HR: 16:45h
AN: H52E-04    [PDF]
TI: Escarpment Erosion and Landscape Evolution in Southeastern Australia
AU: * Heimsath, A
EM: Arjun.Heimsath@Dartmouth.edu
AF: Department of Earth Sciences, Dartmouth College, Hanover, NH 03755 United States
AU: Chappell, J
EM: john.chappell@anu.edu.au
AF: RSES, Australian National University, Canberra, ACT 0200 Australia
AB: Passive margin escarpments are extensively studies around the world and understanding their evolution continues to present one of the more compelling interdisciplinary challenges tackled by Earth scientists today. Escarpments typically separate regions with different climatic and tectonic histories and the inferred rapid rates of escarpment retreat have been at odds with actual measurements of land surface exhumation and erosion. In this paper we present results from extensive cosmogenic 10Be and 26Al analyses across the escarpment of southeastern Australia to quantify the erosional processes evolving the highland, lowland, and scarp face landscapes. We document new relationships between soil production rates and soil thicknesses for the highland and lowland landscapes and compare these soil production functions with those published in two earlier studies from the highlands and at the base of the escarpment. Both new functions define exponential declines of soil production rates with increasing soil depths with inferred intercepts of 65 and 42 m/Ma for the highland and lowland sites, respectively, and slopes of -0.02. Exposed bedrock at both sites erodes more slowly than the maximum soil production rates, at 22 +/- 3 and 9 +/- 2 m/Ma, respectively, thus suggesting a "humped" soil production function. We suggest, instead, that lithologic variations set the emergence of bedrock, which evolves into tors, found extensively across the highlands. Compared to soil production rates from previous work using 10Be and 26Al measurements from two different sites, these results show remarkable agreement and specifically quantify a soil production function for the region with an intercept of 53 m/Ma and a slope of -0.02. Erosion rates determined from 10Be concentrations from outcropping tors, bedrock and saprolite from a main spur ridge perpendicular to the escarpment, and sediments from first and zero order catchments draining the main ridges, show a clear linear decline with elevation, from about 35 m/Ma near the escarpment base to about 3 m/Ma at the escarpment crest. This order of magnitude difference in erosion rates may be due to increases in stream incision with distance downslope on the escarpment, or to decreases in precipitation with elevation, neither of which we quantify here. The rates do agree, in general, with our soil production functions, suggesting that the biogenic processes actively eroding soil mantled landscapes are shaping the evolution of the escarpment despite our observations of block fall and debris flow processes across the steep regions near the scarp crest. Our results support recent results from studies using apatite fission track thermochronology, suggesting that the escarpment is relatively stable after having retreated rapidly immediately following rifting. Differences between our rates of surface erosion caused by processes active today and the scarp retreat rates needed to place the escarpment in its present position need to be explained by future work continuing to untangle the mysteries of escarpment evolution.
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1886 Weathering (1625)
DE: 8105 Continental margins and sedimentary basins
SC: Hydrology [H]
MN: 2003 Fall Meeting