Observational and Experimental Constraints on Radial Miscible Viscous Fingering of Icelandic Plume

Throughout Cenozoic times, the Icelandic mantle plume has had a significant influence on the geologic evolution of the North Atlantic Ocean. Direct manifestations of this major convective upwelling include positive residual depth anomalies of oceanic lithosphere and long wavelength free-air gravity anomalies, both of which reach from Baffin Island to Norway and from Newfoundland to Svalbard. Published full-waveform seismic tomographic imaging shows that the Icelandic plume is associated with a slow, irregular-shaped shear-wave velocity anomaly at 100200 km depth. Five or six horizontal fingers with anomalously slow shear-wave velocities radiate away from the plume conduit on Iceland, extending beneath the fringing continental margins. The clearest fingers occur beneath the British Isles and beneath western Norway where significant residual topographic anomalies occur. They are inferred to be hotter than ambient mantle and they spatially correlate with positive gravity and residual depth anomalies. The region between these two fingers has a fast velocity structure that coincides with the North Sea basin and is characterized by rapid Neogene subsidence. The radial fingers appear to have been generated by a phenomenon known as the Saffman-Taylor instability. Experimental and theoretical analyses show that fingering occurs when a less viscous fluid is injected into a more viscous fluid. In radial miscible fingering, the wavelength and number of fingers are controlled by the Peclet number (i.e. the ratio of advective and diffusive transport rates), by the mobility ratio (i.e. the ratio of viscosities), and by the thickness of the channel into which fluid is injected. Observational estimates for the Icelandic plume suggest the Peclet number is O(10 4 ), mobility ratio is 2050 and asthenospheric channel width is 10020 km. For these channel widths, gravity has an important moderating influence upon finger generation. A detailed comparison of scaled laboratory experiments and geologic observations suggests that the fluid dynamics of the Icelandic plume planform is more complex than the Saffman-Taylor instability.