Seismic structure of the Icelandic crust above layer three and the relation between body wave velocity and the alteration of the basaltic crust
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Abstract
Seismic refraction profiles from Iceland are studied with the aid of synthetic seismograms. The classical layered model of the Icelandic crust is shown to be an unacceptable interpretation of the available data. This is because the layered model does not satisfy the observed amplitude variation. On the other hand, a model which assumes continuously increasing velocity with depth does not contradict the observations and is therefore acceptable although it is not the only possible interpretation. The model represented here shows that the surface value of the P-velocity is variable from 2.0 km/s to 5.0 km/s, depending primarily on the degree of metamorphism. The P-velocity increases rapidly with depth in the velocity interval 2.0-3.5 km/s followed by an approximately constant gradient of about 0.57 s-1. This constant gradient continues down to the 6.5 km/s isovelocity surface below which the P-velocity becomes nearly constant. In view of this, it is more reasonable to divide the Icelandic crust into two parts: the upper crust with velocity continuously increasing with depth (corresponding to layers 0, 1, 2 in the layered model) and the lower crust with almost constant velocity (corresponding to layer 3 in the layered model). The depth to the lower crust is variable and depends on how deep the crust is eroded. A typical depth to the lower crust is 5-6 km for an uneroded basalt pile but can be considerable less where the basalt pile is deeply eroded, especially below extinct central volcanoes.
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