GEMOC ARC National Key Centre

Gohl, K.¹, Seargent, M.J.¹, Uenzelmann-Neben, G.², and Ehrhardt, A.²

1. GEMOC Macquarie
2. Alfred Wegener Institute for Polar and Marine Research
 

The Agulhas Plateau, only a few hundreds of kilometers off the South African coast, has been for long controversially discussed with respect to its composition and origin. Plate tectonic reconstructions infer that parts of the plateau were conjugate to the Falkland Plateau and Maud Rise. It had, therefore, a key central position during the initial break-up of Gondwana. Series of seismic studies and geological dredging from the 1970-80's brought the first suggestions of two distinct parts of the plateau; the northern plateau as being of overthickened oceanic origin, and the southern plateau consisting of continental fragments with intrusive, ridge-like lineament zones. With the intention to bring a light into the understanding of the plateau's deep structural architecture and to further explain its evolutionary history, a deep crustal wide-angle/refraction survey was recently conducted as part of a comprehensive seismic study of the Agulhas Plateau. An east-west velocity-depth transect across the central plateau from ocean-bottom seismograph recordings reveals an up to 25 km thick crust with P-wave velocities ranging from 4.5-6.8 km/s for the upper crystalline crust and between 7.0 and 7.6 km/s for the lower half of the crust. PmP and Pn arrivals place strong confidence in defining the crust-mantle boundary. The very high velocities for the lower half of the crust, which push the average crustal velocity to about 6.9-7.0 km/s, are comparable with those found in the Ontong Java Plateau of the western Pacific. This evidence in the velocity distribution field rejects any dominant continental origin of the southern-central Agulhas Plateau and rather suggests oceanic crustal thickening by large bulk amounts of mantle material as a result of an active mantle plume. Minor continental fragments, as suggested from dredge samples of early studies, cannot be completely excluded and might have remained as local remnants from times before the phase of major magmatism. We consider the Agulhas Plateau as a Large Igneous Province (LIP), formed during as the result of slow spreading in the vicinity of a triple junction during the Cretaceous.