Constraining the Geometry and Flow of the Iceland Mantle Upwelling

Richard M. Allen and Mei Xue
University of California Berkeley

Chapman Conference: The Great Plume Debate. Fort William, Scotland. August 2005.

The Great Plume Debate is to some extent fueled by incomplete consideration of uncertainties in constraining datasets which allow inconsistencies between hypothetical models and constraining data to be excused. We will present our constraints on the range of low velocity geometries in the mantle beneath Iceland and on the geometry of flow beneath the region which, contrary to previous studies, does show evidence for flow related to the upwelling.

The low-velocity structure beneath Iceland is constrained by a suite of resolution tests designed to determine the range of velocity structures that satisfy the data. These include ray-theoretical squeezing experiments which attempt to force velocity anomalies into specific geometries while still satisfying the dataset, and finite-frequency experiments which use the Spectral-Element Method (SEM) to simulate full waveform propagation through various 3D velocity models. We find that the width of the upwelling conduit beneath Iceland must lie in the range of 100 to 200 km which is generally narrower than ray-theoretical tomography and broader then preferred geodynamic models. Separate tests on the minimum depth extent of the anomaly show that significant low velocities are required to 350 km depth using this regional dataset. Should the true conduit be at the narrower end of the possible range, both compressional and shear wave perturbations greater than 10% would be required to depths of at least 350 km. Two end-member geometries, radial flow and ridge-channeled flow, have been proposed for the dispersion of material upwelling beneath Iceland. We use teleseismic shear-wave splitting observations to constrain the flow geometry beneath the region. The observed anisotropy pattern is inconsistent with radial flow away from the upwelling. Instead we propose a ridge-channeled flow model in which there is horizontal flow of material away from the upwelling axis beneath southeast Iceland toward the southern end of the Kolbeinsey Ridge and the northern end of the Reykjanes Ridge, both of which are west of the upwelling. This geometry is similar to the ridge perpendicular flow predicted for offridge hotspots towards the ridge. We hypothesize that upwelled material then feeds ridge parallel asthenospheric channels beneath the North Atlantic Ridge. Our interpretation is thus consistent with generation of V-shaped ridges by channeling of upwelling material down the Reykjanes and Kolbeinsey ridges.