Walvis Ridge Passive-Source Seismic Experiment - Seismic structures and seismotectonics in the passive continental margin of NW Namibia
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Northwestern Namibia, at the landfall of the Walvis Ridge, was affected by the Tristan da Cunha mantle plume during continental rupture between Africa and South America, as evidenced by the presence of the Etendeka continental flood basalts. Here we have operated an amphibian passivesource seismological network (WALPASS) consisting of 28 land stations and 12 OBS stations in order to investigate the upper mantle structure and to elucidate the Cretaceous mantle plumelithosphere interaction. We analysed the acquired data with a variety of seismic methods including receiver function, body and surface wave tomography and shear wave splitting to investigate the seismic structure in the crust and mantle lithosphere. In addition we identified and located many local earthquakes in Namibia and detected anomalies at the core-mantle boundary. The results obtained represent the first integrated picture of this region where it is possible to observe the effects of the mantle plume in the upper mantle. One of these effects can be seen in the continental crust that has experienced localized magmatic addition and consequent thickening of the cratonic block in NW Namibia. The lower part of the lithosphere has been eroded during breakup and left a relic that can be seen today as a strong converter between 80 km and 120 km in the landfall of the Walvis Ridge. Due to mantle-plume interaction the upper mantle was depleted during the phase of magmatic evolution that produced a large amount of melt that is responsible for the development of the Etendeka flood-basalt province. This depleted upper mantle is responsible for the high velocities that produce the apparent early arrival of the MTZ discontinuities beneath the array and show the local character of this anomaly. These high velocities in the upper mantle are also detected by tomographic and surface waves studies. We observed, by surface wave tomography, a thick lithosphere with a thickness of up to 200 km beneath north Namibia. The shear wave splitting analysis shows a regional coherent distribution of most of the SKS and S-wave derived splitting parameters with NE-SW oriented fast polarization directions. The main source of anisotropy is associated to the “frozen” anisotropic structure of the Pan African mantle lithosphere. However, a similar overlapping between absolute plate motion vectors and regional anisotropic NE-SW orientations also indicate a small contribution from the asthenospheric flow process causing the LPO in the mantle. The seismicity results include the detection of a strong earthquake of magnitude 4.8 and a great number of local earthquakes that could be associated with neotectonic activities. The high number of mining blasts and the strong explosions detected with our network could be suggesting that there might be some induced seismicity near the operating mines. Finally, the African Superplume reaches a height of 1200 km above the CMB and a gap of ca. 300 km width with normal velocity in the middle of the Superplume has been detected suggesting that the African Low-Velocity Zone consists of individual rising plumes and not one large homogeneous body.
Publications
- 2015, Crustal thickness and Vp/Vs ratio in NW Namibia from receiver functions: evidence for magmatic underplating due to mantle-plume - crust interaction, Geophys. Res. Lett., 42, 3330-3337
Heit, B., X. Yuan, M. Weber, W. Geissler, W. Jokat, B. Lushetile, K.-H. Hoffmann
(See online at https://doi.org/10.1002/2015GL063704) - 2017, Complex Deep Structure of the African Low‐Velocity Zone, Bull. Seism. Soc. Am.
Kästle, E.D., M. Weber and F. Krüger
(See online at https://doi.org/10.1785/0120160215) - 2017, Seismic structure of the lithosphere beneath NW Namibia: Impact of the Tristan da Cunha mantle plume, Geochem. Geophys. Geosyst., 18, 125-141
Yuan, X., B. Heit, S. Brune, B. Steinberger, W. Geissler, W. Jokat, M. Weber
(See online at https://doi.org/10.1002/2016GC006645)
