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.

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Summary Gravity data from Iceland and its surroundings are analysed and modelled with respect to seismic data. A Bouguer gravity map of Iceland is recomputed based on admittance between the topography and the gravity and with corrections for glacial ice sheets. From seismic data and with the help of relations between the residual topography and the depth to seismic boundaries we construct maps of the main seismic boundaries, including the Moho. By inversion calculations we recomputed these maps, assuming different density values for Seismic Layer 4 to fit the observed gravity field. We found that the average density of Layer 4 has to be in the range 3050–3150 kg m−3 in order to fit both seismic and gravity data. Thus we conclude that Layer 4 is a transition zone between the mantle and the oceanic crust in Iceland. Furthermore by assuming that the upper‐mantle density variations necessary to compensate for the gravity effect of crustal layers, are caused by thermal variations in the upper mantle, we calculate the depth to the 1200 °C isotherm to be at 30–50 km depth below Iceland but rising up to less than 20 km below parts of the volcanic zone in Northern Iceland. We conclude that the temperature within the Seismic Layer 4 is close to 600 °C at its top, increasing to approximately 950 °C at its bottom (Moho), which makes a widespread layer of partially molten material within Layer 4 unlikely. By use of cross spectral analysis of the gravity field and the external topographic load at short wavelengths, we conclude that the elastic plate thickness in Iceland can hardly exceed 6 km. In addition we point out that the residual isostatic anomalies have circular forms east of the eastern volcanic zone but are near parallel to the ridge axis on the western side. This form of the anomalies may be caused by pressure from the eastward moving mantle plume below the eastern volcanic zone.