JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. BI1, PAGES 17,475-17,502,OCTOBER 10, 1990 Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System' Implicationsfor African and South American Plate Motions Over Plumes JOHN M. O'CONNOR AND ROBERT A. DUNCAN College of Oceanography,Oregon State University,Corvallis Crystallizationages of volcanicrocks, dredged or drilled from the Walvis Ridge (ten sites)and the Rio GrandeRise (one site), have been determined by the 40Ar/39Ar incremental heating technique. The fundamentallyage-progressive distribution of thesebasement ages suggests a commonhot spot sourcefor volcanismon the island of Tristan da Cunha, along the Walvis Ridge and Rio Grande Rise, and for the formation of the continental flood basalts located in Namibia (Africa) and Brazil (South America). The WalvisRidge-Rio Grande Rise volcanic system evolved along a sectionof the SouthAtlantic spreading-axis, as the African and South American plates migratedapart, astride,or in closeproximity to, an upwelling plume. Reconstructionsof the spatialrelationship between the spreading-axis,the Tristan hot spot, and the evolvingWalvis Ridge-Rio Grande Rise volcanic feature show that, at about70 Ma, thespreading-axis began to migratewestward, away from the hot spot. The resultingtransition to intraplatehot spotvolcanism along the Walvis Ridge (and associatedtermination of Rio GrandeRise formation)also involveda northward migrationof previouslyformed African seafloor over the hot spot. Rotationparameters for Africanmotion overfixed hot spots(i.e., absolutemotion) have been recalculated such that the predictedtrail of the Tristan hotspot agrees with the distribution of radiometricand fossil basement ages along the Walvis Ridge. African absolutemotion has been extended to the Southand North American plates, by the additionof relativemotion reconstructionpoles. INTRODUCTION island [McDougall and Ollier, 1982], with the most recent volcaniceruption having occurred in 1962 [Barker,1964]. Apartfrom the mid-Atlanticspreading-axis, the Walvis Ridge This paperreports the resultsof sixteen40Ar-39Ar (Figurela) and Rio GrandeRise (Figure lb) are the two most incrementalheating studies of volcanicrocks recovered from ten prominentbathymetric features in the South Atlantic basin. locationsdistributed along the Walvis Ridge and a singlesample Broadlyspeaking, the Walvis Ridge and the Rio Grande Rise site located on the Rio Grande Rise (Figure 2). The changing form a V-shaped pair of volcanic lineaments, whose axis of spatialrelationship between the spreading-axis,the Tristan hot symmetryis the South Atlantic spreading-center(Figure 2). spot,and the evolvingstructure of the Walvis Ridge-RioGrande Morgan [1971, 1972], following Wilson [1963], related the Rise system,has been reconstructedfor selectedtimes. These formationof thesevolcanic trails to hot spotvolcanism, presently reconstructionsprovide new insights into the influenceof hotspot active beneath the ocean island of Tristan da Cunha. and spreading-axis interactions on the structural and The Walvis Ridge can be subdivided into three compositionalevolution of the Walvis Ridge-RioGrande Rise morphologicallydistinct regions, two of which appearto have a volcanicsystem. Previously estimated finite reconstructionpoles comparableor "mirror image" region on the Rio Grande Rise. for African plate motionover hot spots(i.e., absolutemotion) Thebroad eastern plateau of the Walvis Ridge projectsseaward havebeen adjusted such that the predicted track of theTristan hot from the African coast to about 5øE. This plateau initially spoton the Africanplate reconstructs the distributionof the new evolvedin conjunctionwith the TorresArch, later with the broad WalvisRidge basement ages. The absolutemotions of the South plateauof the Rio GrandeRise, andfinally with the northeastern and North Americanplates have, in turn, beencalculated by the shoulder of the Rio Grande Rise. The central section of the additionof appropriaterelative motion reconstruction poles. Walvis Ridge (about 5øE to 2øW) includes three distinct, subparallelNE-trending ridges, which formed in conjunctionwith GEOLOGICAL SETTING the eastern,N-S ridge of the Rio Grande Rise. Finally, the WalvisRidge mergesinto a line of large seamounts,which The Parana (southeastBrazil) and the Etendeka(southwest extends southwestward from the central section of the Walvis Africa) continentalflood basalts(Figure 2) representthe earliest Ridgetoward Tristan da Cunha.A correspondingseamount chain known manifestationof the hot spotresponsible for formingthe doesnot appearto existon the SouthAmerican plate; however, Walvis Ridge-Rio GrandeRise. Theseflood basaltfields are theZapiola Seamount Complex, located to the southof the Rio shownto have formedas adjoiningneighbors, on reconstruction GrandeRise may be relatedin some•vay to theTristan hot spot. of the SouthAtlantic for early Cretaceoustime [Rabinowitzand The island of Tristan da Cunha is located N550 km to the east LaBrecque,1979] and Figure 9a. Geochemicaland isotopic data of thepresent day spreading-axis,on C6 oceaniccrust (19 Ma supporta commonsource for the Parana-Etendekaand Walvis after the time scale of Kent and Gradstein [1986], used Ridgebasalts [Hawkesworth et al., 1986]. K-Ar agesranging throughoutthis discussion). Apparent K-Ar agesranging from from 114 to 196 Ma have beenreported for the Etendekalavas 0.01 + 0.02 Ma to 0.21 + 0.01 Ma have been determined for this [Siednerand Miller, 1968;Siedner and Mitchell, 1976]. Erlank et al. [1984] have recognizedthree petrographicunits within thesebasaltic lavas: the coastalAlbin type, the widespreadand Copyright1990 by the American Geophysical Union. voluminousTafelberg type, and in the faultedcoastal zone, late Papernumber 90JB00782. stagecross-cutting dolerite dikes which have been grouped as the 0148-0227/90/90JB-0078255.00 Horingbaaitype. K-Ar agesfor these three units range from 88 to 17,475 17,476 O'CONNORAND DUNCAN: WALVIS RIDGE AND RIO GRANDE RISE HOT SPOTSYSTEM ETENDEKA 24' 34' Fig.1. (a)Bathymetry forthe Walvis Ridge, contoured in1000 m intervals[after Needham etal., 1986]. (b) Bathymetry forthe Rio GrandeRise, contoured in 1000m intervals[after Cherkis et al., 1989]. Mercatorprojections. O'CONNORAND DUNCAN:WALVIS RIDGEAND RIO GRANDERISE HOT SPOT SYSTEM 17,477 I I 1 i i I I 10S Africa South America Parana Spreading-Axis 13 f -' 34 i / s_•.o...u,o '•! !..• 31 25 21 13 6 a 13 II, ,•, ß • 40S •1 13I I I I I I ! I 50 s 6O W 50 w 40 w 30 w 20 w 10 w 0 10 E 20 E Fig. 2. Solidcircles show the locationsof the samplesites from whichthe rocksanalyzed in thisstudy were recovered. Seafloor spreadinganomalies and tYacture zones are from Candeet al. [ 1989]. A fossilspreading center south of the Rio GrandeRise is shownby a "railroadtrack". The locationto whichthe spreading-axisjumped, between C32 andC30 times,is shownas a lineof opencircles on boththe Rio GrandeRise and the Walvis Ridge. Mercatorprojection. 377 Ma. The authorsattribute these anomalously high K-Ar ages continentalcrust, as shownby seismicreflection studies [Sibuet et (as indicated by stratigraphic relationships) to excess argon al., 1984]. This portionof the ridge is characterizedby internal "inherited"at the time of crystallization.Feldspar separates taken basementreflectors which dip to the west, while still further to from two fresh Horingbaai dolerites yielded fairly convincing the west (10øE to 8øE) the acousticbasement changes to that of 40Ar-39Arplateau ages of 125,and 125 to 130Ma [Erlanket al., typical oceanic crust [Sibuet et al., 1984]. On the South 1984]. These late stage intrusives may represent a final American margin, the Parana lavas stretch seaward toward the upwelling of continental hot spot volcanism immediately Vema Channel in the form of the Torres Arch •,Figure lb) precedingthe initiation of Walvis Ridge construction. In Brazil, [Kowsmann et al., 1977; Kumar, 1979]. Paranavolcanism also appearsto have peaked at between 120 and The presentday positionof the spreading-axisin the latitudes 130Ma [Amaralet al., 1966;McDougall and Ruegg, 1966; Melfi, of the Walvis Ridge and Rio Grande Rise is asymmetric with 1967; Siedner and Mitchell, 1976; Cordani et al., 1980]. respect to the coasts of Africa and South America; i.e., the Basedon theresults of seismicreflection profiling, Austin and spreading-axisis locatedcloser to the African than to the South Uchupi[1982] havesuggested that the SouthAtlantic began to American coast (Figure 2). The southern boundary of this openin the form of a northwardpropagating rift. The oldest spreading-axis asymmetry would appear to be the unnamed seafloorspreading anomaly identified in the SouthAtlantic at the fracturezone locatedto the north of Tristan da Cunha (Figure 2). latitudeof Capetownis M11 (133 Ma), while the oldestanomaly This asymmetryhas been attributedto a successionof easterly recognizedoff of the Namibian coastis M4 (126 Ma) [Cande et and southeasterlyspreading-axis migrations [Mascle and Phillips, al., 1989]. As the Etendekaand Paranaflood basaltsappear to 1972; Sclater and McKenzie. 1973; Ladd, 1974, 1976], which have erupted at about the same time that the northward- began prior to C34 (84 Ma) time. The first recordedeastward propagatingSouth Atlantic rift reachedthe latitudeof Namibia, a migration of the spreading-axisoccurred to the north of the closeassociation may haveexisted between hot spotvolcanism Walvis Ridge-TorresArch lineamentat early Albian-late Aptian andcontinental rifting. However,due to uncertaintyin locating time