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Escarpment erosion and landscape evolution in southeastern Australia

Arjun M. Heimsath, John Chappell, Robert C. Finkel, Keith Fifield and Abaz Alimanovic
Escarpment erosion and landscape evolution in southeastern Australia (in Tectonics, climate, and landscape evolution, Sean D. Willett (editor), Niels Hovius (editor), Mark T. Brandon (editor) and Donald M. Fisher (editor))
Special Paper - Geological Society of America (2006) 398: 173-190


Passive margin escarpments are extensively studied around the world, and understanding their evolution continues to present one of the more compelling inter-disciplinary 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 (super 10) Be and (super 26) Al 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+ or -3 and 9+ or -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 (super 10) Be and (super 26) Al 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 (super 10) Be 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 approximately 35 m/m.y. near the escarpment base to approximately 3 m/m.y. at the escarpment crest. This order of magnitude difference in erosion rates may be due to the 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.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 398
Title: Escarpment erosion and landscape evolution in southeastern Australia
Title: Tectonics, climate, and landscape evolution
Author(s): Heimsath, Arjun M.Chappell, JohnFinkel, Robert C.Fifield, KeithAlimanovic, Abaz
Author(s): Willett, Sean D.editor
Author(s): Hovius, Nielseditor
Author(s): Brandon, Mark T.editor
Author(s): Fisher, Donald M.editor
Affiliation: Dartmouth College, Department of Earth Sciences, Hanover, NH, United States
Affiliation: University of Washington, Department of Earth and Space Sciences, Seattle, WA, United States
Pages: 173-190
Published: 2006
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Meeting name: Penrose conference on Tectonics, climate, and landscape evolution
Meeting location: Taroko Gorge, TWN, Taiwan
Meeting date: 20032003
References: 85
Accession Number: 2006-061383
Categories: Geomorphology
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch maps
S36°40'00" - S36°00'00", E149°00'00" - E150°00'00"
Secondary Affiliation: University of Cambridge, GBR, United KingdomYale University, USA, United StatesPennsylvania State University, USA, United StatesAustralian National University, AUS, AustraliaLawrence Livermore National Laboratory, USA, United StatesAustralian National University, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200635
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