Cookies

We use cookies to ensure that we give you the best experience on our website. You can change your cookie settings at any time. Otherwise, we'll assume you're OK to continue.

Durham University

Department of Earth Sciences

Postgraduate Students

Publication details for Prof Gillian Foulger

Foulger, G.R, Pritchard, M.J, Julian, B.R, Evans, J.E, Allen, R.M, Nolet, G, Morgan, W.J, Bergsson, B, Erlendsson, P, Jakobsdottir, S, Ragnarsson, S, Stefansson, R & Vogfjord, K (2001). Seismic tomography shows that upwelling beneath Iceland is confined to the upper mantle. Geophysical Journal International 146(2): 504-530.

Author(s) from Durham

Abstract

We report the results of the highest-resolution teleseismic tomography study yet performed of the upper mantle beneath Iceland. The experiment used data gathered by the Iceland Hotspot Project, which operated a 35-station network of continuously recording, digital, broad-band seismometers over all of Iceland 1996–1998. The structure of the upper mantle was determined using the ACH damped least-squares method and involved 42 stations, 3159 P-wave, and 1338 S-wave arrival times, including the phases P, pP, sP, PP, SP, PcP, PKIKP, pPKIKP, S, sS, SS, SKS and Sdiff. Artefacts, both perceptual and parametric, were minimized by well-tested smoothing techniques involving layer thinning and offset-and-averaging. Resolution is good beneath most of Iceland fromy60 km depth to a maximum ofy450 km depth and beneath the Tjornes Fracture Zone and near-shore parts of the Reykjanes ridge. The results reveal a coherent, negative wave-speed anomaly with a diameter of 200–250 km and anomalies in P-wave speed, VP, as strong as x2.7 per cent and in S-wave speed, VS, as strong as x4.9 per cent. The anomaly extends from the surface to the limit of good resolution at y450 km depth. In the upper y250 km it is centred beneath the eastern part of the Middle Volcanic Zone, coincident with the centre of the y100 mGal Bouguer gravity low over Iceland, and a lower crustal low-velocity zone identified by receiver functions. This is probably the true centre of the Iceland hotspot. In the upper y200 km, the lowwave-speed body extends along the Reykjanes ridge but is sharply truncated beneath the Tjornes Fracture Zone. This suggests that material may flow unimpeded along the Reykjanes ridge from beneath Iceland but is blocked beneath the Tjornes Fracture Zone. The magnitudes of the VP, VS and VP/VS anomalies cannot be explained by elevated temperature alone, but favour a model of maximum temperature anomalies <200 K, along with up to y2 per cent of partial melt in the depth range y100–300 km beneath east-central Iceland. The anomalous body is approximately cylindrical in the top 250 km but tabular in shape at greater depth, elongated north–south and generally underlying the spreading plate boundary. Such a morphological change and its relationship to surface rift zones are predicted to occur in convective upwellings driven by basal heating, passive upwelling in response to plate separation and lateral temperature gradients. Although we cannot resolve structure deeper than y450 km, and do not detect a bottom to the anomaly, these models suggest that it extends no deeper than the mantle transition zone. Such models thus suggest a shallow origin for the Iceland hotspot rather than a deep mantle plume, and imply that the hotspot has been located on the spreading ridge in the centre of the north Atlantic for its entire history, and is not fixed relative to other Atlantic hotspots. The results are consistent with recent, regional full-thickness mantle tomography and whole-mantle tomography images that show a strong, lowwave-speed anomaly beneath the Iceland region that is confined to the upper mantle and thus do not require a plume in the lower mantle. Seismic and geochemical observations that are interpreted as indicating a lower mantle, or core–mantle boundary origin for the North Atlantic Igneous Province and the Iceland hotspot should be re-examined to consider whether they are consistent with upper mantle processes.

References

Aki, K., Christoffersson, A. & Husebye, E., 1977. Determination of the
three-dimensional seismic structure of the lithosphere, J. geophys.
Res., 82, 277–296.
Allen, R.M. et al., 1999. The thin hot plume beneath Iceland, Geophys.
J. Int., 137, 51–63.
Anderson, D.L., 1989. Theory of the Earth, Blackwell Scientific
Publications, Boston.
Anderson, D.L., 1998a. The EDGES of the mantle, in The
Core–Mantle Boundary Region, pp. 255–271, eds Gurnis, M.E.,
Wysession, E.K. & Buffett, B.A., AGU, Washington, DC.
Anderson, D.L., 1998b. The helium paradoxes, Proc. Nat. Acad. Sci.,
95, 4822–4827.
Anderson, D.L., 1998c. A model to explain the various paradoxes
associated with mantle noble gas geochemistry, Proc. Nat. Acad. Sci.,
95, 9087–9092.
Anderson, D.L. & Bass, J.D., 1984. Mineralogy and composition of the
upper mantle, Geophys. Res. Lett., 11, 637–640.
Anderson, D.L. & Sammis, C., 1970. Partial melting in the upper
mantle, Phys. Earth planet. Inter., 3, 41–50.
Bernauer, F., 1943. Junge Tektonik Auf Island und Ihre Ursachen, in
Spalten Auf Island—Geologische, Geodaetische und Geophysikalische
Forschungsarbeiten der Deutschen Island-Expedition Des Jahres 1938,
ed. Niemczyk, O., Verlag von Konrad Wittwer, Stuttgart.
Bijwaard, H. & Spakman, W., 1999. Tomographic evidence for a
narrow whole mantle plume below Iceland, Earth planet. Sci. Lett.,
166, 121–126.
Bjarnason, I.T., Wolfe, C.J., Solomon, S.C. & Gudmundson, G., 1996.
Initial results from the ICEMELT experiment: body-wave delay
times and shear-wave splitting across Iceland, Geophys. Res. Lett.,
23, 459–462.
Bott, M.H.P., 1985. Plate tectonic evolution of the Icelandic transverse
ridge and adjacent regions, J. geophys. Res., 90, 9953–9960.
Chevrot, S., Vinnik, L. & Montagner, J.P., 1999. Global-scale analysis
of the mantle Pds phases, J. geophys. Res., 104, 20 203–20 219.
Darbyshire, F.A., Bjarnason, I.T., White, R.S. & Flovenz, O.G., 1998.
Crustal structure above the Iceland mantle plume imaged by the
ICEMELT refraction profile, Geophys. J. Int., 135, 1131–1149.
Davies, G.F., 1988. Ocean bathymetry and mantle convection 1, Largescale
flow and hotspots, J. geophys. Res., 93, 10 467–10 480.
Du, Z.J. & Foulger, G.R., 1999. The crustal structure beneath
the Northwest Fjords, Iceland, from receiver functions and surface
waves, Geophys. J. Int., 139, 419–432.
Du, Z. & Foulger, G.R., 2001. Variation in the crustal structure across
central Iceland, Geophys. J. Int., 145, 246–264.
Du, Z. et al., 2001. Crustal structure beneath western and eastern
Iceland from surface waves and receiver functions, Geophys. J. Int.,
submitted.
Engdahl, E.R., van der Hilst, R.D. & Buland, R.P., 1998. Global
teleseismic earthquake relocation from improved travel times and
procedures for depth determination, Bull. seism. Soc. Am., 88, 722–743.
Evans, J.R. & Achauer, U., 1993. Teleseismic velocity tomography
using the ACH method: theory and application to continentalscale
studies, in Seismic Tomography: Theory and Applications,
pp. 319–360, eds Iyer, H.M. & Hirahara, K., Chapman & Hall,
London.
Faul, U.H., Toomey, D.R. & Waff, H.S., 1994. Intergranular basaltic
melt is distributed in thin, elongated inclusions, Geophys. Res. Lett.,
21, 29–32.
Feighner, M.A. & Kellogg, L.H., 1995. Numerical modeling of
chemically buoyant mantle plumes at spreading ridges, Geophys.
Res. Lett., 22, 715–718.
Fitton, J.G., Saunders, A.D., Norry, M.J., Hardarson, B.S. &
Taylor, R.N., 1997. Thermal and chemical structure of the Iceland
plume, Earth planet. Sci. Lett., 153, 197–208.
Foulger, G.R. & Pearson, D.G., 2001. Is Iceland underlain by a plume
in the lower mantle? Seismology and helium isotopes, Geophys. J.
Int., 145, F1–F5.
Foulger, G.R. et al., 2000. The seismic anomaly beneath Iceland
extends down to the mantle transition zone and no deeper, Geophys.
J. Int., 142, F1–F5.
Fukao, Y., Obayashi, M., Inoue, H. & Nenbai, M., 1992. Subducting
slabs stagnate in the mantle transition zone, J. geophys. Res., 97,
4809–4822.
Goes, S., Govers, R. & Vacher, P., 2000. Shallow mantle temperatures
under Europe from P and S wave tomography, J. geophys. Res., 105,
11 153–11 169.
Graham, D.W., Larsen, L.M., Hanan, B.B., Storey, M., Pedersen, A.K.
& Lupton, J.E., 1998. Helium isotope composition of the early
Iceland mantle plume inferred from Tertiary picrites of west
Greenland, Earth planet. Sci. Lett., 160, 241–255.
Grand, S.P., 1994. Mantle shear structure beneath the Americas and
surrounding oceans, J. geophys. Res., 99, 11 591–11 621.
Gudmundsson, M.T., Sigmundsson, F. & Bjornsson, H., 1997.
Ice–volcano interaction of the 1996 Gjalp subglacial eruption,
Vatnajokull, Iceland, Nature, 389, 954–957.
Gutenberg, B., 1959. The asthenosphere low-velocity layer, Ann.
Geofis., 12, 439–460.
Hager, B.H. & Clayton, R.W., 1989. Constraints on the structure of
mantle convection using seismic observations, flow models, and the
geoid, in Mantle Convection, Plate Tectonics and Global Dynamics,
pp. 657–764, ed. Peltier, W.R., Gordon & Breach, New York.
Halbert, S.E., Buland, R. & Hutt, C.R., 1988. Standard for the
Exchange of Earthquake Data (SEED), Version V2.0, USGS,
Albuquerque Seismological Laboratory, NM.
Harvey, D. & Quinlan, D., 1996. Datascope Seismic Application
Package (DSAP), Rept Joint Seismic Processing Center, University
of Colorado, CO.
Helmberger, D.V., Wen, L. & Ding, X., 1998. Seismic evidence that the
source of the Iceland hotspot lies at the core–mantle boundary,
Nature, 396, 251–255.
Hilton, D.R., Gronvold, K., Macpherson, C.G. & Castillo, P.R.,
1999. Extreme 3He/4He ratios in northwest Iceland: constraining the
common component in mantle plumes, Earth planet. Sci. Lett., 173,
53–60.
Holmes, A., 1931. Radioactvity and Earth movements, Trans. geol.
Soc. Glasgow 1928–29, 18, 559–606.
Houseman, G.A., 1990. The thermal structure of mantle plumes:
axisymmetric or triple-junction?, Geophys. J. Int., 102, 15–24.
Ito, G., Lin, J. & Gable, C.W., 1996. Dynamics of mantle flow and
melting at a ridge-centered hotspot: Iceland and the mid-Atlantic
ridge, Earth planet. Sci. Lett., 144, 53–74.
Iwamori, H., McKenzie, D. & Takahashi, E., 1995. Melt generation
by isentropic mantle upwelling, Earth planet. Sci. Lett., 134,
253–266.
Julian, B.R., Evans, J.R., Pritchard, M.J. & Foulger, G.R., 2001. A
geometrical error in some versions of the ACH method of teleseismic
tomography, Bull. seism. Soc. Am., 90, 1554–1558.
Karason, H. & van der Hilst, R.D., 2001a. Tomographic imaging of the
lowermost mantle with differential times of refracted and diffracted
core phases (PKP, Pdiff), J. geophys. Res., 106, 6569–6587.
Karason, H. & van der Hilst, R.D., 2001b. Mantle P-wave speed from
seismic tomography: advances in methodology and data integration,
J. geophys. Res., submitted.
Karato, S., 1993. Importance of anelasticity in the interpretation of
seismic tomography, Geophys. Res. Lett., 20, 1623–1626.
Keller, W.R., Anderson, D.L. & Clayton, R.W., 2000. Difficulties in
seismically imaging the Icelandic hotspot, Geophys. Res. Lett., 27,
3993–3996.
Kendall, J.-M., 1994. Teleseismic arrivals at a mid-ocean ridge: effects
of mantle melt and anisotropy, Geophys. Res. Lett., 21, 301–304.
Kennett, B.L.N. & Engdahl, E.R., 1991. Travel times for global
earthquake location and phase identification, Geophys. J. Int., 105,
429–466.
King, S.D. & Anderson, D.L., 1995. An alternative mechanism of flood
basalt formation, Earth planet. Sci. Lett., 136, 269–279.
King, S.D. & Anderson, D.L., 1998. Edge-driven convection, Earth
planet. Sci. Lett., 160, 289–296.
Korenaga, J., 2000. Magmatism and dynamics of continental breakup
in the presence of a mantle plume, PhD thesis, MIT, Cambridge, MA.
McKenzie, D. & Bickle, M.J., 1988. The volume and composition of
melt generated by extension of the lithosphere, J. Petrol., 29, 625–679.
Megnin, C. & Romanowicz, B., 1998. The effect of theoretical
formalism and data selection scheme on mantle models derived
from waveform tomography, Geophys. J. Int., 138, 366–380.
Megnin, C. & Romanowicz, B., 2000. The three-dimensional velocity
structure of the mantle from the inversion of body, surface and
higher-mode waveforms, Geophys. J. Int., 143, 709–728.
Morgan, W.J., 1971. Convection plumes in the lower mantle, Nature,
230, 42–43.
Morgan, W.J., 1972. Plate motions and deep mantle convection, Geol.
Soc. Am. Bull., 132, 7–22.
O’Nions, R.K. & Tolstikhin, I.N., 1994. Behaviour and residence times
of lithophile and rare gas tracers in the upper mantle, Earth planet.
Sci. Lett., 124, 131–138.
Pritchard, M.J., 2000. A seismological study of the mantle beneath
Iceland, PhD thesis, University of Durham, Durham.
Pritchard, M.J., Foulger, G.R., Julian, B.R. & Fyen, J., 2000.
Constraints on a plume in the mid-mantle beneath the Iceland
region from seismic-array data, Geophys. J. Int., 143, 119–128.
Ramberg, H., 1981. Gravity, Deformation and the Earth’s Crust,
Academic Press, London.
Ribe, N.M., Christensen, U.R. & Theissing, J., 1995. The dynamics of
plume–ridge interaction, 1: Ridge-centered plumes, Earth planet. Sci.
Lett., 134, 155–168.
Richards, M.A. & Hager, B.H., 1988. Dynamically supported geoid
highs over hotspots: observations and theory, J. geophys. Res., 93,
7690–7708.
Ritsema, J., van Heijst, H.J. & Woodhouse, J.H., 1999. Complex shear
wave velocity structure imaged beneath Africa and Iceland, Science,
286, 1925–1928.
Saemundsson, K., 1979. Outline of the geology of Iceland, Jokull, 29,
7–28.
Saemundsson, K., Kristjansson, L., McDougall, I. & Watkins, N.D.,
1980. K-Ar dating, geological and paleomagnetic study of a 5-km
lava succession in northern Iceland, J. geophys. Res., 85, 3628–3646.
Schilling, J.-G., 1973. Iceland mantle plume: geochemical study of
Reykjanes ridge, Nature, 242, 565–571.
Schilling, J.-G., 1991. Fluxes and excess temperatures of mantle plumes
inferred from their interaction with migrating mid-ocean ridges,
Nature, 352, 397–403.
Schilling, J.-G., Thompson, G., Kingsley, R. & Humphris, S., 1985.
Hotspot–migrating ridge interaction in the south Atlantic, Nature,
313, 187–191.
Schmeling, H., 2000. Partial melting and melt segregation in a convecting
mantle, in Physics and Chemistry of Partially Molten Rocks,
pp. 141–178, eds Bagdassarov, N., Laporte, D. & Thompson, A.B.,
Kluwer, Dordrecht.
Shen, Y. & Forsyth, D.W., 1995. Geochemical constraints on initial
and final depths of melting beneath mid-ocean ridges, J. geophys.
Res., 100, 2211–2237.
Shen, Y., Solomon, S.C., Bjarnason, I.T. & Wolfe, C.J., 1998. Seismic
evidence for a lower-mantle origin of the Iceland plume, Nature, 395,
62–65.
Shen, Y. et al., 2001. Seismic evidence for a tilted mantle plume and
north–south mantle flow beneath Iceland, Science, submitted.
Sigmundsson, F., Einarsson, P., Bilham, R. & Sturkell, E., 1994. Rifttransform
kinematics in south Iceland: deformation from GPS
measurements 1986–1992, J. geophys. Res., 100, 6235–6248.
Sigvaldason, G.E., Steinthorsson, S., Oskarsson, N. & Imsland, P.,
1974. Compositional variation in recent Icelandic tholeiites and the
Kverkfjoll hot spot, Nature, 251, 579–582.
Sleep, N.H., 1990. Hotspots and mantle plumes: some phenomenology,
J. geophys. Res., 95, 6715–6736.
Sleep, N.H., 1996. Lateral flow of hot plume material ponded at
sublithospheric depths, J. geophys. Res., 101, 28 065–28 083.
Sleep, N.H., 1997. Lateral flow and ponding of starting plume material,
J. geophys. Res., 102, 10 001–10 012.
Spetzler, H. & Anderson, D.L., 1968. The effect of temperature and
partial melting on velocity and attenuation in a simple binary system,
J. geophys. Res., 73, 6051–6060.
Steck, L.K. & Prothero, W.A., 1991. A 3-D raytracer for teleseismic
body-wave arrival times, Bull. seism. Soc. Am., 81, 1332–1339.
Stefa´nsson, R. et al., 1993. Earthquake prediction research in the south
Iceland seismic zone and the SIL project, Bull. seism. Soc. Am., 83,
696–716.
Tackley, P.J., 1998. Self-consistent generation of tectonic plates
in three-dimensional mantle convection, Earth planet. Sci. Lett.,
157, 9–22.
Thorbergsson, G., Magnusson, I.T. & Palmason, G., 1990. Gravity
Data and Gravity Map of Iceland, Rept OS-90001/JHD-01, National
Energy Authority, Reykjavik.
Tryggvason, K., Husebye, E.S. & Stefa´nsson, R., 1983. Seismic
image of the hypothesized Icelandic hot spot, Tectonophysics, 100,
94–118.
van der Hilst, R. & Karason, H., 1999. Compositional heterogeneity
in the bottom 1000 kilometers of Earth’s mantle: toward a hybrid
convection model, Science, 283, 1885–1888.
van der Hilst, R., Widiyantoro, S. & Engdahl, E.R., 1997. Evidence for
deep mantle circulation from global tomography, Nature, 353, 37–42.
Vink, G.E., 1984. A hotspot model for Iceland and the Voring Plateau,
J. geophys. Res., 89, 9949–9959.
Vogt, P.R., 1976. Plumes, sub-axial pipe flow, and topography along
mid-oceanic ridges, Earth planet. Sci. Lett., 29, 309–325.
White, R.S. & McKenzie, D.P., 1995. Mantle plumes and flood basalts,
J. geophys. Res., 100, 17 543–17 585.
White, R.S., Bown, J.W. & Smallwood, J.R., 1995. The temperature
of the Iceland plume and origin of outward-propagating V-shaped
ridges, J. Geol. Soc. Lond., 152, 1039–1045.
Wilson, J.T., 1963. A possible origin of the Hawaiian Islands, Can. J.
Phys., 41, 863–870.
Wolfe, C.J., Bjarnason, I.T., VanDecar, J.C. & Solomon, S.C., 1997.
Seismic structure of the Iceland mantle plume, Nature, 385, 245–247.
Woodhouse, J.H. & Dziewonski, A.M., 1984. Mapping the upper
mantle—3-dimensional modeling of Earth structure by inversion of
seismic waveforms, J. geophys. Res., 89, 5953–5984.
Zhou, H.W., 1996. A high resolution P-wave model for the top
1200 km of the mantle, J. geophys. Res., 101, 27 791–27 810.