Reinterpretation of the RRISP-77 Iceland shear-wave profiles

Two shear-wave profiles, E and G, collected during the 1977 Reykjanes Ridge Iceland Seismic Experiment have played an important role in models of the Icelandic crust. They were originally interpreted as indicating very low shear-wave velocities and abnormally low shear-wave quality factors in the 10–15 km depth range. These attributes, which are indicative of near-solidus temperatures, were used to support the hypothesis that the crust of Iceland is relatively thin (10–15 km) and underlain by partially molten material. More recent seismic data, however, contradict this hypothesis and suggest that the crust is thicker (20–30 km) and cooler. A re-examination of the RRISP-77 data indicates that the low shear-wave velocities are artefacts arising from source static anomalies (in the case of profile G) and misidentification of a secondary shear phase, SmS, as S (in the case of profile E). Furthermore, the attenuation occurs at ranges when rays from the shots pass near the Askja (profile E) and Katla and Oraefajokull (profile G) volcanoes. It may therefore have a localized source, and not be diagnostic of Icelandic crust as a whole. This new interpretation of the RRISP-77 shear-wave data is consistent with models having a thick, cold crust.We thank 0. Flovenz, one of the principal investigators of the SIST experiment, G. Foulger and B. Julian, principal investigators of the Hengill experiment, and the Incorporated Research Institutions for Seismology for providing us with copies of the data. Lamont Doherty Contribution Number 5513Peer Reviewe

Seismic evidence for a lower-mantle origin of the Iceland plume

Iceland, one of the most thoroughly investigated hotspots1,2,3, is generally accepted to be the manifestation of an upwelling mantle plume4. 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 depth5,6 remains unconstrained. Tomographic inversions of body-wave delay times show that low seismic velocities extend to at least 400 km depth beneath central Iceland7,8, 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 Earth9 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.This work was supported by the US NSFPeer Reviewe

Tomographic image of the Mid-Atlantic Plate Boundary in southwestern Iceland

Publisher's version (útgefin grein)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 km. 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 tomographic 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.This research was supported by the U.S. National Science Foundation, the Iceland National Science Foundation, the National Energy Authority of Iceland, the Incorporated Research Institutions for Seismology, IcelandAir, and the Lamont-Doherty Geological Observatory of Columbia University.Peer Reviewe