QUANTIFYING SOUTH AUSTRALIAN LANDSCAPE EVOLUTION

Quantitative estimates of erosion rates, tectonic uplift rates, mean elevation changes and magnitudes of relief generation are determined for the Flinders Ranges of South Australia. Cosmogenic nuclide (CN) 10Be determined hilltop erosion rates of 4-6 m Myr -1 are exceeded by CN-determined incision rates and catchment erosion rates of 8-12 m Myr -1, suggesting relief of the ranges is increasing. Longer-term estimates of erosion rates from mass balance calculations of the Pliocene-Pleistocene Wilkatana alluvial fans adjacent to the range front yield catchment erosion rates of 5-9 m Myr -1, supporting the hypothesis that Quaternary climatic fluxuations have increased the rates of erosion and sedimentation and in part generated the “youthful” topography characteristic of the present landscape. OSL dating of deposits at the apex of the alluvial fans places the last major climatically-driven aggradation events at ~60 ka and ~30 ka, corresponding to periods of regional climatic change. These deposits are offset by the Wilkatana Fault System (WFS), which traverses the western range front and is associated with reverse left-lateral thrusting in response to ~E-W contraction. The WFS is one of the youngest prehistoric fault systems mapped in Australia, and yields in-situ fault slip rates of 106-229 m Myr -1 and longer-term fault slip rates of >23-26 m Myr -1, relatively high for a supposedly quiescent intraplate setting. V-shaped valleys incised into broader u-shaped valleys attest to the likelihood of rapidly headward propagating knickpoints generated during reverse faulting at the range front, as observed at the Knickpoint Fault. Subtracting spatially averaged erosion rates from favored estimates of tectonic uplift rates suggests a mean elevation uplift of ~125 m since 5 Ma. Modeling suggests as much as 70% of the present catchment relief was generated over this interval. Tectonic uplift alone, while providing additional erodable material to a landscape, will not independently increase catchment erosion and range front sedimentation. Instead, intermittent periods of rapid catchment erosion and sedimentation within the Flinders Ranges has occurred during transitions from soil mantled to rock-dominated landscapes in response to climatic oscillations, superimposed on a ~5 Ma record of tectonic uplift.