Phase velocities of teleseismic Rayleigh waves have been measured
in the central North Atlantic on both sides of the Azores-Gibraltar
Ridge (AGR) by means of a specially designed long-period station
network. The dispersion data obtained were regionalized and then
subjected to a ?hedgehog? inversion, which gives a set of upper mantle
models compatible with the observational data within specified error
bounds.
Reasonable model solutions were selected by using regional body-wave
observations, such as Pn- and Sn-wave velocities determined from
earthquakes along the AGR. The S(itn) velocities measured indicate
that the shear-wave velocity in the mantle part of the lithosphere
is much higher on the northern side of the AGR. Strongly negative
P-wave residuals in this area indicate faster seismic propagation
than implied by the Jeffreys-Bullen travel-time tables, while propagation
is much slower in the Gulf of Cadiz area. Furthermore the residuals
show a clear difference for paths through oceanic and continental
domains and suggest that the transition between these two domains
extends much further into the ocean on the southern side of the AGR
than on the northern side.
The proposed model for the structure of the upper mantle in that region
shows that there exists a pronounced velocity contrast across the
AGR. Thickening of the lithospheric plate with increasing plate age
is indicated to the south of the ridge. The greatest thickness is
reached close to the continental margin within a zone about 500 km
wide, whose velocity close to the Canary Islands and Madeira is significantly
lower, probably due to the well-known volcanic activity there. These
observations together with the travel time residuals reveal that
this zone seems to be of a transitional nature somewhere between
a continental and oceanic structure.