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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. B6, PAGES 7685-7729, JUNE 10, 1989

Magmatism at Zones: The Generation of Volcanic Continental Margins and Flood

ROBERT WHITE AND DAN MCKENZIE

Bullard Laboratories, Department of Sciences, Cambridge, Enyland

When continents rift to form new ocean basins, the rifting'is sometimes accompanied by mas- sive igneous activity. We show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying . The igneousrocks are generated by decompressionmelting of hot asthenosphericmantle as it rises passivelybeneath the stretched and thinned lithosphere. Mantle plumes generate regions beneath the lithosphere typically 2000 km in diameter with temperatures raised 100-200øC above normal. These relatively small mantle temperature increases are sufficient to cause the generation of huge quantities of melt by decom- pression: an increase of 100øC above normal doubles the arnount of melt whilst a 200øC increase can quadruple it. In the first part of this paper we develop our model to predict the effects of melt generationfor varying amounts of stretchingwith a range of mantle temperatures. The melt generated by decompressionmigrates rapidly upward, until it is either extruded as flows or intruded into or beneath the crust. Addition of large quantities of new to the crust considerably modifies the subsidence in rifted regions. Stretching by a factor of 5 above nor- real temperature mantle produces immediate subsidenceof more than 2 km in order to maintain isostatic equilibrium. If the mantle is 150øC or more hotter than normal, the same amount of stretching results in uplift above sea level. Melt generated from abnormally hot mantle is more magnesian rich than that produced from normal temperature mantle. This causesan increase in seismicvelocity of the igneousrocks eraplacedin the crust, from typically 6.8 km/s for normal mantle temperaturesto 7.2 km/s or higher. There is a concomitantdensity increase. In the second part of the paper we review volcmaiccontinental margins and flood basalt provinces globally and show that they are always related to the thermal anomaly created by a nearby . Our model of melt generation in passivelyupwelling mantle beneath rifting continental lithosphere can explain all the major rift-related igneousprovinces. These include the Tertiary igneonsprovinces of Britain and Greenland and the associated volcanic continental maxgins caused by opening of the North Atlantic in the presence of the Iceland plume; the Paran& and parts of the Karoo flood basalts together with volcanic continental margins generated when the South Atlantic opened; the Deccan flood basalts of India and the Seychelles-Saya da Malha volcanic province created when the Seychellessplit off India above the l•union hot spot; the Ethiopian and Yemen Traps created by rifling of the Red Sea and Gulf of Aden region above the Afar hot spot; and the oldest and probably originally the largest flood basalt province of the Karoo produced when Gondwana split apart. New continental splits do not always occur above thermal anomalies in the mantle caused by plumes, but when they do, huge quantities of igneous materiM are added to the . This is an important method of increasing the volume of the continental crust through geologic time.

Massiveoutbursts of igneousactivity sometimesac- Not only were huge quantities of igneousrock gen- companythe rifting of continentsas they are stretched erated when some continents rifted, but it appears that prior to breaking to form new oceanic basins. Much the bulk of it in any one area was emplaced in rather of the evidence for this is now deeply buried on conti- short periods of time. For example, when the north- nental margins under thick piles of sediment and, fur- ern North Atlantic opened during the early Tertiary, thermore, is hidden underwater. In some places, huge up to 10 millionkrn a of igneousrock wasproduced on areas of flood basalts were extruded onto the surface of the rifted margins in as little as 2-3 m.y. [White et the adjacent continental areas at the same time as the al., 1987b].The bulk of the DeccanVolcanic Province continentswere breaking apart: examples include the of 1-2.5 milhon km3, to cite a continentalflood basalt Deccan of India, the Karoo of southern Africa, and the example,was extruded in probablyonly 0.5 m.y. [Cour- Paran• of SouthAmerica (Figure 1). tillot and Cisowski,1987]. Thesetwo examplesare not insignificant:they are amongstthe two largest volcanic events on Earth that have occurred since 200 Ma. They Copyright 1989 by the American Geophysical Up_ion. may as a consequencehave had considerableeffect on Paper number 88JB03912. the biosphere, and may have been contributory factors 0! 48-0227/89/88 JB-03912505.00 to mass extinctions.

7685 7686 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

NORTH

CENTRAL ATLANTIC

Ethiopia

INDIAN OCEAN iil•ea WEST• o•=,• )'=•t

ATLANTICSOUTH

(•GONDWANABREAKUP

Maud

Fig. 1. Location of voluminousextrusive volcanicrocks on rifted continentalmargins (hatched), and extent of associatedcontinental flood basalts (solid) with circlednumbers showing section of this paper where eachregion is discussed. Projection is Airoff equal are•

Yet, whereas was widespread at some con- riffs in these hot areas and consider the effects of ac- tinental , at others(such as the Bay of Biscaymar- creting the melt to the crust. gin) it was virtually absent. Both volcanicallyactive In section 1 of this paper we discussbriefly the ther- and nonvolcanicmargins occur along the length of in- mal structure produced by mantle plumes as a prelude dividual ocean basins. The Bay of Biscay margin, for to our assertion that the distribution of volcanic mar- example, lies along strike from the volcanically active gins and continental flood basalts can be explained by margins off Rockall Bank and the Norwegian continen- their associationwith nearby plumesthat were active at tal shelf. the time of rifling. In section 2 we discussthe physical In this paper we develop a simple model to explain processesinvolved in melt migration and the volumes the occurrenceof volcaniccontinental margins and flood of melt that are generated by asthenosphereas it wells basalts. We explain the igneous activity as a conse- up and decompresses,and we summarize the specific quenceof relativelysmall (100-200øC)increases above predictionsof our model which can be tested by obser- normal in ,•_Abll• ßb•lllp•[i•l,b...... ß.....UI-• UI t,•_bll• unuenyir,J • ß g astheno- vational data from volcanicmargins and flood basalts. sphericmantle acrosswhich the rift runs. The igneous These predictions relate to the area over which mag- rocksare assumedto be producedby of matism occurs,the volume of igneousrock accretedto the asthenospheredue to decompressionas it wells up the crust, the timing of its emplacement,its geochem- passivelyto fill the space generated by the stretching ical and geophysical characteristics, and its effect on and thinning lithosphere. This model has been applied the subsidenceof the margin. In the third, and ma- by White et al. [1987b]and White[1988a] to the devel- jor part of the paper we summarize the observational opment of the North Atlantic: in this paper we develop constraints from all known examples of volcanic conti- the basic theoretical model and show how it can explain nental margins and show that our model can explain all known occurrencesof volcanic continental margins these observations. around the world. There are two crucial components to our model. First, we require asthenospheretemperatures to be in- 1. THERMAL ANOMALIES CAUSED BY MANTLE PLUMES creasedby 100-150øCover large (typically 1500 to 2000- The interiors of oceanicplates are peppered with nu- km diameter)regions of the earth by heat advectedup- merousvolcanic islands and swellsformed subsequently ward in mantleplumes (commonly called "hot spots"). to the oceanic lithosphere on which they sit. These Second,we calculate the amount of partial melt that is have long been consideredto mark the locations of as- generated by the asthenosphereas it wells up beneath cending mantle plumes beneath the plate and are com- WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7687

TABLE 1. Oceanic Swells

Swell Crustal Age, Residual Depth Anomaly Width of Swell, Source at Center,* ]Via m km

Bermuda 110 1000 800 Derrick et al. [1986] CapeVerde 125 1900 1550 Costtheyand White [1986] Cook-Austral 46 650 1250 Crough[1978] Crozet 67 ~ 800 ) 1000 Coatthey and Recq[1986] Hawaii 90 1200 1200 •on Herzen et al. [1982] Iceland 0 ~ 2000 2000 this study Marquests 45 ~ 800 ~ 1500 Croughand Jarrard[1981] Reunion 67 1050 Croagh[1978] Society 70 1100 Crough[1978]

*Residualdepth anomalywith respectto normaloceanic depth [fromParsons and Sclater,1977] attributed to thermal uplift.

monly called hot spots [Wilson, 1963, 1965; Morgan, spherecools [e.g., Parsonsand Sclater,1977], we can 1971,1981]. Recent heat flow measurementsacross the readily determinethe depth anomaly above an oceanic Hawaiianswell Iron Hersenet al., 1982],the Bermuda plume. By contrast, on land the effectsof subaerial swell[Deirick et al., 1986],and the Cape Verde swell erosionquickly modify the uplift causedby underlying [Courtheyand White, 1986]have confirmed that they plumes,and a variety of other causescan produceele- do indeed have a deep thermal origin. The heat flow vation anomalies. It is harder therefore to identify the acrossthe Cape Verdeswell, for example,reaches 25% positionand size of a subcontinental hot plume. abovenormal at the center comparedwith that through The effect of suboceanicmantle plumes is to uplift lithosphereof the same age away from the swell. the existing seafloorby typically 1000-2000 m at the Although mantle plumes undoubtedlyexist under center. The seafloor is elevated over a 1000 to 2000-km- continentstoo, we shall for the moment considerthose diameter region. The swellsthat coverthe largest area under oceanic lithosphere because the thermal struc- tend to exhibit the greatest uplift. In Table I we show ture of the oceaniclithosphere is more straightforward a compilation of hot spot swell statistics drawn from to predictthan that underold continentsand the distri- a number of different sourcesto show their typical di- bution of suboceanicplumes can be mapped usingsatel- mensions.All these swellsalso exhibit prominent geoid lite altimetry. Furthermore, seafloorelevations caused anomaliesand are causedby deep thermal anomalies. by hot spot uplift are generallypreserved underwater The way in which a mantle plume modifies the up- becausethere is little erosion. Since the age of oceanic per mantle temperaturesis well displayedby studiesof crust is easy to determineusing mag- the Cape Verde swell publishedby Courtney and White netic anomalies and the seafloor depth increasesmono- [1986],so we use this as an example. The Cape Verde tonicallywith age in a well-knownmanner as the litho- swellis a good exampleto model becausethe overlying

_/,.•-•oo•---'------'•oo •oo• -•oo---•_ 200 -200

km _ km thenosphere 400 - 400

600- __ -600

500 km I •

Fig. 2. Temperaturevariations seen in cross-sectionthrough the CapeVerde swell from the bestfitting axisymmet- ric convectionmodel of Courtheyand White [1986]. Temperatureanomalies are labeledin degreesCelsius with respectto the mean asthenospheretemperature. Note the narrowcentral rising plume and the broad mushroom- shapedhead of hot material deflectedlaterally by the overlyingplate. 7688 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

Atlantic plate is moving only slowly with respect to the the core-mantle boundary. However, the temperature underlying hot plume, and the plume has been active structure in the upper mantle where melting occursis since at least 20 Ma and probably since 40-50 Ma. This insensitiveto the depth at which the mantle instability means that the thermal anomalies are fully developed, initiates. So it does not matter for our model. and in a steady state, so heat flow measurementsat We are here concernedwith magmatism which oc- the surface can be used to constrain the upper mantle curs when continental lithosphere is stretched, so we temperature distribution. Courtney and White used a need to know the normal temperature conditions and steady state axisymmetric convectionmodel in the up- the anomaliescaused by mantle plumes under continen- per mantle to model the heat flow variations acrossthe tal lithosphere. The thickness of oceanic lithosphere is swell, together with the other main observationalcon- governedby the loss of heat from the vigorously con- straints available, namely the geoid anomaly and the vecting,adiabatic upper mantle [Richter and Parsons, bathymetric relief. 1975;Parsons and MCKenzie,1978]. Upper mantlecan The temperature distribution acrossthe Cape Verde be convertedfrom asthenosphereto lithospheresimply swell for the best fitting convectionmodel is shown in by coolingand without any changeof chemicalcomposi- Figure 2. The main point to notice is that there is a tion. The temperature structure beneath oceans in the narrow (150-km diameter) plume of hot mantle rising absenceof hot spot plumes can be readily parameterized upward at the center, and that this is deflected later- [Richterand MCKenzie,1981]. It is characterizedby an ally by the overlying plate to form a mushroom-shaped upper rigid layer, the mechanicalboundary layer, with a head of anomalously hot mantle some 1500 km across. conductivetemperature gradient which lies abovea thin A common misconceptionabout hot spots is that they thermal boundary layer in which a transition occursto are only relatively narrow features. Although plume- the well-mixed adiabatic temperature of the upper man- generated melts will normally only penetrate through tle [Parsonsand MCKenzie, 1978; MCKenzie and Bickle, the crust immediately above the rising mantle plume, 1988; White, 1988b]. Under old oceaniclithosphere, the central plume feedshot mantle into a huge area be- such as that beneath the Cape Verde swell, the litho- neath the plate. As we shall see in section2, if the litho- sphereattains a thicknessof about 125 km [Parsons sphere is rifted anywhere above this mushroom-shaped and Sclater,1977]. head, then enhanced melt production will result from The thickness of continental lithosphere is princi- the abnormally hot asthenosphereas it wells up and pally controlled by its age. Beneath Archaean shields decompresses. the lithospheric thickness may reach 200 km or more, The convective flow dynamically uplifts the plate basedon the stabilityfield for diamondformation [Boyd above it. This explains the broad swells observed et al., 1985], on heat flow [Pollack and Chapman, aroundhot spots (Table 1). Crustal thickeningby the 1977],and on seismicvelocity structure [Jordan, 1975, addition of igneousrock to the crust can also causeup- 1978, 1981]. However,beneath Phanerozoic crust the lift, as can the reduction in density of the upper mantle lithospheric thickness seems to be similar to that of depleted by melt removal, and a proportion of the up- old oceanic lithosphere, and it behaves thermally in lift of the hot spots listed in Table I may be due to this a similar way to oceaniclithosphere [MCKenzie, 1978; effect. In the case of the Cape Verde swell, there has Sclater et al., 1980; Barton and Wood, 1984; Richter, been little crustal thickening, and the majority of the 1988; White, 1988b].The magmaticallyactive rifts re- uplift is dynamically supported by the flow. The same ported from around the world have all formed on rifted is probably true of all the swells listed in Table 1. Phanerozoiclithosphere, and thus justify use of the pa- It is also worth noting the magnitude of the tem- rameterization of the thermal structure of the litho- perature anomaliesgenerated by the hot spot. Beneath spheredeveloped by Richter and MCKenzie [1981]in most of the swell the temperaturesjust under the plate our calculationsof melt generation. are of the order of 100øC above normal, with hotter Geoid anomaliessuggest that upwelling regionsin temperatures only in the core of the central plume. the mantle occur with a spacing of 2500-4000 km be- The constraintsimposed by the surfaceobservations are neath the oceans[MCKenzie et al., 1980; Watts et al., suchthat the best resolutionon the underlyingtemper- 1985a].The geometryof the mantle circulationbeneath ature structure is in the region just beneath the plate continents is much more difficult to study, since the [Parsonsand Daly, 1983; Courlneyand While, 1986]. geoid cannot be obtained from satellite altimetry. Nor For our purposesthis is fortunate, becausethe amount is the topography much help, because any variations of melt generated is critically dependent on the tem- producedby mantle flow are superimposedon larger peratures in just this region near the base of the plate variations resulting from changesin crustal thickness. [MCKenzieand Bickle,1988]. Though active intraplate vulcanism givessome indica- We note in passingthat we can say little about how tion aboutwhere the mantleplumes may be [e.g.,Mor- deep in the Earth the convectionpenetrates. Courlney gan, 1981],it is lessreliable for this purposethan are and While[1986] modeled convection in the upperman- the geoid or residual depth anomalies. tle only,whilst Olsonel al. [1987],for example,assume An important questionis whether or not the plumes that mantle plumes are generated in the D • layer at are steady. At present we know more about the time de- WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7689 pendenceof two-dimensional numerical convectionthan earlier, we use the thermal model developedfor the we do about that of the mantle. Vigorously convecting Cape Verde swell as a good analogof conditionsprior to systemsshow various forms of time dependencein three rifting abovea continentalmantle plume. Not only does dimensions. Most studies have been concerned with the Cape Verde swell lie over old oceanic lithosphere constant viscosity fluids heated from below. When the which therefore probably has a thermal structure simi- Rayleighnumber is large(larger than about 105) the lar to Phanerozoic continental lithosphere, but it is also convective planform consists of hot and cold vertical producingonly minor volcanicproducts, despiteits ma- sheetsradiating from points. The flow becomestime ture age. We therefore take it to be representativeof the dependent when hot and cold blobs are carried round thermal structure prior to continental rifting and be- the cells. The planform of mantle circulation more often fore the production of voluminous melt. We would not consistsof plumes than of sheets,probably becausethe expect every mantle plume to be identical in thermal viscosityof the material dependsso strongly on tem- structure to the Cape Verde model. Indeed the range perature. of sizesof the swellslisted in Table 1 givessome idea of When constant viscosity fluids are heated inter- their variability. But we do expect them to exhibit sev- nally, the planform consistsof narrow sinking plumes eral general features in common. We summarize these in three dimensions..[Whileheadand Chen, 1970; Craig as follows. and MCKenzie,1987]; or sinkingsheets if the flow is re- 1. Hot spots comprisea narrow (150-200 km wide) strictedto two dimensions[MCKenzie el al., 1973].Such central plume of rising mantle with abnormally high circulationsbecome time dependentin a different way. temperatures. Existing plumes slowly approach each other and then 2. The convectedmaterial spreadsout under the plate join suddenly. As this processreduces the number of to form a mushroom-shapedhead of asthenospherewith plumes, and therefore the heat transport, new plumes temperatures raised typically 100-200øC above normal. must form. They do so through boundary layer instabil- 3. The lateral extent of the hot asthenospherein the ities at placeswhich are furthest from existing plumes. mushroom head is 1000-2000 km in diameter. A similar process has recently been described in two- 4. The convectingmantle in the hot spot causesdy- dimensionalconvection heated from below [Jarvis, 1984] namic uplift which reachesa maximum of 1000-2000 m. at a Rayleighnumber of 1.5 x 109with freeboundaries. 5. The start of a new plume is associated with the The necessaryRayleigh number for this processto occur arrival of a large blob containingabout 5 x 106 km3 is likely to be lower for three-dimensional circulations, of material 100-200øC hotter than the mantle interior and is likely to be further reduced if one or both of the temperature at the base of the plate. The rate of mass boundariesis rigid and if the viscosity is temperature transport and the temperature of the initial blob are dependent. All these effects reduce the stability of the higher than those of the subsequentsteady state man- boundary layers. The time scale for plumes to move tle plume, providing transient conditions particularly together is likely to be at least 100 m.y., whereas the favorable to excessmagmatism at the initiation of a time taken for an instability of the boundary layer to new plume. develop is unlikely to exceed 10 m.y. There is as yet little information about the time de- pendenceof mantle convection. That of the large-scale 2. MELT GENERATION BENEATH flow is controlledby the movementof the large plates. RIFTING LITHOSPHERE Studiesof hot spots[Molnar andAiwalet, 1973;Molnar When lithosphere is thinned tectonically the under- and $1ock,1987; Sager and Bleil, 1987]show that they lying asthenospherewells up to fill the space. As the move relative to each other at rates of 10-30 mm/yr, asthenospheredecompresses, it partially melts. The but do not yet provide evidence that they attract each amount of melt generated can be calculated readily, other in the manner seen in the experiments. The be- provided the upwelling is assumedto occur at constant ginning of a hot spot has also not been observed,but entropy[MCKenzie, 1984]. numerical experimentssuggest that the boundary layer In the last few years there has been a great deal of instability will produce a roughly spherical blob 100- interestin the separationof melt from a partially molten 200øC hotter than the averageinterior temperature of rock. This work has led to a better understandingof the mantle and will contain about 5 x 106 km 3 of ma- the processesinvolved. Two principal resultsare of con- terial. As the blob approachesthe upper boundary, its cern to this discussion. The first concerns the amount shapeis changedinto a fiat circular pillow by the stag- of melt which fails to separate from the residue. Those nation point flow. This blob will start a mantle plume who have consideredthis problem agreethat this melt whose radius will be about half that of the blob. The fractionis about 1% or 2% for basalts[Ahem and Tur- rate of heat transport by the blob is likely to be about cotte,1975; MCKenzie,1985], or considerablyless than a factor of 5 greater than the steady heat transport of the uncertaintiesin the melting calculations.M. Chea- the plume. die (personalcommunication, 1988) has recentlystud- In the absence of direct measurements across a con- ied this problem in more detail, and has shownthat the tinental hot spot, becauseof the difiqcultiesmentioned earlier calculationshave overestimatedthe quantity of 7690 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES melt that remains behind with the residue. It is there- fore sufficientlysmall to be neglectedin the presentcal- culations. 30I MELT The second result involves the question of whether !THICKNESS ...p=50 km melt can flow significanthorizontal distances as it sepa- rates from the residual matrix. The extent of horizontal 20 flow is controlledby the pressuregradient in the matrix, is=5 which is in turn controlled by the matrix viscosity.This viscosityis not likely to exceedthe valueof 102•Pa s usedby Spiegelmanand MCKenzie[1987],who found a deflectionof only 30 km at oceanicridge axes. Ribe and Smooke[1987] have shown that the influenceof the ma- trix viscosity on melt separation from plumes is even p=2 ... smaller, and Scott and Stevenson[1989] have argued that the matrix viscosity is i or 2 orders of magnitude less than 102• Pa s, and thereforedeflection of melt as it separates is even less important than Spiegelman 1200 1300 1400 1500 and MCKenzie[1987]believed. For our presentconcerns these arguments are of little interest: all authors are POTENTIAL TEMPERATURE *C agreed that the horizontal deflection of melt between Fig. 3. Thickness of melt generated by adiabatic decompression of asthenosphericmantle over a range of potential temperatures. its sourceand the surfaceis no greater than 30 km. Curves are shown for initial thicknesses of the mechanical bound- These calculations therefore allow us to assume that ary layer of 70, 100, and 130 km, with thinning by factors of 2, all melt produced by decompressionmelting separates 5, and 50. Corresponding lithosphere thicknessesin the thermal from its residue and moves directly upward. It is then plate model are somewhat thicker than the mechanical bound- ary layer [MCKenzieand Bickle, 1988; White, 1988b].Annotated either erupted at the surface or emplaced in the crust. mechanical boundary layer thicknesses of 70, 100, and 130 km These assumptionsare now soundly based and greatly correspond to plate thicknesses of 87, 118, and 149 kin, respec- simplify the calculations. tively. Cross shows limits on normal temperatures We calculate the melt production using a param- calculated from the range of measured oceanic igneous crustal thickness from spreading centers. eterization of the melt fraction as a function of pres- sure and temperature derived by MCKenzie and Bickle [1988].Their parameterizationwas calculatedfrom all available published data on melting obtained by labo- warduntil it is addedto the overlyingcrust [MCKenzie, ratory experiments. For our purposesthe most impor- 1985]. Thereforealthough a small fractionof perhaps tant result is that an increase of 200øC above its normal 1-2% of melt is likely to remain with the solidresidue in value in the potential temperature of the asthenosphere the mantle, for simplicity we assumethat all the melt can more than triple the amount of melt generatedbe- that is generated is extracted. The principal error in neath rifting lithosphere. So when a rift cuts acrossa the calculations arises from uncertainty in the latent region of hot asthenospherecaused by material carried heat of basalt, which produceserrors of perhaps 30% up in a thermal plume, large amounts of melt can be in the amount of melt produced. In Figure 3 we show generated. In the followingsections we amplify our pre- the thicknessof melt generated by uniformly stretch- dictions of the volume of melt produced, the timing of ing the lithosphereby factors of 2, 5, and 50. A factor its emplacement, and its chemical and physical prop- of 2 is a typical value found in intracontinental sedi- erties. In conjunction with the thermal model of hot mentary basinswhich have not subsequentlydeveloped spots, we show how the addition of new material to the into ocean basins. A factor of 5 is roughly the point at crust affects the subsidenceand subsequentevolution of whichstretched continental crust breaksto fully igneous the rifted region. ; and the factor of 50 is representative of Following MCKenzie and Bickle, we define the po- the upwelling which occursbeneath oceanicspreading tential temperature of the asthenosphereas the tem- centers. For each case we show a range of mechani- perature it would have if brought to the surface adi- cal boundary layer thicknessesfrom 70 to 130 km for abatically without melting. To convert to actual as- the undisturbed continental lithosphere: this coversthe thenosphere temperatures, the increase appropriate to range we are likely to encounteralong rifting margins. the depth due to the adiabatic temperature gradient of The variation in melt production with astheno- about 0.6øC/km has to be addedto the potentialtem- sphericpotential temperature providesa sensitivemea- peratures we quote. sure of temperatures under oceanic spreading centers. Since the oceanicspreading centersextend around the 2.1. Volume of Igneous Rocks world, they sample asthenosphereacross a wide geo- Once partial melt has been generated, we assume graphicarea, with a stretchingfactor of infinity (closely that it separatesrapidly from the matrix and movesup- approximatedby • = 50 in our calculations). WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7691

Apart from crustal thickening causedby underplat- aration. However, once the continents start to separate, ing beneathmantle plumes[e.g., Watts et al., 1985b], they often do so rapidly. It is during this main phase and thinning near fracture zones[White et al., 1984], of rifting that the voluminous outburst of magmatism the thickness of the igneous portion of oceanic crust occurs. We expect most of the continental margin vol- is remarkably consistent the world over. This is true canism and continental flood basalt flows to be erupted regardlessof age or of spreading rate. Compilations during a short period contemporaneouswith the main based only on travel time inversionsof seismic refrac- rifting. tion profiles indicate a mean igneouscrustal thickness of 6.5 km [Hill, 1957; Raitt, 1963]. More recent de- 2.3. Properties of the Igneous Rocks terminations based on synthetic seismogrammodeling Even with high asthenospheretemperatures and provide similar conclusions,suggesting normal oceanic large amounts of stretching, the degreeof partial melt- thicknessof 6-7 km, with an outer range from 4.5 to ing does not exceed about 30%. Infinite stretching 8.5 km [$pudichand Orcutt,1980; White, 1984]. From with asthenosphereat a normal potential temperature this we infer that the normal asthenospherepotential of 1280øC produces melt with the composition of mid- temperatureis 1280øC,with a rangeof q-30øC(shown oceanridge basalts(MORB). As the potentialtemper- by a crossin Figure 3). Foucheret al. [1982]came to ature of the asthenosphereis increased up to 1480øC, a similar conclusion:using a simple analytic expression some 200øC above normal, the percentageof MgO in- for the degreeof partial melting as a function of temper- creasessystematically from about 10% to 18%, and the ature and depth [fromAhem and Turcotte,1979], they percentageof Na20 simultaneouslydecreases. Thinning found that the potential temperature of the astheno- the lithosphere by a factor of 2 above asthenosphere sphere required to produce average thickness oceanic at 1480øC producesalkali basalts passingto tholeiitic crust is between 1262 and 1295øC. basalts as the stretching is increasedfurther. The volume of melt generated dependsonly on the These systematicchemical changes in the melt are a amount of lithosphere thinning and the temperature of sensitive indication of the asthenospheretemperature. the underlying asthenosphere. Thickest igneous sec- Thus picritic basalts with a high percentageof MgO tions are found on rifted continentalmargins where they foundon DiskoIsland in westGreenland [Clarke, 1970] have cut acrossthe 'mushroom heads' of abnormally hot are indicative of abnormally high asthenospheretem- mantle brought up in a plume. However, any minor rift- peratures. Similarly, the increasein percentageof Na20 ing above the hot asthenospherewill produce localized of oceanicbasalts as one movesaway from the hot spot igneousactivity on a smaller scale, and this explains, underIceland [Klein and Langmuir,1987] is a sensitive for example, the broad occurrenceof similarly agedvol- measureof decreasingasthenosphere temperature away canic rocks across a 2000-km-wide area in the British from the central mantle plume. Tertiary igneous province. These chemicalchanges in melt, and particularly the The area over which volcanic rocks are found de- systematicchange in the MgO content of the melt with pends not just on the location of the rift but also on asthenospheretemperature, cause systematic changes how far the flow. Huge areas of flood basalts are in the seismicvelocity and the density of the igneous often found on the continents adjacent to rifts. Their rocks that are formed on rifted margins. The seis- outward flow may be assistedby the fact that the rifted mic velocity of the igneousrock was calculated by tak- region which forms the source is uplifted by typically ing the theoretical compositionof the melt, finding its 1- 2 km (alsosee section 2.4). CIPW norm, the Voight-Reuss-Hill averagesfor each mineral and then for the aggregate. All hypersthene 2.2. Timing of Volcanism is assumedto be converted to olivine and quartz for Partial melt is generatedas the asthenospherewells these calculations. The seismic velocities were cor- up passivelybeneath the thinning lithosphere. The tim- rected to the value they would have at 10-km depth ing of volcanismseen at the surfacethus dependson the in the crust, using the changeof velocity with temper- time and rate of lithosphericthinning and on the time ature and pressure for individual minerals from Fur- it takes for the melt to move upward from the zone long and Fountain [1986]. The overall correctionsare of melting in the mantle to the surface. The basalts about-0.45 x 10-3 km/tsper øC for temperatureand that are formed by asthenospheredecompression sepa- 16 x 10-3 km/s per kbar for pressure.Since the pres- rate out from the matrix very rapidly, and the bulk of sure and temperature correctionswork in opposite di- the melt will reach the surface in less than I m.y. af- rections, the net result is very small. At 10-km depth ter its formation[MCKenzie, 1985]. On geologicaltime the seismicvelocity is about 0.05 km/s lower than at scalesit is essentiallysimultaneous with the rifting. the surface due to pressureand temperature effects. When continentsbreak apart, there is often a long The results of these calculations show that as the as- precursoryperiod of small-scalerifting, sometimesdis- thenospheremantle potential temperature is increased tributed over a broader region than the final split. This from normal (1280øC),the seismicvelocity of the ig- is likely to cause minor volcanismover a broad region neousrocks formed by partial melting increasesfrom for severalmillions of years prior to the continental sep- about6.9 km/s up to 7.2 km/s (Figure4). Clearlythis 7692 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

7.5 main factor is the thinning of the lithosphere caused SEISMIC by extension. This was the only cause consideredby VELOCITY (km/s) MCKenzie[1978],and it providesa goodmodel for rifted regionssuch as the North Sea where there is little vol- canism.Above normal temperature asthenosphere (Tp 7.0 ß = 1280øC),the surfacesubsides by 2.3 km when the lithosphereis thinnedby a factorof 5 (1280øCcurve in Figure6). Where the underlying asthenosphericmantle is hot- ter than normal due to the presenceof a nearby thermal

6.5 I I plume, three additional factors work in concertto pro- 1300 1400 1500 1600 duce uplift. Depending on the temperature anomaly POTENTIAL TEMPERATURE oc in the mantle, this can severely reduce the amount of subsidencecaused by lithospheric thinning, or can even Fig. 4. Theoretical compressional wave seismic velocity at a depth of 10 km of igneous rock formed by decompression melting of produce uplift above sea level. In reducing order of mantle at a range of potential temperatures(see text for details importance, the three effects are the addition to the of calculation). crust of igneousmaterial producedby adiabatic decom- pression,the dynamic support produced by the mantle plume, and the reduction in density of the residual man- tle after removal of melt. calculationis only approximate becausethe preciseseis- The modification to the subsidencecaused by accre- mic velocity of the rocks in the lower crust will depend tion of melt to the crust can be readily calculatedusing on the mineralogy that is present, the actual pressure the appropriatevolume of igneousrock (from Figure3) and temperature, the porosity of the rock caused by and its density(from Figure 5) for any givencombi- cracks,and the degreeof alignment of the crystals. But nation of stretchingfactor and asthenospheretempera- it is sufficient to indicate that the lower crustal layers ture. with seismicvelocities of about 7.2 km/s foundon conti- The accretion of melt to the crust makes a major nental margins in associationwith evidenceof extrusive differenceto the subsidence. When the asthenosphere volcanicactivity probably representnew igneousrock. temperature is 100øC above normal, the initial subsi- The density of the igneousrock at 10-kin depth cal- dence for 1• - 5 is reduced from 2.3 to 1.5 km, and culated in a similar manner varies from 2.99 to 3.07 when the temperature is 200øC above normal, it sub- Mg/m3 asthe potentialtemperature of theparent man- sidesby only 0.5 km (Figure 6). Note that our calcu- tle is increasedfrom 1280øC to 1480øC (Figure 5). lations are different from those made by Foucher et al. Again, this takes no account of cracks,but it does al- [1982]when they consideredthe effectof melt produc- low us to use a self-consistentdensity in our calculations tion on the subsidenceof the Biscay continentalmargin. of subsidenceof the margin discussedin the next sec- In our calculations of initial subsidence we assume that tion. The corrections for pressure and temperature are almost all the melt separatesfrom the matrix and cal- small, decreasingthe density at 10-km depth by only culate the densitiesafter solidification, whereas Foucher 0.01Mg/m 3 comparedwith the densityat the surface. The density of the igneous rock generated by mantle decompressionlies midway between that of the mantle and that of continental crust, and thus explains why 3.2 a considerable volume of melt is trapped as it moves DENSITY upward by the lower-density crust and is underplated (Mg/m3) beneathit [e.g., Herzberget at., 1983]. 2.4. Subsidenceof Rifted Continental Margins When continental lithosphere is thinned by exten- 3.0 sion, the elevation of the surface changesimmediately to maintainisostatic equilibrium [MCKenzie, 1978]. For normal crustal thickness and asthenospheretempera- tures the surface subsides. Since stretched continental crusthas very little flexuralrigidity [Barton and Wood, 1984; Fowler and MCKenzie,in press],the subsidence 1300 1400 1500 1600 can be calculated easily, assuming that local isostatic POTENTIAL TEMPERATURE Oc equilibrium is maintained. Fig. 5. Theoretical density at a depth of 10 km of igneous rock There are four main factors that control the amount formed by decompressionmelting of mantle at a range of poten- of subsidence, which we will consider in turn. The tial temperatures(see text for details of calculation). WHITEAND MCKENZIE: MAGMATISM ATRIFT ZONES 7693

STRETCHING FACTOR p Upliftof about 500 m isproduced on the rifted mar- ginby the density change ofmantle that is 100øC hotter 3 4 5 thannormal (1380øC curves in Figure7, comparedwith Figure6). If themantle is 200øC above normal, the up- 7O lift is about twice as much. Directly abovethe central plumeof thehot spotthe upliftmay be evengreater. 1480øC 0.5 The upliftcaused by the densitychange of theman- tle as it becomesdepleted by the removalof melt is the most uncertainof the corrections,but is also the smallest.The effectincreases as the amountof melt ex-

1.0 tractedincreases. For a stretchingfactor of 5, whichis aboutthe mostwe might expect to seeon a continental 7O marginand an asthenospheric temperature anomaly of

1380øC 1.5 ,.5[ ELEVATION residual

2.0 70 km

SUBSIDENCE 1480øC km 1280øC

2.5 0.5 Fig.6. Amountof subsidence at the time of rifting as a function of thedegree of stretching.Subsidence calculation includes the effectof lithosphere thinning and the effect of addingto the crust 3 4 5 meltgenerated by decompressionof the asthenospheric mantle. 0.0 Subsidenceis shownfor three representativeasthenosphere po- STRETCHING FACTOR tentialtemperatures of 1280øC(normal), 1380øC, and 1480øC. Foreach temperature the subsidencecurves are shownfor three + residual initial thicknessesof the mechanicalboundary layer of 70, 100, mantle and130 kin. Correspondinglithosphere thicknesses in the ther- 0.s matplate model are 87, 118,and 149, respectively. 1380øC + thermal anomaly

1.0 et al. allow melt fractionsof up to 10% to remain in the mantle and use the densityof the liquid melt. The subsidencecurves in Figure6 are calculatedfor 1.5 a rangeof initiallithosphere thicknesses which encom- passthe likely values prior to riftingof thecontinental margins.For each different lithosphere thickness the

surfaceis assumedto be at sea level prior to rifting, 2.0 and the subsequentsubsidence calculated by isostati- SUBSIDENCE + residualmantle callybalancing the stretched section against the prerift km 1280oc section. Superimposedon the considerableuplift caused by 2.5 -- addition of melt to the crust is the dynamicuplift gen- Fig.7. Amountof subsidence at the time of rifting as a function eratedby mantlecirculation in the underlyingplume. of the degreeof stretching.To thesubsidence caused by litho- To calculatethis correctly,the completeflow should spherethinning and the addition of melt to thecrust (taken from be modeled[e.g., Courthey and White, 1986], but the Figure6) isadded the uplift caused by reduced density in theab- maineffects can be approximatedby assumingthat hot normallyhot asthenospheric mantle (solid lines), and the further upliftresulting from reduced density in theresidual mantle from mantlereplaces the normalmantle down to somecho- whichmelt hasbeen extracted (broken lines). Curvesare calcu- sendepth of compensation.An appropriatedepth is, latedassuming aninitial mechanical boundary layer thickness of in therange 150-200 km [Courthey and White, 1986; 100 km (equivalent toa 118-km-thick thermal plate), which has Kleinand Langmuir, 1987], sowe show curves forthese its unstretched surfaceatsea levd prior tothe introduction of twocases inFigure 7(solid lines) ' The coefficient of acurvesthermal refer anomalyto the depth inthe of asthenosphere. compensation inThe kilometersnumbers and onspan the thermalexpansion for the mantlewas taken as 3.4 x thetypical depth range over which the anomalous mantle extends 10-s øC-• [afterCochran, 1982]. inthe region surrounding a mantle plume. 7694 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

100øC, the additional uplift causedby mantle depletion sidencemay be very different on the two sides of a de- is about 350 m (brokenlines, Figure 7). When the tem- velopingoceanic rift; second,the magmatismcaused by perature anomaly is 200øC, the uplift is about 500 m. decompressionof upwelling asthenospheremay be off- For lower amounts of stretching the effect is smaller set laterally from the main region of crustal thinning (Figure 7). In principleit is possibleto calculatethe [Bosworth,1987]. The rift basinsformed in continen- compositionof the residual mantle left after removal of tal crust are often asymmetric[Bally, 1981; Bosworth, any desired amount of partial melt, and hence to de- 1985; Klitgord et al., 1988], and detachmentfaulting rive the densityof the residualmantle [Bickle, 1986]. has been invokedto explain the asymmetry[Lister et But in practice the uncertainties in the mineralogy at al., 1986],the variabihtyin the basinsalong the East depth make this an unreliable method for small melt Coast U.S. continentalmargin [Klitgord et al., 1988], fractions, and we have adopted the simpler method fol- the crustal thinning on the Biscay continental margin lowedby Klein and Langmuir[1987] of usingthe den- [Le Pichon and Barbier, 1987], and the exposureof sity differencebetween lherzolite and residual harzbur- mantlerocks on the Iberianmargin [Boillot et al., 1987]. gite [Oxburghand Parmentier,1977] as indicative of the In our model we assumethat stretching the litho- density changesto be expected. sphereresults in uniformthinning (pure shear), and cal- In our subsidencecalculations we have ignored the culate the melt produced accordingly. This approachis density decreaseof the crustal rocks causedby heating justified becausewe are dealing with the large quan- from the newly intruded melt. We have also neglected tities of melt generated by extreme thinning of the the density decreaseof the igneousrocks that results lithosphere, where the ultimate break to a new oceanic from their initially high temperatures. Both these ef- spreadingcenter occurs in the same location as the ma- fects would tend to reduce still further the initial subsi- jor thinning. The minor, but neverthelesswidespread denceshown in Figure 7. Elevated temperaturesin the syn-rift volcanism that commonly accompaniesthe ini- lower crust would be lost by conduction on a timescale tial stages of intra-continental rifting is likely to re- of a few to 10 million years, so the reduced densities re- flect localized thinning and asymmetric stretchingand sulting from high temperatureswould only be effective may well be asymmetricallydistributed [e.g., Bosworth, in the initial stagesof rifting. 1987]. But the large volumesof melt with whichwe are We conclude that relatively small temperature in- concernedwill be produced close to the location of the creasesin the asthenosphericmantle have a dramatic ultimate break to fully igneousoceanic crust. effect on the initial subsidence of rifted basins and In general,the transition from normal thicknesscon- continental margins. For typical ranges of lithosphere tinental crust to oceanic crust occurs over a relatively thicknessesand compensationdepths, a temperature in- short distance, typically 50-80 km, on volcanic rifted crease of 100-150øC above normal causes the surface to margins[White et al., 1987b;Mutter et al., 1988].The remainnear to or abovesea level as the regionrifts (Fig- transition on nonvolcanicmargins such as the Bay of ure 7). Continentaluplift hasoften beenreported prior Biscay margin is often over a greater width. We at- to rifting [e.g., Le Bas, 1971; Kinsman, 1975]. As we tribute this to the weakeningeffect of large quantities shall see in section 3, the majority of the volcanic rocks of hot intruded igneousrock: once the lithospherehas found on rifted continental margins were erupted sub- started to rift this will weaken it considerablyin the re- aerially. The main rift may be elevated so much that gion of greatestthinning. We also note in passingthat the basalt generatedby mantle decompressionis able it is difficult to define a precise"continent-ocean bound- to flow out laterally and downhill over huge regionsto ary" in the heavily intruded region of faulted, thinned generateextensive areas of flood basaltson the adjacent continentalcrust, where there are possiblydisaggre- continents. gated continentalblocks in a matrix of young igneous Once a continental margin has ceasedrifting and a rock [White, 1987]. new oceanic spreading center has developed,the mar- gin will begin to subsidethermally in a normal manner for the amount of stretching as the underlying astheno- 2.6. Comparison With Other Models sphere cools. of Riff Magmatism It may be useful to draw out some of the differ- encesbetween our model for rift magmatism and some 2.5. Pure or Simple Shear? of some of those proposed by others, although this is There has been much debate about the possibility by no means intended to be a detailed critique of other that in extensionalregions there may be major detach- models. ment faults extending through the crust and possibly The model of melt generation by passiveastheno- through the entire lithosphere which allow the upper sphere upwelling under rifting continental lithosphere crust to thin in one location whilst transferringthe ma- proposedby Foucheret al. [1982]comes closest to our jor lithosphericthinning laterally to another[e.g., Bally, model, although they only consideredstretching above 1981; Wernicke,1985]. There are two major conse- normal temperature asthenospheresuch as is found on quencesof this idea: first, the crustal rifting and sub- the Biscaymargin. They did not considerrifting above WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7695 abnormally hot asthenospherewhich generatesvolumi- to move melt generated in that central plume laterally nous magmatism. In detail, they used a simpler an- 1000 km before extruding it. alytic expressionfor melt generation as a function of A rather different model to explain volcanic conti- pressure and temperature, and they assumedthat up nental margins which requires no increasein the tem- to 10% liquid melt remains in the matrix in their cal- perature of the asthenospherehas been proposed by culation of subsidence, in contrast to our belief that Mutter et al. [1988].They suggestthat small-scalecon- almost all of the melt migrates upward rapidly. Fur- vection beneath the rift will locally enhance the melt long and Fountain [1986]suggested that 10-15 km of generation. The controlling factor in their models as crustal underplating could be seen in high seismicve- to whether or not small-scale convection commences is locity lower crust, but they arguedthat the high seismic the sharpnessof the lithosphere break. Beneath the velocitieswere causedby phasechanges rather than by V0ring Plateau they believe that the continental litho- compositionalchanges as in our model. sphere breaks almost vertically and so promotes small- Hot spots have long been consideredas a source of scale convection whilst the broader stretched transition weaknessin the lithosphere which controls the location on the Biscay margin does not trigger small-scalecon- of subsequentrifting [e.g., Hyndman, 1973; Kinsman, vection. 1975;Morgan, 1981, 1983]. We have no particularrea- In our model it is unnecessaryto proposethat small- son to believe that the location of oceanic rifts is con- scale convectionoccurs to enhance melt production be- trolled solely by hot spots as in $pohn and $chubert's causesufficient melt is producedby passiveupwelling of [1982]model, or in Sengotand Burke's[1978] "active hot asthenosphere,and there is such an obviouscorrela- mantle hypothesis," since there are examples of hot tion betweenmagmatism on the rifted margins and the spotswithout subsequentrifting and many other exam- extent of the thermal anomaly in the mantle from ad- plesof continentalrifting awayfrom the influenceof any jacent mantle plumes. This correlation is documented known hot spots. Nevertheless,if a hot spot is present from rifted margins around the world in the secondhalf prior to rifting, it will causeconsiderable uplift, of the of this paper. The relatively sharp break in the conti- order of i km or more across a 1500 to 2000-km broad nental crust on volcanic margins commented upon by region. This providessignificant gravitational potential Mutter et al. [1988]may be a natural consequenceof which will greatly assistrifting acrossthe region. The the weakeningcaused by heavy igneousintrusion, rather difference between the contribution from the elevation than a prima facie cause of the magmatism. Several and the earlier notions of the effectsof hot spots on con- other observationson volcanicmargins are also at vari- tinental rifting is largely one of scale:the 2000-km-wide ancewith Mutter et al's model, suchas the grossasym- thermal anomaly generatesstresses across a broad re- metry of the conjugate Hatton Bank and east Green- gion, whilst earlier authors were thinking only in terms land volcanicmargins [White et al., 1987b;Larsen and of the localized thermal effects beneath the rift itself. Jakobsdottir,1988], the extreme rapidity of melt pro- Sometimesthe onset of continental breakup may be duction documentedfrom some volcanic margins and causeddirectly by the initiation of a new mantle plume. flood basalts, and the presenceof continental crust be- In other cases the plume may have existed for some neath the seaward dipping reflectors. time but rifting may not start until the plate has moved acrossthe plume to a place where the plate can be bro- 2.7. Summary of the Mantle Plume- ken. Lithosphere Rifling Model Once rifting does occur acrossthe mushroom head Our model is intended to explain the presenceof vo- thermal anomaly around a mantle plume, then we ex- luminous magmatic activity on some continental mar- pect it to modify the continentalmargins by producing gins, producing thick intrusive and extrusive igneous extensivemagmatism along the rifted margins with a sectionson the rifted margins and sometimesextensive thick volcanic ridge such as the Iceland-FaeroesRidge flood basalts on the adjacent continents. The main un- or the Walvis Ridge directly above the central plume certainty in calculating the volume of partial melt that itself. Note that our model is different from that pro- is generated comesfrom uncertainty in the latent heat posedby Vink [1984],who suggestedthat not only was of melting,which is takento be 3.8 x 105J/kg. This the Iceland-FaeroesRidge produced by melt fed from uncertainty may causeerrors of up to 20øC in the deter- the central hot spot plume, but also that the marginal mination of the asthenospheretemperature. The partial Voring Plateau to the north was fed directly from the melt calculations are least certain for small melt frac- plume. We do not believe that Vink's suggestionis tions because not only are experimental data sparse, correct becausewe can show that passiveupwelling of but the uncertainty in the small amount of melt left the mushroomhead of hot asthenosphereis adequateto in the matrix will affect how much melt reaches the explain the igneoussection not only under the V0ring crust. However, this is not a severe problem because Plateau, but also along the entire 2000-km-long vol- it is unlikely that more than 1% of melt remains in the canicmargin of the northernNorth Atlantic [White et mantle, and in any caseour main objective is to explain al., 1987b;White, 1988a]. The centralmantle plume is the production of massive quantities of melt for which only about 150 km in diameter, and it is not possible our calculations are reliable. 7696 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

The main events in the development of a volcanic 3. VOLCANIC CONTINENTAL MARGINS margin are as follows. AND FLOODBASALTS 1. A mantle plume, which may initiate abruptly, forms 3.1. Worldwide Distribution a mushroom-shapedanomMy of hot mantle beneath un- broken continental lithosphere. The thermal anomaly Thick outpouringsof volcanic rock have been iden- extends acrossa 1000 to 2000-km diameter region and tified on rifted continental margins around the world reaches typically 100-200øC. Dynamic uplift of up to (Figure1). Minor synriftvolcanism has been found on 1000-2000 m accompaniesthe hot spot. The thermal almost all rifted continental margins, but our main in- anomaly is especiallylarge at the onset of a plume. terest in this paper is in those margins with unusually 2. If the continental lithosphere rifts acrossthe thermal voluminousmagmatism. Many of the regionsof exten- anomaly created by the hot spot, passivelyupwelling sive volcanismon rifted margins were first identified by asthenospheregenerates large quantitiesof partial melt Hinz [1981]. Othershave been added by Mutter et al. by decompressionas it rises to fill the spacecreated by [1988]and by numerousother local sourceswhich we the thinning lithosphere. The melt segregatesquickly discuss further in the context of each individual area. and risesupward until is accretedto the overlyingcrust. In the followingsections we discussin detail the de- Part of the melt penetrates to the surface to produce velopmentof the variousrift marginswhere voluminous voluminous basaltic flows; the remainder is accreted be- magmatism has been found. We start with the north- neath or intruded into the thinned continental crust. ern North Atlantic, which is perhaps the best exam- 3. As melt is added to the crust and the parent mantle ple of the interaction of a developingoceanic rift with becomesdepleted, the uplift of the surfaceremains large the thermal anomaly created by a mantle plume. The to maintain isostatic equilibrium. Most of the basaltic North Atlantic is a goodexample because both margins section is likely to be erupted subaerially from an ele- of the basin have been widely studied on land and at vated regionabove the rift and flowslaterally awayfrom sea, there is little sediment to obscure the deep struc- the rift onto the adjacent continent. ture, and the timing of the igneousevents is reason- 4. Once the continental lithosphere has been thinned ably well controlled. Other regionswe shall discussare by a factor in excessof about 5, it breaks completely progressivelyless well documented,sometimes owing to to generate a new oceanic spreading center. If the rift their geographicalsetting, which means that the for- has passedacross, or near to the central mantle plume, merly conjugatemargin is no longer availablefor study as often seemsto be the case, huge quantities of melt (vide the westAustralian margin), sometimes because generatedin the upwellingmantle plume are fed directly the deep structure is obscuredby thick overlying sedi- to the surface and create a 15 to 30-km thick igneous ments, and often becauseonly reconnaissancework has ridge acrossthe openingocean (such as the Iceland- so far been undertaken. FaeroesRidge or the WalvisRidge). The distribution of voluminous continental flood 5. Subsequent to breakup, the volcanic continental basalts will be discussedalong with the rift margin margins exhibit rates of thermal subsidencethat are which was responsiblefor the melt generation. In Table appropriate for the amount of lithospherethinning they 2 we list the major flood basalt provinces,together with have undergoneas the elevated asthenospherebeneath their extents and ages. The Columbia River basalts are them cools. However, unlike nonvolcanicmargins, their included for completeness,though rifting has not there thermal subsidence commences from near or above sea continuedto develop an oceanicspreading center. The level rather than from greater depths, so they remain volumes of the extrusive rocks in individual flood basalt abnormally shallow. provincesreach 2.5 millionkm 3. The datingof individ- 6. The thicknessof the oceanic crust not directly above ual provinces is quite variable for two distinct reasons. the mantle plume generated following breakup is often First, minor early and late volcanic events tend to cause lessthan the maximum thicknessof the igneoussection a wide spread in overall agesof a province and perhaps on the adjacent volcanicrifted margins, althoughit still are preferentially sampled because they tend to be in- remainsgreater than normal. This reductionin igneous teresting petrologically. The volumetrically important production may be attributed partly to a decreasein ex- basaltic flows are often much less densely sampled in cessasthenospheric mantle temperature resulting from proportion to their volume and so have relatively fewer the enormousloss of heat advected out of the mantle by age determinations. Second, radiometric dating tech- melt generated in the central plume once breakup has niques are subject to all the well-known sourcesof er- occurred and the melt is able to bleed to the surface. ror, both from experimental technique and from geo- It may also be due to breakup shortly after the initia- logical effects such as reheating or loss of one of the tion of a mantle plume, becausetransient anomalously radiogeniccomponents. Potassium-argon dating is par- high temperatures are produced acrossa wide area by ticularly difficult for these basalts becausethey contain the initial mantle blob which heralds the onset of a new so little K20. In Table 2 we have tried to give not only plume. The breakup of the North Atlantic, for example, the span of dates for each province but also the best was probably triggered by the initiation of the Iceland estimates, where they exist, of the age and duration of plume [White, 1989]. the peak of volcanicactivity. WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7697

TABLE 2. Major Flood Basalts and Continental Breakup

Ocean Basin Onset of Seafloor Igneous Province Flood Basalts Spreading, Ma Age Span, (Main Age) Duration, Area, (Magnetic Chron) (Ma) Ma m.y. x1000 km2

NorthAtlantic 58-56a BritishTert. Ig. Prov. 63-52b (59)c 2-3d 500 (24r) East Greenland 57-53e,! ~ 3! 54g WestGreenland 62-53b (58-54)h 3-4d 55i

SouthAtlantic 130-117 Paran• 130-120k (~ 120)l '1 to a few'm 1200m (M9-M4)J Etendeka 128-113n (~ 120)ø 2n 15p

Indian Ocean- 64 Deccan 67-60r (66) s • 0.5s > 1000t,u Seychelles (27r) q

RedGulf Sea-of Aden EthiopianAden and YemenFlood basalts traps 30-1529-20 (~(~ 27)25) w'x v 'mostearlier intense time'"at 750"

Gondwana Karoo 200-175 (193)y 'a few m.y.'z > 150z Breakup Antarctic 179-16288'bv

ColumbiaRiver 17-6 cc (17-13.5) cc <3.5 m.y.cc 200c• aAnomaly24r, Berggrenet al. [1985]time scale(Hatland et al. [1982]give 56-53 for 24r) time scale. VFromcompilation in Figure10. CMussettet al. [1988]. dThis paper. ½Nobleet al. [1988]. ! Berggrenet al. [1985]. • Clarke and Pederson[1976]. hThispaper after Clarkeand Upton[1971], Klose et al. [1981],Srivastava [1983], Rolle [1985]. i Clarkeand Pedersen [1976]. 3Anomalyidentifications from Austin and Uchupi[1982]. M9-M4 time from Hartand et al. [1982] (130-127 Ma); van Hinte [1976] (126-122 Ma), and Larson and Hilde [1975](121-117 Ma). • Cordantet al. [1980]. l Araaral et al. [1966]. mBellieni et al. [1984]. n128-113 Ma [Siednerand Mitchell, 1976]. OErlank et al. [1984]. PEales et al. [1984]. qSchlich [1982]. r Courtillot et al. [1986]. s Courtillot and Cisowski[1987]. t Pascoe[1964]. uDeshraukh [1982]. "ohr [1983]. w Civetta ei al. [1978]. x Moseley[1970]. YFi•ch and Miller [1984]. ZEales et al. [1984]. aaScrutton [1973]. bbBehrendtet al. [1980];Ford and Kistler [1980]. CCHooper[1982].

In general,the main peak of volcanismoften occurs relative timings are consistentwith our model of melt near the beginning of a volcanically active period, of- generation. Details of the relative timing of uplift, of ten just a few million years after the onset of igneous continental separation, and of igneousactivity provide activity. The flood basalts often overlie, are interca- a crucial test of our hot spot rifting model, so we dis- lated with, or are overlain by subaerial or freshwater cussthe timing in detail in the sectionsthat follow. sediments.This indicatestheir subaerial origin. There are often indicationsof regionaluplift prior to the ef- 3.2. Identification of Igneous Rocks fusive volcanism. Where the timing of the start of on Seismic Profiles rapid seafloor spreadingis good, it can be seen that Before commencing our survey of magmatism on the peak of igneousactivity is coincident with the on- continental margins, we shall discussthe criteria by set of spreading(Table 2; see also section4). These which we identify extrusive and intrusive igneousrocks 7698 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES on the margins. On land there is little difficulty in map- The flow direction of the basalts was toward the land ping the extent of the volcanicrocks other than in es- from vents retreating oceanwardwhich later developed timating how much has been lost by erosion,or is ob- into oceanicspreading centers: thus reflectorsequences scured by sediment or by ice cover. Under water it is dipping in the oppositegeographic direction shouldbe, quite a different matter, as there has been very little and indeed are, found on the conjugate margin which direct sampling and we must rely on indirect methods lay on the oppositeside of the rift, [e.g., see White et such as seismicreflection profiling, wide-angleseismic al., 1987b;Larsen and Jakobsdottir,1988]. velocity determinations, and gravity or magnetic map- The seawarddipping reflectorshave often been used ping. as an indication of where the continent-oceanboundary Although it has been known for a long time that lo- is to be defined. Different authors, however, have de- calized synrift volcanismoften accompaniesthe breakup fined the continent-oceanboundary as lying at a wide of continents, it is only recently that it has been realized range of differentpositions with respectto the dipping that somemargins exhibit very considerableigneous ac- reflector wedge. Definitions range from the landward tivity [e.g., Hinz, 1981; Robertset al., 1984b; White et margin of the wedge, through a position midway be- al., 1987b; White, 1988a]. The presenceof large quan- neath it, to one at the seawardedge. We believe that tities of volcanic rocks on some margins was first pos- much(though not all) of the extrusivevolcanic wedge is tulatedby Hinz [1981],who suggested that characteris- underlainby heavily intruded and stretchedpre-existing tic patterns of seawarddipping reflectorsseen on many continental crust. It then becomes a matter of seman- seismicreflection profiles were causedby thick extrusive tics whether to call the isolated blocks of continental basalticflows. Sinceseismic profiles only give the shape crust in a matrix of new igneousmaterial a "continen- of reflectors and not the compositionof the rocks, this tal" or an "oceanic" crust. It is better not to expect conclusionwas just one interpretation, and others held to define a precise point which is the "continent-ocean that the dipping reflectors were mainly sedimentary boundary." deltaic deposits[e.g., Robertset al., 1979]. However, Recent detailed wide-angle seismicexperiments on direct sampling by drilling of seaward dipping reflec- rifted continental margins in the North Atlantic have tors on the Rockall and Norwegianmargins has shown shown that accompanyingthe extruded basalts which that they are indeed composedof basaltic rocks as pos- form the seaward dipping reflectors there is an even tulated by Hinz [Robertsel al., 1984a, b; Eldholmel greater volume of new underplatedor intruded igneous al., 1986; ODP Leg 10• Scientific Party, 1986; Parson rockin the lowercrust [e.g., Whiteet al., 1987b;Mutter et al., 1985]. Detailed seismicvelocity measurements et al., 1988]. As we haveshown in section2, we expect using wide-angle profiles have confirmedthat the dip- these lower crustal igneousrocks to exhibit seismicve- ping reflectors exhibit seismic velocities characteristic locitiesof around 7.2 km/s, and so we shouldlook for of volcanic rocks and too high for sedimentary deposits velocities of this type on rift margins that we suspect [Mutter et al., 1984; White et al., 1987b; Whirmarsh have been affected by a mantle plume. However, al- and Miles, 1987;Spence et al., 1989]. though high-velocitylower crustal rocks are causedby Once it has been acceptedthat seawarddipping re- magmatism related to continental breakup, their pres- flectors are causedby volcanic flows, it is easy to map ence is not sufficient evidence on its own, since un- the distribution of volcanism along continental mar- stretched continental crust can sometimes exhibit sim- gins where good seismic profiling exists. Care must ilar high seismicvelocities [e.g., Meissner,1986]. It is still be taken, however, not to interpret automatically only where the adjacent unstretched continental crust any package of reflectors that dips seaward as neces- doesnot have a high-velocitylayer that high velocities sarily caused by volcanic flows. For example, Klitgord developedon the margins can be attributed with confi- et al. [1988]show a seriesof planar dippingreflectors denceto igneousaccretion. on United States Geological Survey line 28 acrossthe southern Baltimore Canyon Trough on the U.S. con- 3.3. The northern North Atlantic and tinental shelf which are interpreted as volcaniclastic the Tertiary Igneous Province sedimentary wedges. Volcanic flow dipping reflectors, This region includesthe northern Labrador Sea, the by contrast, generally exhibit convexupward curvature Greenland-NorwegianBasin, and the Iceland Sea, all of with dips that increase in a seaward direction. which were opened in the early Tertiary by the spread- Similar dipping patterns seenin piles of basalt flows ing of Greenland away from northwest Europe, and on land in Iceland have been explained by Pallmason North America awayfrom Greenland. The Tertiary ig- [1980]as due to the loadingeffects of flowsfed from a neousprovince of Britain, northern Ireland, the Faeroes retreatingvent. By analogy,Mutter et al. [1982]sug- and Greenland was formed at the same time and is also gestedthat the seawarddipping reflectorson the Norwe- related to rifting abovethe thermal anomaly surround- gian margin were formed by "subaerialseafloor spread- ing the mantle plume which is now beneath Iceland. ing," a phrasealso usedby Smythe[1983] in describ- Rifting during the Mesozoicopened a seriesof sedimen- ing similar reflectorson the Faeroescontinental margin. tary basins off northwest Europe, such as the More and WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7699

2000 NORTHATLANTIC 2øøø

0 500 1000 i km

Fig. 8. Reconstruction of the northern North Atlantic region at magnetic anomaly 23 time, just after the onset of oceanic spreading. Position of extrusive volcanic rocks is shown by solid shading, with hatching to show the extent of early Tertiary igneous activity in the region. The inferred position of the mantle plume beneath east Greenland at the time of rifting and the extent of the nmshroom-shaped head of abnormally hot asthenosphere are superimposed. Projection is equal area centered on the mantle plume.

Voring basinsoff Norway, Rockall Trough off northwest Britain and Norway (Figure 8). The extent of these Britain and Ireland, and the Hatton-Rockall Trough early Tertiary basalts givesa good indication of the area west of the continental fragment on which Rockall itself underlain by the mushroom head of hot mantle carried sits. None of these basins developed into full-fledged up by the plume. The volcanism was causedby small- oceans, and the rifting that eventually continued into scale tensional rifting, perhaps induced by the regional oceaniccrust occurred along their western margins. uplift resulting from the mantle plume itself. It was Extent of igneous activity. On the rifted continental only when Greenland started separating from Europe margins there was considerableigneous activity dur- along the line of the present ocean that volcanism be- ing the initial stages of separation. Sequencesof sea- came focusedon the continental rift margins. There is ward dipping reflectors produced by extrusive basalts no evidence of contemporary volcanism in the interior are found in a band up to 100 km wide along both of Greenland, although much of the area is inaccessible the east Greenland margin and the Rockall Plateau- due to ice cover. We do not take this to mean that cen- Faeroes-Norwegianmargin. In Figure 8 we show the tral Greenland was not underlain by anomalously hot extent of these offshore extrusives on a reconstruction of asthenosphere,but rather that the thick lithosphere be- the continentsat anomaly24 time [fromSrivastava and neath the Archaean and early Proterozoic crust was not Tapscott,1986] just subsequentto the onsetof spread- subject to the same small-scalerifting as the thinner ing. lithosphere beneath the Mesozoic basins of northwest Volumetrically smaller, but widespread basaltic vol- Europe. canism extended onshore in both east and west Green- The offshore basaltic volcanism on the eastern side land, through northwest Britain and Ireland, and of the North Atlantic has been well documented by throughout the offshoreMesozoic basins off northwest drilling [Robertset al., 1979, 1984a, b; Eldholm et al., 7700 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

HATTON-ROCKALL HATTON ICELAND LINE BASIN BANK BASIN SE a 4.0 A B C D E F G H NW 0 I•:.•:.•::•.!•:•:•...:i:•:i•7•.:.•:.•.:..:::•.•:!•:•?:.•:::•:•..•:•::•:•:•:•:•:::• ß water -•ß ""'•"•' "••...-••'...::':.":'• .... . •-.5.•. ß ...... 5.0 -. ß -...-. ß- •---•_•wa.•.erß ß ß waterß ß "' .....•.•'k' ' ' ': ...... s._•.s•.'• .•,•'• ß .... ?.•s' ' ß . • . ß 6'S •'-:/ • • 20- •o.o • • . . •/ eo•O 8.07 •' 7.2___•___•• 8.1

40-- I HATTONBANKMARGIN

VeRING PLATEAU ROST ESCAR P. NO RW EGIAN SE 9 SYNCLINE 10 [ 11 12 BASIN 13 NW 0 ß ß ß ß ß ß I...::.:.:•:::.::..:,...:,:...::..:.::...... ,...-,..:..:...*.,...-,..:...... •.,.,.,...- ...... •.•...... ,..,,.....:.• ...... • , -, ..... ,_.,-.-.,....,.- ...... water '•'"'-'•'-'--"-•-' s-' ., '. ß ...•'•'.'"• "-','•,•,•,• I:..•.0":..." ' ..... ' " . ' " ./'•' 7.s- "'......

km 20 - t•o•O

i

40 J 2 V•RINGMARGIN -

0 50 100 V.E. = 2.3x I km

Fig. 9. Cross sectionsshowing deep structure from wide-angle seismicexperiments and multichannel seismicpro- files across two sections of the volcanic continental margins in the northern North Atlantic. Profile I is from the Hatton Bank margin [from White et al., 1987b; Scrutton,1972]. Profile 2 is from the Vcring Plateau (redrawn and reinterpretedfrom Mutter et aL [1988]). The dashedline on profile2 showsMutter et aL's [1988]interpreta- tion of the boundary between continental and oceanic crust. Diagonal shading shows extent of extrusive basalts generating seaward dipping reflectors. Dense stipple showssediment, and open stipple showsinferred extent of preexisting continental crust. Triangles along the top of the profiles indicate the location of velocity control points.

1986;ODP Leg10•, 1986]and by seismicsurveys [Mul- adjacent continental shelf where they are constrained ler el a!., 1984, 1988; $mylhe, 1983; Parson el a!., 1985; by synthetic seismogrammodeling are found never to Hinz el a!., 1987; While el a!., 1987a, b; Whilmarsh and rise above 7.0 km/s. The prism of accretedigneous Miles, 1987]. Seawarddipping reflectorsreach 8 km rock apparently lies beneath greatly thinned preexist- in thickness; are found above both thinned continen- ing continental crust. tal crust and thick, fully igneous, ocean crust; show On the Voring Plateau, similarly high seismicve- isotopicevidence of continentalcontamination [Morton locities are found in the lower crust. Muller e! al. and Taylor, 1987];were extruded close to or abovesea [1988]interpret the entire sectionas thickenedoceanic level; and show geochemicalanomahes consistent with crust with the boundary between preexisting continen- generationfrom abnormallyhot mantle [While, 1989]. tal crust and the new igneous crust near vertical, as Detailed wide-angle seismic experiments made shown by the broken line in Figure 9. However, an al- acrossthe continental margins of Hatton Bank and the ternative interpretation of the seismicdata is that there V0ring Plateau (see Figure 8 for locations)have de- is a thinned wedgeof old continental crust in the upper fined the presenceof a thick prism of accreted igneous section,as we showin Figure 9. In either casethere was rock in the lower crust with seismic velocities in ex- clearly an immense addition of intrusive igneousrock cessof 7.2 km/s (Figure 9). White e! hi. [1987b], during the early stagesof rifting on the Voring mar- show that the 7.3-km/s prism on the Hatton margin gin. A prism of high velocity lower crustal rocks has is new material, because lower crustal velocities on the also been reported from seismicprofiles acrossthe east WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7701

Greenlandmargin [Mutter and Zehnder,1988], indicat- continentalcrust [Manderscheid,1980], or abnormally ing similar igneousadditions on the margin conjugate thick oceaniccrust [Keen and Barrett, 1973;$rivasiava, to the V•ring Plateau. 1983]. Both the extensivevolcanism and the elevation If a wedgeof intrusiveigneous rock underliesthe ex- at the time of rifting can be explained by our model trusive basalts found elsewhere all along the northern of thinning above anomalouslyhot asthenospherepro- North Atlantic margins, as seemslikely, then an enor- duced by the mantle plume then under east Greenland mous volume of melt must have been generatedduring (Figure 8). In contrast,Morgan [1983] postulated the the early rifting stages. We estimate the total volume presenceof a separate mantle plume centeredunder the as up to 10 million km•. Davis Strait, and $rivasiava[1983] considered that be- On the east Greenland margin, seaward dipping cause the lavas of both east and west Greenland thin reflectors are found from the southern tip of Green- toward central Greenland, they could not be caused by land northward along the entire coast [Larsen, 1984; the same plume. Roberts ½t al., 1984a; Parson et al., 1985; Hinz et al., We believe that one singleplume under east Green- 1987; Uruski and Parson, 1985]. Offshorethe extru- land was responsiblefor bringing to the base of the sive basalts attain similar thicknesses to those found plate the hot asthenospherethat caused excessivevol- on the conjugateEuropean margin [Larsenand Jakob- canism off both east and west Greenland, but that vol- sdottir, 1988]. The onshoretholeiitic plateau basalts canism only occurred when the overlying lithosphere reach 6 to 7-km in thicknessin the vicinity of Kangerd- was actually stretchedand thinned. The openingof the lugssuaq[Larsen, 1984]. They thin landwardand were Labrador Sea is thus a crucial test of our model. As extruded from sourcesto seawardthat were parallel to we show in the next section, the Labrador Sea com- the presentcoast [Noble et al., 1988]: this coincides mencedopening in the south at about 75 Ma [$rivas- with the axis of continental rifting. Gabbroic plutons iava, 1978] when the hot spot was absent. It did not in Skaergaard,in Kap Edvard Holm, and farther south therefore produce any rift-margin volcanism, and the in the Kialineq complex continue the earlier tholeiitic margins and newly formed oceanic crust subsidedto basic magmatism. The onshore basalts were extruded normal depths. But as the rift extended northward into over shallow marine and freshwater sediments and re- Baffin Bay at around 60 Ma, it intersected the hot as- mained above sea level during most of their formation thenospheremushroom either becausethe hot spot had [Larsen,1984]. migrated southward or, more likely, becausethe hot Along the west Greenland coast, no evidence is spot had only then been initiated. Extensive volcanism found of extensive volcanism associated with the rift- was then produced onshore and offshore in the vicinity ing of the southern Labrador Sea, until one is as far of the stretched and thinned lithosphere, and the newly north as the Davis Strait. There is then a large area formed margins and oceanic crust were formed and re- of basalticvolcanism covering an areaof 55,000km • on mainedanomalously shallow [Hyndman, 1973, 1975]. west Greenland around Svartenhuk Halve, Ubekendt The remaining area of excessivevolcanism in the Ejland, Nfigssuaq,and DiskoIsland [Clarkeand Peder- North Atlantic is Iceland itself, with its 15-kin-thick sen, 1976],of the sameage and of similar composition crust and the Greenland-Iceland-Faeroes Ridge, where to that found 700 km away in east Greenland. These the crustalthickness reaches 35 km [Boli and Gunnars- basaltsreach 8-km thicknessin places[Denham, 1974]. son, 1980]. The ridge lay above the central mantle They were extruded over shallow-marine or freshwater plume and so was fed by the considerablequantities of sedimentsand extend offshoreas seawarddipping layers partial melt producedwithin the upwellingmantle. Lat- [Park et al., 1971;Ross and Henderson,1973]. Almost erally, the crustal thicknessdecreases down the spread- identical basalts are found on Cape Dyer, Baflin Island ing center of the Reykjanes Ridge, although the pres- on the conjugatemargin [Clarke,1970; Clarke and Up- ence of abnormally hot asthenosphereis indicated by ton, 1971]. The Cape Dyer basaltswere alsoextruded anomalousseismic velocities in the upper mantle close over terrestrial sediments from an offshore source and to Iceland [Ritzerl and jacoby, 1985], by the anoma- are now tilted seaward. The picritic and olivine-rich lously shallow seafloor [Haigh, 1973], by the rather compositions[Clarke and Upton, 1971] are typical of thicker than normal oceaniccrust generatedalong the melt formed by decompressionof anomalouslyhot ReykjanesRidge [Bunch and Kennell, 1980; While el thenosphericmantle. al., 1987b]and by geochemicalanomalies in the basalts The crust of Baffin Bay, the northward continuation formed at the spreadingcenter [Klein and Langmuir, of the LabradorSea, is oceanic[tfeen and Barrett, 1973] 1987]. and was formed by continentalrifting about the same Timing of igneous activity. The timing of basaltic time as the northern North Atlantic. It is abnormally volcanismin the North Atlantic and Baffin Bay regions shallow, as is the northern North Atlantic, again in- shows that it is tied closely to the onset of oceanic dicative of formation in the vicinity of a mantle plume. spreading. There are three distinct methods of dat- The shallowDavis Strait has very thick crust reaching ing the volcanism,namely, radiometric dating, identi- 20 km, and could be either heavily intruded stretched fication of magnetic anomaly reversals, and biostrati- 7702 WHITE AND MCKENZIE:MAGMATISM AT RIFT ZONES

AGE (Ma)

65 60 55 50 I I I 26 24B 24A 23 22 Berggren et al (1985)

I ...... Mull [1] I--- Ardnamurchan [1] rr'C) I Skye Muck I• Eigg [1] I--i I Rhum I• Canna [1] o• I ..... 4 wO• Arran I• Alisa Crag [1] •o_ Ireland [1] Carlingford, Ireland [1] Mourne, Ireland [2] Lundy [1] St. KiIda Rockall [t] Blackstone Rock British Province [4]

Faeroe - Greenland [4] East Greenland [5] East Greenland [5] Kap Brewster [6] Biosseville extruslves [6]

Helleflsk [7] Gloa [8] Disko Island [9] Baffln Island [10] West Greenland Svartenhuk Halve [lZ]

27 2G 2L•5 24B24A2• 22 Harmandet am(1982)

65 60 55 50

XXX Onset of spreading F---• Igneous intrusives

m Basaltic extrusives i.... • Ash layers Fig.10. Bestestimates ofages of basalticvolcanism (solid lines) and intrusive activity (broken lines) in British TertiaryIgneous Province, east Greenland, and west Greenland. The time of onsetof spreadingin thenorthern NorthAtlantic is shownusing both the Berggren et al. [1985]and the Hatland et al. [1982]magnetic reversal timescales. Note that many of thesources used in thiscompilation are themselves reviews and critiques of earher work,with theirown best estimate of the correctages from the accumulateddata. Key to sourcesis as follows: [1]Musseft et al. [1988];[2] Gibson etal. [1987];[3] Mitchell et al. [1976];[4] Knox and Morton [1988]; [S] Noble etal. [1988];[6] Berggren etal. [1985];[7] nolle [1985]; [8] Ix'lose et al. [1981]in Srivastava[1983]; [9] Srivastava [1983];[10] Clarke and Upton [1971]; [11] Soper et al. [1982];and [12] Parroft and

graphicmethods. Of these,biostratigraphy is of least placedduring magnetic chron 24r, a periodof lessthan use except in the North Sea, where numerousash flows 3 m.y., the absolutetiming of this period is uncertain areintercalated with the sediments[Knox and Morton, by about 3 Ma. In Figure 10 we showfor compari- 1988].Most of the onshoreigneous outcrops have been son two of the main time scales,those by Harland et datedby radiometricmethods and by reversalstratigra- al. [1982]and by Berggrenet al. [1985],together with phy,while the onsetof volcanismon the riftedmargins summariesof radiometricages from the early Tertiary iswell constrained by identification of seafloorspreading igneous rocks generated during rifting. magneticanomaly reversals. Unfortunately, the calibra- The best constrained dates onshore are from the tionof thereversal time scale is disputedin thevicinity BritishTertiary igneous province. The timinghas been of the Eocene-Palaeoceneboundary when breakup oc- reviewedby Musset!e! al. [1988],who concludethat curred. For example,magnetic anomaly 24 is taken to the majority of the volcanismoccurred at about 59 Ma. occurat about51 Ma by Curry and Odin[1982], while This precedesthe onset of voluminous offshorevolcan- Berggrenet al. [1985]use 56.1-55.1 Ma and Harland et ism by between0 and 4 m.y., dependingon whichre- al. [1982]adopt 53.1-52.4 Ma. So althoughwe know versaltime scaleis used. In general,the extrusionof that offshore the bulk of the volcanic rocks were em- basalticvolcanic rocks (solid lines in Figure10) occurs WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7703 before the emplacementof the more acidic central com- Palaeomagneticmeasurements suggest that in most plexes(broken lines in Figure 10). This may be due placesthe basalts were erupted very rapidly, within the partly to a changein the compositionof the melts with space of a single palaeomagneticreversal. Tarling et time, but mostly to the formation of the acidic rocks al. [1988]suggest that the east Greenlandbasalts were by crustal melting resulting from local heating by the extruded during a singlechron 24r. Sincethe maximum intruded basaltic melts. interval without a magnetic field reversal was 3 m.y., The very earliest record of volcanism is from the this is further evidencethat the basalts were eraplaced islands of Muck and Eigg off the coast of Scotland, very rapidly, in less than 3 m.y., and probably much at about 62 Ma [Musseftet al., 1988]. Most of the faster. plateau lavas had been emplaced in the British Ter- Elevation and subsidence.A key difference between tiary igneousprovince by 58 Ma, before the main off- the elevationof volcanicand nonvolcanicrifted margins shore volcanism. The agesof igneousactivity are simi- is that the former remain near or abovesea level during lar throughoutnorthwest Scotland, the offshoreislands, the initial phasesof rifting, while the latter sink im- and northern Ireland with no temporal trends evident. mediately. There is abundant evidence from the rifted We suggestthat the volcanismwas causedby localized margins around the entire northern North Atlantic that rifting above the mushroom head of anomalouslyhot the region of the initial split rose well above sea level, mantle producedby the plume centeredbeneath Green- not only extruding basalts in subaerial conditions, but land. The compositionof all the basic in this allowing them to flow outward onto the adjacent re- province is consistentwith ultimate derivation from as- gions. thenosphericsources [Thompson and Morrison, 1988]. In east Greenland, the plateau basalts erupted con- It is likely that hot asthenosphericmaterial reached the temporaneouslywith the initiation of spreadingflowed baseof the lithosphere at about 63 Ma, sincethere is no onshore, where they were intercalated with fluvial and evidenceof it prior to this time anywhere in the North lacustrinesediments [Larsen, 1984]. In westGreenland Atlantic region. Magmatism in the central complexes over 8000-m thicknessof subaerial lavas were erupted of the British Tertiary igneousprovince continued for in the Nfigssuaqernbayment, with the northern region about 5 m.y., with the latest events dying out at about being overlain subsequently by nonmarine sediments 52 Ma (Figure 10). [ltolle, 1985]. Clarke and Pealerson[1976] report that In east Greenland the production of extensive after initial subaqueousbreccias were produced in west plateau basalts again precededthe emplacementof in- Greenland, the basalt flows became subaerial. Across trusive rocks such as those at Skaergaard,Kap Edvard the presentBariin Bay on the conjugatemargin of Cape Holm, and Kangerdlugssuaq. The main outburst of Dyer, the basalts were extruded over terrestrial sedi- flood basalts is the first sign of Tertiary igneousactiv- ments and were fed from a source to seaward which ity in east Greenland, occurringbetween 57 and 53 Ma must thereforehave been even higher [Clarke and Up- accordingto the reviewof Nobleet al. [1988]. Berggren ion, 1971]. In someof the now offshorewells in the re- et al. [1985]came to a similar conclusion,suggesting gion of Davis Strait, such as HellefiskI, the basalts were that the Kap Brewster flow can be dated as 56.5 Ma also depositedsubaerially and much weathered[Rolle, and that the duration of plateau basalt volcanismwas 1985]. Uplift of the Davis Strait at the time of rift- from 57 to 54 Ma for the Biosseville extrusives. The ing has been reportedby Hyndman[1973] and Srivas- sourceregion of the flood basalts was offshore,contem- [1983],and evennow Bariin Bay, thoughoceanic, poraneouslywith the onsetof spreading[Larsen, 1984; remainsabnormally shallow [Keen and Barrett, 1973]. Nobleet al., 1988]. The fossilspreading ridge in the northern Labrador Sea Last, in west Greenlandthe dates, althoughsparser, wasalso elevated by the presenceof the hot spot [Hyn- are almost the same as those in the British Tertiary dinah,1973]. Province(Figure 10). The basaltsof west Greenland As discussedearlier the presenceof well-developed were emplaced at 61-58 Ma according to Soper et al. zonesof seaward dipping reflectorsover a 100-kin-wide [1982],which is in closeagreement to the age of 58 4- swath on both the Greenland and the European mar- 2 Ma suggestedfor the basalts of Cape Dyer on Bariin ginsof the northern North Atlantic indicatesthat at the Island [Clarkeand Upton,1971]. Age determinations time of rifting the central rift area was shallow marine on basalts recovered from the offshore wells are similar: or subaerialand stood higher than the continentalsur- 56 Ma from the Gjoa well (Klose et al., 1981, quoted faceson either side[e.g., $kogseid and Eldholm,1987]. by Srivastava,[1983]), and 54-53 Ma from the Hellefisk The subsequentsubsidence history of the southwestern I well [Rolle,1985]. This closeagreement in agefor the rifted margin of the Rockall Plateau has been studied basalts from throughout the European, east and west from a number of Deep Sea Drilhng Project holesacross Greenland, and Bariin Island regions, and the abrupt the margin [Murray, 1984; Robertset al., 1984; Hynd- initiation of volcanism at 62-61 Ma are evidence that a man and Roberts,1987]. new plume reached the baseof the lithosphere at about The southwestRockall margin is a good exampleof 63 Ma. subsidenceon a rifted volcanicmargin. We showin Fig- 7704 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

Onset of seafloor center [Klein and Langmuir, 1987]. For example,the sprea_ding o •l =•=' ß , I I , Na20 content increasesaway from Iceland as the tem- \• ß403-404 peratureanomaly decreases [White, 1989]. / • [] 552-553 Summary. The volcanicrifted marginsof the north- / I ern North Atlantic and Baffin Bay and the associated onshoreTertiary igneousprovinces form a well docu- mented example of the influence of a mantle plume on igneousactivity when the overlying lithosphere is • 2000 stretched and rifted. Seafloorspreading in the North Atlantic to the south of Rockall Plateau and in the southernend of the Labrador Sea commencedduring 3000 - 70-65 Ma, with the pole of rotation for the Labrador Sea opening lying near the north end of Baffin Bay. Rift- ing occurred without excessivevolcanism, and ocean 4000 I I I 60 50 40 30 20 10 0 depths were normal, indicating that the mantle plume Age (Ma) was not present at that time; either it was farther north Fig. 11. Subsidence•sto• data from those deep sea drilling sites assuggested by Morgan[1983], Duncan [1984], and Vink on the west m•gin of RochH Plat•u whi• lie over regiom where [1984],or elseit did not at that time exist. b•t w• ext•ded dgin[ rifting [from Hyn•man By 62 Ma, extensive small-scale volcanism com- 1987]. The p•obathymet• data •e prim•Hy from benthic [or••era [M•rray, 1984] •d from other data su•arized in mencedacross a broad region coveringthe British Ter- •04erts et al. [19•a]. Agows on some of the poin• repre- tiary igneousprovince, extending shortly after to both sent s•ples for w• the• ge •te•te inte•retations giving east and westGreenland, and the then contiguouscoast greater pdaeod•ths. So•3 c•ve shows subsidence calculated of Baffin Island. We attribute this to the mantle plume ass•ng norm• temperathe •th•osph•e (Tp = 128•C) and • = S for a nonvoE•c m•gin [Mt• MCKenzie,1978]. Follow- centered beneath the continental lithosphere of east ing Hyndman an• R04erts[198?], the subsidencecurve is 0.8S km Greenland(Figure 8). It createda mushroom-shaped shallower th• •o•e3 b•ement to Eow for the e•ects of the head of hot mantle over a 1000-krn-radiusregion of 2 • of se•ment-vo]c•c sequents deposited shortly Mter rift- ing. Dashed cg• is subsidencefor •thenmphere whi• is 1S• C much the same size as the extent of its present influ- hott• th• norad (i.e. Tp = 1430øC), with • = S, and initi• ence from its current position beneath Iceland. elevation above sewell. Wherever the lithosphere was rifted, extensive vol- canismresulted from decompressionmelting of the pas- sively upwelling, abnormally hot asthenosphere. The regional small-scalerifting which generatedthe British Tertiary igneous province may have been caused by ure 11 the depth-agedata from the portion of the mar- stresses induced by uplift from the hot spot itself, gin exhibitingextrusive volcanic rocks [from Hyndman or may have had other, external origins[e.g., Knox and Roberts,1987]. Duringrifting the marginremained and Morton, 1988]. When the Greenland-European in shallow marine conditions with barren intervals indi- continent finally split in its present location, dur- cating emergenceabove sea level [Murray, 1984]. This ing chron 24r, massiveigneous activity was produced is in stark contrast to the initial rift subsidence of over along the margins. This generated thick extrusive 2 km whichoccurs on nonvolcanicmargins (solid curve basalt sequences,massive underplating, and concomi- in Figure 11; seealso Figures6 and 7). Severalmillion tant elevation of the rifted margins. Subsequentlyas years after the onset of seafloorspreading, and after the seafloorspreading proper commencedand melting was cessationof intense magmatic accretion on the margin, restrictedto the oceanicspreading centers, the cessation it started to subside thermally beneath sea level as the of magmatism on the margins allowed them to start to underlyingelevated asthenosphere cooled (e.g., dashed subsidethermally. The particularly thick sequencesof curveon Figure11 for/3 - 5). But the marginremained igneousrocks on the rifted margins suggestthat the abnormally shallow throughout its subsequenthistory. mantle temperature was of the order of 50øC hotter The Iceland mantle plume is still dynamically sup- during the initial stagesof rifting than after the onset porting abnormally shallowseafloor over an area with a of seafloor spreading [Morgan et al., 1989].This is con- radiusof about 1000km [Andersone! al., 1973]. Most sistent with continental breakup starting shortly after of the Reykjanes Ridge, for example, is about I km the initiation of the Icelandplume [White, 1989]. shallowerthan normal oceaniccrust, rising to sea level In the northern Labrador Sea the rift between near the central plume beneath Iceland itself. Evidence Canada and Greenland intersected the hot mushroom for the anomalous mantle temperatures in the vicinity head created by the same mantle plume under Green- of Iceland and their decrease with distance from Iceland land and immediately produced extensivevolcanism in can be seenin geochemicalanomalies of recentlyformed Davis Strait, spilling out onto the adjacent west Green- ocean basalts along the Mid-Atlantic Ridge spreading land and Baffin Island crust. WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7705

ARMORICAN BAY OF SHELF BISCAY OBS OBS PUBS PUBS NE L4 JULIE ISIS 2 L2 COB 4 L1 SW o -.•s..•_•.•. -'. ,,., .•;..v ;:-::.::.:::::.•.:'..::.>...... v v v ß v ß I

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5 Line 85-2

Fig. 12. Cross sections showing deep structure of the Biscay margin, the Western Approaches margin and New- foundlandmargins. For locationssee Figure 13. Biscaymargin (profile 3) is redrawn from Ginzburget al. [1985]. Western Approaches-FlemishCap compositellne (profile 4) is redrawn from Keen et al. [1989] Newfoundland margin (profile 5) is from Keen and de Voogd[1988] line 85-2. Key to symbolsand scalesare the same as for Figure 9.

Once the continental lithosphere had split apart, the canic rifted margin is found not far to the south, in the melt generated by decompressionin the central mantle Bay of Biscay. The Biscay margin which was formed in plume itself was able to extrude freely to the surface the mid-Cretaceous exhibits classic tilted fault blocks to generate the 25- to 35-km-thick igneous crust be- acrossa broadstretched region [Montaderl el al., 1979]. neath the FaeroesRidge. As the North Across most of the stretched continental margin there continued to spread, the central plume generated the is no evidence from the seismic velocity structure or ocean-bridgingGreenland-Iceland-Faeroes Ridge of ab- from the deep seismicreflection profiles of any igneous normally thick igneous crust. At present the central accretion to the lower crust, or volcanic activity in the plume has drifted to a position beneath eastern Iceland uppercrust [Foucherel al., 1982; Ginzburgel al., 1985; where it continuesto generate thick igneouscrust. Whdmarshel al., 1986]. The transition from stretched and thinned continen- 3.4. The Norlh A llanlic tal crust to fully igneousoceanic crust occursover a nar- If the archetypalvolcanic margins lie in the northern row transition zone (profile 3, Figure 12). Subsidence North Atlantic, one of the best examples of a nonvol- is rapid at the initial time of rifting and decreasessub- 7706 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

SOUTH

AMERICA

o lOOO 2000 -! km

Fig. 13. Reconstructionof central Atlantic during Middle Jurassic(170 Ma) shortly after the onset of oceanic seafloor spreading. Locations of profiles illustrated in Figures 12 and 14 are superimposed. Inferred path of the Cape Verdeshot spot during 180-150 Ma is from Morgan [1983]. Shadedarea showsbasalt flows and diabase intrusives contemporaneous with rifting of the continental margin.

sequently in the manner predicted by simple astheno- Grand Banks. These stretch over a broad region and spherecooling models [Montader! e! al., 1979]. All precede the eventual onset of ocean spreading in the this evidenceis consistentwith stretchingand breakup Middle to Late Cretaceous. Differing styles of the indi- of the continental lithosphere above normal tempera- vidual basinsindicate underlying detachmentsat vary- ture asthenosphere.There is no evidenceof any mantle ing levelsin the middle to lower crust, although Keen plume in the vicinity of the Biscaymargin at the time of and de Voogd comment that they do not require the rifting, so we would expect normal temperature mantle presenceof detachment zoneswhich cut the entire litho- there. sphereas in the modelsof Wernicke[1985], Lister el al. Slightly to the north of the Biscaymargin there are [1986],Le Pichonand Barbier[1987], and on the Iberian good, deep seismic reflection profiles acrossthe West- margin,Boillot el al. [1987]. ern Approaches margin and the conjugate margin of Even in "cold" continental rifts where the astheno- Newfoundland[de Voogdand Keen, 1987; Keen and sphere temperature is not unduly elevated above nor- de Voogd,1988; Keen et al., 1989]. Like the Biscay mal, extreme lithosphere thinning prior to complete margin, these profilesshow the presenceof tilted fault breakagewill causea certain amount of partial melt to blocks across the stretched continental crust with ap- be generatedin the passivelyupwelling asthenosphere parently normal-depthoceanic crust offshore(Figure (Figure 3). This can be seen on the reflectionpro- 12) and without excessivevolcanism such as that found files acrossthe Newfoundland margin, where astheno- in the northern North Atlantic. sphere temperatures must have been close to normal. To the landward side of the continent-ocean bound- On profile5 (Figure12), landwarddipping reflectors are ary on the Newfoundland margin, Mesozoic extension present near the base of the crust closeto the continent- created many basins on the continental shelf of the oceanboundary. Keen and de Voogd[1988] identify the WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7707 landward dipping reflector as the top of new igneous Extrusive volcanism. In a review of the structure rock which forms the ocean crust. It may extend a short of the continental margin along the east coast of the way under the feather edgeof the thinned continental UnitedStates, Klitgord e! al. [1988]show that thereare crust, giving localizedunderplating. It is possiblethat some weakly developed seaward dipping reflectors off- the layeringseen in the lowerhalf of the strongly atten- shoreGeorge's Bank which are indicativeof volcanicac- uated continental crust just landward of the continent- tivity at the time of rifting. The seawarddipping reflec- oceanboundary on the WAM line (profile4, Figure 12) tors are relatively limited, being developedonly across is also causedby igneoussills intruding the crust. a 25-km-wide zone and reaching a maximum thickness Keen and de Voogd suggestthat the lithosphere of only 0.5 to I s two-waytime (approximatelyI to 3- probably underwent rupture by necking. They com- km thickness). They are found seawardof the main ment that thinning of the lithospheremay have been stretched and rifted continental crust, in the vicinity of nonuniformacross the margin, with the lower litho- the East Coast Magnetic Anomaly and adjacent to the spherethinning being greater in the region of the ulti- new oceaniccrust (Figure 14). There are indications mate break, thus enhancingmelt segregationfrom up- elsewherealong the margin, such as beneath the Car- welling asthenospherein that area. olina trough in the samestructural setting, of small de- Across all these relatively nonvolcanic margins in velopmentsof seawarddipping reflectors. Klitgord et al. the North Atlantic, the region of the stretched and comment, however, that a wedge of planar dipping re- thinned continental crust is very broad. On the Bis- flectorsfarther landward beneath the Baltimore Canyon cay margin, for example, it extends acrossmore than Trough,which was identified by Hinz [1981]as seaward 200km (Figure12). As Mutter et at. [1988]comment, it dipping reflectorsindicative of extrusivebasalts, are in seemsto be a commonfeature for nonvolcanicmargins fact only volcaniclasticsediment wedges. On the north- to be broader than volcanicmargins. We attribute the westAfrican margin, no seawarddipping reflectors have narrowness of the transition from continental to oceanic been identified, but the extensive salt diapirism would crust on volcanicmargins to the weakeningeffect of the in any caseobscure them if they were present[von Rad large quantitiesof hot melt intruded into the thinned ½tal., 1982]. crust; this acceleratesthe breakage. Along the adjacent continent, igneous activity oc- curred from Florida in the south to Nova Scotia in the 3.5. The Central Atlantic north. Much of the volcanism was of tholeiitic basalts The continentalmargins around the central Atlantic filling rift basinsor creating large flows as on the Car- generatedduring the Jurassicwhen North America sep- olina shelf, with some intrusive complexessuch as in the arated from Africa and South America show signs of White Mountainsmagma series [de Boer e! al., 1988]. limited volcanic activity and possiblysome igneousun- To summarize, there are indications along the east derplatingwhich is indicativeof slightlyraised astheno- coast of the United States of extrusive basaltic vol- spheretemperatures. However, these margins did not canismgenerating seaward dipping reflectorsjust prior developinto full-blooded volcanicmargins like those in to continental separation. But nowhere does this ap- the northern North Atlantic discussed in section 3.3. proachthe magnitudeof the volcanicactivity found in There was no mantle plume immediately beneath the the northern North Atlantic. site of the oceanicrift, and therefore no volcanic ridge Intrusive magmatism. A similar picture of limited suchas the Iceland-FaeroesRidge wasformed. There is igneousactivity emergesfrom the deep seismicveloc- however,evidence on the adjacent U.S. continent that a ities of the lower crust beneath the continental mar- hot spot may have been presentsome distancefrom the gins. Velocitiesof 7.1-7.6 km/s, but most commonly rift zoneshortly prior to rifting, which would haveraised around7.2 km/s havebeen reportedfrom beneathsev- asthenospheretemperatures sufficiently in the distal re- eral locations on the east coast North American margin gionstraversed by the oceanicrift to producethe ig- [Keen et al., 1975; Ausiin, 1978; Sheridan½! al., 1979; neousactivity we observeon the continental margins. LASE Study Group, 1986; Trehu et al., 1986; Mirhal The east coast of the United States of America and and Diebold,1987; Trehu, 1989]. Similarly, there are Canada and the conjugatemargins off northwestAfrica indications of a deep crustal layer with a velocity of (Figure 13) are arguablythe best studiedcontinental 7.1-7.3 km/s on the northwestAfrican margin off Mo- margins in the world. Unfortunately, the great thick- rocco[Hinz e! al., 1982]and off Mauritania [ Wetgele! nessesof postrift sediments, often exceeding 10 km, al., 1982]. These are the sort of velocitieswe would whichhave accumulatedon the margin obscurethe deep expect from underplatingor from igneousintrusions in structure. So it can be difficult to differentiate between the lowercrust (section2.3). differentmodels of basementsubsidence [$t½ckler and The thicknessof the high-velocitylower crustal layer Watts,1982]. Extensivesalt diapirism,particularly on rangesfrom a typical value of about 5 km up to about the African margin, makesit even more diflqcultto dis- 10 km (Figure 14). AlthoughKlitgord e! al. [1988] cern rift-stage structure at the base of the sedimentary comment that the crust with a high-velocity layer is pile. found almost exclusivelywithin the highly stretchedre- 7708 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

ATLANTIC NW coast 1 2 3 4 5 GS-4 OCEAN SE ß'i ' i" """"' ' '.....' :':'...... •..?...:::::::•:•.:•..•.•:;.•:..•:•?::•:...•`:?•.•:•::•?•i•i•::•:•`:i•;•.•ii' '""":':':::::.:::':::::':.'7.":.::.)::?•!::i:.:::'..>":." ....:.-..•.::•..-.:::...."•:•i:..'•..':.':•:..'•_.'•:'.•;.:;•;...... '•"'"' -('•'"'.======....:..:.::::.:: ...... ::.:.v..;: ...... ::CoNklNEN'TAI: ':':' :' :': .:6:6:.: .: .: .6.2. 6.5 ,.?_•.•--'"-7.z _1 7 2 7.2 /.2 7.5 MOHO•- -I- :::: :: i: i: i: i: !: i: i: i: i: .____•_•-.•o.o 7.3•-4•.2 / .-.'.'.-.-.-.' .-.ß:.:..-" 8.1 8.18.1 / 4o ':':':':':':-::-:>• 6 BALTIMORECANYON TROUGH L

ATLANTIC NW ECOOEH L1 L2 L3 OCEAN SE o

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o 50 lOO

I km

Fig. 14. Crosssections showing deep crustal structure of the east coastU.S. margin across(profile 6) Baltimore Canyontrough (redrawnfrom LASE $•udy Group[1986]), and (profile7) Carolinatrough (from Trehu[1989]). Key to symbols and scalesis the same as for Figure 9. Locations of profiles are shown in Figure 13.

gionnear the outer edgeof the margin(termed by them nental crust is tied closelyto the openingof the central the rift-stage and marginal oceaniccrust), it is not so Atlantic. The first rifting of the shelf basinswas dur- clear as, for example, on the Hatton margin (section ing the late Triassicand earliestJurassic [Manspeizer, 3.3) that the high-velocitylayer is new material not 1985; Manspeizerand Cousininet,1988]. This rifting present in the adjacent unstretched continental crust. was widespread, but did not produce associated vol- Lower crustal layersof 7.2 km/s are also found in the canism. During the Early Jurassicthe rifting became continentalcrust beneath the Appalachians[Taylor et focusedon the marginal basins, and it was during this al., 1980]. On the conjugateMauritanian margin off time that most of the volcanismon the margin occurred. Africa, Weigelet al. [1982]report that they can follow The continentalrifting evolvedinto full-fledgedseafloor a 7.1 to 7.3-km/s layer continuouslyfrom oceaniccrust, spreadingin the early Middle Jurassic. This occurred under the shelf, and perhaps to 25 km and more under during the JurassicQuiet Zone, so to calculatethe time the continent. of the split, it is necessary to extrapolate back from The high-velocitylower crustal layers are indicative the oldest observedseafloor spreading anomaly, which of limited igneousaccretion to the base of the crust on is M-25 (156 Ma). By this methodthe onsetof spread- the stretched continental margins. Observationsof the ing was calculatedas between 190 and 170 Ma by Vog! subsequent subsidencehistory of the margin, though and Einwich[1979], as 180 Ma by Kldgord and Grow not very good discriminants of the possibility of rift- [1980],with the most recentestimate by Klitgorde! al. stage igneousunderplating or intrusion, can neverthe- [1988]refining it to 175 Ma alongthe marginnorth of less be shown to be consistent with limited addition the Blake-Spur fracture zone and about 5 m.y. later to of igneousrock to the crust [Roydenand Keen, 1980; the south of the fracture zone. Beaumontet al., 1982]. At the same time there was considerableproduction Timing of igneousactivity and rifling. Igneous ac- of tholeiitic basalt flowsand diabaseintrusives through tivity both on the margin and on the adjacent conti- the U.S. continentalcrust inland from the split in Mary- WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7709 land, Connecticut,New Jersey,and Massachusetts[Sut- most, namely, on the outer edgesof the continentalmar- ter and Smith, 1979; Puffer et al., 1981; de Boer et gin adjacent to the first oceaniccrust. al., 1988](also see Figure 13). The first estimatesof To summarize, the central North Atlantic margins the agesof this onshoreigneous activity are that there are an example of a continental split above astheno- were three main phases,one near 230 Ma, one at 200- spherethat was just a little warmer than normal. The 175 Ma, and oneat 125-100Ma [Folandand Faul, 1977; split probably occurred acrossthe distal edge of a re- Sutter and Smith, 1979; Sheridan,1983]. Reevaluation gion of anomalouslyhot mantle producedby a distant of the geologicsources of imperfect dates, such as ar- mantle plume. The resultant magmatism was limited gon lossor gain, hasled Sutter [1985]to concludethat in volume, with most melt being generated in the re- the most that can be said is that the igneousrocks in gions adjacent to the ultimate split where lithosphere the eastern U.S. Mesozoicprovince appear to have crys- thinning was greatest. tallized in the general time span of 200-175 Ma. The 3.6. The South Atlantic youngestage igneousrocks, of 125-100 Ma are prob- ably caused by passagebeneath New England of the The openingof the South Atlantic, like that of the mantle plume which subsequentlygenerated the off- northern North Atlantic, is a good example of a con- shorechain of New England [Crough, 1981; tinental split which broke right acrossa mantle plume Duncan, 1984], and is not of interestto us here, as it and its associatedthermal anomaly. It generated ex- postdatescontinental breakup. tensive synrift igneousactivity on the conjugate mar- Extensive basalt flows were also produced beneath gins of both southern Africa and South America, cre- the presentcoastal plain and continental shelf landward ated massive volumes of continental flood basalts which of the volcanicallyactive Carolina Trough [Behrendtet flowed out over the adjacent continents, the Parang in al., 1983; Hamilton et al., 1983; Schil! et al., 1983]. South America which is the most extensive exposed Their agesare about 185 Ma [Lanphere,1983], shortly flood basalt in the world (Table 2), and parts of the prior to the first seafloorspreading in this area. Karoo in Namibia and southwestern Africa; and built The evidenceof widespreadigneous activity onshore the thick volcanicridges of the Rio Grande Rise and the and on the continental shelf during 200-175 Ma over Walvis Ridge straddling the ocean basin. The mantle the same time period as the continental margin rifting plume responsiblefor this is still active, at present ly- and volcanismsuggests that there was an asthenosphere ing beneathTristan da Cunha some300 km east of the temperature anomaly acrossa 1000-km-broad region of Mid-Atlantic Ridge spreadingcenter. the eastern United States. Although igneous activity Continental Margins. Seafloor spreading between was widespread, it was not excessivein volume, indi- Africa and South America started in the southern Cape cating only slightly raised asthenospheretemperatures Basin at about the time of anomaly M9 (130 Ma beneath the site of the continental split. A possiblecan- [Harlandet al., 1982] 1982 time scale, 126 Ma [van didate for a mantle plume which might have generated Hinte, 1976] 1976 time scale,or 121 Ma [Larsonand this broad temperature anomaly is the Cape Verde hot Hilde,1975] 1975 time scale)and propagated northward spot. Accordingto Morgan[1983], during 200-175 Ma [Austinand Uchupi,1982]. Off Walvis Bay, over 1000 it would have lain about 1000 km northwest of the east km to the north, seafloorspreading did not commence coastU.S. continentalsplit (Figure 13). We note, how- until anomaly M4 time, some 3-5 m.y. later. The onset ever, that this position is not well constrainedbecause of seafloorspreading was precededby a long period of Morgan[1983] assumed that differenthot spotsdo not intracontinental rifting of the Precambrian to Permo- drift with respect to one another, whereas recent work Carboniferousgranitic, metamorphic,and sedimentary has indicated that they do, albeit at relatively small ve- rocks, possiblyextending over severaltens of millions locities[Molnar andStock, 1987; Sager and Bleil, 1987]. of years. The large extent of the thermal anomaly surrounding Seawarddipping reflectors can be seenon multichan- the mantle plume meansthat the preciselocation of the nel seismicprofiles off southwestAfrica, reachingthick- plume is not critical to our interpretation, provided it nessesin excessof 4 km. They are found in a seaward was landward of the present continental margin. Since thickeningwedge, which pinches out near the hingezone the rift crossed the distal portion of the mushroom- at the landward end and extends up to 100 km onto shapedthermal anomaly, it would causesomewhat en- oceaniccrust at the seawardend [Austin and Uchupi, hanced partial melting in the upwelling asthenosphere 1982].This wedgeof seawarddipping reflectors overlies beneath stretched lithosphere, but not the massiveig- blocky, probably faulted and rotated continental crust neousoutburst and the volcanicridges generated in the over much of its width. Seawarddipping reflectorshave northern North Atlantic where the split crossedclose also been reported off the margin of southwestAfrica to the central plume. The widespread broadly contem- by Hinz [1981]and by Gerrardand Smith[1982], and poraneousigneous activity indicatesthe extent of the they probably extend about 1500 km along the eastern mantle thermal anomaly. As we predict from our pas- margin of the South Atlantic. A large proportionof sive asthenosphereupwelling model, most partial melt the Cape Basin dipping reflectorsare found landward was generatedwhere the lithospherewas stretchedthe of anomalyM10 [Hinz, 1981],which suggests that they 7710 WHITEAND MCKENZIE: MAGMATISM ATRIFT ZONES

SOUTHAMERICA

•Etendeka

uvet H.S.

;)ooo

0 1000 2000

Am Fig.15. Reconstruction ofSouth Atlantic atanomaly M4time (approximately 120Ma) shortly after the onset of seafloorspreading. Solidshading shows areas ofextrusive basalts. Extent ofParan• basalts from Hawkesworth et al.[1986], Etendeka basaltsfrom Eales etal. [1984], off'shore areasfrom seaward dipping reflectors reported by Hinz[1981], Gerrard andSmith, [1982] andAustin andUchupi [1982]. Shaded areaaround Walvis shows extentofmushroom headof abnormally hotmantle. Equal area projection iscentered onthe hot-spot location.

wereproduced immediately prior to, andduring the gatethe possible presence of underplating revealed by firststages of seafloorspreading. high-velocitylower crust. On the South American side, Thereare no published multichannel profiles across the only availabledeep seismic refraction information the conjugateSouth American margin, although comes from sonobuoys mostly with airgun sources. Few Robertsand Ginzburg [1984] report examples ofdipping of these profiles penetrated tothe lower crust. However, reflectorsfrom the Argentine margin and they are prob- ably equally commonthere. ofthose that did, there is a suggestionofigneous accre- tionfrom the lower crustal velocities of7.1-7.5 km/s Thefeather edge of the wedge containing dipping re- recordedby a handfulof airgunSohobuoy profiles over flectorshas been drilled off southwest Africa, where 692 theouter edge of theArgentinian continental margin m of mainlybasic lavas were encountered in wellAC-1 [Ludwiget al., 1979]. [Gerrardand Smith, 1982]. Off the Abutment Plateau There is much disagreementover where the furtherto thenorth, dredge hauls of basalt [Hekinian, continent-ocean boundary should be placedon the 1972]show that there, also, the dipping reflectors prob- Africanmargin [e.g., Rabinowitz, 1976, 1978; $crut- ablyarise from basaltic material. The wedge ofdipping ton, 1978].The positionof the continent-oceanbound- reflectorsthickens in a seawarddirection off southwestary variesby over150 km accordingto differentau- Africa;although we cannotbe certainthat it consiststhors (summarized in Figure 17 of Gerrardand Smith, entirelyof igneous rock, it islikely that in largepart it does so. 1982]. In the absenceof deepseismic data, the argumentshinge around the interpretationof high- Unfortunatelythere are no availableseismic refrac- amplitudeanomalies onmagnetic and gravity anomaly tion data on the Africanside with whichto investi- maps;some authors consider these to representoceanic WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 77| | crust, and others say they are causedby stretched con- almost all the volcanicswere erupted within a period as tinental crust. We suggest that the large anomalies shortas oneto a few million years[Belltent et al., 1983, are probably causedby considerableigneous intrusion 1984]. on this magmatically active margin as it was stretched The Paran• continental tholeiites cover 1.2 million prior to seafloorspreading. kma (Figure15). The SerraGeral Formation comprises The evidence of extensive igneous activity on the 780,000km a of relativelyuniform flood basalts,with margins of the South Atlantic is consistentwith our 150,000km s of acidiclava flows [Bellieni et al., 1984]. mantle plume model. We alsoexpect the margin to have Geochemical and isotopic anomalies point to a com- remained abnormally elevated during rifting. There is mon origin for the Parana, Etendeka,and Walvis Ridge evidenceof this from the highly amygdaloidal basalts basalts[Hawkesworth et al., 1986],in keepingwith our in the dipping reflectors and from the presenceof shal- model that they stem from lithosphere stretching over low water sediments and even continental red beds in a hot spot. the sediments that are contemporaneouswith the on- Mantle plumes. As the South Atlantic opened, the set of seafloorspreading overlying the dipping reflectors plume responsiblefor the elevated mantle temperatures [Gerrardand Smith,1982]. poured out vast quantities of melt to form two thick vol- Continental flood basalts. Massive outpourings of canicridges, the Rio Grande Rise and the Walvis Ridge flood basalts were produced contemporaneouslywith (Figure 16). For the first 40 m.y. after opening,the the ocean split. On the African continent they are mantle plume was under the spreading center, midway foundaround Etendeka (Figure 15) andoutcrop sporad- between the separating continents. Like Iceland at the ically along the west coastof South Africa and Namibia present time, the partial melt generated in the plume [Ealeset al., 1984;Erlank et al., 1984].Their maximum produced thick igneous crust which was carried away thicknessreaches 900 m with about 70% of the igneous laterally on both the South American and the African rocks comprising basic to intermediate lavas. In the plates. At about 80 Ma, the plume drifted away from more easterly outcrops they are horizontal, but toward the spreading center onto the African side and there- the coast they become faulted and tilted by up to 20ø . after stopped feeding melt to the Rio Grande Rise on The faults are subparallel to the coast, showextensional the South Americanplate [Morgan, 1971, 1983; Dun- movements,and exhibit both preextrusionand postex- can, 1984]. This led to an abrupt terminationof the trusion movement. This is consistentwith the origin Rio Grande Rise and a change of trend between the of the lavas in the main episodeof continentalrifting. east and west Walvis Ridge (Figure 16). At present, The Etendeka lavas are not nearly so extensive as the the plume lies beneath Tristan da Cunha, which is still contemporaneousParana extrusives in South America volcanically active. (Table 2). This may be explainedby the existenceof The thickenedcrust of the Walvis Ridge is typically considerablerelief prior to lava eruption [Eales et al., 200-300 km wide. Gravity and surface wave model- 1984],which limited the widespreadflow of lavaseast- ing suggeststhat the crust is thickened some 10-15 km ward into the South African interior. comparedwith normal oceaniccrust, giving total thick- The timing of the main episode of lava extrusion nessesof 15-25 km [Goslin and Sibuet, 1975; Kogan, in the Etendeka Province is tied closely to the onset 1976; Chave,1979; Derrick and Watts, 1979]. The sedi- of seafloorspreading. $iedner and Mitchell [1976]date ment history showsthat when the ridgeswere produced, most of the igneous activity as 121 • i Ma, and Er- they were subaerial or in shallow water and that they lank et al. [1984]come to a similar conclusionthat subsequentlysubsided thermally along with the oceanic it occurred at about 120 Ma. The onset of seafloor lithosphereon which they sit [Thiede,1977; Moore e! spreadingin this region was at magnetic anomaly M4 ß al., 1984]. The magnitudeand behaviorof thesevol- time. The uncertaintyof dating M4 rangesfrom 117 Ma canic ridges formed above a mantle plume are similar [Larsonand Hilde, 1975], through 122 Ma [vanHinte et to that of the Iceland-FaeroesRidge. al., 1976]to 127 Ma [Harlandet al., 1982],but despite The extent of the thermal anomaly prior to rifting this uncertainty in the calibration of the magnetic re- produced by the Walvis plume is probably similar to versal time scale, it is clear that the lava production is that of Iceland. In Figure 15 we have drawn around closelycorrelated to the splitting of the continents. the position of the Walvis plume at 120 Ma a 1200- In South America the Parana flood basaltsgive iden- km-radius circle, the same size as the circle around tical dates for their production. Reported agesspan the Iceland in Figure 8, to show the area that was proba- period135-120 Ma [Amaralet al., 1966;M•Dougall and bly underlain by abnormally hot mantle. This explains Ruegg, 1966; Melfi, 1966; Sattort et al., 1975; Cor- well both the continental flood basalts and the offshore dant et al., 1980],but the bulk of the basaltswere em- basalts on the continental margin around Walvis Bay. placed toward the end of this period at about 123 Ma However, the offshore basalts produced further south [MCDougalland Ruegg,1966] or 120 Ma [Amaralet al., off Cape Basinreported by Hinz [1981]and by Austin 1966]. Detailed examinationof palaeomagneticrever- and Uchupi[1982] are probablytoo far from the Walvis sals, which were frequent at this time, suggeststhat plume to have been influencedby it (Figure 15). We 7712 WHITE AND MCKENZIE ß MAGMATISM AT RIFT ZONES

80øW 60 ø 30 ø 0 o 30øE 10øN " I I

ß

o, 0 o .. AFRICA

,,, SOUTH AMERICA

Parana Etendeka 20 ø 130-120 o130-120 (---120) (120)

Rio Gra Rise 2 o %0 Tr ista n 40"

60øS

0 1000 2000 3000

Am

Fig. 16. Present configuration of the South Atlantic showing the thick volcanic ridges of the Rio Grande Rise and the Walvis Ridge produced above the mantle plume as the ocean opened. Hot-spot track is from Duncan [1984].

suggestinstead that the thermal anomaly created by with respect to other hot spots, it is quite possiblethat the Bouvet hot spot was responsiblefor the Cape Basin at the time of rifting of the South Atlantic it was a little extrusives. further west. Its thermal anomaly would then embrace In Figure 15 we showthe positionfor the Bouvet hot the South Atlantic rift. spot at 120 Ma, inferred assumingthat it was fixed with Like the northern North Atlantic, the splitting of respectto the Walvis hot spot [Morgan,1983; Duncan, the South Atlantic is an excellent example of the effect 1984]. This is severalhundred kilometers further away of the thermal anomaly surrounding a mantle plume than the maximum thermal influence around, for ex- on the generation of igneous rocks. Massive volumes ample, the Iceland hot spot. However, two factors may of volcanic rock were produced along the rift, spilling have served to make the Bouvet hot spot responsible. out as continental flood basalts on either side. The First, reconstructionssuggest that prior to 120 Ma the plume left a trail of thick volcanic rock directly above Bouvet hot spot had traveled in an easterly direction it, generatingthe Rio Grande Rise and the Walvis Ridge acrossthe site of the SouthAtlantic split [Morgan,1983] as the ocean continued to open. and so may have left a thermal anomaly in its wake. Its positionat 120 Ma shownin Figure 15 is the most east- 3.7. Northwest Indian Ocean: erly it achievedaccording to hot spot reconstructions, Deccan Traps with a subsequentchange to a southwesterlymovement The Deccan Traps are one of the best known regions [Morgan,1983; Duncan, 1984]. Second,recent evidence of continental flood basalts in the world: they extend shows that hot spots do move with respect to one an- acrossa millionsquare kilometers of westernIndia [Pas- other, at rates of the order of 10-30 mm/yr [Molnar coe,1964; Deshmukh,1982] and are exceptionallyfiat and Stock,1987; Sager and Bleil, 1987]. Sincethere are lying with dips mostly lessthan 1ø . They were extruded few constraints on the position of the Bouvet hot spot extremely rapidly at the same time as the continental at 120 Ma other than the assumption that it was fixed block of the Seychellesrifted away from western India WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7713

I

/ \ / •\ AFRICA:..• /

ß SEYCHELLES

•000•

1000 2000

km

Fig. 17. Reconstructionof the northwestIndian Oceanat magneticchron 28 (approximately65 Ma), shortly after the onset of rifting. Solid areas show known extent of Deccan plateau basalts and of contemporaneous offshore basaltic volcanism. Offshorevolcanism was probably continuousalong the west Indian continental margin. Circle shows extent of anomalously hot, advected mantle around plume at time of rifting. Equal area projection is centered on the plume location.

closeto the time of the Cretaceous-Tertiary boundary 300 m deep, as a result of the massiveoutpourings of (Figure 17). The l•union mantle plume was at that basalt. In the upper and lower series some freshwater time situated centrally beneath the volcanic outpour- sedimentsare found betweenthe lavas [Pascoe,1964]. ings and subsequentlybuilt the Chagos-Laccadiveand The sourceregion to the west must have been elevated Mascareneridges as the separatingplates drifted above if it was to feed lavas inland, and this is consistentwith it (Figure 18). the uplift the mantleplume wouldhave generated (Fig- The Deccan Traps. The thickness of the Deccan ure 7). The subsequentreversal of dip to giveseaward traps is greatest near the coast, reaching more than dipping lavas was caused by the movement southward 2000 m near Bombayand thinning eastward[Pascoe, of the plume and by thermal relaxation of the stretched 1964]. Dips are lessthan 1ø over most of the inland lithosphere near the coast. region but increaseup to 5ø seawardtilt near the coast, The timing of the flood basalt emplacementis par- with minor normal faulting roughly parallel to the coast ticularly well constrained in the Deccan by palaeo- in the western regions. In the western Ghats, successive magnetic,palaeontological, and geochronologicaldata. igneousunits overstepprevious units southward[Beane As is usual, the radiometric dates span a fairly broad et al., 1986; K. Cox, personalcommunication, 1988]. range, with the high-quality K-Ar ages clustering be- These observations are consistent with a source in the tween67 and 60 Ma [Courtillotet al., 1986; Courtillot west, perhaps offshore. The locus of igneousactivity and Cisowski,1987]. The youngerdates are probably tended to move southward with respect to India. causedby argon loss. The age histogram peaks closeto Individual lava flowsvary in thicknessfrom 10 to 160 65 Ma, with an uncertainty of around 1 or 2 m.y. More m and were deposited subaerially. The lavas overflowed recent4øAr-a9Ar ages from two studiesof the tholeiitic the preexistingtopography, in placesfilling valleysover basaltsyield bounds of 69-65 Ma [Courtillotet al., 1988] 7714 WHITEAND MCKENZIE: MAGMATISM AT RIFT ZONES

30øN

EURASIA INDIA

ARABIA

ARABIAN SEA

AFRICA

Rodriguez Triple Junction

30øS

30øE 90øE

0 1000 2000

km Fig.18. Present geography of the west Indian Ocean showing the location of thefossil spreading center in the MascaxeneBasin, the active ridge in theArabian Sea and the trail of volcanicridges and islands left by the R4unionplume as it migratedsouthwestward away from its initialposition under India.

and68.5-66.7 Ma [Duncanand Pyle, 1988]. Palaeonto- The link betweenmagmatism on the Seychellesand in logicalevidence gives consistent results, suggesting that the Deccanis emphasizedby reportsthat thegeochem- basaltextrusion began no later than 67-65 Ma. istryof someSeychelles dykes matches exactly with one Palaeomagneticmeasurements suggest that the du- ofthe distinctive types of the western Ghats (K. rationof the volcanismwas very short. The frequencyCox, personal communication, 1988). of reversalswas high during this period, and it is likely ContinentalRifling. The breakupof the Seychelles that the total duration of the volcanismwas less than blockfrom mainlandIndia wascontemporaneous with 3 m.y. [Kono,1973]. Over 80% of the basaltwas em- the timing of the flood basalt production. Prior to placedwithin the samereversed magnetic interval. Of- magneticanomaly 28 therewas an oceanicspreading ricerand Drake [1985] estimate that the majorityof the center southwestof the Seychellesin what is now the volcanismwas produced in 0.4-0.9 m.y., whilst Cour- MascareneBasin. Followingthe time of anomaly28, tillot e! ai. [1986]make the casethat mostof the basalt whichis the oldest identifiable magnetic anomaly in the wasextruded during chron 29r, whichlasted only 0.4 MascareneBasin, the seafloorspreading ridge jumped northwardto split the continentalblock of the Sey- At the sametime asvolcanism was occurring in the chellesaway from the Indian mainland. By doingthis it Deccan,there was volcanicactivity on the continental createdthe Eastern Somali Basin [Schlich, 1982], which Seychelles,which were then close to India(Figure 17). isstill spreading along the Carlsberg Ridge (Figure 18). The peakof igneousactivity on the Seychelleshas been If the bulk of the Deccan flood basalts were extruded reportedas approximately63 Ma by MacIntyreet al. duringanomaly 29r, and seafloor spreading was fully in [1985]and thusis roughlycoincident with the Deccan. progressby anomaly28 or 27, thenthe volcanismpre- WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7715 cededthe onset of drifting by about 2 m.y. This is simi- on the Arabian plate. The major bursts of magmatic lar to the conclusions from the northern North Atlantic activity correlate with the main periods of lithospheric and reflectsthe considerablelithosphere stretching and extension. thinning that must have occurred over a period of 2-3 Flood Basalts. The Ethiopian Tertiary flood basalts m.y. before the continental lithosphere finally ruptured extendacross a regionof about 600,000km 2 (Figure to create a new oceanicspreading center. 19). Prior to erosionduring the Pleistocene,they prob- OffshoreIndia in the vicinity of the Deccan there is ably covered750,000 km 2 [Mohr, 1983]. About 85% abundant evidence of extrusive volcanism on the conti- of the 350,000km 3 of EthiopianCenozoic volcanism is nental margin, generating classicseaward dipping re- of basalticcomposition [Baker et al., 1972]. On the flectors[Hinz, 1981]. Gravity and seismicrefraction Arabian plate the flood basalts cover an area of about data suggestthat the Deccan basalts continueoffshore 130,000km 2 in smallerpatches spread through Saudi into the seawarddipping reflectors[Hinz and Closs, Arabia and South Yemen. Both the Ethiopian flood lOCO]. basalts and the once contiguousYemen Traps were ex- The Rgunion mantle plume. The mantle plume now truded onto an erosional surface. Interbedded fluvial, beneath the volcanic island of Rdunion lay beneath the lacustrine, and subaerialsediments show that they were westernedge of the Deccan at the time the Seychelles depositednear to or above sea level [Geukens,1966; startedto rift awayfrom peninsularIndia (Figure 17). Civetta et al., 1978; Pallister, 1987]. Subsequentmotion of the overlying plates across the The flood basalts extend acrossa region stretching central plume has created a chain of volcanic banks and some1000 km awayfrom the Afar plume(Figure 19). In islands, with ages that gradually decreasetoward the general, both the number and the thicknessof individ- presentposition of the plume beneathR•union [Dun- ual flows in the Ethiopian Province decreasesouthward can, 1981; Morgan, 1981]. The volcanicchain com- and westwardaway from the main rift zones [Mohr, mencedwith the Saya de Malha bank, which comprises 1983]. Similarly,the YemenTraps, which reacha max- volcanicrocks generated at the same time as the Dec- imum thicknessof 1000 m, thin northward into Arabia, can volcanics. As the mantle plume migrated south- wherethey are called the Sirat Plateau Basalts[Cole- ward with respect to India, the Laccadives, Maidives, man, 1974]. Volumetrically,the basalts in Ethiopia and ChagosBank were formed at progressivelyyounger are more important than those on the Arabian plate times(Figure 18). Followingthis the plumepassed be- in Yemen and Saudi Arabia, though the history and neath the Carlsberg Ridge oceanicspreading center and chemical characteristics of the volcanism are similar on continued southward, generating the southern end of both the Arabian and African plates. the Mascarene Plateau, Mauritius and finally Rdunion The main episodeof flood basalt volcanismthrough- [MCDougalland Chamalaun,1969; Fisher et al., 1974; out the region was during the period from 30 to 20 Ma, Duncan,1981; Morgan, 1981]. Many of the basaltswere [Greenwoodand Bleackley,1967; Civetta e! al., 1978; formed in shallow water or even subaerial conditions, as Mohr, 1983; Hempton,1987; Pallister, 1987]. Within is expected for volcanic rocks formed above a plume. this period the volcanism was most intense near the be- The intense volcanism at the time of continental ginning[Mohr, 1983]. After about20-15 Ma the volcan- rifting which resulted in the formation of the Deccan ism ceased on the Arabian side and also died down con- flood basalts, and concurrent igneous activity on the siderablyin Ethiopia. The igneousactivity in Ethiopia Seychelles,the Saya da Malha Bank, and offshoreIndia became localized mainly in the Ethiopian rift valley, were a result of rifting above a region of anomalously wheresome rifting continued[Mohr, 1987]. A second hot mantle around the Rdunion plume. This is a par- major burst of volcanismthroughout the area began at ticularly clear example of the effects of a mantle plume 4.5 Ma, correlating with renewed opening of the Red because not only are the flood basalts formed at the Sea[Hempton, 1987; Pallister, 1987]. time of rifting well preservedbut the plume left behind Compositionally, the Ethiopian flood basalts are it an unmistakabletrail of volcanic islands and ridges mainly on the alkaline-tholeiitic boundary with some as it moved southward away from India. picriticlavas [Mohr, 1971, 1983]. The Yemenand Saudi Arabianbasalts also exhibit alkaline affinities [Coleman, 1974;Civetta et al., 1978;Pallister, 1987], and the prin- 3.8 The Red Sea and Gulf of Aden: cipal rock types in the Aden volcanicsare olivine and pi- Afar Plume criticbasalts [Moseley, 1970]. Since the amountof litho- The Gulf of Aden and the Red Sea are young ocean sphere stretching in the Oligoceneand early Miocene basinscurrently being formed by the separationof Ara- that gave rise to these basaltswas not large, these al- bia and Africa. In the center of the region lies the Afar kaline and picritic compositionsall point to unusually plume. Rifting above the thermal anomaly created by high temperatures in the parent mantle MCKenzie and the Afar plume has given rise to the extensive flood Bickle,1988] and are thereforeconsistent with genera- basalts of the Ethiopian Plateau on the African plate tion within the bounds of the thermal anomaly created and to flood basalts in Saudi Arabia and South Yemen by the Afar plume. 7716 WHITE AND MCKENZIE:MAGMATISM AT RIFT ZONES

ARABIA

AFRICA INDIAN

OCEAN

0 1000 2000

km Fig.19. Map showing present-day configuration ofthe Gulf of Aden-Red Sea region. Solid areas show extent of continentalflood basalts from Moseley [1970], Mohr [1983], and Pallister [1987]. Shading depicts probable extent ofthe thermal anomaly inthe mantle surrounding theAfar plume with a radius of 1000 km. Bathymetric contours arein meters.Equal area projection is centeredon the plumelocation.

Continentalrifling. It haslong been thought that coast[Makris el al., 1975]. To the southof the Afar the Tertiary volcanicactivity on both sidesof the Gulf Trianglethere is normal-thicknesscontinental crust un- of Aden-RedSea rift is relatedto lithospherestretch- der the EthiopianPlateau, but with someindication of ing[Moseley, 1970; Mohr, 1983, 1987; Hemplon, 1987; a smallamount of thickeningtoward the Ethiopian rift Bonalliand $eyler, 1987; Pallislet, 1987]. There has valley,which may represent igneous underplating in the beenless agreement on whetherlarge areas of the Gulf vicinityof theEthiopian rift [Mohr,1987]. of Adenand the Red Seaare flooredby oceaniccrust Underthe Afar Triangleitself, wide-angle seismic or by stretchedand heavily intrudedcontinental crust experimentshave shown that beneathan extensivecara- [Girdlerand Slyles, 1973; Cochran, 1981, 1982; Bobart- pace of volcanic rocks the continental crust is greatly at- non, 1986;Bonalli and$eyler, 1987; Girdler and Soulh- tenuated[Berckhemer el al., 1975].At thebase of the em,1987; Joffe and Gaffunkel, 1987]. For our purposes, stretched crust is a layer of rock with velocitiesof about the differencedoes not mattergreatly because it is the 7.2-7.6km/s. Thebottom of thishigh-velocity crustal lithospherethinning which gives rise to meltproduction layer is not seenin the seismicdata, but evidencefrom as the asthenospherewells up. teleseismicearthquakes [Searle and Gouin, 1971] and Direct evidencefor crustalthinning beneath the fromgravity measurements [Makris el al., 1975]sug- main rift zonecomes from earthquakesand controlled gest that it has a minimum thicknessof 12 km and is sourceseismics and from gravity measurements.On underlainby normalmantle. The basal high-velocity' the Arabianside of the RedSea the crustthins away layeralso exhibits high densities. Thus the thickness, fromthe rift overa distanceof 200-400km [Mooney el seismicvelocity, and density of the basallayer all indi- al., 1985;Prodehl, 1985; Bohannon, 1986]. Similarly, on catethat it is likelyto representunderplated igneous the southernflanks of the Red Sea and the Gulf of Aden materialsimilar to that foundin otherregions which the continentalcrust thins as the oceanicrifts are ap- wererifted above anomalously hot asthenospheric man- proached,decreasing to only 10-14 km at the Red Sea tle. WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7717

The Afar plume. The initiation of volcanism over a thinning and of igneous underplating, and the uplift broad region around the Afar Triangle at 30 Ma sug- of this region are all consistentwith rifting occurring geststhat the Afar mantle plume first reachedthe base above the broad thermal anomaly in the mantle caused of the lithosphere at this time. Over perhaps 10 m.y. by the Afar plume. We note that the Kenyan rift sys- prior to this there is evidenceof somerifting and block tem is not connected with the Afar plume which we faulting in the Red Sea-Gulf of Aden area with only mi- have discussedin this section. The Ethiopian rifts die nor associatedvolcanism [Hempton, 1987]. The lack of out southward before the Kenyan rifts commence, and volcanismin the early rifting is evidencethat the plume apparently there is no connection at depth between the thermal anomaly only impinged on the lithosphere at Ethiopian and Kenyanrifts [Long,1976]. The elevation approximately 30 Ma and only thereafter caused en- and volcanismof the Kenyan rifts and the deep seismic hanced volcanism in rifting regions. Dynamic uplift structure are indicative of the influence of an underly- generated by the plume may well have assisted the ing thermal anomaly in that region, too, but it must be lithosphere rifting which occurred subsequently. The caused by a different plume. thermal anomaly generated by a new plume is espe- cially large at the beginning,and this coupledwith the 3.9. The West Australian Margin relatively small amounts of lithospherethinning, can Seaward dipping reflectorshave been reported from explain the production of alkaline basalts and picritic seismic profiles across the northwest Australian conti- basalts. Stretching continued until approximately 15 nental margin near the Scott Plateau [Hinz, 1981]and Ua. in the Cuvier Basin [Mutter et al., 1988]. They are less From about 15 Ma until 4.5 Ma there was little con- clear than, for example, in the North Atlantic but nev- tinental rifting in the Red Sea and only scattered vol- ertheless are indicative that this was a volcanically ac- canism. Stretching continuedlocally in the Wonji fault tive margin when it rifted. Unfortunately, unlike all the belt of the Ethiopian rift, causinglocal volcanismwithin other volcanic rifted margins that we have considered, the rifts. Vigorous extensionrecommenced in the Gulf the conjugatemargin to Australia is no longer available of Aden and Red Sea at about 4.5 Ma. In the Gulf of for study, as it has been destroyed in plate collision Aden it has now generatedfully oceanicseafloor along a zones. So we are now unable to identify the nature of central oceanicspreading center. The reneweddrifting the conjugate margin or the track of the mantle plume at 4.5 Ma can be related to reconfigurationof plate mo- reponsiblefor creating the thermal anomaly. tionselsewhere [Cochran, 1981; Coleman, 1985; Hemp- There was considerablemagmatism along the west ton, 1987]. Australian margin at the time of breakup [Veeversand The present position of the Afar mantle plume is Cotterill, 1978]. Intensevolcanism is exhibitedon a se- in the centerof the Afar Triangle(Figure 19) near the ries of plateaus along the edge of the continental mar- triple junction between the three rifts representedby gin, including the Scott Plateau, the northwest corner the Gulf of Aden, the Red Sea, and the Ethiopian rift of the Exmouth Plateau, the Wallaby Plateau, and the [Schilling,1973]. It has generatedconsiderable uplift NaturalistePlateau (Figure 20). Seawarddipping re- acrossthe regionof the thermal anomaly. To the south, flectors indicative of extrusive volcanism have been re- the Ethiopian Plateau is elevated more than 2000 In ported from the flanks of the Scott Plateau and the abovesea level. Similar elevationshave beengenerated Cuvier Basin. Although no seawarddipping reflectors to the north in the Yemen Plateau and along the flank have been reported from the oceanward flank of the of the Gulf of Aden. Part of this elevation may be Exmouth Plateau in the interveningregion, there is ev- attributed to crustal thickeningby igneousunderplat- idence there from high seismicvelocities in the lower ing or intrusion, but most of it must result from dy- crust for magmatic underplating beneath the stretched namic uplift by the thermal anomaly surroundingthe continental crust near the continent-ocean transition. Afar plume. Breakup of the region and the onset of seafloor A clear example of the uplift causedby the thermal spreadingstarted at about 150 Ma in the Argo Basin anomalyis seenalong the fully oceanicspreading center to the north of the region[Veevers and Heirtzler, 1974; in the Gulf of Aden. Closeto the center of the plume at Heirtzler et al., 1978], and somewhatlater at about the westernend of the Gulf of Aden the spreadingcenter 125-120 Ma to the south in the Gascoyne, Cuvier, is actually subaerial. Eastward the depth of the spread- and Perth basins [Larson et al., 1979; Johnsonet al., ing center increasesgradually away from the center of 1980; von Rad and E•con,1982]. The volcanismalong the plume. It only attains normal oceanicdepths about the margins was associatedwith this breakup and was 1000km from the centerof the Afar plume(Figure 14). caused,we suggest,by a broad thermal anomaly gen- This behavior is the same as that seen along the Reyk- erated by a mantle plume. Prior to the breakup there janes Ridge spreadingcenter as one moves away from is a long history of rifting and subsidencein a series the mantle plume under Iceland. of basinsalong the margin, but with little concomitant It is clear that the timing and extent of the volcan- volcanism. This provides supportive evidence that no ism, its chemical characteristics,the degree of crustal hot plume was present in this area until shortly before 7718 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

Naturaliste Plateau [Davies et al., 1974; Veeversand Heirtzler,1974]. Although Veeversand Cotterill[1978] Plab 10osI Argø suggestthat the volcanicmarginal plateausmay consist entirelyof igneousrocks ("epiliths" in their terminol- ogy), it seemslikely that mostof them are underlainby • _.}/• Gascoyne sin stretchedcontinental crust [$taggand Exon, 1982; yon •• Basin Rad and Exon, 1983]. The thickestvolcanic sequences, 20ø-r•o•/.• zenith comprisingolivine tholeiites and alkaline basalts, have beenreported from the Wallaby Plateau [von Stackel- ••v(j• Cuvier berge! al., 1980]. • Wallaby We conclude that the uplift and generation of basaltic volcanic rocks on the marginal plateaus of the AUSTRALIA west Australian continental margin at the time of con- 25ø•- P•teau tinental breakup, together with seawarddipping reflec- tors and igneous underplating, can be explained by rifting above a broad thermal anomaly in the mantle 30ø•• PerthBasin causedby a mantle plume. • Naturaliste I• Plateau 3.10. Gondwana Breakup Between Africa and Antarctica 100øE 10 ø 110ø 115ø 120 ø 125 ø E 0 1000 2000 When Gondwanaland began to break up in the ! Jurassic by the separation of Antarctica from Africa, km massivevolcanism accompaniedthe split. This gener- Fig. 20. West Australian continental margin and adjacent oceanic ated on land the Karoo flood basalt province in Africa basins. Open areas show regions of thick volcanic rocks formed at [Cox, 1970; Eales et al., 1984] and the Ferrar flood the time of continentalbreakup [from Veeversand Cotterill, 1978; yon Rad and Eaton,1983]. Shaded areas are regionsof offshore basaltsin Antarctica (Figure 21). Offshorethick se- volcanism[from Hinz, 1981; Mutter et al., 1988]. Bathymetric quencesof volcanicrocks were emplacedalong the con- contours in kilometers. tinental margin of Antarctica in the Explora Wedge and along the Andenes Escarpmentbeneath the Wed- dell Sea. In the following sections we amplify the ev- idence that the Karoo volcanics, and the contempora- breakup occurred. Several kilometers of sediment were neousmagmatism in Antarctica and along the offshore accumulated in basins such as the Browse Basin, the continental margins, were produced when Gondwana Kangaroo Syncline,the Dampier subbasin,and the Ex- began to rift above a mantle thermal anomaly. mouth Plateau during Triassic-Jurassicrifting prior to Extent and nature of igneous activity. We begin breakup. with the Karoo Province. The present outcrop area Shortly before breakup the stretched continental of Karoo lavasis about 140,000km 2, althoughthis is crust of the Exmouth Plateau was uplifted above sea but a remnant left after erosionor subsequentburial of level, giving a widespreaderostonal uncon-formity [yon the originallavas, which probably covered 2 millionkm 2 Rad and Exon, 1983]. This probably was caused [Cox,1970, 1972]. In generalthe lavasare fiat lying, al- by dynamic uplift generated by the hot spot thermal though in the east the dip increasesto about 45ø and anomaly. Coincident with the breakup, basalts were occasionallyto 64ø in the Lebombo monocline. Over- extruded on the marginal plateaus of the Scott Plateau all, the thicknessdecreases toward the west, indicating a [Powell,1976; Hinz et al., 1978], the Wallaby Plateau source off the present southeast coast of South Africa. and northwestExmouth Plateau [Veeversand Cotterill, Tholeiitic basalts form by far the dominant lavas, al- 1978; yon Stackelberg et al., 1980; yon Rad and Exon, though there are outcropsof varied rock types ranging, 1983], and the Naturaliste Plateau [Coleman et al., for example,from picritic basaltsat the beginningof the 1982]. Evidencethat thesevolcanic marginal plateaus igneousactivity in the Nuanetsi region to rhyolites in were above sea level at the time of rifting and only sub- the late stageLebombo extrusives (for a recentreview, sequently subsided is provided by the sediment cover seeEales e! al. [1984]). on the Scott, Wallaby, and Naturaliste plateaus, which Over most of southern Africa the Karoo not only is relatively thin but is also younger than the are extensional in nature, with predominantly normal age of the adjacentocean basins [Veevers e! al., 1974; faults[Eales et al., 1984].The Jurassicvolcanism which $tagg, 1985; Mutter et al., 1988]. Furthermore,sedi- generatedthe Karoo volcanicsoccurred toward the end ments drilled in the Perth Basin have probably been de- of a long history of extensionaltectonics which began rived by erosion off subaerially exposedbasalts on the in the Permian. We suggestthat the sudden onset of WHITE AND MCKENZIE:MAGMATISM AT RIFT ZONES 7719

Explora ( 19 3 Ma)

Queen Maud Land

ANTARCTIC

0 1000 2000

km Fig.21. Reconstruction ofGondwana inthe mid-Jurassic (170Ma) at the time of breakup. Solid areas show present outcropof contemporaneous volcanic rocks. Karoo distribution is from Eales et al. [1984],offshore Antarctica fromHinz and Krause [1982] and Kristoffersen and Haugland [1986], and Ferrar volcanic outcrops on land in Antarcticaarea from Elliot [1976] and Ford [1975]. Projection is equalarea, centered on the continental split between Africa and Antarctica.

Karoo volcanism in the Jurassic indicates that a man- mappedalong the 2000-km-longintervening rift mar- tle plumehad onlythen impinged on the baseof the gin (Figure21). Extrusivelavas were first identified lithosphere.There is alsoevidence from the sedimentsas thicksequences of seawarddipping reflectors in the that southernAfrica was elevatedabove sea level at the ExploraWedge [Hinz, 1981; Hinz andKrause, 1982]. time of Karoo volcanism. For example, thin sandstone The dippingreflector wedge is up to 5 km thickand ex- lensesinterbedded with the lower lavas in the Lesotho hibitsseismic velocities of 3-4 km/s in the uppersection andnortheast Cape Province are probablyplaya lakes, increasing,to about5 km/s in the lowerparts. This andin the AlgoaBasin the volcanicrocks are overlain is similar to the wedgesof seawarddipping reflectors by fiuviatile sediments. foundon the rifted continentalmargins in the northern In the east of the region the Karoo volcanics NorthAtlantic (section 3.3). The totalvolume of extru- dip beneaththe Cretaceous-Tertiarysediments in the sivelavas in the Explora Wedgeis estimatedby Hinz Lebomboflexure which extendsover 1000 km subparal- [1981]as 200,000-300,000 km3. Thereare indications lel to the coast. The Karoo lavasextend at least as far as on somesonobuoy refraction profiles of lowercrustal ve- the coast[Flores, 1970, 1973; Darracott and Kleywegt, locities of about7.4 km/s beneaththe dippingreflector 1974]and probably continue on offshore. The Lebombo sequences [Hinz andKrause, 1982], which may be in- monoclinewas actively flexing down during the period dicativeof igneousunderplating similar to that seenon of lava extrusionand may representthe boundarybe- othervolcanic rifted margins.But detaileddeep seismic tween normal and stretched continental crust. The refractionprofiles are not availableto mapthe possible thicknessof buried lavas beneath the coast has been underplated region. estimatedas between 6 and 13 km [Ealeset al., 1984]. The topsof the dippingreflector sequences of the Prior to breakup,Antarctica lay adjacentto south- ExploraWedge are marked by a distinctand widespread eastAfrica, and largeoutpourings of lavashave been unconformity,called the "WeddellSea Unconformity," 7720 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES or "U9" by Hinz and Kristoffersen[1987]. It is pre- marked differences in the major and minor elemental sumedto be of late Mid-Jurassic age and to be related compositions,and in the strontium isotope and dis- to onset of separation of Africa and Antarctica. persedelement ratios [Brook,1972; Faure et al., 1972, Southwestward the thick extrusive lavas of the Ex- 1979;Juckes, 1972; Ford and Kistler, 1980]. The bound- plora Wedge continue into a 150-km-wide rift zone be- ary between the two provinceslies between the massive neath the southernWeddell Sea [Hinz and Krause, Dufek intrusives, which are related to the Transantarc- 1982]. Though volcanicallyactive, this rift doesnot tic Mountain type, and the Theron Mountains, which mark the site of the break between Africa and Antarc- belong to the Weddell Sea and Queen Maud Land tica becausethat occurred slightly to the north along ProvinceFigure 21). We only considerhere the vol- the 1000-km-long linear Andenes-ExploraEscarpment, canismfrom the latter province, which is related to the which was also volcanicallyactive [Kristoffersenand breakup between Africa and Antarctica. Haugland,1986] (Figure 21). The southernWeddell Sea In summary, there is extensive Jurassic volcanism rifted before the Andenes-Explora rift and was proba- related to the Africa-Antarctic rift, forming thick vol- bly causedby the same set of prebreakup rifting which canic sequencesboth offshoreabove the line of the conti- generated a burst of volcanism in the Karoo prior to nental rupture and onshoreon either side, in both Africa separation of Africa and Antarctica. The Andenes- and Antarctica, where large volumes of flood basalts Explora Escarpment marks the boundary between fully were eraplaced. oceanic crust to the north and stretched, thinned con- Continental Breakup. In a general senseit is clear tinental crust of Antarctica to the south [Kristoffersen that the extensive volcanism in Africa and Antarctica is and Haugland,1986]. It exhibits its own set of sea- related to the breakup of Gondwanaland. However, de- ward dippingreflectors caused by synrift volcanism,and tailed reconstructions show that the break did not occur truncates earlier rift structures. in a single, clean episode;there were many small con- Flood basalt volcanism also occurred on the Antarc- tinental fragmentsinvolved, and there was probably at tic land mass in the Jurassic at the time of the breakup least one abortive period of rifting and possiblytran- of Gondwana, although the extensiveice cover leaves scurrent faulting before a later break to fully oceanic only patchesof it exposed. Jurassictholeiites are found crust occurred in the Mid-Jurassic. in exposuresthroughout Queen Maud Land, and in- Age determinationson Karoo igneousrocks have re- trusivedolerites occur in the Theron Mountains(Fig- cently been reviewedby Fitch and Miller [1984], who ure 21). In the Vestfjella chain of nunataksnear the concludethat there were two main periods of major Weddell Sea coast, between 1-2 km of mostly tholeiitic basalt flood production, one at 193 q- 5 Ma and a sec- basalts were emplaced during the Jurassicin subaerial ond at 178 q- 5 Ma. There were other periodsof minor, conditions[Krisioffersen and Haugland, 1986]. Sim- mostlyintrusive volcanism and a muchyounger episode ilarly in northeastern Heimefrontjella, also in Queen of flood basalt productionin the Etendekaregion at 120 Maud Land, at least 300 m of uniform basaltic lavas q- 5 Ma. The 120 Ma phase was related to the split of were extrudedsubaerially in the Mid-Jurassic[Juckes, the South Atlantic discussed in section 3.6 and is not 1972]. Elsewherein QueenMaud Land, Jurassicvol- relevant to the Africa-Antarctic separation. canics overlie the Beacon Group of sediments in the The Karoo flood basaltswere extrudedvery rapidly. Kirwan Escarpment, and Jurassic dykes intrude them Fitch and Miller [1984]comment that the magmatism in the Theron Mountains and the Whichaway Nunataks around 193 Ma "was undoubtedly one of the most sud- [Brook,1972; Hinz and Kristoffersen,1987]. The com- den, extensive and important events in the Mesozoic positionsand isotoperatios of the igneousrocks are sim- history of Africa." Of the extrusive episode around ilar to those of the Karoo tholeiites, which is consistent 178 Ma, they say that it "occurred over a very short with their having a commonorigin [Ford and Kistler, time interval" and that in both the 193 and 178 Ma 1980]. Furthermore,not only the extensiverift-related cases,the "regionaleruption rates must have been lit- volcanismbut also the elevationof the Antarctic region erally enormous." It is not certain which of the events above sea level are consistent with the presence of a was dominant;Cox [1988]comments that someof the broad thermal anomaly in the mantle caused by a hot formations, for example in the Lebombo, that appear spot. to be diachronousfrom radiometricage determinations There is also extensive Jurassicvolcanism through- appearfrom field relationshipsto belongto a singleex- out the TransantarcticMountains (Figure 21) at ap- trusive episode. proximately the same time as the volcanismin Queen The Jurassiclavas in the Weddell Sea-Queen Maud Maud Land adjacent to the continental rift between Land area of Antarctica all appear to belongto a single Africa and Antarctica. The Transantarctic volcanism is igneousphase. None of them is older than about 179 Ma generallyassigned to the FerrarGroup [Grinalley, 1963]. [Fordand Kistler, 1980],which correlates well with the However, despite being coeval, the Transantarcticig- secondphase of extrusivesin the Karoo discussedabove. neous activity apparently has a different cause to the The tholeiitic basalts in Queen Maud Land are dated volcanism we have considered above, and it exhibits as 179-162q- 6 Ma by Rex [1972],and a wholerock K- WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES 7721

Ar age on basalt from Bj0rnnutane in the same region near the northern edge of the Karoo volcanic outcrops yields 174 4- 7 Ma [Juckes,1972]. Doleritesintruded at the time of Gondwanabreakup. However,given that into the Theron Mountains yield agesof about 169-154 relativemotions of 10-30 mm/yr can occurbetween hot 4. 6 Ma [Rex,1972], although only a few sampleshave spots, it is likely that the Crozet hot spot could have been dated. lain slightly further south in our favored position be- The offshore lavas are not well dated, other than neath the Lebombo monocline at the time Antarctica by the widespreadoverlying Weddell Sea unconformity, broke away from Africa. which is assumedto have a late Middle Jurassic age (about170 Ma) by Hinz andKristoffersen [1987]. The oldest identifiable seafloorspreading anomalies 4. DISCUSSION suggestthat breakup of Gondwanalandoccurred at the In the first part of this paper we have shown that earliestat about 170 Ma [Martin and Hartnady,1986]. thermal plumes in the mantle feed mushrooms of ab- Sincethere was probably a period of rifting prior to the normally hot mantle immediately beneath the litho- onset of fully developed seafloor spreading, the 178 4. sphere. Their diameter is typically 1500-2000 km, and 5 Ma dates of flood basalt volcanism in the Karoo and the temperature is of the order of 100-150øC above 179-162 Ma in Antarctica correlate well with the Gond- normal. When the lithosphere is thinned by rifting, wana breakup. The earlier phase of volcanism around decompressionof the upwelling asthenosphericmantle 193 Ma in the Karoo, and the formation of the vol- producesmelt which rapidly movesupward to the over- canically active failed rift in the Weddell Sea probably lying crust. The relatively small increase of tempera- reflect an earlier phase of rifting that did not develop ture around mantle plumes dramatically enhances the fully into seafloorspreading. As commentedearlier, the amount of melt produced by passiveupwelling. We ex- long phaseof extensionalrifting in southern Africa that plore in detail the predictions of our hot spot rifting was not accompaniedby significantvolcanism until the model concerningthe volume of igneousrocks produced, outburst at 193 Ma is indicative that it was only at that their geochemical,seismic, and density signatures,the time that a mantle plume started to generate anoma- effecton subsidenceof rifted margins,and the timing of lously hot asthenosphericmantle beneath this region. igneousactivity. In the secondhalf of the paper we show We suggest that the most likely location of the how our model can explain all the major volcanic con- thermal plume was beneath the coastal region of the tinental margins and associatedflood basalts. An iden- Lebombo monocline. A 1000-km-radius circle, which tical model can be applied to other considerablyolder is the typical extent of the thermal anomaly created flood basalt provincessuch as the 1100 Ma Keweenawan by a plume, neatly encompassesthe 1700-km-long off- volcanicsof the Mid-Continent Rift System, where we shore volcanically active portion of the continental rift deducean initial potential temperature of 1510-1560øC, and the volcanicsof the Karoo and Antarctica (Figure some 150-200øC above the ambient mantle temperature 21). Supportiveevidence for the presenceof the plume at the time of rifting. beneath this area comes from the very thick igneous The production of voluminous igneous rocks on sequence,which may reach 13-km thickness beneath somerifted continental margins providesa major mech- the coastal region and is either very heavily intruded, anismfor increasingthe volumeof the continentalcrust. stretched continental crust or, like Iceland, consistsen- With the present number and global distribution of hot tirely of new igneousmaterial. Considerableuplift of spots, we expect that at least one hot spot will passun- this central region, as would be expected above a man- der any point on the Earth every few hundred million tle plume, is indicated by the occurrenceof continen- years. The hot spot will not always cause continen- tal conglomeratesabove the volcanicrocks [Martin and tal breakup, nor does continental rifting always occur Hartnady,1986]. Marine sedimentswere not deposited above hot spots. However, the presenceof a hot spot hereuntil the Barremian[Forster, 1975], several tens of itself causesconsiderable dynamic uplift and additional millions of years after breakup. tensional forces in the lithosphere. So if the stressfield The presenceof a thermal plume in this regionis also in any givenregion is suchthat incipient rifting is likely indicatedby the subsequentgeneration of the Madagas- to occur, then the initiation of a new plume beneath car Ridge as the plume movednortheastward away from that area, or the passageof the area above an exist- Africa. The MadagascarRidge comprisesa 25-km-thick ing plume may well lead to lithosphererifting. Fur- igneouspile formed above a hot spot[Sinha et al., 1981]. thermore, if rifting does occur shortly after initiation The neighboringMozambique Ridge may similarly be of a new mantle plume, then the increasedtemperature an igneousridge, particularly as tholeiitic basalts have anomaly in the blob with which the plume commences been drilled from it, although the slop in reconstruc- will lead to the initial generationof particularly large tion would also allow it to be a dislocated continental quantities of melt as the mantle decompresses. fragment[Martin and Hartnady, 1986]. If a fixed hot The interplay of continental breakup and hot spot spotreference frame is used,Morgan [1981] shows that locations over the past 200 m.y. is such that on aver- the hot spot at present beneath Crozet would have lain age a major igneous province is generated once every 7722 WHITE AND MCKENZIE: MAGMATISM AT RIFT ZONES

30 m.y. Those we have discussedin this paper (Fig- known example. Two major mechanismsfor extinctions ure 1) includethe Ethiopianand Yemenregions around have been proposed,one relying on collisionwith large Afar at about 30 Ma, the North Atlantic Tertiary ig- extraterrestrial objects, the other suggestingthat vol- neousprovince, and the Seychelles-Deccanboth at ap- canismis the cause[e.g., Alvares et al., 1980; Officer proximately 60 Ma, the South Atlantic, Parana, and and Drake, 1983, 1985; Courtillot and Cisowski, 1987; Etendeka provincestogether with the west Australian Hallam, 1987;Hut et al., 1987;Officer et al., 1987]. margin at about 120 Ma, and the Karoo-Antarctic and Both mechanismsrely on large quantitiesof dust be- central Atlantic splits during 190-170 Ma. If each split ing thrown up into the upper atmosphereand thereby producedabout 10 million km3 of new igneouscrust, blockingout sunlightwith a concomitantsudden change as deducedfrom the North Atlantic Tertiary Province, in the temperature of the Earth's surfaceleading to the then we mightexpect to add about330 millionkm 3 of death of many organisms. Other factors such as the mantle-derived igneous rocks to the continental crust injection of sulfate aerosolsinto the atmosphere may every1000 m.y.; an averageof around0.3 kmS/yr. also cause potentially damaging environmental conse- To this minimum estimate we should add the con- quences[Stothers et al., 1986].Without wishingto enter tribution from other volcanically active areas of rift- into the protracted argumentsfor and against the two ing above hot spots which have not progressedfully to hypotheses,it is clear that our model providesa method an oceanic split. Presently active examples include the for generatingrapid and intense volcanismas a normal Basinand RangeProvince, where some 3 millionkm s consequenceof rifting above hot mantle. At least some of igneousmaterial has been added to the crust, the of the volcanismis explosive,generating massive ash- Columbia River flood basalts, and the Kenya rift. We falls. In the North Atlantic Tertiary igneousprovince, shouldalso take accountof the small volume of igneous for example, over 100 separate ashfalls have been iden- rock added to the crust alongcontinental margins which tified in the North Sea sediments[Knox and Morton, have rifted above normal-temperature asthenospheric 1988].The intervalbetween major igneousepisodes re- mantle. Although only 1-2 km of igneousrock may sulting from rifting above hot spots is typically of the be generated(Figure 3), this neverthelessadds up to a order of 30 m.y., which falls into the typical timespan significantvolume along the total length of the world's between observedmajor extinctions. nonvolcanicrifted margins. In conclusion,this surveyshows that our simpleno- We estimate the total rate of growth of the conti- tions of rifting abovehot spots can explain all the main nental crust by melt production aboverifted regionsas featuresof the world's major igneousprovinces. It pro- 0.4 kmS/yr. Sincemuch of this materialis addedor videsa powerfulpredictive tool for assessingthe geolog- intruded into the base of the crust, we will not see all of ical and subsidencehistory of rifted regions. There are it in the surface geology. By comparison,if the conti- many detailed studies which can be made to test our nental crust had grown at a constant rate to its present model. One field of investigationupon which we have volumeof 7.6 x 109km s [Rayrnerand Schubert, 1984] touchedonly briefly in this paper is the geochemicalna- over the 4500-m.y. history of the Earth, the average ture and isotopicsignature of the igneousrocks, which growthrate wouldbe 1.7 km3/yr. Mostauthors, how- providea sensitiveindicator of the mantle temperature ever, consider that growth rates were much greater in and conditionsunder which they were generated. An- the early Archaean, and that during the Phanerozoic other valuabletest is to look for geophysicalevidence theyhave been somewhere between zero and I kmS/yr of igneousunderplating, or intrusion in the lower crust (for a reviewof growthrate estimates,see Rayrner and under rifted regions. Last, classicalgeological investi- Schubert[1984]). gationsof the subsidencehistory of rifted regionsand Our estimation of continental growth rates by ig- the preciserelationships between tectonic events,mag- neousaddition in rifted regionscould thus accountfor matism and the onsetof seafloorspreading along rifted the majority of the increaseof continental volume dur- continental margins can provide important tests of our ing the Phanerozoic. We note also that our calcula- model. tion assumesthat rates of plate motion and astheno- spheric mantle temperatures have remained the same Acknowledgments. The bulk of this paper was written while as at present throughout geologictime. If either were R.S.W. was a Cecil and Ida H. Green Scholar at IGPP, Scripps higher during the early history of the Earth, as seems Institution of Oceanography. He gratefully acknowledgessupport likely, then considerablymore melt would have been from the Green Foundation and the provision of facilities, includ- produced by our mechanismof decompressionmelting ing an excellent library, and outstandingly beautiful surroundings at Scripps. The following people kindly provided preprints of beneath rifted regions. their work prior to publication, for which we are grateful: Enrico Another by-product of our model is that it may Bonatti, Keith Cox, Roy Hyndman, Charlotte Keen, Kim Klit- provide a mechanism for explaining some of the gord, Robert Knox, Andy Morton, Alan Mussett, John Mutter, R. H. Noble, Frank Richter, Don Tarling, and Bob Whitmarsh. widespread and catastrophic extinctions that have oc- We thank Phyl Fisher for help with the illustrations and Elaine curred through geologictime. The major extinction at Blackmore and Barbara Dyson for typing the manuscript. 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