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2.1 Subproject
Sub-Project 2.1
The Western (Atlantic) Margin
Participants
*Coordinators
Institutions
Names
Email addresses
GeoForschungsZentrum Potsdam
(GFZ)
M. Weber*
R. Trumbull
A. Schulze
K. Bauer
T. Vietor
H. Kaufmann
U. Wetzel
S. Sobolev
S. Niedermann
mhw@gfz-potsdam.de
bobby@gfz-potsdam.de
robert@gfz-potsdam.de
klaus@gfz-potsdam.de
tvietor@gfz-potsdam.de
charley@gfz-potsdam.de
uwetz@gfz-potsdam.de
stephan@gfz-potsdam.de
nied@gfz-potsdam.de
Bundesanstalt f.
Geowissenschaften und Rohstoffe,
Hannover (BRG)
B. Buttkus*
S. Neben
B. Schreckenberger
B. Cramer
b.buttkus@bgr.de
s.neben@bgr.de
b.schreckenberger@bgr.de
b.cramer@bgr.de
University of Cape Town
Z. Ben-Avraham
G. Smith
D. Reid
zba@geology.uct.ac.za
gcsmith@geology.uct.ac.za
dlr@geology.uct.ac.za
Council for Geoscience
C. deBeer *
J. Mahanyele
coenie@geobell.org.za
josphat@geoscience.org.za
SA Petroleum Agency
McLachlan
mclachlain@petroleumagencysa.com
Requested Funding
Total for the 5-year duration project beginning in 2004: Euros 1242500
2004
2005
2006
GFZ
169500
235500
162500
BGR
120200
135200
72600
RSA
117800
127800
101400
Total
407500
498500
336500
2007
2008
2.1 Subproject
Summary
The goal of this project is to achieve a better understanding of the
mechanisms that cause continentals to break-up and the to track
subsequent development of passive margins in the South Atlantic
large igneous province (LIP). The focus will be the ca. 2000 km long
volcanic rifted margin of western South Africa and Namibia, and will
involve marine and terrestrial geophysical, geological and petrologic
studies. The project will employ state of the art geoscience
methodology to document and interpret the record of continental
break-up and formation of the Atlantic basins, including thermal
conditions, magma genesis, tectonics and sedimentation. The
program will generate geodynamic models of break-up and identify
the key parameters controlling passive margin development. These
results will provide a solid base for evaluation and assessment of the
hydrocarbon potential, which is of major importance to South Africa
and other regional SADC states. Also, our marine research activities
are important to fulfill conditions for defining the extent of exclusive
economic zones.
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2.1 Subproject
Scientific Background
The western margin of southern Africa is a classic example of continental breakup associated
with high heat- and magma-flux due to a mantle plume. The paired hotspot tracks of the
Walvis Ridge and Rio Grande Rise are direct evidence of plume-related volcanism offshore
coeval with formation of the South Atlantic large igneous province (LIP). Geochemical and
thermal modelling studies have demonstrated an active role of a mantle plume in forming the
Paraná-Etendeka flood basalts and intrusive complexes in Namibia and Brazil (Turner et al.,
1996; Trumbull et al., 2003). In addition, deep-crustal seismic images off the Namibian
margin near the Walvis Ridge show up to 20 km- thick layers of high-velocity lower crust at
the continent-ocean boundary that are interpreted as plume-related high-Mg gabbros (Bauer et
al., 2000; Trumbull et al., 2002).
The role of a plume in the breakup process itself remains controversial, since South Atlantic
rifting started some 2000 km south of the inferred hotspot focus, in an area that lacks a strong
magmatic character, and from there progressed northward towards the Walvis Ridge
(Gladczenko et al., 1997). Seaward-dipping reflector sequences (SDRS), interpreted to
represent flood basalts deposited on the stretched and subsiding continental shelf, are
characteristic of the western south Atlantic margin and its South American counterpart. The
SDRS are well developed over the entire 2000 km extent of the margins south of the Walvis
Ridge-Rio Grande Rise hotspot tracks (Fig. 2.1.1) and therefore far beyond the expected
influence of even a large mantle plume. Possible explanations for such a long, linear basaltic
province may be channeling of upwelling plume material into a “trough” of thin lithosphere
established during the proto-Atlantic formation, or a non-plume model of enhanced upper
mantle convection due to temperature contrasts across the break in continental lithosphere
(Boutilier and Keen, 1999). Furthermore, the arrangement of pre-rift and rift-related basins
along the margin and the pattern of tectonics, as revealed by mafic dike swarms suggest that
breakup was strongly influenced by inherited structures of the pre-Gondwana lithosphere
(Reeves, 2000). Therefore, central issues for this subproject are the interplay of plume-driven
and “normal” rift to MOR magmatism, regional tectonics and pre-Gondwana inherited
structures in the continental breakup on the western margin.
The availability of industry geophysical data and well logs (South African Petroleum
Agency), and the extensive datasets of the German proponents, BGR, AWI and GFZ, will
permit a thorough assessment of the SDRS in terms of volume of magmas produced and the
timing of magmatism with respect to the sedimentation and subsidence record. A vital
component is the recent acquisition of deep seismic reflection and refraction data on the South
African margin by the BGR and GFZ in April-June, 2003. This work completes a set of five
onshore-offshore seismic traverses across the African continental margin (Fig. 2.1.1), and
with this dataset, the Namibia – South Africa margin will be the second of only two locations
worldwide (after W. Greenland: Nielsen et al., 2002) where a study of lithospheric properties
and breakup processes at varying distance from a mantle plume can be undertaken. This is a
second major aspect of the work proposed here, and the results will provide a basis for new
geodynamic models of mantle melting and magma emplacement, coupled with lithospheric
extension and basin development. These models will also need to take into account the
geodynamic influence of inherited structures for pre-, syn- and post-rift basin development.
The western African margin and the conjugate margin of South America possess considerable
hydrocarbon resources and potential. While available data reveal a general similarity between
South American and West African marginal basins with respect to depositional sequences and
source rock facies, asymmetric rifting resulted in different burial histories and major
differences in oil occurrence and composition. These differences need to be quantified by new
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85
2.1 Subproject
studies of basin modelling and petroleum systems analysis, and interpreted in terms of the
underlying influences of magmatism, tectonics and sedimentation.
Seaward Dipping Reflector Sequences (SDRS)
Continental flood basalts and regions of thick oceanic crust
ibia
Nam
Rio
is e
al
W
s
vi
R
ge
id
A
RS
Gr
an
de
R
BGR GFZ
survey 2003
BGR survey
2004/2005?
Figure 2.1.1. Conjugate volcanic rifted margins of the South Atlantic and approximate areas covered by seismic data from
BGR and joint BGR-GFZ sources (industry seismic coverage is extensive but not shown).
Key references
Bauer, K., Neben, S., Schreckenberger, B., Emmermann, R., Hinz, K., Fechner, N., Gohl, K.,
Schulze, A., Trumbull, R.B., Weber, K. (2000) Deep structure of the Namibia continental
margin as derived from integrated geophysical studies – the MAMBA experiment.
Journal of Geophysical Research. 105, 25829-25853.
Bauer, K., Schulze, A., Ryberg, T., Sobolev, S.V., Weber, M.H. (2003) Classification of
lithology from seismic tomography: a case study from the Messum igneous complex,
Namibia. Journal of Geophysical Research, 108, 2152, doi: 10.1029/2001JB001073.
Boutilier, R.R., Keen, C.E. (1999) Small-scale convection and divergent plate boundaries. J.
Geophy. Res., 104, 7389-7403.
Gladczenko, T.P., Hinz, K., Eldholm, O., Meyer, H., Neben, S., Skogseid, J. (1997). South
Atlantic volcanic margins. J. Geol. Soc. London, 154, 465-470.
Hinz, K., Neben, S., Schreckenberger, B., Roeser, H.A., Block, M., Goncalves-de-Souza, K.,
Meyer, H. (1999) The Argentine continental margin north of 48 degrees S; sedimentary
successions, volcanic activity during breakup. Marine and Petroleum Geology, 16, 1-25.
Nielsen, T.K., Larsen, H.C., Hopper, J.R.(2002) Contrasting rifted margin styles south of
Greenland: implications for mantle plume dynamics. Earth Planet. Science Letters, 200,
271-286.
Reeves, C.V. (2000) The geophysical mapping of Mesozoic dyke swarms in southern Africa
and their origin in the disruption of Gondwana, Journal of African Earth Sciences, 30,
499-513.
2.1 Subproject
Trumbull, R.B., Sobolev, S.V., Bauer, K. (2002) Petrophysical modeling of high seismic
velocity crust at the Namibian volcanic margin. In: Menzies, M.A., Klemperer, S.L.,
Ebinger, C.J. and Baker, J. (Eds.) Volcanic Rifted Margins GSA Special Paper 362, 221230.
Trumbull, R. B. Bühn, B., Romer, R.L, Volker, F. (2003) The petrology of basanite-tephrite
intrusions in the Erongo complex and implications for a plume source of Cretaceous
alkaline complexes in Namibia. Journal of Petrology, 44, 93-112
Turner, S.P., Hawkesworth, C.J., Gallagher, K., Stewart, K, Peate, D.W., Mantovani, M.S.M.
(1996). Mantle plumes, flood basalts and thermal models for melt generation beneath
continents: assessment of a conductive heating model and application to the Paraná.
Journal of Geophysical Research 101, 11503-11518.
Key questions





What factors determine passive versus active rifting modes along the South Atlantic
margin, what role can be ascribed to deep-mantle (plume) vs. shallow-mantle convection
(edge effects)?
What controls the abruptness of the continent-ocean boundary/tranision, is it the preexisting lithospheric structure or the interplay of magmatism and extension rate?
How does rift-related magmatism affect the thermal and subsidence history of the
developing margin and its hydrocarbon potential?
How does the asthenospheric and lithospheric temperature evolve through time during
margin development pre-, syn- and post-rifting and what are the consequences for
extensional deformation and subsidence?
What aspects of margin development and hydrocarbon systems are symmetric or
asymmetric across the South Atlantic and what factors control any differences?
Scientific Goals







Use high-resolution seismic records and onshore geologic studies to quantify the timing,
rate and volumes of mafic magmas intruded or extruded along the margin at different
distances from the Walvis Ridge / Tristan plume trace.
Develop geochemical and petrologic estimates of the relative contribution to magmas of
asthenospheric and lithospheric mantle sources in breakup-related mafic magmas erupted
in proximal (Namibia) and distal (South Africa) positions relative to the plume.
Define the age and geochemical nature of submerged volcanic rocks in offshore basins
from laboratory studies of drill samples.
Better resolve the magnetic anomaly patterns off the rifted margin and interpret their age
and geometric significance in terms of rift development.
Define the nature of the continent-ocean boundary or transition (COB/COT) along the
volcanic margin far from influence of the Tristan plume and compare this with existing
models from near the plume trace.
Map the continuation of major continental sutures offshore and determine their
relationship to the Atlantic rift. Emphasis is on new data from the South African Gariep,
Namaqua-Natal and Cape Fold Belts. Comparisons will be made with the Damara Belt in
Namibia.
Estimate the sedimentation volumes and rates in offshore basins and their variation with
time from rift to drift stages.
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2.1 Subproject


Perform basin modeling and petroleum systems analysis that considers the interplay of
magmatism, extension and sedimentation rates on the passive margin.
Determine similarities and differences in key features of the South Africa - Argentina
conjugate margins and develop interpretive models to explain their symmetric and
asymmetric aspects.
Methodical approach/Techniques
Geophysical mapping of the continent-ocean boundary
Critical information needed for understanding the continent-ocean transition are crustal
thicknesses and seismic velocity data complemented by magnetic and gravity. We propose an
integrated study of wide-angle seismic and potential field data from two traverses near 30°S
that were acquired by BGR and GFZ teams in April-June 2003. The study will also involve
data from an earlier traverse in southern Namibia near the Orange River that will be processed
by AWI (Fig. 2.1.1). Emphasis of the study will be to determine the nature of the COB/COT
and the extent of high-velocity material (Vp> 7.2 km/s) in the lower crust, as well as the cause
of the magnetic G anomaly and gravity edge effect.
Other targets relate to the depth continuation of major crustal features such as the suture
between the Namaqua-Natal and Gariep Belts, and the Beattie magnetic anomaly. A further
target is to image the deep structure of breakup-related magmatic intrusions, exemplified by
the Koegelfontein ring complex.
Seaward-dipping reflector sequences, volume and rates of magmatism
The spatial and temporal variability of offshore igneous structures, in particular the SDRS,
will be determined on a regional scale by 3D-modelling of existing seismic and magnetic
data, calibrated by well intersections where possible (Kudu gas field and exploratory wells).
Emphasis will be placed on distinguishing volcanic–sedimentary stratigraphy and facies
changes along and across the margins. Interpretation of magnetic data from a deep-tow
magnetometer will constrain the timing and episodicity of basalt emplacement in the SDRS
wedges from their polarity of magnetization. The database used for this study is a
combination of existing industry and academic sources, and data from the BGR and GFZ
experiment in April-June 2003. The new data includes marine MCS, gravity and magnetic
data from 30°-38°S as well as two on/offshore wide-angle traverses near 30°S.
The study will be complemented by new data from samples of volcanic rock intersected by
exploration wells in the Orange Basin (cooperation with SA Petroleum Agency). These will
be studied to constrain ages and compositions of the lavas erupted.
Sedimentary basins and hydrocarbon potential
The available and newly acquired MCS data coupled with well data will allow mapping and
correlations of pre- and synrift basins and grabens on the passive margin and estimation of
sedimentation rates and subsidence. This information, coupled with the data and constraints
on magmatism from studies of the seaward-dipping reflector sequences will provide the input
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2.1 Subproject
to basin models. Wherever possible, data from onshore sedimentation and denudation studies
will be integrated.
The techniques of petroleum systems analysis will be applied to classify hydrocarbon
occurrences along the South African margin and its conjugate in Argentina. Integration with
basin models will allow reconstruction of the timing of hydrocarbon generation and trapping,
and key mechanisms that control the oil or gas potential of different segments of the margins.
Interplay of tectonics and magmatism: timing and emplacement mechanisms
The pattern of dike swarms and intrusive bodies provide a record of crustal extension and
magma emplacement. The approach we propose to use is a combination of remote-sensing
evaluation and field studies to determine the distribution and orientation of dike swarms along
the margin, with laboratory studies in key areas to determine the age of emplacement and to
identify magma types and sources (geochemical and isotopic analysis). Two areas of
emphasis will be: the regional mafic dike swarms to provide a large-scale picture of extension
and magmatism; and detailed study of the Koegelfontein complex which was crossed by the
2003 offshore/onshore seismic traverses in South Africa.
Characterizing primary mantle melts and plume-lithosphere sources
Progress in understanding the role of mantle plumes in breakup-related magmatism has been
slow because primary magmas are rare and the competing effects of lithospheric vs. crustal
processes have not been resolved. We propose laboratory studies of noble gas isotope
composition (He, Ne, Ar) from olivine separates in magmas erupted along the western margin
to discriminate plume and lithospheric mantle sources. We will also investigate melt
inclusions in these olivines to determine the composition of primary melts and estimate the
physical conditions of melting in the mantle. The study will also determine concentrations of
volatile elements in the magma (Cl, S, CO2), which are needed for assessing the climatic
impact of large igneous provinces.
Petrophysical and thermo-mechanical modeling
Petrophysical modelling will provide a lithologic interpretation of the seismic velocity
structures obtained in the wide-angle geophysical experiments. Work will involve calculating
thermodynamically stable mineral assemblages for varying P-T conditions and bulk chemical
compositions, and computing the densities and seismic velocities of the material for
comparison with measured values. With integrated thermo-mechanical modeling constrained
by geologic and geophysical data we can link breakup processes acting within the lithosphere
(extension, convection, melting) and those acting at the earth’s surface (uplift, erosion,
subsidence, sedimentation). Crustal deformation and vertical movement of the earth’s surface
will be treated by finite element modeling on a lithospheric scale whereas surface processes of
erosion and sedimentation will be simulated by basin-scale models.
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2.1 Subproject
Expected outcomes/deliverables





Provide new 3-D images and models of the continent-ocean boundary/transition at the
South African margin with emphasis on basins.
Reveal the physical properties and structures of the deep crust and upper mantle and
evaluate the extent of plume influence in the breakup process along the margin at different
distances from the Walvis Ridge.
Provide detailed stratigraphic and structural data to evaluate basin development during
pre-, syn- and post-breakup stages, and integrate these with hydrocarbon systems analysis.
Estimate the total volume of magmas produced along the volcanic margin, distinguish
compositions and sources of magmas erupted and their variations with time and space on
the margin.
Develop and test conceptual scenarios for margin development thermo-mechanical and
petrochemical modeling.
Estimated Funding Requirements
Overview
The GFZ Potsdam and the BGR in Hannover bore the costs of the geophysical experiments
on- an offshore South Africa in 2003 and will, in followup, commit in-house staff resources
for data preparation and initial processing. In 2003 the GFZ also provides funding for training
a researcher from the Council of Geoscience in seismic interpretation of the new data.
Additional resources are needed to carry out the geophysical interpretation and modelling
(BGR and GFZ), for the petroleum systems analysis (BGR), and for the geochemicalpetrologic studies of magmatic rocks (GFZ). Finally, travel expenses are solicited for annual
progress meetings within the project, and for presentations of results at international
congresses.
Manpower
(salaries based on German civil service scale (BAT))
Geophysics and modelling
1 post-doc for 2 years
marine seismics: processing, modelling and interpretation
Administrative:
BGR
Salary group:
BAT IIa (€ 4800/month)
Duration:
01/2004 – 12/2005
€ 115,200
1 post-doc for 3 years
gravity/magnetics processing and interpretation; third year for synthesis and
modelling of all geophysics data sets
Administrative:
BGR
Salary group:
BAT IIa (€ 4800/month)
Duration:
01/2004 – 12/2006
€ 172,800
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2.1 Subproject
2 PhD students for 3 years:
marine data interpretation and basin modelling
Administrative:
RSA (Smith, UCT)
Salary group:
BAT IIa/2 (€ 2400/month)
Duration:
01/2004 – 12/2006
€ 172,800
1 PhD student for 3 years:
onshore seismic data processing, modelling and interpretation
Administrative:
GFZ
Salary group:
BAT IIa-O/2 (€ 2125/month)
Duration:
01/2004 – 12/2006
€ 76,500
1 MSc student for 2 years:
gravity and magnetic survey of Koegelfontein complex
Administrative:
RSA
Salary group:
€ 1200/month
Duration:
01/2004 – 12/2005
€ 28.800
1 post-doc for 2 years:
thermomechanical modelling of breakup and lithospheric evolution of
western margin
Administrative:
GFZ
Salary group:
BAT IIa-O (€ 4250/month)
Duration:
01/2005 – 12/2006
€ 102,000
Petrology/ Geology
1 PhD student for 3 years
studies of mantle sources and primary melts (He-isotopes, melt inclusions)
Administrative:
GFZ
Salary group:
BAT IIa-O/2 (€ 2125/month)
Duration:
01/2004 – 12/2006
€ 76,500
1 PhD student for 3 years
petrology and origin of Koegelfontein complex and related dykes
Administration
RSA
Salary group:
BAT IIa/2 (€ 2400/month)
Duration:
01/2004 – 12/2006
€ 86,400
1 PhD student for 3 years:
geochemistry and geochronology of offshore volcanics from drillcores
Administrative:
GFZ
Salary group:
BAT IIa-O/2 (€ 2125/month)
Duration:
01/2004 – 12/2006
€ 76,500
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2.1 Subproject
1 post-doc for 2 years:
remote sensing and GIS analysis and support
Administrative:
GFZ
Salary group:
BAT IIa-O (€ 4250/month)
Duration:
01/2004 – 12/2005
€ 102,000
total manpower:
=========
€1,009,500
Consumables
Laboratory equipment for sample preparation and analyses (RSA)
Duration:
07/2004 – 12/2006
€
10,000
Laboratory equipment for sample preparation and analyses (GFZ)
Duration:
07/2004 – 12/2006
€
10,000
Data storage media and equipment for geophysical analysis (BGR)
Duration:
01/2004 – 12/2006
€
10,000
Costs for Ar-Ar age dating (GFZ)
Duration:
07/2004 – 12/2006
total consumables:
€ 20,000
=========
€ 50,000
Travel
Field work
Sampling and geology studies, Koegelfontein complex
Administrative:
GFZ+RSA
Duration:
03/2004 – 06/2004
03/2005 – 06/2005
subtotal:
€ 7,000
€ 7,000
€ 14,000
Gravity and magnetic survey, Koegelfontein complex
Administrative:
RSA
Duration:
03/2004 – 06/2004
€
Sampling and geology studies, mafic dyke swarms and primary magmas
Administrative:
GFZ+RSA
Duration:
03/2004 – 06/2004
03/2005 – 06/2005
subtotal:
€ 7,000
€ 7,000
€ 14,000
5,000
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2.1 Subproject
Congresses and workshops
Participation of project scientists at scientific congresses, GFZ
Basis: 1 international (2000 €), 1 national (1000 €) per year for 5 scientists
Time frame:
2005
€
2006
€
subtotal:
€
15,000
15,000
30,000
Participation of project scientists at scientific congresses, BGR
Basis: 1 international (2000 €), 1 national (1000 €) per year for 5 scientists
Time frame:
2005
€
2006
€
subtotal:
€
15,000
15,000
30,000
Participation of project scientists at scientific congresses, RSA
Basis: 1 international (2000 €), 1 national (1000 €) per year for 5 scientists
Time frame:
2005
€
2006
€
subtotal:
€
15,000
15,000
30,000
Annual project workshops, alternately in Germany and South Africa
Administrative:
GFZ
Basis: 10 participating scientists, 1 week duration
Time frame:
2004
2005
2006
subtotal:
20,000
20,000
20,000
60,000
total travel:
€
€
€
€
=========
€ 183,000
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2.1 Subproject
Work plan
2004
Month 0
Geophysics and
Basin modelling
Seismic processing
and interpretation
Grav. Mag. processing
and interpretation
Integration, modelling
Assessment of SDR
volcanic volumes
Onshore grav./mag.
survey, Koegelfontein
Basin models,
petroleum system
analysis
Petrology/geology
Koegelfontein, dykes
Field sampling
Geochemistry and
interpretation
Offshore volcanics
Geochemistry and age
dating
Mantle sources,
primary melts
Field sampling
He-isotope and melt
inclusion analyses
Synergy/integration
Remote sensing, GIS
Thermo-mechanical
modelling
Workshops
6
2005
12
18
2006
24
30
2007
36
42
48
2008
54
60