When continents rift to form new ocean basins, the rifting is sometimes accompanied by massive 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 mantle. The igneous rocks are generated by decompression melting of hot asthenospheric mantle as it rises passively beneath the stretched and thinned lithosphere. Mantle plumes generate regions beneath the lithosphere typically 2000 km in diameter with temperatures raised 100–200°C above normal. These relatively small mantle temperature increases are sufficient to cause the generation of huge quantities of melt by decompression: an increase of 100°C above normal doubles the amount of melt whilst a 200°C increase can quadruple it. In the first part of this paper we develop our model to predict the effects of melt generation for varying amounts of stretching with a range of mantle temperatures. The melt generated by decompression migrates rapidly upward, until it is either extruded as basalt flows or intruded into or beneath the crust. Addition of large quantities of new igneous rock to the crust considerably modifies the subsidence in rifted regions. Stretching by a factor of 5 above normal temperature mantle produces immediate subsidence of more than 2 km in order to maintain isostatic equilibrium. If the mantle is 150°C 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 causes an increase in seismic velocity of the igneous rocks emplaced in the crust, from typically 6.8 km/s for normal mantle temperatures to 7.2 km/s or higher. There is a concomitant density increase. In the second part of the paper we review volcanic continental margins and flood basalt provinces globally and show that they are always related to the thermal anomaly created by a nearby mantle plume. Our model of melt generation in passively upwelling mantle beneath rifting continental lithosphere can explain all the major rift-related igneous provinces. These include the Tertiary igneous provinces of Britain and Greenland and the associated volcanic continental margins caused by opening of the North Atlantic in the presence of the Iceland plume; the Parana 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 Seychelles split off India above the Reunion hot spot; the Ethiopian and Yemen Traps created by rifting 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 material are added to the continental crust. This is an important method of increasing the volume of the continental crust through geologic time.

Magmatism at rift zones: The generation of volcanic continental margins and flood basalts

Variations in sea-floor depth and heat flow with age provide the main constraints on the thermal structure and evolution of the oceanic lithosphere. Joint fitting of heat flow and bathymetry yields a model with a hotter, thinner lithosphere than in previous models. The new model provides a significantly better fit to the data, including those from older lithosphere previously treated as anomalous. This will facilitate the analysis of lithospheric processes, including the effects of mid-plate volcanism and swells, regional subsidence, and hydrothermal circulation near spreading centers.

A model for the global variation in oceanic depth and heat flow with lithospheric age

Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts Such provinces do not originate at “normal” seafloor spreading centers We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen-Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (70-76 km s?1) compressional wave velocities Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short-lived pulses of increased global production; Ontong Java’s rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid-ocean ridge system The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features

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Large igneous provinces: crustal structure, dimensions, and external consequences

Seismic refraction results show that the igneous section of oceanic crust averages 7.1±0.8 km thick away from anomalous regions such as fracture zones and hot-spots, with extremal bounds of 5.0–8.5 km. Rare earth element inversions of the melt distribution in the mantle source region suggest that sufficient melt is generated under normal oceanic spreading centers to produce an 8.3±1.5 km thick igneous crust. The difference between the thickness estimates from seismics and from rare earth element inversions is not significant given the uncertainties in the mantle source composition, though it is of the magnitude that would be expected if partial melt fractions of about 1% remain in the mantle and are not extracted to the overlying crust. The inferred igneous thickness increases to 10.3±1.7 km (seismic measurements) and 10.7±1.6 km (rare earth element inversions) where spreading centers intersect the regions of hotter than normal mantle surrounding mantle plumes. This is consistent with melt generation by decompression of the hotter mantle as it rises beneath spreading centers. Maximum inferred melt volumes are found on aseismic ridges directly above the central rising cores of mantle plumes, and average 20±1 and 18±1 km for seismic profiles and rare earth element inversions respectively. Both seismic measurements and rare earth element inversions show evidence for variable local crustal thinning beneath fracture zones, though some basalts recovered from fracture zones are indistinguishable geochemically from those generated on normal ridge segments away from fracture zones. This is consistent with a model where the melt generated beneath spreading ridges is redistributed to intrusive centers along the ridge axis, from where it may flow laterally along the axis at crustal or surface levels. The melt may sometimes flow into the bathymetric lows associated with fracture zones. Oceanic crust created at very slow-spreading ridges, and in regions adjacent to some continental margins where rifting was initially very slow, exhibits anomalously thin crust from seismic measurements and unusually small amounts of melt generation from rare earth element inversions. We attribute the decreased mantle melting on very slow-spreading ridges to the conductive heat loss that enables the mantle to cool as it rises beneath the rift.

Oceanic crustal thickness from seismic measurements and rare earth element inversions

The available data, mainly topography, geoid, and heat flow, describing hotspots worldwide are examined to constrain the mechanisms for swell uplift and to obtain fluxes and excess temperatures of mantle plumes. Swell uplift is caused mainly by excess temperatures that move with the lithosphere plate and to a lesser extent hot asthenosphere near the hotspot. The volume, heat, and buoyancy fluxes of hotspots are computed from the cross-sectional areas of swells, the shapes of noses of swells, and, for on ridge hotspots, the amount of ascending material needed to supply the length of ridge axis which has abnormally high elevation and thick crust. The buoyancy fluxes range over a factor of 20 with Hawaii, 8.7 Mg s -1, the largest. The buoyancy flux for Iceland is 1.4 Mg s -1 which is similar to the flux of Cape Verde. The excess temperature of both on-ridge and off-ridge hotspots is around the 200oC value inferred from petrology but is not tightly constrained by geophysical considerations. This observation, the similarity of the fluxes of on-ridge and offridge plumes, and the tendency for hotspots to cross the ridge indicate that similar plumes are likely to cause both types of hotspots. The buoyancy fluxes of 37 hotspots are estimated; the global buoyancy flux is 50 Mg s -1, which is equivalent to a globally averaged surface heat flow of 4 mWm -2 from core sources and would cool the core at a rate of 50 o C b.y. -1. Based on a thermal model and the assumption that the likelihood of subduction is independent of age, most of the heat from hotspots is implaced in the lower lithosphere and later subducted. I.NTRODUCWION ridge plumes using Iceland as an example. The geometry of flow implied by the assumed existence of a low viscosity Linear seamount chains, such as the Hawaiian Islands, are asthenospheric channel is illustrated by this exercise. Then the frequently attributed to mantle plumes which ascend from deep methods for obtaining the flux of plumes on a rapidly moving in the Earth, perhaps the core-mantle boundary. The excessive plate are discussed with Hawaii as an example. These methods volcanism of on-ridge hotspots, such as Iceland, is also often involve determining the flux from the plume from the crossattributed to plumes. If on-ridge and midplate hotspots are sectional area of the swell and taking advantage of the kinematreally manifestations of the same phenomenon, one would ics of the interaction of asthenospheric flow away from the expect that the temperature and flux of the upwelling material plume and asthenospheric flow induced by the drag of the would be similar under both features. In particular, the core- lithospheric plate. The methods for extending this approach to mantle boundary is expected to be nearly isothermal so that the hotspots on slowly moving plates are then discussed which Cape temperature of plumes ascending from the basal boundary layer Verde as an example. An estimate of the global mass and heat should be the same globally provided that cooling by entrain- transfer by plumes is then obtained by applying the methods to ment of nearby material and thermal conduction are minor. 34 additional hotspots. The magnitude of this total estimated Finally, the global heat loss from plumes should imply a reason- flux is compatible with the heat flux expected from cooling the

http://www.mantleplumes.org/WebDocuments/Sleep1990.pdf

Hotspots and Mantle Plumes' Some Phenomenology

Summary.

We describe results from 200 heat flow measurements across the Cape Verde Rise in the North Atlantic. They show that the heat flow through normal, 125 Myr-old crust is 45.5 ± 3.4mWm-2, close to the prediction for a lithospheric plate model. Heat flow increases towards the centre of the Rise, reaching a maximum of 16 ± 4 mW m-2 above the normal oceanic value. The concommitant geoid anomaly at the centre of the Rise is 7.6 ± 0.3 m and the depth anomaly is 1900 ± 200 m.



The anomalous heat flow, geoid and bathymetric values are used to constrain a variety of theoretical models of hot spot mechanisms. Lithospheric reheating models alone, whether involving transient or sustained thinning of the lithosphere, fail to model the broad heat flow anomaly and the long duration of uplift and volcanism observed on the Cape Verde Rise.



Axisymmetric convection models provide a better fit to the observed data, and suggest that the swell is a consequence primarily of dynamic uplift generated by an ascending thermal plume in the underlying mantle. Uplift caused by expansion of the solid lithosphere is of smaller importance. The observed geoid, bathymetry and heat flow anomalies are reproduced well using convection models with upper mantle viscosities of 2-4 times 1020 Pa s and a coefficient of thermal expansion in the range 2.0 - 2.5 times 10-5 K-1.



The effect of melting in the ascending plume is to buffer the heat flow anomalies. In the case of the Cape Verde Rise, partial melting is minimal, but its effect on other hotspots will be to limit observed heat flow anomalies at the surface to less than 20-25 mW m-2.

Robert C. Courtney

Papers

Anomalous heat flow and geoid across the Cape Verde Rise: evidence for dynamic support from a thermal plume in the mantle