Reinterpretation of the RRISP-77 Iceland shear-wave profiles

Abstract

S U M M A R YTwo 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 10-15km 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, S,S, 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.

Primitive off-rift basalts from Iceland and Jan Mayen: Os-isotopic evidence for a mantle source containing enriched subcontinental lithosphere

“…These geographically related heterogeneities could result from different degrees of partial melting related to different crustal thickness (Elliott et al, 1991;Hards et al, 1995;Stracke et al, 1998). The inferred low to moderate degree of partial melting of the mantle under the EVFZ is consistent with the observed crustal thickness of about 30-35 km away from the axial rift zone (Menke et al, 1996;Staples et al, 1997). Alternatively, these trends could also reflect the sampling of a compositionally-zoned Iceland mantle plume (e.g., Fitton et al, 1997;Kempton et al, 2000;Blichert-Toft et al, 2005).…”

Variation in the crustal structure across central Iceland

“…We present our results as a set of velocity-depth profiles for V p . Receiver function analysis yields V s , and we construct our profiles by converting this to V p using a V p /V s ratio of 1.76 (Menke et al 1996), which is a typical value for the Icelandic crust as a whole (Darbyshire et al 1998).…”

“…4). This zone is best developed in the structure obtained from the data from the southwest The V s models derived from the receiver function analysis are converted to V p using a V p /V s ratio of 1.76 (Menke et al 1996).…”

“…These include the SIST (South Iceland Seismic Tomography) (Bjarnason et al 1993), FIRE (Faeroe-Iceland Ridge Experiment) (Staples et al 1997), B96 (Menke et al 1998) and ICEMELT profiles (Darbyshire et al 1998). In the light of these new results, the RRISP profile, shot in 1977, and on which the thin-crust model was heavily based, was reinterpreted by Menke et al (1996). That reinterpretation suggested that the RRISP profile had originally been incorrectly interpreted by Gebrande et al (1980) and that a Moho at 30-35 km depth below central Iceland was in fact detected by the profile.…”

Case studies of seismic tomography and earthquake location in a regional context

“…Measurements of velocity ratio have been used to infer (or rule out) partial melting and magma ponding within the crust and shallow mantle (e.g. Gebrande et al 1980;Menke et al, 1996).…”