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Project Description
Margins of Africa –
Continental breakup: causes & consequences
Goals
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Causes: mantle dynamics, continental
structures (nature, geometry of sutures),
plate boundary forces
Mechanisms: magma production &
emplacement, development of lithospheric
shear, extension, uplift & subsidence
Consequences: sedimentation style
(HC-systems), opening of oceanic
gateways, epeirogenic uplift, post-rift
magmatism, oceanic plateaux
Establish a model for the break-up of
Gondwana
Programme elements
1.
The Western (Atlantic) Margin of
southern Africa
2. Agulhas-Karoo Geoscience Transect: A
land-sea deep crustal seismic, MT and
petrological transect across the Agulhas
Plateau, the Agulhas Fracture Zone, the
Agulhas Bank, the Cape Fold Belt and into
the Karoo Province
3. South-East African coast geophysical and
geological program
4. Dredge sampling of the Walvis Ridge,
Meteor Rise – Shona Ridge, Madagascar
Ridge and Discovery Seamounts, South
Atlantic
Theme Coordinators:
Hartwig Frimmel –
hef@geology.uct.ac.za
Zvi Ben Avraham –
zba@geology.uct.ac.za
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Why Southern Africa?
South Africa has a long history of comprehensive geologic study, and the level of information
on both continental and marine geology is high enough to undertake focused, processorientated research on key targets. Thus, the Southern Oceans around South Africa present a
natural laboratory for developing and testing geodynamic models of passive margins and their
resource potential. Thanks to recent geophysical surveys in the Colorado Basin offshore
Argentina, the opportunity is at hand for the first time to address issues of symmetric vs.
asymmetric development of the South Atlantic volcanic margins.
At the core of this project are questions concerning the role of mantle dynamics and
lithospheric structure in controlling continental break-up, the mechanism and the eventual
consequencs of such break-up. The South Atlantic region is a global key site for studying
passive margins in general, and volcanic rifted margins in particular, and the continental
margins around southern Africa, reflecting the Mesozoic break-up of Gondwana, present
themselves as most suitable areas to address the above questions for the following reasons:
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Southern Africa is located between two inferred Mesozoic mantle plumes of different age
(around 180 versus 130 Ma);
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Continental break-up and seafloor spreading have gone to completion and the geologic
record of these processes, from pre- to post-breakup, is well-preserved both offshore and
onshore;
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Plume tracks can be traced from recent oceanic seamounts in the South Atlantic to the
continental margin, where abundant evidence exists for plume-related magmatism at the
time of break-up;
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Two principally different types of margin define the boundaries of today’s southern
African sub-continent: The margins change in character from north to south, with a
hotspot association in the area of the Walvis-Ridge, followed to the south by typical
volcanic rifted margins with seaward-dipping reflector sequences and large sedimentary
basins with significant hydrocarbon resources and, and still further south and east, a
transition to a non-volcanic sheared margin;
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The two types of margin are separated by a major crustal discontinuity, the Agulhas
fracture zone, which forms one of the world’s largest transform faults;
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The continental crust consists of an Archean (>3 Ga) cratonic core that is surrounded by a
number of progressively younger tectonic belts. This offers a unique opportunity to study
not only the origin and growth of continents but also the different types of sutures that can
weld together continental and oceanic crustal blocks and, in turn, might influence the
nature and position of subsequent continental break-up.
Of particular importance is the geodynamic influence of inherited lithospheric structures, i.e.
the arrangement of old cratons separated by younger fold belts. We need to explore the
influence of the continental mosaic structure on the geodynamics of break-up, particularly in
terms of the development of pre-, syn- and post-rift sedimentary basins with their potential for
hydrocarbon resources. The catastrophic magmatism associated with volcanic rifted margins
has a strong environmental impact (mass extinctions) and investigations of the extent and
origin of breakup-related magmatism can offer important boundary constraints for palaeoenvironmental models.
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Southern Africa and its surrounding oceans are an ideal area for studying the respective roles
of mantle dynamics and the nature and geometry of major lithospheric structures in the breakup of larger continental masses, such as Gondwana. Comparison with findings from the
former southwestern (in today’s coordinates) margin of Gondwana will assist in also
assessing the significance of plate boundary forces in the break-up of continents.
Magmatism is both a driving force for and a consequence of break-up, as are the development
of lithospheric shear, extension, and uplift as well as subsidence. Prime evidence of all of
these processes exist in and around South Africa. Understanding the development of
morphological features, such as oceanic plateaux or channels, as well as the relationship
between extension, uplift and sedimentation, has a major bearing on the welfare of mankind.
The opening or closing of oceanic gateways is one of the single most important factors
controlling climate, whereas the style of sedimentation controls the formation and distribution
of important resources, such as those of hydrocarbons. Neotectonic activity, which influences
our daily life most obviously in the form of seismic hazards, is a direct consequence of the
interplay between mantle processes and lithospheric structures following continental breakup. These aspects, which deal with the consequences for life in Africa, will be dealt with by
Working Group 3.
Scientific Goals
The main goals of this project are to establish the causes (mantle dynamics, continental
structure, plate boundary forces), mechanisms and consequences (sedimentation style,
hydrocarbon systems, opening of oceanic gateways, epeirogenic uplift, post-rift magmatism,
oceanic plateaux formation) of continental break-up. From this, a model for the break-up of
central Gondwana will be established, which might serve as model for the break-up of
supercontinents in general.
In order to achieve these goals, it is planned to:
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Conduct a series of offshore/onshore geophysical transects across different margin types,
with high resolution geophysical studies of key targets;
Recover samples by dredging of the Agulhas Plateau and hot spot-related
seamounts/aseismic ridges;
Drill the Agulhas Plateau and the Beattie Anomaly;
Supplement the seismic data with petrological, geochemical, geochronological and
structural data; and
Provide an integrated interpretation and geodynamic modelling
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For logistic reasons and in order to optimise integration with existing, currently running
programmes, the project will be carried out in four phases (Sub-projects 1 to 4) as detailed
below (Fig. 1):
Figure 1. Position of the proposed four sub-projects: 1 – Western Margin, 2 – Agulhas-Karoo Transect,
3 – Eastern Margin, 4 - Walvis Ridge, Meteor Rise–Shona Ridge and Discovery Seamounts; black lines
– existing seismic profiles, red lines – proposed new seismic profiles.
Expected Outcomes
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Training of young South African scientists in modern geoscience techniques, both in
South Africa and Germany;
Documentation of the architecture of continental margins around southern Africa;
A considerably improved understanding and quantification of asthenosphere-lithosphere
mass transfer;
Reconstruction of Mesozoic palaeogeography;
An advanced understanding of the nature of pre-breakup lithospheric structures, and in
particular of their respective roles in continental break-up.
A new integrated model for break-up of Gondwana.