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

Abstract

A B S T R A C TThe 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 480 km 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 18 km and 12 km and the lower crustal velocities are 7.2 km/s and 7.4 km/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.

A global reference model of the lithosphere and upper mantle from joint inversion and analysis of multiple data sets

“…For instance, LithoRef18 predicts deeper Moho depths than CRUST1.0 beneath oceanic features such as the Walvis and Ninetyeast ridges, in closer agreement with recent high-resolution seismic studies (e.g. Fromm et al 2017).…”

Structure and evolution of the Atlantic passive margins: A review of existing rifting models from wide-angle seismic data and kinematic reconstruction

“…The profiles, located in the vicinity of the Walvis ridge and a proposed hotspot trail (Duncan, 1984, Perez-Diaz andEagles, 2014), show high lower crustal velocities of up to 7.5 km/s. In contrast to the neighboring profiles at the southwestern African margin where these high velocities do not extend underneath the continent (Bauer et al, 2000;Schinkel, 2007;Hirsch et al, 2009), here, they extend several tens of kilometers underneath the African continent, and might therefore be connected to a mantle thermal anomaly or hot spot (Fromm et al, 2015;Fromm et al, 2017). However, the authors find it unlikely that this thermal anomaly triggered the initial opening of the South Atlantic Ocean due to the missing large-scale intrusions within the continental crust in the region.…”

“…Two transform marginal plateaus (TMP) are located along this segment, the Sao Paulo PLATEAU north of the Florianopolis Fracture Zone and the Walvis PLATEAU south of it. The Walvis PLATEAU consists of a 26 to 35 km-thick heavily intruded crust, probably of continental origin with a more-than-300 km-wide HVLC and the western part is proposed to consist of "Icelandic" type crust (Fromm et al, 2017;Planert et al, 2017). The Sao Paulo PLATEAUwas found to consist of thinned continental crust; however, small scale volcanic activity also took place during its construction (Evain et al, 2015.…”

Formation and geophysical character of transitional crust at the passive continental margin around Walvis Ridge, Namibia

“…4, 0 to ~200 km) exhibits a decreasing influence of breakup related magmatism. The crust is still significantly thicker than normal oceanic crust (up to 20 km, Fromm et al, 2017a;Fromm et al, 2017b) demonstrating the impact of the Tristan plume tail, but it lacks the previously discussed typical upper crustal cluster cru-A representing interlayered sediments and surface volcanic flows. This indicates the transition to submarine spreading and the development of crust corresponding to pillow basalts, sheeted dykes, and gabbros of oceanic layers 2 and 3 but in a thickened, Icelandic type form (Foulger et al, 2003;Fromm et al, 2017a).…”