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2.2 Subproject

Sub-Project 2.2

AgulhasKaroo Geoscience Transect: A landsea 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

Participants* Coordinators
Institutions / Names / Email addresses
Alfred Wegener Institute (AWI) / K. Gohl*
G.Uenzelmann-Neben /

GeoForschungs-Zentrum Potsdam
(GFZ) / O. Ritter*
U. Weckmann
M. Weber
A. Schulze
T. Ryberg /




University of Cape Town
(UCT) / H.E. Frimmel*
G. Viola
A.P. le Roex
Z. Ben-Avraham
M.J. de Wit /




Council for Geoscience / E. Stettler
C. de Beer /

Univ. Witwatersrand / S. Webb /
Univ. Port Elizabeth / R. Shone
P. Booth /

Univ. Natal / M. Watkeys /
Univ. Stellenbosch / R. Scheepers /
Petroleum Agency of SA / I. McLachlan /
Fugro Airborne Surveys / L. Ameglio /
Green’s Geophysics / R. Green / -
Requested Funding

Total for the 5-year duration project beginning in 2004: Euros 2381300

2004

/

2005

/

2006

/

2007

/

2008

AWI

/

0

/

461600

/

74600

/

72600

/

72600

GFZ

/

101000

/

618000

/

418000

/

168000

/

40500

RSA

/

62600

/

131400

/

99600

/

45800

/

15000

Totals

/

163600

/

1211000

/

592200

/

286400

/

128100

Summary

Southern Africa and its southern continental margin offer an unrivalled region, where continental accretion processes over a period of more than 3.5 billion years can be studied. Along a geoscientific transect, stretching from the offshore Agulhas Plateau across the Agulhas Fracture and the Outeniqua Basin, continuing across the Cape Fold Belt, the Namaqua-Natal Belt and into the Karoo Province and the southern Kaapvaal Craton, geophysical and geological/ geochemical data and samples will be collected in order to build a model of the evolution and crustal accretion as well as the continental break-up of this region. With such a single transect, multifold significant objectives can be addressed such as the Mesoproterozoic accretion processes along the southern margin of the Kaapvaal Craton, the extent of Pan-African inliers in the Cape Fold Belt, the extent and formation of the Cape Fold Belt, the sources for the Beattie Magnetic Anomaly and the Southern Cape Conductivity Belt, the continental/ oceanic origin of the Agulhas Plateau, the formation of the Agulhas Fracture Zone and its consequences for basin formation and uplift processes relevant for the hydrocarbon producing provinces of the Outeniqua Basin. A combined land-sea deep crustal seismic reflection and refraction survey as well as a magnetotelluric survey along the transect will provide detailed structures and constraints for physical parameters from the upper crust to the upper mantle which will be integrated with geological, petrological and geochemical analysis on rock composition, age and alteration history to form an overarching geodynamic model for the evolution of the region and its tectonic units.

Scientific Background

Continents are known to be made up of a collage of displaced terranes of various origins that accumulate or accrete when continental margins are active. Today, oceanic plateaux, which can be extinct island arcs, extinct spreading ridges, seamounts, or disrupted fragments of continental crust, are passive features that are embedded within the oceanic lithosphere. When large enough, they resist subduction, do not return to the mantle but accrete to existing continents. A prime example of such an oceanic plateau is the Agulhas Plateau situated offshore southern Africa. It can be expected that this plateau will collide with the present African continent when the southern African margin becomes active again.

Southern Africa offers an unrivalled region, where continental accretion processes over a period of more than 3.5 billion years can be studied: A composite Archean (>2.5 Ga) craton (Kaapvaal Craton) is flanked by progressively younger crust (Fig. 2.2.1), recording four main stages of continental lithosphere formation: 2.06 - 1.80 Ga (Eburnian) Kheis Belt, 1.2 - 1.0 (Grenvillian) Namaqua Natal Belt, 0.58 - 0.48 (Pan-African) Saldania Belt, and the 0.25 (Gondwanide) Cape Fold Belt. Large areas of the Namaqua-Natal Belt are covered by Mesozoic Karoo sediments, and most of the Saldania Belt is overlain by rocks of the younger Cape Fold Belt. Only a few exposures in form of basins such as the Kangoo and the Kaaimans Inlier can provide reliable data on tectonism which are vital for the understanding of accretion and collision in southern South Africa.

The eastern margin of the craton has been reworked by Mesozoic Gondwana tectonic processes. This left a sharp break, manifest at the surface by the Lebombo monocline (a priority target of Subproject 2.3, see below). The best region to study the successive accretions of continental lithosphere onto the Kaapvaal Craton since its stabilisation around 2.0 Ga is its southern margin. The proposed geophysical (seismic and magnetotelluric) transect cuts across all of the above <2.0 Ga tectonic belts and thus also at least three major continental suture zones of vastly different age. The same area also hosts two of Earth’s largest known geophysical anomalies, the Beattie Magnetic Anomaly (BMA) and the Southern Cape Conductive Belt (SCCB). Both anomalies extend for almost 1000 km in east-west direction across the whole of southern South Africa (Fig. 2.2.2). The surface expressions of these anomalies seem to coincide with the mapped boundaries of the Cape Fold Belt and the Namaqua Natal Belt. The nature of both geophysical anomalies remains enigmatic; they have been interpreted as a slice of paleo-oceanic lithosphere or alternatively as thrust zones, but neither their relative age nor their extent to depth and internal structure are known. A better understanding of their geometry and origin is essential for any meaningful reconstruction of the subsequent break-up processes.

By extending the transect offshore to the Agulhas Plateau (Fig. 2.2.3), not only can the question of continent formation, but also that of the demise of continents be addressed. Late Mesozoic basins, both onshore and offshore, formed subsequent to Gondwana break-up. Today they are important exploration targets for the petroleum industry. The nature and origin of the Agulhas Plateau are poorly known and this project promises to provide major new insights into this oceanic plateau. Finally, the southern tip of South Africa, though surrounded by passive continental margins, displays a considerable relief. This gives rise not only to spectacular landscapes, but also to the recognition that southernmost Africa is affected by active neotectonic activity. Both causes and hazards as well as the utilization potential of this neotectonic activity are not fully understood. The proposed project, in conjunction with the other Working Groups, offers an opportunity to address these issues.

Figure 2.2.1. Distribution of tectonic units around the Archaean Kaapvaal Craton in southern Africa. Large parts of the Namaqua Natal Belt and the southern segment of the Kaapvaal Craton are covered with Karoo sediments.
Fig. 2.2.2. The Beattie Magnetic Anomaly (red and magenta colours and white dots) and the Southern Cape Conductive Belt (SCCB, black and white dashed lines) are continental scale geophysical anomalies. They stretch from east to west for more than 1000 km across the whole of southern South Africa (after Harvey et al., 2001).

Fig. 2.2.3. (Top) Proposed offshore seismic transects across the Agulhas Plateau and the southern margin of the African plate. (Bottom) Proposed geophysical transects across the SCCB and the BMA. (Yellow: onshore/offshore seismic experiment, blue: near vertical reflection seismic, red: magnetotelluric profiles)

Key references

Allen, R.B., Tucholke, B.E. (1981). Petrography and implications of continental rocks from the Agulhas Plateau, southwest Indian Ocean. Geology,9, 463-468.

Bauer, K., Schulze, A., Ryberg, T., Sobolev, S. V. and Weber, M. (2003): Classification of lithology from seismic tomography. A case study from the Messum igneous complex, Namibia. Journ. Geophys. Res., 108(B3), 2152, doi: 10.1029/2001JB001073 .

Ben-Avraham, Z., Hartnady, C.J.H., Malan, J.A. (1993) Early tectonic extension between the Agulhas Bank and the Falkland Plateau due to the rotation of the Lafonia microplate. Earth Planet. Sci. Lett. 117, 43-58.

Ben-Avraham, Z., Hartnady, C.J.H., le Roex, A.P. (1995) Neotectonic activity on continental fragments in the southwest Indian Ocean: Agulhas Plateau and Mozambique Ridge. J. Geophys. Res., 100, 6199-6211.

Ben-Avraham, Z. (1995) Neotectonic activity offshore southeast Africa and its implications, S. Afr. J. Geol., 98, 202-207.

Ben-Avraham, Z., Hartnady, C.J.H., Kitchin, K.A. (1997) Tectonics of the Agulhas-Falkland transform. Tectonophys.,282, 83-98.

Frimmel, H.E., Hartnady, C.J.H., Koller, F. (1996) Geochemistry and tectonic setting of magmatic units in the Pan-African Gariep Belt, Namibia. Chem. Geol., 130, 101-121.

Frimmel, H.E., Frank, W. (1998) Neoproterozoic tectono-thermal evolution of the Gariep Belt and its basement, Namibia/South Africa. Precambr. Res., 90, 1-28.

Frimmel, H.E., Fölling, P.G., Diamond, R. (2001) Metamorphism of the Permo-Triassic Cape Fold Belt and its basement, South Africa. Mineral. Petrol., 73, 325-346.

Frimmel, H.E., Fölling, P.G., Eriksson, P. (2002) Neoproterozoic tectonic and climatic evolution recorded in the Gariep Belt, Namibia and South Africa. Basin Research, 14, 55-67.

Frimmel, H.E. (in press a) Neoproterozoic sedimentation rates and timing of glaciations a southern African perspective. - In: Condie, K C, Veenstra, F (eds.) Tempos and Events in Precambrian Time, Elsevier (Amsterdam).

Frimmel, H.E. (in press b) Formation of a late Mesoproterozoic supercontinent: The South Africa East Antarctica connection. In: Condie, K.C., Veenstra, F. (eds.) Tempos and Events in Precambrian Time, Elsevier (Amsterdam).

Gohl, K. & Uenzelmann-Neben, G. (2001). The crustal role of the Agulhas Plateau, southwest Indian Ocean: evidence from seismic profiling. Geophys. J. Int., 144, 632-646.

Hartnady, C.J.H., Ben-Avraham, Z., Rogers, J. (1992) Deep-ocean basins and submarine rises off the continental margin of southeastern Africa: new geological research developments. South African J. Sci., 88, 534-539.

Harvey, J.D., de Wit, M.J., Stankiewicz, J., and Doucouré, M. (2001). Structural variations of the crust in the southwestern Cape derived from seismic receiver functions. S A Journal Geology, 104, 231-242.

Lyakhovsky, V., Ben-Avraham, Z., Reznikov, M. (1994) Stress distribution over the Mozambique Ridge. Tectonophys., 240, 21-27.

McMillan, I.K., Brink, G.J., Broad, D.S. & Maier, J.J. (1997). Late Mesozoic sedimentary basins off the south coast of South Afrika. In: Selley, R.C. (Ed.), Sedimentary Basins of the World: TheAfrican Basins. Elsevier, Amsterdam, 319-376.

Niemi, T.M., Ben-Avraham, Z., Hartnady, C.J.H. (2000) Post-Eocene seismic stratigraphy of the deep ocean basin adjacent to the southeast African continental margin: a record of geostrophic bottom-current systems. Marine Geology, 162, 237-258.

Raith, J.G., Cornell, D.H., Frimmel, H.E., de Beer, C.H. (2003) New insights into the geology of the Namaqua Tectonic Province, South Africa, from ion microprobe dating of detrital and metamorphic zircon. J. Geol., in press.

Ritter, O., Weckmann, U., Vietor, T. and Haak, V. (2003) A magnetotelluric study of the Damara Belt in Namibia 1. Regional scale conductivity anomalies, Phys. Earth Planet. Inter., 138:71-90, 2003, doi:10.1016/S0031-9201(03)00078-5

Ritter, O., Ryberg, T., Weckmann, U., Hoffmann-Rothe, A., Abueladas, A., and Garfunkel, Z., (2003) Geophysical images of the Dead Sea Transform in Jordan reveal an impermeable barrier for fluid flow, Geophys. Res. Lett., 30(14):1741-1744, 2003, doi:10.1029/2003GL017541

Tucholke, B.E., Houtz, R.E., Barrett, D.M. (1981). Continental crust beneath the Agulhas Plateau, southwest Indian Ocean. Journal of Geophysical Research, 86, 3791-38061.

Uenzelmann-Neben, G., Gohl, K., Ehrhardt, A. & Seargent, M.J. (1999). Agulhas Plateau, SW Indian Ocean: New evidence for excessive volcanism. Geophys. Res. Lett., 26, 1941-1944.

Uenzelmann-Neben, G. (2001). Seismic characteristics of sediment drifts: An example from the Agulhas Plateau, southwest Indian Ocean. Marine Geophysical Researches, 22, 323-343 .

Uenzelmann-Neben, G. (2003). Contourites on the Agulhas Plateau, SW Indian Ocean: Indications for the evolutions of currents since Paleogene times. In: Stow, D., Pudsey, C., Howe, J., Faugeres, J.C., Viana, A.R. (eds), Deep-water contourite systems: Modern drifts and ancient series. Geological Society of London, Memoir, 22, 271-288.

Weckmann, U., Ritter, O. and Haak, V. (2003) A magnetotelluric study of the Damara Belt in Namibia 1. MT phases over 90° reveal the internal structure of the Waterberg Fault/Omaruru Lineament, Phys. Earth Planet. Inter., 138:91-112, 2003, doi:10.1016-S0031-9201(03)00079-7

Key questions arising are:

  • Does the Mesoproterozoic crust continue from the Namaqua Belt in the west to the Natal Belt in the east? If yes, what is the geometry of this belt along the southern flank of the Kaapvaal Craton?
  • How much and what type of Neoprotererozoic crust exists to the south of the Kaapvaal Craton?
  • Are some (and if yes, which) of the Pan-African inliers in the Cape Fold Belt syn-orogenic intramontane molasse basins?
  • What is the Pan-African kinematic history of these inliers and how does it relate to that of the Gariep Belt and the Ross Orogen?
  • What is the subsurface geometry of major tectonic and stratigraphic boundaries within the Cape Fold Belt; how do the features mapped on the surface continue to depth?
  • To what extent can the Cape Fold Belt be followed off-shore to the south?
  • Do the Beattie magnetic anomaly and the Southern Cape Conductivity Belt (SCCB) have the same source?
  • Can we resolve structural details within the SCCB and its depth extent?
  • Is the SCCB caused by partially serpentinized rocks or is it connected with tectonic processes of the orogenic front (Cape Fold Belt)?
  • Is the Agulhas Plateau oceanic or continental?
  • What is the age and the source of the volcanics of the Agulhas Plateau and how can the magmatic volume added to the Agulhas Plateau and its surroundings be quantified?
  • Under what geodynamic process did the sheared margin of South Africa evolve?
  • What is the origin of the Agulhas Fracture Zone?
  • Has any magmatism been associated with break-up along the Agulhas Fracture Zone, and has the Fracture Zone been reactivated through present times?
  • How did the Outeniqua Basin form and what is the nature of the underlying basement and deep crust?
Objectives

Ten main objectives to be addressed within a number of sub-projects are briefly explained below in a sequence that reflects the progressively younger ages of geological phenomena of interest:

  • Nature of the basement to the Karoo Supergroup rocks and the extent to which it fits into a postulated contiguous Namaqua-Natal Belt: The new geophysical data are expected to provide new insights into the geometry of the main Mesoproterozoic crustal structures and thus yield crucial information for the modelling of Mesoproterozoic accretion processes along the southern margin of the Kaapvaal Craton.
  • A combination of petrological, structural and geochemical studies, combined with the geophysical data, are aimed at assigning the various tectono-stratigraphic units of the Saldania Belt (Fig. 2.2.4) to the different stages of Pan-African basin development and to place them into a regional, intercontinental framework of Gondwana assembly. Particularly their relationship to other Pan-African units along the western margin of southern Africa (Gariep, Damara, Kaoko Belt and equivalents in South America) and the Ross Orogen in Antarctica will be tested.
  • The expected geophysical data will be supplemented by systematic ground gravimetric measurements on the George Pluton and surrounding rocks in order to delineate the shape of the pluton at depth, calculated by gravity data inversion, and also its spatial relationships with the surrounding rocks.
  • The accretion process that gave rise to the Cape Fold Belt remains enigmatic, as no high-grade metamorphic core complexes or syn-orogenic igneous activity are known. The new geophysical data, combined with extensive structural and stratigraphic studies, will serve as basis on which to establish the geometry of major tectonic features by tracing tectonic and stratigraphic boundaries from surface outcrops into depth through the crust.
  • The up to 200 km wide SCCB was mapped in the late 1960s by means of magnetometer-array studies with stations separated by tens to hundreds of kilometers. A common source for the BMA and the SCCB has been proposed by several authors but remains uncertain because direct observations are missing. Preliminary seismic work across the BMA indicates zones of high reflectivity in the upper crust, while receiver functions studies identify sharp crustal discontinuities at depths of 8 and 18 km which are interpreted as a 10km thick block of altered Mesoproterozoic mafic-ultramafic rocks. However, this is by no means the only possible interpretation and we note that in some cases, low-resolution geophysical data sets are more a hindrance than a help for geodynamic understanding. For example the interpretation of the SCCB, as a serpentinized slice of paleo-oceanic crust is doubtful in view of recent magnetotelluric investigations with a much higher resolution from the Damara Mobile Belt in Namibia (Ritter et al., 2003). This new data set resolves the internal structure of a comparable high conductivity belt and shows clearly that conductive zones correlate with deep reaching fossil shear zones.
  • Thermo-mechanical forward modelling of the terrane accretion/ continental collision will investigate the geodynamic significance of the BMA and SCCB geophysical anomalies. One process we wish to model is the progressive magnetisation of crustal rocks as they pass the Curie isotherm during syntectonic and posttectonic exhumation. Additionally, the distribution of strain within the models will shed light on the distribution of the shear zones and their final geometry in the eroded orogen. The results will be applicable to collisional settings in general.
  • The extensional history of the Cape Fold Belt related to the break-up of Gondwana during the Mesozoic will form part of the study, where normal faults and related structures can be identified along the planned transect through the Cape Fold Belt.
  • The nature of the Agulhas Plateau (oceanic versus continental) and the amount of magmatic material added to it at various times is a premier target of the offshore studies. Recent deep seismic surveying of the southern central Agulhas Plateau indicates very high P-wave velocities (>7 km/s) in the lower two-thirds of the 25 km thick crustal column, suggesting an oceanic crustal affinity. However, this is in conflict with the interpreted continental origin of quartzo-feldspathic samples dredged from the plateau. It is uncertain if the observed velocity-depth distribution is symptomatic for the entire plateau or if continental crustal fragments are embedded in the plateau. An important aim is to understand the role of young volcanism near the northern Agulhas Plateau.
  • Origin of the Agulhas Fracture Zone and its relationship to the separation of the Falkland Plateau. Furthermore, the question of any magmatism having been associated with break-up and the tectonic as well as magmatic consequences of possible repeated re-activation of the fracture zone along the continental margin will be addressed.
  • On the Outeniqua Basin of the Agulhas Bank, the nature and position of the crust-mantle boundary will be examined, as well as the development of the oil-producing sedimentary basin and its deep-seated fractures during break-up.