional base level lowering associated with recently proposed continental-scale tilting, as opposed to differential uplift along discrete faults. Cosmogenic 10Be analysis provides a useful tool for interpreting the palaeoseismic history of intraplate near-fault landforms over 105 to 106 years. Show more