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Fromm, T., L. Planert, W. Jokat, T. Ryberg, J. H. Behrmann, M.H. Weber, and Christian Haberland, South

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Fromm, T., L. Planert, W. Jokat, T. Ryberg, J. H. Behrmann, M.H. Weber, and Christian Haberland, South Downloaded from geology.gsapubs.org on September 15, 2015 South Atlantic opening: A plume-induced breakup? T. Fromm1*, L. Planert2†, W. Jokat1*, T. Ryberg3, J. H. Behrmann2, M.H. Weber3,4, and Christian Haberland3 1Alfred-Wegener-Institut für Polar- und Meeresforschung, Am Alten Hafen 26, D-27568 Bremerhaven, Germany 2GEOMAR, Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, D-24148 Kiel, Germany 3Helmholtz-Centre Potsdam–German Research Centre for Geosciences (GFZ), Telegrafenberg, D-14473 Potsdam, Germany 4University of Potsdam, Institute of Earth and Environmental Science, Golm, D-14471 Potsdam, Germany ABSTRACT data examples are available in the GSA Data Upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are Repository1. considered to be drivers of active continental breakup. The formation of the Walvis Ridge dur- ing the opening of the South Atlantic is related to a putative plume-induced breakup. We inves- RESULTS tigated the crustal structure of the Walvis Ridge (southeast Atlantic Ocean) at its intersection Our P-wave velocity models (Fig. 2) show with the continental margin and searched for anomalies related to the possible plume head. The that the edifice of the Walvis Ridge consists overall structure we identify suggests that no broad plume head existed during opening of the of closely spaced seamounts and up to 35-km- South Atlantic and anomalous mantle melting occurred only locally. We therefore question the thick crust. Crustal velocities of 5.5–7.2 km/s importance of a plume head as a driver of continental breakup and further speculate that the point to a gabbroic composition resembling hotspot was present before the rifting, leaving a track of kimberlites in the African craton. thickened oceanic crust. The ridge is covered by extrusive rocks with velocities of 3.8–5.5 km/s, INTRODUCTION possible end member, and global observations which we interpret as hyaloclastites and basalt The processes of lithospheric weakening from continental margins with and without lava flows. The transition from the Walvis Ridge that finally allow continents to break are still flood basalt provinces suggest a very different to the adjacent basins reveals drastic differences poorly understood and geophysical data con- explanation: i.e., preexisting weak zones and a between the northern and the southern flanks as straints are sparse. Various ideas exist about the prior history of rifting in combination with gen- well as along the axis of the ridge. underlying mechanisms that cause continental eral plate movements might be more important While the southern flank gradually converts breakup, ranging from changing plate bound- factors for breakup (e.g., Armitage et al., 2010; into the continent ocean transition of the volca- ary forces to mantle dynamics. A much-debated Buiter and Torsvik, 2014). nic margin (Fig. 2B), the northern flank is char- model involves the arrival of a deep mantle Here we use seismic refraction data to image acterized by a sharp transition from 35-km-thick plume (e.g., Storey, 1995). Mantle plumes are the crustal structure associated with a hotspot crust below the ridge to 5–6-km-thick oceanic deep-seated thermal anomalies carrying hot and track and the proposed site of the Tristan plume crust in the Angola Basin (Figs. 2B and 2C). buoyant material from the core-mantle bound- head impact (Duncan, 1984), the easternmost This strong lateral variation is limited to the area ary to the lithosphere-asthenosphere boundary Walvis Ridge (southeast Atlantic Ocean), in- close to the continental margin. Further offshore (LAB). The LAB forms a rheological barrier cluding the junction with the Namibian coast (Fig. 2A) both flanks transfer smoothly into oce- to the plume’s further ascent, and so the man- (Fig. 1). The area is well covered by four mostly anic crust, but with some additional volcanism tle material spreads out as a large disk (e.g., amphibious deep seismic sounding profiles. The and thickened crust at the northern flank. Griffiths and Campbell, 1991). In the original data image 2490 km of crust and upper mantle This surprising jump in crustal thickness on model, Morgan (1971) postulated that regional along profiles varying in length from 470 to 730 the Angola Basin side can be explained by the ki- uplift and stress induced by thermal doming km. We used 166 ocean bottom stations, 99 land nematic evolution of the South Atlantic (Fig. 3). cracked the continents and pushed them apart. receivers, 12,864 airgun shots, and 13 dynamite The Angola Basin is considerably younger than More recent simulations show that plumes also shots. One profile is located along the ridge axis the Cape Basin (up to 20 Ma; Gee and Kent, have the potential to thermally and chemically and continues onshore, and the other three cross 2007; Shipboard Scientific Party, 1984), and the erode the base of the lithosphere (Sobolev et al., the Walvis Ridge at different angles and loca- 2011) and promote the accumulation of melt tions. The traveltimes of refracted and reflected 1 GSA Data Repository item 2015311, seismic P-phases were used to derive two-dimensional data and model descriptions, is available online at that further exacerbates lithospheric weaken- www.geosociety.org/pubs/ft2015.htm, or on request ing. This melt intrudes the crust, partly accu- velocity models using standard modeling pro- from [email protected] or Documents Secre- mulates at the crust-mantle boundary (Moho), cedures. Further details of the analysis and tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. which can be mapped by seismic methods, and partly erupts at the surface as large flood basalt 0° 2°E 4°E 6°E 8°E 10°E 12°E 14°E 16°E provinces (e.g., Ridley and Richards, 2010). RyL4berg et al. (2015) P3 The formation of flood basalt provinces is often P2 Kaoko Belt Figure 1. Location of the 18°S Angola Basin in close spatial and temporal proximity to con- deep crustal seismic pro­ 102.5 Ma files at the Walvis Ridge, tinental breakup, which has led to the contro- southeast Atlantic Ocean. 20°S P150 versial concept that the impact of plume heads Etendeka Magnetic anomalies with arriving at the base of the lithosphere initiates P100 the ages (Gee and Kent, 2007): C34, 83.5 Ma; M0, continental breakup (e.g., Cande and Steg- 22°S man, 2011). However, this model is only one 120.6 Ma; M4, 125.7 Ma. Florianopolis Cape Basin Red square marks the Fracture Zone dated Deep Sea Drilling *E-mails: [email protected]; Wilfried.Jokat@ 24°S Walvis Ridge Project Site 530 Leg 75 in awi.de C31 C34 M0 M4 the Angola Basin (Ship­ †Current address: Bundeswehr Technical Center board Scientific Party, Legend WTD71, Research Department for Underwater Acous- Topography / m Scale / km 1984). tics and Marine Geophysics, Berliner Strasse 115, Large igneous province (Con et al., 2006) Faults (Foster et al., 2009) 0 100 200 D-24340 Eckernförde, Germany. -4000 -2000 0 GEOLOGY, October 2015; v. 43; no. 10; p. 931–934 | Data Repository item 2015311 | doi:10.1130/G36936.1 | Published online 28 August 2015 GEOLOGY© 2015 Geological | Volume Society 43 | ofNumber America. 10 Gold | www.gsapubs.org Open Access: This paper is published under the terms of the CC-BY license. 931 Downloaded from geology.gsapubs.org on September 15, 2015 A across WR, (~600 km oshore) file 2 (Fig. 2C), onshore seismological experi- km/s ments (Heit et al., 2015), and gravity models NW FFZ ? SE 2 0 3 (Maystrenko et al., 2013). Close to the coast, the 5.5 5.5 5.5 5.56.5 10 6.5 4 models show high seismic velocities (as high as 7.87 7 5 7.5 km/s) in the lower crust of the Walvis Ridge. Depth (km) 20 7.8 P150 VE 1:3 6 Velocity This high-velocity lower crustal body is partly 050 100 150 200 250 300 350 400 450 7 8 Distance (km) constrained by reflections from the top and oth- B across WR, (~200 km oshore) FFZ erwise defined by the 7.0 km/s contour line. The N P100 S 0 5.5 5 3.5 high-velocity lower crustal body tapers out ~300 5.5 5.55 6.5 6.5 10 6.5 6.5 km offshore, much like others found along the 7 7 7 20 7 southwestern African coast (Bauer et al., 2000; 7.8 7.3 7.3 7.3 7.3 7.8 7.8 Hirsch et al., 2009; Schinkel, 2006). Compared Depth (km) 30 to these models, where the high-velocity lower 40 P3 VE 1:3 crustal bodies terminate 50 km offshore from 050 100 150 200 250 300 350 400 450 500 550 600 the coast, the Walvis body continues a few tens C across Angola Basin, continued onshore Distance (km) Coast P100 SE of kilometers beneath the continental interior 0 NW 5.5 (Fig. 4). Independent onshore seismic profiles 5 5 6.5 10 6.7 7.87.3 indicate that this eastern promontory of the Wal- 20 7 vis high-velocity lower crustal body is only 100 6.5 6.5 7.3 km wide (Ryberg et al, 2015); this is consider- 30 7.8 Depth (km) 7 7 3 ably narrower than further offshore at P3 (Fig. 40 7. 2B), where its width is almost equivalent to the 50 P2 VE 1:3 bathymetric expression of the Walvis Ridge 050 100 150 200 250 300 350 400 450 500 550 600 650 (160 km). Therefore, compared to the southern D along WR, continued onshore Distance (km) volcanic margin, the additional area of intrusive W P3 P2Coast E 0 lower crust at the landfall of the Walvis Ridge is 3.52.5 5.5 5 5 5.5 5.5 2 10 6.5 6.5 at most 100 × 100 km (Fig.
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