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Research paper thumbnail of Imaging the mantle beneath Iceland using integrated seismological techniques

Journal of Geophysical Research, 2002

1] Using a combination of body wave and surface wave data sets to reveal the mantle plume and plu... more 1] Using a combination of body wave and surface wave data sets to reveal the mantle plume and plume head, this study presents a tomographic image of the mantle structure beneath Iceland to 400 km depth. Data comes primarily from the PASSCAL-HOTSPOT deployment of 30 broadband instruments over a period of 2 years, and is supplemented by data from the SIL and ICEMELT networks. Three sets of relative teleseismic body wave arrival times are generated through cross correlation: S and SKS arrivals at 0.03-0.1 Hz, and P and PKIKP arrivals at 0.03-0.1 and 0.8-2.0 Hz. Prior to inversion the crustal portion of the travel time anomalies is removed using the crustal model ICECRTb. This step has a significant effect on the mantle velocity variations imaged down to a depth of 250km.Inversionofrelativearrivaltimesonlyprovidesinformationonlateralvelocityvariations.SurfacewavesarethereforeusedtoprovideabsolutevelocityinformationfortheuppermostmantlebeneathIceland.TheaveragewavenumberfortheLovewavefundamentalmodeat0.020and0.024HzismeasuredandusedtoinvertfortheaverageSvelocity.Combinationofthebodywaveandsurfacewaveinformationrevealsapredominantlyhorizontallow−velocityanomalyextendingfromtheMohodownto250 km. Inversion of relative arrival times only provides information on lateral velocity variations. Surface waves are therefore used to provide absolute velocity information for the uppermost mantle beneath Iceland. The average wave number for the Love wave fundamental mode at 0.020 and 0.024 Hz is measured and used to invert for the average S velocity. Combination of the body wave and surface wave information reveals a predominantly horizontal low-velocity anomaly extending from the Moho down to 250km.Inversionofrelativearrivaltimesonlyprovidesinformationonlateralvelocityvariations.SurfacewavesarethereforeusedtoprovideabsolutevelocityinformationfortheuppermostmantlebeneathIceland.TheaveragewavenumberfortheLovewavefundamentalmodeat0.020and0.024HzismeasuredandusedtoinvertfortheaverageSvelocity.Combinationofthebodywaveandsurfacewaveinformationrevealsapredominantlyhorizontallow−velocityanomalyextendingfromtheMohodownto250 km depth, interpreted as a plume head. Below the plume head a near-cylindrical lowvelocity anomaly with a radius of $100 km and peak V P and V S anomalies of À2% and À4%, respectively, extends down to the maximum depth of resolution at 400 km. Within the plume head, in the uppermost mantle above the core of the plume, there is a relatively high velocity with a maximum V P and V S anomaly of +2%. This high-velocity anomaly may be the result of the extreme degree of melt extraction necessary to generate the thick (46 km) crust in central Iceland. Comparison of the plume volumetric flux implied by our images, the crustal generation rate, and the degree of melting suggested by rare earth element inversions, suggests that (1) mantle material must be flowing horizontally away from the plume core faster than the overlying lithosphere and (2) the bulk of the plume material does not participate in melting beneath Iceland. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 7203 Seismology: Body wave propagation; 7255 Seismology: Surface waves and free oscillations; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; Citation: Allen, R. M., et al., Imaging the mantle beneath Iceland using integrated seismological techniques,

Research paper thumbnail of Plume-driven plumbing and crustal formation in Iceland

Journal of Geophysical Research, 2002

1] Through combination of surface wave and body wave constraints we derive a threedimensional (3-... more 1] Through combination of surface wave and body wave constraints we derive a threedimensional (3-D) crustal S velocity model and Moho map for Iceland. It reveals a vast plumbing system feeding mantle plume melt into upper crustal magma chambers where crustal formation takes place. The method is based on the partitioned waveform inversion to which we add additional observations. Love waves from six local events recorded on the HOTSPOT-SIL networks are fitted, S n travel times from the same events measured, previous observations of crustal thickness are added, and all three sets of constraints simultaneously inverted for our 3-D model. In the upper crust (0-15 km) an elongated low-velocity region extends along the length of the Northern, Eastern and Western Neovolcanic Zones. The lowest velocities (À7%) are found at 5-10 km below the two most active volcanic complexes: Hekla and Bárdarbunga-Grímsvötn. In the lower crust (>15 km) the low-velocity region can be represented as a vertical cylinder beneath central Iceland. The low-velocity structure is interpreted as the thermal halo of pipe work which connects the region of melt generation in the uppermost mantle beneath central Iceland to active volcanoes along the neovolcanic zones. Crustal thickness in Iceland varies from 15-20 km beneath the Reykjanes Peninsula, Krafla and the extinct Snaefellsnes rift zone, to 46 km beneath central Iceland. The average crustal thickness is 29 km. The variations in thickness can be explained in terms of the temporal variation in plume productivity over the last $20 Myr, the Snaefellsnes rift zone being active during a minimum in plume productivity. Variations in crustal thickness do not depart significantly from an isostatically predicted crustal thickness. The best fit linear isostatic relation implies an average density jump of 4% across the Moho. Rare earth element inversions of basalt compositions on Iceland suggest a melt thickness (i.e., crustal thickness) of 15-20 km, given passive upwelling. The observed crustal thickness of up to 46 km implies active fluxing of source material through the melt zone by the mantle plume at up to 3 times the passive rate.

Research paper thumbnail of Seismic tomography shows that upwelling beneath Iceland is confined to the upper mantle

Geophysical Journal International, 2001

We report the results of the highest-resolution teleseismic tomography study yet performed of the... more 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 1996Iceland -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 from y60 km depth to a maximum of y450 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, V P , as strong as x2.7 per cent and in S-wave speed, V S , 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 V P , V S and V P /V S 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 Geophys.

Research paper thumbnail of The thin hot plume beneath Iceland

Geophysical Journal International, 1999

We present the results of a seismological investigation of the frequency-dependent amplitude vari... more We present the results of a seismological investigation of the frequency-dependent amplitude variations across Iceland using data from the HOTSPOT array currently deployed there. The array is composed of 30 broad-band PASSCAL instruments. We use the parameter t*, de¢ned in the usual manner from spectral ratios , to compare observed S-wave amplitude variations with those predicted due to both anelastic attenuation and di¡raction e¡ects. Four teleseismic events at a range of azimuths are used to measure t*. A 2-D vertical cylindrical plume model with a Gaussian-shaped velocity anomaly is used to model the variations. That part of t* caused by attenuation was estimated by tracing a ray through IASP91, then superimposing our plume model velocity anomaly and calculating the path integral of 1/oQ. That part of t* caused by di¡raction was estimated using a 2-D ¢nite di¡erence code to generate synthetic seismograms. The same spectral ratio technique used for the data was then used to extract a predicted t*. The t* variations caused by anelastic attenuation are unable to account for the variations we observe, but those caused by di¡raction do. We calculate the t* variations caused by di¡raction for di¡erent plume models and obtain our best-¢t plume, which exhibits good agreement between the observed and measured t*. The best-¢t plume model has a maximum S-velocity anomaly of {12 per cent and falls to 1/e of its maximum at 100 km from the plume centre. This is narrower than previous estimates from seismic tomography, which are broadened and damped by the methods of tomography. This velocity model would suggest greater ray theoretical traveltime delays than observed. However, we ¢nd that for such a plume, wave-front healing e¡ects at frequencies of 0.03^0.175 Hz (the frequency range used to pick S-wave arrivals) causes a 40 per cent reduction in traveltime delay, reducing the ray theoretical delay to that observed.

Research paper thumbnail of The seismic anomaly beneath Iceland extends down to the mantle transition zone and no deeper

Geophysical Journal International, 2000

A 3-D teleseismic tomography image of the upper mantle beneath Iceland of unprecedented resolutio... more A 3-D teleseismic tomography image of the upper mantle beneath Iceland of unprecedented resolution reveals a subvertical low wave speed anomaly that is cylindrical in the upper 250 km but tabular below this. Such a morphological transition is expected towards the bottom of a buoyant upwelling. Our observations thus suggest that magmatism at the Iceland hotspot is fed by¯ow rising from the mantle transition zone. This result contributes to the ongoing debate about whether the upper and lower mantles convect separately or as one. The image also suggests that material¯ows outwards from Iceland along the Reykjanes Ridge in the upper 200 km, but is blocked in the upper 150 km beneath the Tjornes Fracture Zone. This provides direct observational support for the theory that fracture zones dam lateral¯ow along ridges.

Research paper thumbnail of Crustal structure beneath western and eastern Iceland from surface waves and receiver functions

Geophysical Journal International, 2002

We determine the crustal structures beneath 14 broad-band seismic stations, deployed in western, ... more We determine the crustal structures beneath 14 broad-band seismic stations, deployed in western, eastern, central and southern Iceland, using surface wave dispersion curves and receiver functions. We implement a method to invert receiver functions using constraints obtained from genetic algorithm inversion of surface waves. Our final models satisfy both data sets. The thickness of the upper crust, as defined by the velocity horizon V s = 3.7 km s −1 , is fairly uniform at ∼6.5-9 km beneath the Tertiary intraplate areas of western and eastern Iceland, and unusually thick at 11 km beneath station HOT22 in the far south of Iceland. The depth to the base of the lower crust, as defined by the velocity horizon V s = 4.1 km s −1 is ∼20-26 km in western Iceland and ∼27-33 km in eastern Iceland. These results agree with those of explosion profiles that detect a thinner crust beneath western Iceland than beneath eastern Iceland. An earlier report of a substantial low-velocity zone beneath the Middle Volcanic Zone in the lower crust is confirmed by a similar observation beneath an additional station there. As was found in previous receiver function studies, the most reliable feature of the results is the clear division into an upper sequence that is a few kilometres thick where velocity gradients are high, and a lower, thicker sequence where velocity gradients are low. The transition to typical mantle velocities is variable, and may range from being very gradational to being relatively sharp and clear. A clear Moho, by any definition, is rarely seen, and there is thus uncertainty in estimates of the thickness of the crust in many areas. Although a great deal of seismic data are now available constraining the structures of the crust and upper mantle beneath Iceland, their geological nature is not well understood.

Research paper thumbnail of Imaging the mantle beneath Iceland using integrated seismological techniques

Journal of Geophysical Research, 2002

1] Using a combination of body wave and surface wave data sets to reveal the mantle plume and plu... more 1] Using a combination of body wave and surface wave data sets to reveal the mantle plume and plume head, this study presents a tomographic image of the mantle structure beneath Iceland to 400 km depth. Data comes primarily from the PASSCAL-HOTSPOT deployment of 30 broadband instruments over a period of 2 years, and is supplemented by data from the SIL and ICEMELT networks. Three sets of relative teleseismic body wave arrival times are generated through cross correlation: S and SKS arrivals at 0.03-0.1 Hz, and P and PKIKP arrivals at 0.03-0.1 and 0.8-2.0 Hz. Prior to inversion the crustal portion of the travel time anomalies is removed using the crustal model ICECRTb. This step has a significant effect on the mantle velocity variations imaged down to a depth of 250km.Inversionofrelativearrivaltimesonlyprovidesinformationonlateralvelocityvariations.SurfacewavesarethereforeusedtoprovideabsolutevelocityinformationfortheuppermostmantlebeneathIceland.TheaveragewavenumberfortheLovewavefundamentalmodeat0.020and0.024HzismeasuredandusedtoinvertfortheaverageSvelocity.Combinationofthebodywaveandsurfacewaveinformationrevealsapredominantlyhorizontallow−velocityanomalyextendingfromtheMohodownto250 km. Inversion of relative arrival times only provides information on lateral velocity variations. Surface waves are therefore used to provide absolute velocity information for the uppermost mantle beneath Iceland. The average wave number for the Love wave fundamental mode at 0.020 and 0.024 Hz is measured and used to invert for the average S velocity. Combination of the body wave and surface wave information reveals a predominantly horizontal low-velocity anomaly extending from the Moho down to 250km.Inversionofrelativearrivaltimesonlyprovidesinformationonlateralvelocityvariations.SurfacewavesarethereforeusedtoprovideabsolutevelocityinformationfortheuppermostmantlebeneathIceland.TheaveragewavenumberfortheLovewavefundamentalmodeat0.020and0.024HzismeasuredandusedtoinvertfortheaverageSvelocity.Combinationofthebodywaveandsurfacewaveinformationrevealsapredominantlyhorizontallow−velocityanomalyextendingfromtheMohodownto250 km depth, interpreted as a plume head. Below the plume head a near-cylindrical lowvelocity anomaly with a radius of $100 km and peak V P and V S anomalies of À2% and À4%, respectively, extends down to the maximum depth of resolution at 400 km. Within the plume head, in the uppermost mantle above the core of the plume, there is a relatively high velocity with a maximum V P and V S anomaly of +2%. This high-velocity anomaly may be the result of the extreme degree of melt extraction necessary to generate the thick (46 km) crust in central Iceland. Comparison of the plume volumetric flux implied by our images, the crustal generation rate, and the degree of melting suggested by rare earth element inversions, suggests that (1) mantle material must be flowing horizontally away from the plume core faster than the overlying lithosphere and (2) the bulk of the plume material does not participate in melting beneath Iceland. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 7203 Seismology: Body wave propagation; 7255 Seismology: Surface waves and free oscillations; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; Citation: Allen, R. M., et al., Imaging the mantle beneath Iceland using integrated seismological techniques,

Research paper thumbnail of Plume-driven plumbing and crustal formation in Iceland

Journal of Geophysical Research, 2002

1] Through combination of surface wave and body wave constraints we derive a threedimensional (3-... more 1] Through combination of surface wave and body wave constraints we derive a threedimensional (3-D) crustal S velocity model and Moho map for Iceland. It reveals a vast plumbing system feeding mantle plume melt into upper crustal magma chambers where crustal formation takes place. The method is based on the partitioned waveform inversion to which we add additional observations. Love waves from six local events recorded on the HOTSPOT-SIL networks are fitted, S n travel times from the same events measured, previous observations of crustal thickness are added, and all three sets of constraints simultaneously inverted for our 3-D model. In the upper crust (0-15 km) an elongated low-velocity region extends along the length of the Northern, Eastern and Western Neovolcanic Zones. The lowest velocities (À7%) are found at 5-10 km below the two most active volcanic complexes: Hekla and Bárdarbunga-Grímsvötn. In the lower crust (>15 km) the low-velocity region can be represented as a vertical cylinder beneath central Iceland. The low-velocity structure is interpreted as the thermal halo of pipe work which connects the region of melt generation in the uppermost mantle beneath central Iceland to active volcanoes along the neovolcanic zones. Crustal thickness in Iceland varies from 15-20 km beneath the Reykjanes Peninsula, Krafla and the extinct Snaefellsnes rift zone, to 46 km beneath central Iceland. The average crustal thickness is 29 km. The variations in thickness can be explained in terms of the temporal variation in plume productivity over the last $20 Myr, the Snaefellsnes rift zone being active during a minimum in plume productivity. Variations in crustal thickness do not depart significantly from an isostatically predicted crustal thickness. The best fit linear isostatic relation implies an average density jump of 4% across the Moho. Rare earth element inversions of basalt compositions on Iceland suggest a melt thickness (i.e., crustal thickness) of 15-20 km, given passive upwelling. The observed crustal thickness of up to 46 km implies active fluxing of source material through the melt zone by the mantle plume at up to 3 times the passive rate.

Research paper thumbnail of Seismic tomography shows that upwelling beneath Iceland is confined to the upper mantle

Geophysical Journal International, 2001

We report the results of the highest-resolution teleseismic tomography study yet performed of the... more 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 1996Iceland -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 from y60 km depth to a maximum of y450 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, V P , as strong as x2.7 per cent and in S-wave speed, V S , 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 V P , V S and V P /V S 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 Geophys.

Research paper thumbnail of The thin hot plume beneath Iceland

Geophysical Journal International, 1999

We present the results of a seismological investigation of the frequency-dependent amplitude vari... more We present the results of a seismological investigation of the frequency-dependent amplitude variations across Iceland using data from the HOTSPOT array currently deployed there. The array is composed of 30 broad-band PASSCAL instruments. We use the parameter t*, de¢ned in the usual manner from spectral ratios , to compare observed S-wave amplitude variations with those predicted due to both anelastic attenuation and di¡raction e¡ects. Four teleseismic events at a range of azimuths are used to measure t*. A 2-D vertical cylindrical plume model with a Gaussian-shaped velocity anomaly is used to model the variations. That part of t* caused by attenuation was estimated by tracing a ray through IASP91, then superimposing our plume model velocity anomaly and calculating the path integral of 1/oQ. That part of t* caused by di¡raction was estimated using a 2-D ¢nite di¡erence code to generate synthetic seismograms. The same spectral ratio technique used for the data was then used to extract a predicted t*. The t* variations caused by anelastic attenuation are unable to account for the variations we observe, but those caused by di¡raction do. We calculate the t* variations caused by di¡raction for di¡erent plume models and obtain our best-¢t plume, which exhibits good agreement between the observed and measured t*. The best-¢t plume model has a maximum S-velocity anomaly of {12 per cent and falls to 1/e of its maximum at 100 km from the plume centre. This is narrower than previous estimates from seismic tomography, which are broadened and damped by the methods of tomography. This velocity model would suggest greater ray theoretical traveltime delays than observed. However, we ¢nd that for such a plume, wave-front healing e¡ects at frequencies of 0.03^0.175 Hz (the frequency range used to pick S-wave arrivals) causes a 40 per cent reduction in traveltime delay, reducing the ray theoretical delay to that observed.

Research paper thumbnail of The seismic anomaly beneath Iceland extends down to the mantle transition zone and no deeper

Geophysical Journal International, 2000

A 3-D teleseismic tomography image of the upper mantle beneath Iceland of unprecedented resolutio... more A 3-D teleseismic tomography image of the upper mantle beneath Iceland of unprecedented resolution reveals a subvertical low wave speed anomaly that is cylindrical in the upper 250 km but tabular below this. Such a morphological transition is expected towards the bottom of a buoyant upwelling. Our observations thus suggest that magmatism at the Iceland hotspot is fed by¯ow rising from the mantle transition zone. This result contributes to the ongoing debate about whether the upper and lower mantles convect separately or as one. The image also suggests that material¯ows outwards from Iceland along the Reykjanes Ridge in the upper 200 km, but is blocked in the upper 150 km beneath the Tjornes Fracture Zone. This provides direct observational support for the theory that fracture zones dam lateral¯ow along ridges.

Research paper thumbnail of Crustal structure beneath western and eastern Iceland from surface waves and receiver functions

Geophysical Journal International, 2002

We determine the crustal structures beneath 14 broad-band seismic stations, deployed in western, ... more We determine the crustal structures beneath 14 broad-band seismic stations, deployed in western, eastern, central and southern Iceland, using surface wave dispersion curves and receiver functions. We implement a method to invert receiver functions using constraints obtained from genetic algorithm inversion of surface waves. Our final models satisfy both data sets. The thickness of the upper crust, as defined by the velocity horizon V s = 3.7 km s −1 , is fairly uniform at ∼6.5-9 km beneath the Tertiary intraplate areas of western and eastern Iceland, and unusually thick at 11 km beneath station HOT22 in the far south of Iceland. The depth to the base of the lower crust, as defined by the velocity horizon V s = 4.1 km s −1 is ∼20-26 km in western Iceland and ∼27-33 km in eastern Iceland. These results agree with those of explosion profiles that detect a thinner crust beneath western Iceland than beneath eastern Iceland. An earlier report of a substantial low-velocity zone beneath the Middle Volcanic Zone in the lower crust is confirmed by a similar observation beneath an additional station there. As was found in previous receiver function studies, the most reliable feature of the results is the clear division into an upper sequence that is a few kilometres thick where velocity gradients are high, and a lower, thicker sequence where velocity gradients are low. The transition to typical mantle velocities is variable, and may range from being very gradational to being relatively sharp and clear. A clear Moho, by any definition, is rarely seen, and there is thus uncertainty in estimates of the thickness of the crust in many areas. Although a great deal of seismic data are now available constraining the structures of the crust and upper mantle beneath Iceland, their geological nature is not well understood.