ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

PETER VAN DER BEEK\ MIKE SUMMERFIELD2, JEAN BRAUN l
& ROD BROWN 3
Modelling large-scale long-term landscape evolution
across the eastern margin of South Africa
1 Research School of Earth Sciences, Australian National University,
Canberra ACT 0200, Australia
2 Department of Geography, University of Edinburgh,
Edinburgh ER8 9XP, United Kingdom
3 Victorian Institute of Earth and Planetary Sciences,
La Trobe University, Bundoora VIC 3083, Australia
The eastern margin of South Africa is a classical area for
studying the morphological evolution of high-elevation rifted
continental margins. Traditionally, the evolution of the
margin has been reconstructed by correlating onshore erosion
surface remnants with offshore sedimentary sequence
boundaries. Later, attempts have been made to date recognised
erosion surfaces using weathering deposits, as well
as to derive denudation histories from offshore sedimentation
rates. All these approaches, however, faced serious
problems of either correlation, quantitative dating control
or demarcation of sediment source areas.
Recently, two of us (Rb & Ms) have collected a large
amount of apatite fission track data from the Lesotho highlands
and the Natal coastal area, in order to better constrain
the denudation history of the south-eastern African
margin. These data provide evidence for substantial post­
Jurassic (i.e, post-break-up) cooling and denudation, both
to the west and to the east of the Lesotho highlands crest.
Seaward of the escarpment, >4 (and possibly up to 7) km
of crustal section have been removed since -100-80 Ma.
Inland of the escarpment, the data indicate -2 km of denudation
since -80-60 Ma. The data are in partial disagreement
with traditional models of landscape development in
eastern South Africa. For instance, the absence of a younging
trend of fission track ages from the coastline towards
the escarpment is inconsistent with a classical model of
escarpment retreat. Also, the fission track ages inland do
not corroborate the ages previously assigned to specific
erosion surfaces.
We employ a numerical surface processes model in order
to quantitatively assess the controls on landscape evolution
and denudation history of the margin. The model adopts
two main types of processes: «short-range» hill-slope processes
(including weathering, mass wasting, slope wash and
soil creep) and «long-range» fluvial processes. Hill-slope
processes are modelled by a simple linear diffusion law;
fluvial transport is controlled by the carrying capacity of rivers
and an erosion length scale, which is a measure of the
«erodibility» of the substratum and is included to model
supply-limited behaviour. We do not aim to reproduce
exactly the observed morphology of the south-east African
margin. Rather, we aim to constrain the relative importance
of factors such as antecedent topography, lithological
control, evolution of inland drainage and flexural rigidity
of the lithosphere on landscape evolution.
The model tracks the evolution of topography and the denudation
history for each point in the grid. From the modelled
denudation histories we predict the pattern of fission
track ages across the margin. Modelling results are
compared with the present-day margin morphology (eg.,
position and elevation of the top of the escarpment, escarpment
slope erc.), as well as with the amounts of denudation
inferred from the exposed stratigraphy and the fission
track data. The predicted amount of post-break-up isostatic
uplift, in response to erosion of the margin, is compared
to the elevation of remnants of uplifted Cretaceous-Palaeogene
marine sediments within the coastal zone.
Modelling results indicate that the initial (pre-break-up)
morphology of the area exerts a key control on the subsequent
evolution of the margin. A model in which a pre-existing
drainage divide is located 100-150 km W of the present-day
coastline gives results that compare best with the
observations. It predicts rapid denudation of the entire
area seaward of the initial drainage divide during the first
30-50 My after continental break-up, consistent with the
denudation histories inferred from the fission track data.
Additional first-order controls appear to be exerted by the
flexural rigidity of the lithosphere, which directs the
amount and distribution of isostatic rebound, and the onset
of inland (westward draining) drainage. Best-fitting
models are characterised by remarkably low flexural rigidities:
they have equivalent elastic thicknesses of the lithosphere
on the order of 10 km, which is substantially smaller
than elastic thicknesses inferred from geophysical studies.
Denudation from the top of the Lesotho highlands
appears to have been initiated by an inland drop in base level
that occurred at around 80-90 Ma. Although the regional
stratigraphy (consisting of a cap of resistant basalts
overlying softer Karroo sediments, which in turn overly the
hard crystalline basement) has strongly influenced the
morphology in detail, the models suggest lithological control
on a regional scale to be less important.
W. VAN HUELE, F. PATTYN & H. DECLEIR
Glacial valley form revised
Department of Geography, Vrije Universiteit Brussel, Pleinlaan 2,
B-I050 Brussel, Belgium
A common tool to describe the cross profile of a glacial
valley is a curve fitting analyses by means of a parabolic or
power-law equation. Although the parabolic equation offers
a unique and unbiased solution, it is not able to describe
the valley form. On the other hand, the power law
equation (y = a x b) can describe the valley form through its
exponentb but is not suitable as a curve fitting function
(see discussion by Harbor & Wheeler, 1992).
Pattyn & Van Huele proposed a general power law: y - Yo
= a Ix - xol b taking into account the determination of the
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