Crustal structure of the Agulhas Ridge (South Atlantic Ocean): Formation above a hotspot?

Highlights

• We present detailed deep seismic sounding data across the Agulhas Ridge

• The data indicate no major modification of the oceanic crust by massive hotspot volcanism

• The data support models that hotspot material migrated into the fracture zone.

Abstract

The southern South Atlantic Ocean contains several features believed to document the traces of hotspot volcanism during the early formation of the ocean basin, namely the Agulhas Ridge and the Cape Rise seamounts located in the southeast Atlantic between 36°S and 50°S. The Agulhas Ridge parallels the Agulhas-Falkland Fracture Zone, one of the major transform zones of the world. The morphology of the ridge changes dramatically from two parallel segments in the southwest, to the broad plateau-like Agulhas Ridge in the northeast. Because the crustal fabric of the ridge is unknown relating its evolution to hotspots in the southeast Atlantic is an open question. During the RV Polarstern cruise ANT-XXIII-5 seismic reflection and refraction data were collected along a 370 km long profile with 8 Ocean Bottom Stations to investigate its crustal fabric. The profile extends in NNE direction from the Agulhas Basin, 60 km south of the Agulhas Ridge, and continues into the Cape Basin crossing the southernmost of the Cape Rise seamounts. In the Cape Basin we found a crustal thickness of 5.5–7.5 km, and a velocity distribution typical for oceanic crust. The Cape Rise seamounts, however, show a higher velocity in comparison to the surrounding oceanic crust and the Agulhas Ridge. Underplated material is evident below the southernmost of the Cape Rise seamounts. It also has a 5–8% higher density compared to the Agulhas Plateau. The seismic velocities of the Agulhas Ridge are lower, the crustal thickness is approximately 14 km, and age dating of dredge samples from its top provides clear evidence of rejuvenated volcanism at around 26 Ma. Seismic data indicate that although the Cape Rise seamounts formed above a mantle thermal anomaly it had a limited areal extent, whereas the hotspot material that formed the Agulhas Ridge likely erupted along a fracture zone.

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.