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Summary The crustal structure of central Iceland is modelled using data from a 310 km long refraction profile shot during summer 1995. The profile traversed Iceland from the Skagi Peninsula on the north coast (surface rocks of age 8.5–0.8 Myr) to the southeast coast (surface rocks of age 8.5–3.3 Myr), crossing central Iceland (surface rocks of age 3.3–0 Myr) over the glacier Vatnajökull, below which the locus of the Iceland mantle plume is currently centred. The crustal thickness is 25 km at the north end of the profile, increasing to 38–40 km beneath southern central Iceland. The upper crust is characterized by seismic P‐wave velocities from 3.2 to approximately 6.4 km s‐1. At the extreme ends of the profile, the upper crust can be modelled by a two‐layered structure, within which seismic velocity increases with depth, with a total thickness of5–6 km. The central highlands of Iceland have a single unit of upper crust, with seismic velocity increasing continuously with depth to almost 10 km below the surface. Below the central volcanoes of northern Vatnajökull, the upper crust is only 3 km thick. The lower‐crustal velocity structure is determined from rays that turn at a maximum depth of 24 km below central Iceland, where the seismic velocity is 7.2 km s‐1. Below 24 km depth there are no first‐arriving turning rays. The Moho is defined by P‐and S‐wave reflections observed from the shots at the extreme ends of the profile.P‐ to S‐wave velocity ratios give a Poisson’s :of 0.26 in the upper crust and 0.27 in the lower crust, indicating that, even directly above the centre of the mantle plume, the crust is well below the solidus temperature.

The 170 km South Iceland Seismic Tomography (SIST) profile extends from the west and across the Mid-Atlantic Ridge spreading center in the Western Volcanic Zone and continues obliquely through the transform zone (the South Iceland Seismic Zone) to the western edge of the Eastern Volcanic Zone. A total of 11 shot points and 210 receiver points were used, allowing precise travel times to be determined for 1050 crustal P wave rays and 180 wide-angle reflections. The large amplitudes of the wide-angle reflections and an apparent refractor velocity of 7.7 km/s are interpreted to be from a relatively sharp Moho at a depth of 20-24 kin. This interpretation differs from the earlier models (based on data gathered in the 1960s and 1970s), of a 10-15 km thick crust underlain by a upper mantle with very slow velocity of 7.0-7.4 km/s. Nevertheless, these older data do not contradict our new interpretation. Implication of the new interpretation is that the lower crust and the crust-mantle boundary are colder than previously assumed. A two-dimensional totoographic inversion of the compressional travel times reveals the following structures in the crust: (1) a sharp increase in thickness of the upper crust ("layer 2A") from northwest to southeast and (2) broad updoming of high velocity in the lower crust in the Western Volcanic Zone, (3) depth to the lower crust ("layer 3") increases gradually from 3 km at the northwestern end of the profile to 7 km at the southeastern end of the profile, (4) a low-velocity perturbation extends throughout the upper crust and midcrust into the lower crust in the area of the transform in south Iceland (South Iceland Seismic Zone), and (5) an upper crustal high-velocity anomaly is associated with extinct central volcanos northwest of the Western Volcanic Zone. The travel time data do not support the existence of a large (> 0.5 km thick) crustal magma chamber in this part of the Western Volcanic Zone but do not exclude the possibility of a smaller one.

Iceland, one of the most thoroughly investigated hotspots 1-3 , is generally accepted to be the manifestation of an upwelling mantle plume 4 . Yet whether the plume originates from the lower mantle or from a convective instability at a thermal boundary layer between the upper and lower mantle near 660 km depth 5,6 remains unconstrained. Tomographic inversions of body-wave delay times show that low seismic velocities extend to at least 400 km depth beneath central Iceland 7,8 , but cannot resolve structure at greater depth. Here we report lateral variations in the depths of compressional-to-shear wave conversions at the two seismic discontinuities marking the top and bottom of the mantle transition zone beneath Iceland. We find that the transition zone is 20 km thinner than in the average Earth 9 beneath central and southern Iceland, but is of normal thickness beneath surrounding areas, a result indicative of a hot and narrow plume originating from the lower mantle.Observational constraints on the deep structure of plumes are sparse, with only one report of the detection of a narrow plume conduit at a depth of ϳ700 km (ref. 10). A new approach to address the question of the depth of origin of mantle plumes is to map the mantle seismic discontinuities near 410 and 660 km depth, features that have been identified with the transitions from the ␣-phase to the ␤-phase of (Mg,Fe) 2 SiO 4 and from ␥-(Mg,Fe) 2 SiO 4 to perovskite plus magnesiowustite 11 , respectively. The depths to the 410-and 660-km discontinuities respectively increase and decrease with increasing temperature, and thus provide information on lateral temperature variations and associated mantle circulation patterns. As sketched in Fig. 1, lateral variations in the discontinuity depths can be diagnostic of the depth of origin of mantle plumes.The data used here are receiver functions 12 derived from bodywave records of teleseimic earthquakes from the broadband ICE-MELT seismic network 13 and the permanent Global Seismographic Network (GSN) station BORG on Iceland (Fig. 2). The calculation of receiver functions follows procedures previously described 14 .To image lateral variations in seismic discontinuities, we use geographical binning of receiver functions 15 . At a given binning depth beneath Iceland and the surrounding area, we divide the horizontal plane into overlapping square patches (200 ϫ 200 km, Fig. 2), comparable in dimension to the Fresnel zone of a P660s phase (Pds denotes a P-to-S conversion at depth d) of frequency 0.1-0.3 Hz at the conversion depth. Each patch overlaps two-thirds of its area with adjacent patches. For every 10-km increment in binning depth from the surface to 1,200 km, receiver functions having Pds paths that pierce the same patch are gathered (Fig. 2), corrected for moveout to a reference ray parameter of 0.0573 s km −1 (ref. 14), and stacked using an nth-root method 16 (n ¼ 2) in the time window corresponding to the given binning depth interval.The stacked receiver functions clearly reveal arrivals corresponding to the ...

Shear waves converted from compressional waves at mantle discontinuities near 410-and 660-km depth recorded by two broadband seismic experiments in Iceland reveal that the center of an area of anomalously thin mantle transition zone lies at least 100 km south of the upper-mantle low velocity anomaly imaged tomographically beneath the hotspot. This offset is evidence for a tilted plume conduit in the upper mantle, the result of either northward flow of the Icelandic asthenosphere or southward flow of the upper part of the lower mantle in a nonet rotation reference frame.