Dynamic models for mantle flow and seismic anisotropy in the North Atlantic region and comparison with observations

Abstract

Dynamic mantle flow and temperature models for the North Atlantic based on a regionalized P‐wave and a global S‐wave tomography model were derived under the constraint of a maximum fit to the observed gravity field. For the regional flow model Cartesian geometry, temperature‐ and depth‐dependent viscosity and a free slip surface were assumed, while the global model assumed a radially dependent viscosity and kinematic plate velocity boundary conditions. Both models show pronounced upwellings within the upper mantle beneath the Iceland area and the lower mantle beneath, the regional model containing a lateral shift associated with a horizontal flow near 660 km depth. The upper mantle temperature field of the regional model shows two distinct anomalies, one beneath Iceland and the westerly adjacent regions with a connection to a deep mantle root and an excess temperature of 200°C, and a second one below 300 km at the Kolbeinsey Ridge with an excess temperature of 120°C. These anomalies do not appear to be connected. An essentially radial flow pattern is found south of Iceland with ridge parallel flow along Reykjanes and divergent flow at the Kolbeinsey Ridge. The long‐wavelength global model does not show such details but is characterized by a NE‐SW elongated upwelling flow beneath Iceland and a ridge perpendicular flow within the upper mantle. From the modeled flow fields, lattice preferred orientations (LPO) of olivine are calculated. For the regional model, azimuthal seismic anisotropy is predicted with fast directions diverging away from Iceland and the Kolbeinsey ridge. The global model predicts roughly ridge perpendicular fast directions. Comparison of predicted with observed seismic anisotropy models shows regions of good agreement north, east, and SE of Iceland, as well as for Iceland. No agreement is found beneath the Reykjanes Ridge area, leading to the speculation that the fast directions are perpendicular to the flow due to a change in the LPO generating mechanism. Regarding geochemical findings, the regional flow model can explain plume‐related geochemical signatures observed on the Reykjanes Ridge and predicts a deep, hot melt zone beneath the Kolbeinsey Ridge without plume tracers.