Crustal structure of the Agulhas Ridge (South Atlantic Ocean): Formation above a hotspot?
Introduction
The rifting of the South Atlantic started some 136 Ma with the eruption of massive flood basalts. Today remnants of this early magmatic event are found in Brazil and Namibia, namely the Parana and Etendeka flood basalts provinces. Seismic data along the southwestern African margin show the presence of seaward dipping reflectors indicating that the South Atlantic rift-drift transition was accompanied by massive subaerial magmatism along its margins (Becker et al., 2014). Today, these thick volcanic sequences mark the transition to the onset of oceanic crust and are covered by thick sediments. Most of the Angolan and Namibian continental shelves are typical rifted margins, which most likely opened somewhat obliquely (Jokat et al., 2003, Eagles, 2007, Heine et al., 2013). Further offshore the South Atlantic hosts also a variety of submarine, magmatic features, which document an ongoing interaction between deep mantle sources and the South Atlantic system of spreading ridges and fracture zones.
In the very south of the Atlantic, the Cape Basin (Fig. 1) is bordered by a large strike slip system, the Agulhas-Falkland Fracture Zone (AFFZ) (LaBrecque and Hayes, 1979, Ben-Avraham et al., 1997), which has been active since the opening of the South Atlantic. For reasons that are still unclear the Falkland Islands, together with the Falkland Plateau (Fig. 1), remained attached to the South American plate during the dispersal of Gondwana. Comparing geological units in southern Africa and the Falkland Islands suggests that the islands were at one time located close to Cape Town (Hartnady and le Roex, 1985, König and Jokat, 2006). Thus, the entire east and southeast coast of South Africa represents a huge sheared margin. The Natal and Agulhas basins, as well as the Mozambique Ridge, all formed in a complex stepwise pattern (König and Jokat, 2010, Leinweber and Jokat, 2012). While strike slip movement along the AFFZ was not accompanied by massive onshore volcanism, offshore volcanism in the Southern Ocean is documented by numerous submarine features such as the Agulhas Plateau, the Meteor Rise, the Shona Ridge, the Cape Rise seamounts, and the Agulhas Ridge. Most of these structures are interpreted as being the consequence of hotspot volcanism based mainly on kinematic modeling. Hartnady and le Roex (1985) speculate that the Cape Rise seamounts, the Agulhas Ridge, the Meteor Rise as well as the southern extension of the Shona Ridge were all formed by the interaction with the same hotspot. According to these authors, the Shona hotspot was located below the Cape Rise seamounts during the Late Mesozoic and that it interacted at a later stage with the AFFZ resulting in the intrusion of hotspot material into the already existing fracture zone system. The Shona hotspot weakened the crust sufficiently such that the spreading axis jumped westwards and formed the present-day Meteor Rise. In the Hartnady and le Roex (1985) model this rise is the conjugate feature to the westernmost part of the Georgia Basin. Thus, several authors (Hartnady and le Roex, 1985, Martin, 1987) explain the rather complex “Zigzag” topography of these submarine features by continuous interaction of the Shona hotspot with local plate kinematics. According to this scenario the formation age of the Agulhas Ridge is likely to be Late Mesozoic to Early Cenozoic.
The main question we address here is whether the AFFZ has been affected by the Shona hotspot. Basement rock samples provide clear evidence that Shona hotspot material erupted along the AFFZ (LeRoex et al., 2010, Hoernle et al., 2016). Geophysical experiments reported here help to establish whether hotspot material has modified the crustal fabric of the oceanic crust. In such a scenario, typical observations are over-thickened oceanic crust, and a high seismic velocity (≥ 7.0 km/s) lower oceanic crust. A first deep seismic sounding line acquired at approximately 42°30'S 009°E (Fig. 1) in order to provide insights into the crustal variations across the AFFZ (Uenzelmann-Neben and Gohl, 2004) shows that the oceanic crust north and south of the AFFZ has a normal thickness of 7 km. The AFFZ splits where it crosses this profile into two topographic highs. The valley between these AFFZ ridges is 40 km wide and filled with well-stratified sediments with a maximum thickness of 1 km (Uenzelmann-Neben and Gohl, 2004) in the westernmost part of the Agulhas Ridge. The crust below the northern high has a thickness of 13 km, while there is no evidence for crustal thickening in the central graben and southern high. The generally low seismic velocities are interpreted as an indication of fragments of continental crust (Uenzelmann-Neben and Gohl, 2004). These fragments might have been sheared off from the Maurice Ewing Bank during its westwards drift. Our line is located 350 km further to the east in an area of rougher topography and crosses the Agulhas Ridge. It is unclear if the Agulhas Ridge, located further to the east (41–43°S/009–016°E), has a completely different origin or is part of the AFFZ that is characterized by two parallel topographic highs.
Section snippets
Methods and data acquisition
The geophysical data were acquired during a research cruise with RV Polarstern (ANT-XXIII/5). The deep seismic transect (Fig. 1) has a length of 368 km orientated in north-south direction. As seismic source we used 8G-Guns (8.1 l each) and a single 32 l BOLT airgun totaling to 96.8 l. The airguns were fired every 60 s at a pressure of 130 bar. Together with a ship speed of 5 ktns, this resulted in a shot spacing of 150 m. Parallel to the deep seismic sounding profile we acquired conventional seismic
Error analysis
One class of error is due to the fact that we could only estimate the exact position of the OBS/OBH on the seafloor. However, far larger errors are due to low signal/noise ratios and the rough topography, which makes the phase identification difficult. This is especially true for the Pn mantle phases, which are in general difficult to identify. The error bars for the picks are automatically calculated by ZP and manually modified as necessary. To estimate the depth and velocity errors we varied
Swath bathymetry
We continuously acquired swath bathymetry parallel to the ship track and offset the track where possible to achieve a better coverage. The resulting data set is shown in Fig. 6. Strong depth differences are observed between the basins (5000 m) and the top of the Agulhas Ridge (3500 m). In general, the topography of the ridge's plateau is flat but occasionally this is disrupted by small seamounts with a maximum altitude of 550 m (Table 4) and diameter of 2.3 km. The seismic reflection data (Fig. 7)
Implications and conclusions
Finally, we wish to summarise the implications of our observations on the crustal fabric of the oceanic crust/seamounts/ridges in our research area for current understanding of hotspot volcanism in the southeast Atlantic as discussed previously. The Agulhas Ridge extends from 41 to 43°S to 9–16°E and parallels the AFFZ. The southwestern part has two topographic highs at approximately 13.5°E, while the northeastern part of the ridge broadens and forms a topographic plateau. Uenzelmann-Neben and
Acknowledgements
We thank the captain and the crew R/V Polarstern as well as the seismic team during the expedition ANT-XXIII/5 for their excellent support. We acknowledge the German Instrument Pool for Amphibian Seismology (DEPAS-Pool) for providing the OBS. The English of the manuscript was greatly improved by J. O′Connor. We thank Edi Kissling and an anonymous reviewer for their helpful comments.
References (40)
Vertical tectonism in oceanic fracture zones
Earth Planet. Sci. Lett.
(1978)- et al.
Neogene crustal emersion and subsidence at the Romanche fracture zone, equatorial Atlantic, earth planet
Sci. Lett.
(1977) - et al.
Vertical crustal movements at the Vema fracture zone in the Atlantic: evidence from dredged limestones
Tectonophysics
(1983) - et al.
Southern Ocean hotspot tracks and the Cenozoic absolute motion of the African, Antarctic, and South American plates
Earth Planet. Sci. Lett.
(1985) - et al.
Seafloor spreading history of the Agulhas Basin
Earth Planet. Sci. Lett.
(1979) - et al.
The Jurassic history of the Africa-Antarctica corridor - new constraints from magnetic data on the conjugate continental margins
Tectonophysics
(2012) Plate reorganisations around southern Africa, hotspots and extinctions
Tectonophysics
(1987)- et al.
The tectonic evolution of the South Atlantic Late Jurassic to present
Tectonophysics
(1991) - et al.
The great meteor seamount: seismic structure of a submerged intraplate volcano
Geodynamics
(1999) - et al.
Asymmetry of high velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental break-up
Solid Earth
(2014)
Structure and tectonics of the Agulhas-Falkland fracture zone
Tectonophysics
Plate kinematics of the South Atlantic: Chron C34 to present
J. Geophys. Res.
Mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites
Geophys. Res. Lett.
Seismic imaging of hotspot related crustal underplating beneath the Marquesas Islands
Nature
Spatial distribution of hotspot material added to the lithosphere under La Reunion, from wide angle seismic data
J. Geophys. Res.
Plume-ridge interactions of the discovery and Shona mantle plumes with the southern mid-Atlantic ridge (40°–55°S)
J. Geophys. Res.
New angles of South Atlantic opening
Geophys. J. Int.
Application of three-dimensional interactive modeling in gravity and magnetics
Geophysics
Crustal architecture and deep structure of the Ninetyeast ridge hotspot trail from active-source ocean bottom seismology
Geophys. J. Int.
Krustenstruktur Des Agulhas Rueckens Und Der Cape Rise Seamount Zwischen 38°–42°S Und 12°–16°E
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Now at ION Geophysical, Chertsey, Surrey, United Kingdom.