NICOLE PARSIEGLA, KARSTEN GOHL AND GABRIELE UENZELMANN-NEBEN 393 Deep crustal structure of the sheared South African continental margin: first results of the Agulhas-Karoo Geoscience Transect Nicole Parsiegla, Karsten Gohl and Gabriele Uenzelmann-Neben Alfred Wegener Institute for Polar and Marine Research, PO BOX 12161, 27515 Bremerhaven, Germany e-mail: [email protected]; [email protected] [email protected] © 2007 September Geological Society of South Africa ABSTRACT The southern margin of South Africa developed as a consequence of shear motion between the African and South American plates along the Agulhas-Falkland Transform Fault during the Early Cretaceous break-up of Gondwana. The Agulhas-Karoo Geoscience Transect crosses this continental margin, and includes two combined offshore-onshore seismic reflection/refraction profiles. We present results from the western offshore profile. Using ocean-bottom seismometers and a dense airgun shot pattern, a detailed image of the velocity – depth structure of the margin from the Agulhas Passage to the Agulhas Bank was derived. Modelling reveals crustal thicknesses of between 7 km and 30 km along the profile. The upper crust has P-wave velocities of between 5.6 and 6.6 km/s and the lower crust has velocities that lie between 6.4 and 7.1 km/s. Uppermost mantle velocities range from 7.8 to 8.0 km/s. The 52 km wide continent-ocean-transition zone, where the Moho rises steeply, occurs at the Agulhas-Falkland Fracture Zone. Beneath the Southern Outeniqua Basin and the Diaz Marginal Ridge, a zone of relatively low velocities (~5 km/s) with a thickness of up to 3 km, can be discerned in the upper crust. We interpret this zone as an old basin filled with pre-break-up metasediments, which may be related to the Cape Fold Belt. We suggest that the origin of the Diaz Marginal Ridge is bound up with the tectonic history of this basin as it exhibits a similar velocity structure. Almost no stratified sediment cover exists in the Agulhas Passage because of strong erosion due to ocean currents. The velocity structure and seismic reflection results indicate the presence of alternating layers of volcanic flows and sediments, with a mean velocity of about 4 km/s. We suggest these volcanic flows were an accompaniment of possible re-activation events of parts of the Agulhas-Falkland Fracture Zone. Tectonic motion seems to be sub vertical instead of strike-slip along the re-activated part of the fracture zone. Therefore, a relation to the uplift of the South African crust may be possible. Introduction model of the architecture and dynamics of this margin. Continental accretion and break-up processes can be This model supplies new insights into the break-up traced back over more than 3.5 billion years in southern of Gondwana in this region, the possible causes of Africa and along its margins. The Agulhas-Karoo the epeirogenic uplift of southern Africa, and the Geoscience Transect is part of the Inkaba ye Africa geodynamic processes that shaped the sheared framework project (de Wit and Horsfield, 2006) and is continental margin. an onshore-offshore transect crossing the southern Using seismic refraction and reflection data from the continental margin of South Africa starting in the Karoo western of two offshore seismic lines (Figure 1), this Province, passing through the Cape Fold Belt (CFB), the contribution provides new detailed information on the Agulhas Bank, the Agulhas-Falkland Fracture Zone crustal structure of the sheared South African continental (AFFZ) and the Agulhas Plateau (Figure 1). This transect margin. We discuss results with reference to the deep offers an unprecedented opportunity to address many structure of the Outeniqua Basin, the composition of the different objectives concerning South Africa and its Diaz Marginal Ridge, and the tectonomagmatic evolution southern continental margin, applying seismic reflection of the AFFZ. and refraction, magneto-telluric, petrological, geological and geochemical methods. The main objectives of the Tectonic framework onshore part of the survey are the identification of The South Atlantic formed due to rifting and seafloor the sources of the Beattie Magnetic Anomaly and the spreading during the break-up of Gondwana in the Early Southern Cape Conductivity Belt, (Weckmann et al., this Cretaceous (e.g. Barker, 1979, Ben-Avraham et al., 1997). issue) and an explanation of the history of the CFB At the same time, shear processes along the Agulhas – (Stankiewicz et al., this issue). Offshore, the structure Falkland Transform (AFT) caused the development of and formation of the AFFZ and adjacent basins, and the the southern margin of South Africa. Right-lateral strike- evolution of the Agulhas Plateau and its crustal structure slip motion separated the African from the South including its volume, age and magmatic source are American continent along this transform, which had a addressed. The entire transect is a unique opportunity to maximum ridge-ridge offset of 1200 km (Ben-Avraham use multidisciplinary data to create an overarching et al., 1997). Due to a series of ridge jumps (Barker, SOUTH AFRICAN JOURNAL OF GEOLOGY, 2007,VOLUME 110 PAGE 393-406 doi:10.2113/gssajg.110.2/3.393 394 DEEP CRUSTAL STRUCTURE OF THE SHEARED SOUTH AFRICAN CONTINENTAL MARGIN 1a 1b Figure 1. (a) In the overview map (left) the satellite derived topography (Sandwell and Smith, 1997) of the area of investigation is shown. Offshore (AWI-20050100, AWI-20050200) and onshore (FRA, GRA) seismic lines of the Agulhas-Karoo Geoscience Transect are plotted as black lines. The Agulhas-Falkland Fracture Zone (AFFZ) is marked with a white dashed line. The Cape Fold Belt (CFB) is drawn as a yellow area and the Karoo Province (KP) in light red on the map. The position of the Southern Cape Conductivity Belt (SCCB) is outlined by a blue dashed line and the location of the Beattie Magnetic Anomaly (BMA) is sketched by a green line (positions after de Beer et al., 1982). Abbreviations are OB=Outeniqua Basin, SOB=Southern Outeniqua Basin, which is part of the OB. (b) In the enlargement (right) the shot profile AWI-20050100 is shown as black line and positions of ocean-bottom seismometers (OBS) on the profile are marked with points. Thick points mark OBS whose data are shown in Figures 4, 5, and 6. Yellow points mark OBS which did not record data. Red bars illustrate the positions of coincident seismic reflection sections on the OBS profile, which are shown in Figures 11, 12, and 13. 1979; Tucholke et al., 1981) the offset has been reduced The margin-parallel Southern Outeniqua Basin (SOB) to about 290 km at the present day (Figure 2). lies between these sub-basins and the Diaz Marginal The present Agulhas-Falkland Transform is an Ridge (DMR) (Figure 3). From the observation of great intraoceanic feature located between 46.3°S, 10.0°W and depths to acoustic basement (Figure 3), Ben-Avraham 46.9°S, 13.3°W (Figure 2) that separates the South et al. (1997) concluded that the Outeniqua Basin is American and African arms of the Agulhas-Falkland either underlain by oceanic or highly stretched Fracture Zone from each other. South of the African continental crust. Reconstructions of the Falkland continent, the AFFZ can be divided into four parts, from Plateau to its former position at the South African margin southwest to northeast: the Mallory Trough segment, show that the Southern Outeniqua Basin and the Diaz Ridge segment, East London segment and Durban Falkland Plateau Basin between Maurice Ewing Bank segment (Ben-Avraham et al. 1997) (Figure 2). Ben- and the Falkland Island platform (Figure 2) were Avraham et al. (1995) interpreted disturbed seafloor in originally juxtaposed across the AFFZ (Martin et al. 1981; the eastern part of the Diaz Ridge segment as evidence Ben-Avraham et al., 1997). for a renewal of tectonic activity in the Quaternary. The The DMR, which borders the Outeniqua Basin to the causes of the initial development of such a long offset south, is usually described as a basement ridge transform boundary are still poorly understood. (e.g. Scrutton, 1979) although it has no strong magnetic Complex basins and marginal ridges are typical intensity field signature (Ben-Avraham et al., 1993). features of sheared margins (Bird, 2001). North of the Ben-Avraham et al. (1993) discuss three possible AFFZ the Outeniqua Basin is a complex network of sub- compositions of the DMR; upward faulted continental basins (Figure 3). From west to east, the northern part of basement, sedimentary rocks deformed during the Outeniqua Basin comprises the Bredasdorp, Infanta, movement along the AFFZ, and extruded volcanic rocks. Pletmos, Gamtoos and Algoa sub-basins (Figure 3). Ben-Avraham et al. (1993) prefer the second These basins are separated from each other by basement explanation, based on interpretation of seismic reflection arches and faults (McMillan et al., 1997) (Figure 3). results. SOUTH AFRICAN JOURNAL OF GEOLOGY NICOLE PARSIEGLA, KARSTEN GOHL AND GABRIELE UENZELMANN-NEBEN 395 Figure 2. Satellite derived topography map (Sandwell and Smith, 1997) illustrating the tectonic setting. The white dashed line encircles the African Superswell (ASS). Tracks of the Shona hotspot (filled squares with solid lines according to Martin, 1987; open squares with solid lines according to Hartnady and le Roex, 1985) and the Bouvet hotspot (filled circles with dashed lines according to Martin, 1987; open circles with dashed lines according to Hartnady and le Roex, 1985) are shown and numbers refer the time in Ma when the hotspot is assumed to have reached that position. Abbreviations are FP=Falkland Plateau, FB=Falkland Basin, FI=Falkland Islands, MEB=Maurice Ewing Bank, AFT=Agulhas-Falkland Transform, MT=Mallory Trough segment, DR=Diaz Ridge segment, EL=East London segment, D=Durban segment.