Cenozoic evolution of the Icelandic Plume and its influence upon the topographic evolution of Northwest Europe

The Icelandic mantle plume is probably the largest convective upwelling on Earth. It is generally agreed that its growth and evolution have had a significant influence on the geologic and oceanographic evolution of both the North Atlantic Ocean and Northwest Europe during Cenozoic times. At the present day, three significant observations testify to the existence and size of this plume. First, residual depth anomalies prevail in the oceanic lithosphere surrounding Iceland. These anomalies show that the oceanic plates are 1-2 km shallower than expected in a region that stretches from Baffin Bay to the coast of Norway, and from Svalbard to Newfoundland. Secondly, an irregular-shaped long wavelength free-air gravity anomaly with an amplitude of 30-50 mGal is centred upon Iceland. Thirdly, full-waveform tomographic imaging of the North Atlantic region shows that the planform of the Icelandic plume has a complex irregular shape with significant shear wave velocity anomalies lying beneath the lithospheric plates at a depth of 100-200 km. Distribution of these anomalies suggests that about five horizontal fingers extend radially beneath the fringing continental margins. The best-imaged fingers lie beneath the British Isles and beneath western Norway where significant departures from crustal isostatic equilibrium have been measured. It has been suggested that these radial fingers are 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. For radial, miscible fingering, the wavelength and number of fingers are controlled by the mobility ratio (i.e. the ratio of viscosities), by the Péclet number (i.e. the ratio of advective and diffusive transport rates), and by the thickness of the horizontal layer into which fluid is injected. Shear wave velocity estimates have been combined with residual depth measurements around the Atlantic margins to estimate the planform distribution of temperature and viscosity within a horizontal asthenospheric layer beneath the lithospheric plates. These calculations yield mobility ratios, Péclet numbers, and asthenospheric channel thicknesses that are compatible with Saffman-Taylor fingering. A useful rule of thumb is that the wavelength of fingering is ~5 times the thickness of the horizontal layer. Across the Northwest European shelf, the pattern of mapped residual topography and subsidence anomalies is remarkably consistent with the planform of asthenospheric fingering. In conclusion, a combination of disparate observations supports the notion that Cenozoic dynamic topography of Northwest Europe is generated by fast, irregular horizontal flow within thin, but rapidly evolving, asthenospheric fingers of the Icelandic plume.