93
2.1 Subproject
Sub-Project 2.1
The Western (Atlantic) Margin
Participants *Coordinators
Institutions / Names / Email addressesGeoForschungsZentrum Potsdam (GFZ) / M. Weber*
R. Trumbull
A. Schulze
K. Bauer
T. Vietor
H. Kaufmann
U. Wetzel
S. Sobolev
S. Niedermann
/
Bundesanstalt f. Geowissenschaften und Rohstoffe, Hannover (BRG) / B. Buttkus*
S. Neben
B. Schreckenberger
B. Cramer /
University of Cape Town / Z. Ben-Avraham
G. Smith
D. Reid
/
Council for Geoscience / C. deBeer *
J. Mahanyele
/
SA Petroleum Agency / McLachlan /
Requested Funding
Total for the 5-year duration project beginning in 2004: Euros 1242500
2004
/2005
/2006
/2007
/2008
GFZ
/169500
/235500
/162500
/ /BGR
/120200
/135200
/72600
/ /RSA
/117800
/127800
/101400
/ /Total
/407500
/498500
/336500
/ /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.
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 studies of basin modelling and petroleum systems analysis, and interpreted in terms of the underlying influences of magmatism, tectonics and sedimentation.
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.
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, 221-230.
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 pre-existing 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.
· 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 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.