The Walvis Ridge hotspot track in the South Atlantic represents ~130 Myr of continuous intra-plate volcanism (OʹConnor & Duncan 1990). It is considered one of the few “primary hotspots” consistent with the thermal plume model (Courtillot et al. 2003; Campbell & Kerr 2007). It also is a rare example of a seamount trail that positively can be connected with a massive large-igneous province (LIP; Richards et al. 1989), the once conjunct Paranŕ-Etendeka continental flood basalt (OʹConnor & Duncan 1990; OʹConnor & Le Roex 1992; Renne et al. 1996). However, around 60 Ma, the morphological expression of the Walvis Ridge changed drastically from a robust 200 km wide aseismic ridge into a 400 km wide region of diffuse and diminished volcanism (OʹConnor & Le Roex 1992). This “young” end of the Walvis Ridge may include two or more subtracks that have distinct geochemical signatures (Class et al. 2008), one ending at the Tristan da Cunha Islands and another ending at Gough Island more than 400 km to the SSE. Because of the sparse and uneven sampling in this part of the Walvis Ridge, it is unclear what caused this transition to diffuse volcanism and what it means for the evolution of the Walvis Ridge, the plate tectonic history of this Atlantic region, and intra-plate volcanism in general.
Intensive mapping and dredging will be conducted with the end goal of obtaining high-precision 40Ar/39Ar ages and geochemical analyses for each seamount. These data will help address the following key questions:
- What is the detailed age progression of the young end of the Walvis Ridge?
- Is diffuse volcanism in the young Walvis Ridge related to local extension, fracturing of the oceanic crust, or proximity to the Mid-Atlantic Ridge spreading center?
- Has the hotspot been geochemically zoned over all its history?
- Where is the current location of the Walvis Ridge hotspot?
- Is the Walvis Ridge hotspot failing or dying?
- What are the consequences of the answers to the above questions for hotspot fixity in the Atlantic, the Euler pole model for the African Plate, and global tectonic modeling?
The 49-day cruise will depart from and return to Cape Town, South Africa, on the R/V Melville. The primary activities will be multibeam mapping and dredging of seamounts, with several magnetic survey lines in locations where previous data is scarce. The initial plan is to sample 40 seamounts of varying sizes and to conduct magnetic anomaly mapping for 9 transit sections, including the 3-day transit from port to the first seamount target and also on the return trip to port. Additional seamounts will be added or removed from the cruise track as time and weather permits.
Each seamount will first be mapped with the Melville’s EM122 multibeam system to collect high-resolution maps that will help identify a suitable dredge site. As soon as a site is identified, the dredge will be deployed and monitored from the ship for up to 10 hours. After retrieval of the dredge basket, all samples will be cleaned, described, labeled and sorted for further shore-based analyses.
Campbell, I.H. and Kerr, A.C. (2007). The Great Plume Debate: Testing the plume theory. Chemical Geology 241(3-4), 149-152.
Class, C., le Roex, A.P., Rollins, N., Long, D., Goldstein, S.L., Cai, Y., van de Flierdt, T. and Perfit, M.R. (2008). Is plume shape variability real? Evidence from Tristan-Gough-Discovery. Geo-chimica Et Cosmochimica Acta 72(12), A167-a167.
Courtillot, V., Davaille, A., Besse, J. and Stock, J. (2003). Three distinct types of hotspots in the Earth's mantle. Earth and Planetary Science Letters 205(3-4), 295-308, doi: 10.1016/S0012-821X(02)01048-8.
O'Connor, J.M. and Duncan, R.A. (1990). Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot system: implications for African and South American plate motions over plumes. Journal of Geo-physical Research 95(B11).
O'Connor, J.M. and Le Roex, A.P. (1992). South Atlantic hot spot-plume systems: 1. distribution of volcanism in time and space. Earth and Planetary Science Letters 113(3), 343-364.
Renne, P.R., Glen, J.M., Milner, S.C. and Duncan, A.R. (1996). Age of Etendeka flood volcanism and associated intrusions in southwestern Africa. Geology 24(7), 659-662.
Richards, M.A., Duncan, R.A. and Courtillot, V.E. (1989). Flood basalts and hot-spot tracks: plume heads and tails. Science 246(4926), 103-107.