Interaction between a hotspot and a fracture zone: The crustal structure of Walvis Ridge at 6° E

Abstract

The Walvis Ridge is one of the major hotspot trails in the South Atlantic and a classical example for volcanic island chains. Two models compete about the origin of the ridge: It is either the result of a deep mantle plume or active fracture zones above mantle inhomogeneities. Among other things crustal information is needed to constrain the models. Here, we provide such constraint with a 480km long P-wave velocity model of the deep crustal structure of the eastern Walvis Ridge at 6° E. According to our data the Walvis Ridge stretches across the Florianopolis Fracture Zone into the Angola Basin. Here, we observe a basement high and thick basaltic layers covering the oceanic crust and the fracture zone. We found two crustal roots along the profile: one is located beneath the ridge crest, the other one beneath the northern basement high in the Angola Basin. The crustal thickness reaches 18km and 12km and the lower crustal velocities are 7.2km/s and 7.4km/s, respectively. The bathymetric expression of the ridge along the profile is less pronounced than closer to shore, which is mainly attributable to the absence of a thick layer of volcanic debris, rather than to reduced crustal thickness below the basement surface. Therefore, this part of the ridge was never or only briefly subaerially exposed. The crustal structure suggests that the ridge and the fracture zone formed independently of each other. The oceanic crust north of the fracture zone, which is buried underneath the basalt layer, is younger than the reconstructed age of hotspot volcanism of the Walvis Ridge. We interpret these structures north of the fracture zone to be at least partly a product of late stage volcanism.