tracking 200 million years of uplift, exhumation, erosion -


22 févr. 2014 (il y a 7 années et 5 mois)

307 vue(s)

1.3 Subproject


Project 1.3

Epeirogenic history of Southern Africa: tracking 200 Ma of uplift,

exhumation, erosion and influence on climate


* Coordinator(s)



Email address

y of Cape Town


G. Viola *

M. de Wit



Nuclear Waste Systems


M. Andreoli

GFZ Potsdom


J. Erzinger

S. Niedermann *



Requested Funding

Total for the 5
year duration project beginning in 2004:








UCT and







1.3 Subproject



Africa's topography is unique in a globa
l perspective in two ways. Whereas
elevated topography of most continents can be related to horizontal forces
across compressional plate tectonic margins (e.g. Andes, Cordilleras,
Himalayas, Tibet), this is not so for Africa. Africa is surrounded mostly by

extensional plate margins in the form of spreading ridges. Yet Africa is
host to some of the world's greatest elevated regions; these are thus truely
epeiorogenic in origin, most likely related to vertical dynamic forces in the
underlying mantle. There is
, however, considerable debate about the local
origin of these highlands, including that of South Africa.

By balancing erosion and deposition on and around South Africa since
Gondwana break
up, key questions about the geodynamic uplift history of
South A
frica, its connection to mantle convection and its effect on climate
change can be monitored.

By combining low
T geochronological methods (apatite fission track
analysis, apatite U
Th/He analysis, cosmogenic isotopes and exposure
dating) with detailed fie
ld investigations we aim at addressing key question
such as when South Africa first underwent significant epeiorogenic uplift
and what the uplift rates were thereafter; a series of related subquestions

Can we quantify rates of erosion and deposition
in southern Africa?

Where, how and when did erosional products transported in river
systems end up on the continental shelves around southern Africa?

How did the paleo
drainage system evolve?

How did climate
change influence southern Africa's geomorpho
evolution (or vice versa)?

The proposed research will to link onshore and offshore processes with
rates of erosion, sediment transport and deposition. Precise dating of paloe
upliftments and base
level changes around southern Africa will be
ated by the detailed analysis of the sedimentary record of the
continental shelves (and beyond) of southern Africa. Understanding how,
where and when sediment is transported offshore hinges on a detailed
seismic and sequence stratigraphic study of selected

transects around the
South African coast. The ultimate aim will be to quantify sediment erosion
and depositional fluxes and thus link the terrestrial with marine
environment over the last 200 million years. This project can make an
important contribution
to the exploration for oil and alluvial diamonds.

1.3 Subproject


Scientific motivation and State of the Art

The importance of tectonic factors in influencing modes of long
term landscape development
both in passive margin settings and associated intraplate environment
s has been increasingly
recognized (e.g. Summerfield, 1985; Bishop, 1988; Gilchrist and Summerfield, 1994). Also,
there is a wider appreciation among geophysicists and geologists that the morphological
evolution of passive margins, in addition to their the
rmal, structural and stratigraphic
development, must be accounted for if a comprehensive understanding of their tectonic
evolution is to be achieved (Gilchrist et al., 1994; Kooi and Beaumont, 1994; van der Beek et
al., 1995; Brown et al., 2000).
topography of most continents can be directly related
to crustal shortening linked to destructive processes across active plate margins (e.g. Andes,
Cordilleras, Alps, Himalayas). In a global tectonic framework the African continent stands out
for it is mostly surrounded (> 90%) by extensional plate margins in the form of
spreading ridges and no high topography should thus shape the African plate. Yet Africa is
host to some of the world's greatest elevated regions. Southern Africa in particular,

being very
distant from the elevation of the Atlas Mountains, Africa’s only exception that can be linked
directly to active processes in the diffuse convergent margin between it and Europe, is
remarkable for the very high average topographic elevation. Th
e highlands of Southern Africa
are thus truly epeirogenic in origin, most likely related to dynamic forces in the underlying
mantle (e.g. Lithgow
Bertelloni and Silver, 1998; Gurnis et al., 2000). A further complication
is due to the fact that the relation
ship between rifting/drifting processes and relief evolution
along passive margin shoulders is still poorly known. High
elevation passive margins and their
associated major escarpments are indeed the most prominent landforms resulting from
continental brea
up in Southern Africa.

In spite of a recent strong interest in understanding these morphotectonic features (e.g. Brown
et al., 2002 and references therein), there is still a lack of adequate answers to some of the
basic phenomena observed in southern A
frica. A considerable debate about the local origin and
even age of these prominent highlands and physiographic features is still ongoing. Some
believe the highlands of South Africa to be mainly Cenozoic in age (possibly as young as 30
Ma, e.g. Burke, 1996
), specifically related to the present
day tomographically imaged
Superswell in the mantle below the region. Present
day topography would thus be a dynamic
feature formed in response to vertical stresses at the base of the southern African lithosphere
rated by flow in the lower mantle or positive buoyancy in the mid
lower mantle beneath
southern Africa. However, others are convinced that much of the topography is at least in part
inherited from pre
Cretaceous times, possibly associated with geodynamic p
accompanying the birth of Africa during the break
up of Gondwana between 120
200 Ma
(King, 1962; de Wit et al., 1988; Brown et al., 1990, 2000; Doucoure and de Wit, 2003). Some
of the elevation may even be related to earlier events such as isostat
ic uplift following rapid
deglaciation of the great continental ice sheets that covered much of central and southern
Africa between about 300 and 350 Ma (du Toit, 1936; King, 1962; Crowell, 1999) and even to
the earlier widespread Pan
African orogenesis (c
a. 500
700 Ma) during which major cratons
were welded together to form Gondwana (e.g. de Wit et al., 2001).

It is clear that a holistic approach is needed in order to address these basic questions. Only b
balancing erosion and deposition on and around so
uthern Africa since Gondwana break
key questions about the geodynamic uplift history of southern Africa, its connection to mantle
convection and its effect on climate change can be monitored.

1.3 Subproject


Scientific Goals

The ultimate aim of the research will be
to date and quantify the epeirogenic uplift of southern
Africa, to quantify sediment erosion and depositional fluxes and thus link the terrestrial with
the marine environment over the last 200 million years.
We will be able to answer questions
such as:

w have tectonic, climatic and oceanographic processes affected the geomorphological
evolution of southern Africa from the Cretaceous to the present?

How have these processes sculptured our present landscape?

How have they influenced the concentrated natu
ral resources like oil, gas, diamonds and
heavy minerals around southern Africa's continental margins?

How does mantle upwelling couple to the lithosphere uplift and exhumation?

How does mantle upwelling tie in with climate changes?

The scientific goals o
f this ambitious research proposal can be summarized in a series of
interlinked activities that, if fully and holistically addressed, will help clarify some of the
debated scientific issues.

When did South Africa first undergo significant epeirogenic upli
ft and what were the
uplift rates thereafter? Was uplift episodic or linear?

The initiation of the phase of epeirogenic uplift and its rate through time are to be
precisely determined. Without precise temporal constraints no further investigations and
ingful interpretations are possible.

Can we quantify rates of erosion and deposition in southern Africa?

By balancing erosion and deposition in and around South Africa since Gondwana break
up, key questions about the geodynamic uplift history of South Afr
ica, its connection to

convection and its effect on climate change can be monitored.

We need to link
onshore and offshore processes and rates of erosion, sediment transport and deposition.
Below southern Africa, seismic tomography has identified the

“African Superswell”, a
region in the lower mantle that some believe represents a bulge of the core
boundary (e.g. Nyblade and Robinson, 1994; Lithgow
Bertelloni and Silver, 1998; Gurnis
et al., 2000). The only other comparable lower mantle upwelli
ng is found at Hawaii.
Southern Africa therefore is the only region in the world where a real link between mantle
upwelling (and downwelling) and lithospheric topography can be observed. The proposed
scientific approach would investigate a unique natural l


did the paleo
drainage system of southern Africa evolve in response to uplift?


understanding of this aspect is crucial in linking the onshore evolution to the
offshore processes and the origin of southern Africa’s diamond and ore de


did southern Africa’s geomorphological evolution influence climate
changes and
what are the feed
back processes?

Work Plan

The above questions relate to unravelling the rates of erosion and fluxes of sediments from
the southern African cont
inent. They can be constrained by two scientific methods:

1.3 Subproject


Precise dating of paleo
upliftments and base
level changes around southern

Dating of uplift and subsequent exhumation will involve localised mapping of onshore river
systems and paleo
r terraces, together with careful sampling and application of
cosmogenic nuclide, Ar
Ar, U
Pb, and U/Th
He dating as well as fission track analysis.

Sedimentary record of the continental shelves (and beyond) of southern Africa.

Understanding how, where

and when sediment is transported offshore hinges on a detailed
seismic and sequence stratigraphic study of selected transects around the South African coast.

The ultimate aim will be to date and quantify the epeirogenic uplift of southern Africa,

sediment erosion and depositional fluxes and thus link the terrestrial with the marine
environment over the last 200 million years.


In order to carry out the outlined research in an integrated and holistic study (and within a
reasonable time f
ramework), we plan 2 PhD students and a post
doctoral scientist to be
directly involved in the research.

Because it is important that the project starts as soon as possible and benefits from the
participation of a large number of scientists, UCT has init
iated the funding for the salary of a
second post
doctoral student. Dr Kounov, the appointed scientist, will begin his research
activity at UCT in January 2004. The requested funding is also meant to cover part of his
research activity that is integratin
g closely for the present proposal.

Analytical Methods

To understand how the topography, drainage patterns and sediment source areas of the
subaerial parts of continental margins have changed over geological time scales we clearly
need information on va
riations in rates of denudation over time spans of 10

years. Our
understanding of landscape evolution is compromised by a lack of data on rates of landscape
change over appropriate geologic time scales. Traditional approaches to establishing long

denudational histories for passive margins and adjacent intraplate terrains relied on the
landward extrapolation off offshore chronosequence boundaries to erosion surface remnants,
and the use of (rarely well
) dated sedimentary deposits inland in the rar
e instances where
these are present (King, 1967). More recently these have been supplemented by the
employment of weathering deposits and duricrusts to characterize land surfaces interpreted to
be of a particular age (Partridge and Maud, 1987). Problems wi
th dating control inherent in
these models, especially where correlation criteria are limited (since erosional residuals lack
dated coeval deposits) coupled with the growing availability of information on offshore
sedimentary sequences have led to attempts

to derive denudational histories from offshore
sediment volumes deposited within known time intervals.

Thermochronologic techniques provide a robust, independent and location
specific means of
quantifying histories of crustal stripping, and in doing so e
stablish denudational histories. The
following section provides a brief description of the thermochronological analytical methods
that we will be using to date paleo
uplifments and base level changes.

1.3 Subproject


Fission Track and (U/Th)
He analyses

Fission track ana
lysis is based upon the natural, slow, but statistically constant, spontaneous
radioactive fission decay of the more abundant isotope of uranium,
U. Tracks are damage
trails in the apatite atomic lattice due to the explosive process of fission in which
two highly
charged fragments fly apart from each other, stripping electrons from atoms lying in their path.
Tracks accumulate within the crystal over time and, under suitable conditions, they may be
revealed and counted. The number of tracks per unit area
depends on the rate at which fission
occurs, the length of time during which tracks have been accumulating and the uranium
content of the crystal. For fission
track systems, there are no discrete “closure” temperatures
beyond which tracks are either preser
ved or destroyed as in other radiogenic systems. A
transition zone where tracks are essentially unstable is instead recognised

this is termed the
partial annealing zone and is defined by upper and lower temperature limits. The effective
closure of the sy
stem lies within these bounds, and is dependent on cooling rates. The partial
annealing zone for apatite lies between 60 to 120° C (Green and Duddy, 1989; Corrigan, 1993)
with a mean effective closure temperature constrained at 100 ± 10°C. Hence, apatite f
track analysis is particularly useful for evaluating low temperature thermal histories,

affecting the upper 3
4 km of the crust.

Since tracks are produced continuously, each track in a sample will have been exposed to a
different portion

of the time and temperature history of its host rock and the distribution
pattern of confined fission
track lengths is an integrated cooling history. The time taken for a
rock to pass through the partial annealing zone is reflected in the track

length di
Further, if a sequence undergoes burial and/or heating, pre
existing tracks are shortened to a
length determined by the maximum temperature and the duration of burial. At temperatures
greater than the upper limit of the partial annealing zone,
all tracks are erased and the ¨clock¨
is reset when the rock cools again through the partial annealing zone. Using the random
Monte Carlo and Genetic Algorithm approach (Gallagher, 1995), the sample age and the
length parameters are compared to those

determined through experimental annealing in
order to assess some possible T
t paths.

up in the South Atlantic occurred at about 120 Ma, preceded by a period of continental
rifting starting at about 160 Ma (Brown et al., 2000). As the South Americ
an and African
plates drifted apart, the rifted continental margins were subjected to a phase of major
denudation immediately following breakup. The results of some fission
track studies as well
as preliminary studies of some offshore basins indicate that
the bulk of the denudation of the
south west African margin occurred during the early post
rift phase (Brown et al., 1990, 2000;
Rust and Summerfield, 1990). The total amount of denudation generally decreases from 3 to 5
km in the coastal sector to less th
an 1 in the continental interior.

As with apatite fission
track dating, U/Th
He dating has been used to study tectonic processes
that cause rock cooling. However, the lower apatite He closure temperature (ca. 60

C) makes
it possible to detect and quantif
y degrees of tectonically induced cooling that are too small to
be recorded by higher temperature systems. Apatite He ages are thus strongly influenced by
perturbations in the thermal field of the shallow crust and their sensitivity is such that they can
e used to reconstruct the evolution of topography in the past. U/Th
He analysis studies on
passive margins are still very scarce and published material is not yet available for the African
margin. However, preliminary results by Viola et al. (in prep) acro
ss the Namibian sector of
the margin confirm very early denudation even as for the U/Th
He system is concerned.

1.3 Subproject


Cosmogenic isotopes and exposure dating

Cosmogenic nuclides (e.g.
Al) are generated by nuclear interactions of
tic cosmic
ray particles with target elements (such as O, Mg, Al, Si) in the
uppermost layer of the Earth’s surface (e.g. Gosse and Phillips, 2001; Niedermann, 2002). As
the cosmic ray flux and, hence, production rates decrease rapidly and approximately
ponentially with depth (half depth ~0.4 m), the concentration of a cosmogenic nuclide in a
rock provides a measure for the duration of its surface residence. Over the last decade a
wealth of applications of this dating tool have been developed, providing n
ovel insights into
various fields of geosciences, such as geomorphology, glaciology, neotectonics, climate
change research, etc.

Due to the rapid decrease of production rates with depth, the concentrations of cosmogenic
nuclides do not only depend on the
age of a geomorphic surface, but also on the rate at which
it erodes. For very old surfaces they reach an equilibrium value that is directly related to the
erosion rate. On the other hand, the cosmogenic nuclides contained in river sediment can be
used to
derive basin
wide mean erosion rates (e.g. Schaller et al., 2001).

Rates of tectonic uplift can also be quantified using cosmogenic nuclides. For example, the
surface exposure ages of fluvial terraces, in combination with their elevations above the activ
riverbed, provide a measure for the uplift rate assuming that river incision keeps pace with
uplift (e.g. Hetzel et al., 2002).

The feasibility of cosmogenic nuclides as a tool to unravel the denudation history of southern
Africa has been demonstrated
in earlier investigations (Fleming et al., 1999; Cockburn et al.,
2000; Van der Wateren and Dunai, 2001), which have shown that the rates of escarpment
retreat at both the south
west African margin and the Drakensberg are 1
2 orders of
magnitude lower than

previously suggested based on the assumption that the escarpments
originated at the continental margin during Gondwana break

Research implementation and time framework

The 2 PhD students will concentrate on two main transects stretching from the
coast to the
continent interior (Figure 1). Samples for FT and (U/Th)
He analyses will be collected at
regular intervals along the sections. In order to cover southern Africa geographically and to
integrate this new work with some already ongoing projects
(Justine Tinker: PhD project at
UCT), a first transect will be sampled from the West Coast of South Africa, across the main
escarpment to Namaqualand and Griqualand on the Archean craton at Kimberley. The second
will instead cover a transect stretching fro
m coastal Mozambique via Barberton and then
across the escarpment to Johannesburg on the Archean craton. Sampling and dating along the
proposed transects will significantly increase the already existing dataset (especially in
eastern southern Africa) and,
even more importantly, will offer for the first time the
possibility of a multiple integrated geochronological approach, whereby different dating
techniques will be applied to the same samples. A number of land

and offshore seismic
are available and

will be used to link these transects. Moreover, a vast number of borehole
samples are available and will be used to construct true, vertical crustal cooling profiles.

Selected samples along the transects will be collected for cosmogenic nuclide exposure

dating. Integration of these results with the other low
T geochronological data will provide an
extremely solid basis for a proper interpretation in a regional scheme.

1.3 Subproject


Figure 1


te Fission
Track Age
Map of Southern Africa (from Brown, 2002) and location of
suggested transects for fission
track and (U
Th)/He dating. The transect from Mossel Bay to Upington
is currently being investigated by Justine Tinker for her PhD project at UCT

1.3 Subproject


The following table summarizes a suggested time f
ramework for the research activity.







Mapping of morpho
tectonic features
along selected
transects. Sampling
of material for
exposure dating.
Focus on specific
problems such as
erosional rates a
different sites to
evaluate the influence
of tectonic and
climatic factors.

Analytical work at GFZ
for exposure ages

Write up results in
publication format

student 1

Literature review.

Familiarization with FT

Field work and sampling

along a transect

Collection of borehole
samples from the Council
for Geosciene.

Sample preparation and
mineral separation for FT
and (U/Th)
He dating. FT
analyses in Cape Town.

He and selected
cosmogenic dating

analysis and

Writing of the

student 2

Literature review.

Familiarization with FT

Field work and
sampling along a
transect West coast
Kimberley. Collection
of borehole samples
from the Co
uncil for

Sample preparation and
mineral separation for
FT and (U/Th)
dating. FT analyses in
Cape Town.

and selected
dating at GFZ

analysis and

Writing of the

Funds Requested


2 PhD students scholarships:

Salary for living in SA, 2 years: 17.000
€ p.a. each

Salary for living in Potsdam, 1 year: 20.000 € p.a. each

1 Post
Doctoral student:

Salary: 40.000 € p.a.


228.000 €

1.3 Subproject



Refurbishment of an existing fume hood for heavy
liquid use: 2000 €

Upgrading of the FT laboratory in Ca
pe Town: New microscope: 35.000 €

Hot Plate: 1.000 €


38.000 €

Research expenses (at UCT):

Irradiation fees for FT: about 110 samples: 5000 €

Heavy liquids, hot plates, glass sections and micas, standard glasses: 8.000 €

General consumables (p
aper, printer toners, postage expenses, sample bags, markers, etc.),
Basic running expenses: 11.000 €

Computers and printers for the students: 11.000 €

Sampling of 10
15 boreholes for FT and (U
Th)/He analysis: 10.000 €


45.000 €

Travel and Subs

Field work for 2 PhD students and supervisors (including vehicle rental, petrol,
accommodation and food): 13.000 €

Field work for Post
doc (s): 10.000 €

Introductory field trip for GFZ scientists: 9.000 €

Two international conferences for student
s, post
doc and supervisors: 28.000 €


60.000 €































99.000 €

78.000 €

98.000 €

58.000 €

38.000 €

Existing Infrastructure:

Sample preparation will be carried out both at the University of Cape Town and at the GFZ.
Fission track analysis will be carried out in Cape Town, where Dr. Viola set up a l
aboratory in
2001. U/Th
He and surface exposure dating will be carried out at GFZ Potsdam, where a
thermal ionization mass spectrometry (TIMS) and a noble gas laboratory are available for
these analyses. In addition, depending on the results obtained from
cosmogenic He and Ne it
may be necessary to determine cosmogenic radionuclides (
Al) in a few samples, which
will be done in collaboration with Dr. P. Kubik at ETH Zurich.

1.3 Subproject


operation with other INKABA ye AFRICA projects:

The proposed research p
rogram represents a natural integration to several others proposals of

It links very tightly with the studies aimed at a better understanding of the present mode of
mantle dynamics beneath southern Africa. The present
day lithospheric re
sponse to the
mantle dynamics, which is the goal of other projects and will be investigated using long
term geodetic and vertical GPS measurements, is a natural integration of our research. The
answer to the question posed in turn may aid better understand
ing of the core/mantle heat
flux and its possible feedback on geomagnetism.

This project will make an important contribution to the exploration for oil and alluvial
diamonds on the continental shelf and offshore sedimentary basins around southern

he project will integrate closely with the proposed seismic studies across the West and
East coasts of southern Africa.

Potential Impact on HR Development

First, the project will be important for student training. Two PhD students and a post
udent will be directly involved in the research. Every possible effort will be made to
guarantee that at least one of the students will be a non
white South African. The application
and perfection of modern geochronological techniques and tools (as those e
mployed to carry
out the research for this project) is recognized as being the necessary key to unravel any
complex tectonic evolution. The cutting
edge training component the students would benefit
from during this project (at UCT and during exchange peri
ods overseas at GFZ) is such that
they will have more chances of obtaining a position to continue research in this field at other
universities or industries.

Active cooperation is being established at the moment with the University of Western Cape,
in the

hope of exposing, attracting and involving even more students from previously
disadvantaged backgrounds in the project.

1.3 Subproject



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281, John Wiley, New York.

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term landscape development. In Process Models and Theoretical
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84, John Wiley, New York.

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using geological evidence of surface

uplift rates: the case of the African Superplume.
Geochemistry, Geophysics, Geosystems 1.

Hetzel, R., Niedermann, S., Tao, M., Stokes, S., Kubik, P.W., Ivy
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superswell. Nature 395, 269


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, 2002. Cosmic
produced noble gases in terrestrial rocks: dating tools for surface
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1.3 Subproject


Nyblade, A.A., Robinson, S.W., 1994. The African superswell. Geophysical Research Letters 21,


Partridge, T. C., and R
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merfield, M. A., 1985. Plate tectonics and landscape development on the

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and J. T. Hack, pp. 27

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