Contemporary versus long-term denudation along a passive plate margin: the role of extreme events.

Short-term (contemporary) and long-term denudation rates were determined for the Blue Mountains Plateau in the Western Sydney Basin, Australia, to explore the role of extreme events (wildfires and catastrophic floods) in landscape denudation along a passive plate margin. Contemporary denudation rates were reconstructed using 40 years of river sediment load data from the Nattai catchment in the south-west of the basin, combined with an analysis of hillslope erosion following recent wildfires. Long-term denudation rates (10 kyr-10 Myr) were determined from terrestrial cosmogenic nuclides, apatite fission track thermochronology and post-basalt flow valley incision. Contemporary denudation rates average several times lower than the long-term average (5(.)5 +/- 4 rum kyr(-1) versus 21(.)5 +/- 7 tmm kyr(-1)). Erosion of sediment following wildfires accounts for only a small proportion (5%) of the contemporary rate. Most post-fire sediment is stored on the lower slopes and valley floor, with the amount transported to the river network dependent on rainfall-run-off conditions within the first few years following the fire. Historical catastrophic floods account for a much larger proportion (35%) of the contemporary erosion rate, and highlight the importance of these events in reworking stored material. Evidence for palaeofloods much larger than those experienced over the past 200 years suggests even greater sediment export potential. Mass movement on hillslopes along valleys incised into softer lithology appears to be a dominant erosion process that supplies substantial volumes of material to the valley floor. It is possible that a combination of infrequent mass movement events and high fluvial discharge could account for a significant proportion of the discrepancy between the contemporary and long-term denudation rates. © 2007, Wiley-Blackwell. The definitive version is available at www3.interscience.wiley.co