Crustal and uppermost mantle structure of the NW Namibia continental margin and the Walvis Ridge derived from ambient seismic noise

Abstract

The Walvis Ridge is the most prominent hotspot track related to the opening in the South Atlantic Ocean. Several hypotheses have been developed to explain its origin and evolution. The presence of a massive magmatic structure at the landfall of the Walvis Ridge (WR) in Northwest Namibia raised speculation about the role of a hotspot during the opening of the South Atlantic ocean. To investigate its deeper velocity structure at the junction of the Walvis Ridge with the African continent was the focus of the amphibious seismological WALPASS experiment. In total 12 ocean-bottom seismometers and 28 broadband land stations were installed between 2010 and 2012 to acquire seismological data. Here, we present the results of seismic ambient noise tomography to investigate to which extent the Tristan hotspot modified the crustal structure in the landward prolongation of the ridge and in the adjacent oceanic basins. For the tomography, vertical and hydrophone component cross correlations for >300 days for OBS stations and between one and two years for land stations data were analyzed. More than 49,000 velocity measurements (742 dispersion curves) were inverted for group velocity maps at 75 individual signal periods, which then had been inverted for a regional 3-D shear wave velocity model. The resulting 3-D model reveals structural features of the crust related to the continent-ocean transition and its disturbance caused by the initial formation of the WR ∼130 Ma ago. We found relatively thick continental crust below Northwest Namibia and below the near-shore part of the Walvis Ridge, a strong asymmetry offshore with typical, thin oceanic crust in the Namibe Basin (crossing over into the Angola Basin further offshore) to the North and a wide zone of transitional crust towards the Walvis Basin south of the Walvis Ridge.

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

“…From our results, we suppose that Walvis Ridge is an outcome of the combination of halted spreading and the impingement of the Tristan plume. However, the 3D extent of magmatism imaged in our models (which we do not discuss in this article, but in Franz et al, 2021 andJegen et al, 2016), confirms the observation, that the area of magmatic overprint is too small to justify the arrival of a large plume head (Fromm et al, 2017a;Ryberg et al, 2022) . An alternative model explains this smaller scale surface display of the plume by a large, lower mantle upwelling, which tops out at ~1000 km, from where smaller, secondary plumes rise to the surface (Courtillot et al, 2003;French & Romanowicz, 2015;Homrighausen et al, 2019;Zhao, 2007).…”

“…Our identifications derived from MT and gravity data are corroborated by seismic surveys (e.g. by Fromm et al, 2017;Planert et al, 2017;and Ryberg et al, 2022), pointing to the interesting possibility to derive large scale 3D models through acquisition of 3D MT and joint inversion with satellite gravity data and guiding 2D seismic acquisition.…”

“…CC BY 4.0 License. crust (Heit et al, 2015;Ryberg et al, 2022;Ryberg et al, 2015;Yuan et al, 2017), and interpret it to be an effect of magmatic underplating from the thermal anomaly associated with the Tristan mantle plume arrival. In resistivity studies, cratonic crust is usually expected to have very high electrical resistivities of several thousand Ωm and more (Jones et al, 2009;Moorkamp et al, 2022;Van Zijl, 1977), while mobile belts can show significantly reduced electrical resistivities of ~10 -50 Ωm (de Beer et al, 1982;Moorkamp et al, 2022;Van Zijl, 1977).…”

“…Many regional geophysical studies around Walvis Ridge have investigated typical breakup related geological features such as flood basalts, thickened crust, magmatic underplating, and various crustal intrusions (e.g. Bauer et al, 2003;Fromm et al, 2017a;Gladczenko et al, 1998;Goslin & Sibuet, 1975;Kapinos et al, 2016;Planert et al, 2017;Ryberg et al, 2022), which are reviewed in detail in the next section. While Walvis Ridge is often referred to as a classic plume example (White & McKenzie, 1989), different authors have also argued, that the amounts of magmatism and thermal alteration of the crust at the Namibian coast are too small to justify a continental breakup induced by the arrival of a large plume head (Fromm et al, 2017a;Koopmann et al, 2016;Ryberg et al, 2022).…”