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Escarpment erosion and landscape evolution in southeastern Australia

By
Arjun M. Heimsath
Arjun M. Heimsath
1
Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA
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John Chappell
John Chappell
2
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
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Robert C. Finkel
Robert C. Finkel
3
Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA
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Keith Fifield
Keith Fifield
4
Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia
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Abaz Alimanovic
Abaz Alimanovic
5
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
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Published:
January 01, 2006

Passive margin escarpments are extensively studied around the world, and understanding their evolution continues to present one of the more compelling interdisciplinary challenges tackled by earth scientists today. Escarpments reflect the morphotectonic development of passive margins and can separate regions with different climatic histories, but the inferred rapid rates of escarpment retreat have been at odds with actual measurements of land surface denudation. 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 our 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/m.y. for the highland and lowland sites, respectively, and slopes of –0.02. Exposed bedrock at both of the new sites erodes more slowly than the maximum soil production rates, at 22 ± 3 and 9 ± 2 m/m.y., respectively, thus suggesting a “humped” soil production function. We suggest that instead of a humped function, lithologic variations set the emergence of bedrock, which evolves into the tors that are found extensively across the highlands and at the crest of the escarpment by eroding more slowly than the surrounding soil-mantled landscape. 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 where soil production rates decline exponentially with increasing soil thickness, with an intercept of 53 m/m.y. 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 ∼35 m/m.y. near the escarpment base to ∼3 m/m.y. 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 low-temperature thermochronology, which suggest 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 to untangle the mysteries of escarpment evolution.

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Contents

GSA Special Papers

Tectonics, Climate, and Landscape Evolution

Sean D. Willett
Sean D. Willett
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Niels Hovius
Niels Hovius
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Mark T. Brandon
Mark T. Brandon
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Donald M. Fisher
Donald M. Fisher
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Geological Society of America
Volume
398
ISBN print:
9780813723983
Publication date:
January 01, 2006

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