THE AEGIR RIFT - CRUSTAL STRUCTURE OF AN EXTINCT SPREADING AXIS

Two seismic refraction and gravity lines were obtained along and norma l to the axis of the Aegir Rift, an extinct spreading centre in the No rway Basin. Velocity-depth solutions and crustal structure models are derived from ocean-bottom records using two-dimensional ray tracing an d synthetic seismogram modelling techniques. Gravity data are used to generate models consistent with the lateral variations in thickness of the layers in the crustal models. The resulting models require consid erable degree of lateral inhomogeneity along and perpendicular to the rift axis. Crust within the extinct spreading centre is found to be th inner and of low P-wave velocity when compared with the crust sampled off-axis. To explain reduced velocities of the lower crust we suggest that, due to the relationship between fracturing and seismic velocity, the decreasing spreading rate leading up to extinction let the mechan ically strong layer thicken, so that faulting and fracturing extended to greater depths. Low velocities are also observed in the uppermost m antle underlying the extinct spreading ridge. This zone is attributed to hydrothermal alteration of upper mantle peridotites. Furthermore, a fter spreading ceased 32-26 my ago, ongoing passive hydrothermal circu lation was accompanied by the precipitation of alteration products in open void spaces, thereby decreasing the porosity and increasing the v elocity. Consequently the typical low velocities of layer 2 found at a ctive mid-ocean ridges have been replaced by values typical of mature oceanic crust.