Ultrafast mantle plumes and implications for flood basalt volcanism in theNorthern Atlantic Region

Recent geochemical and geochronological data on Paleocene flood basalts fro m West Greenland, SE Greenland and the British Isles show that volcanic act ivity at these widely separated locations commenced nearly simultaneously a round 61-62 Ma ago, and that the duration of this initial phase of flood ba salt magmatism was of the order of a few million years or less. A small, fa st moving upper mantle plume which rapidly spreads out horizontally on enco untering the base of the lithosphere, appears to be a viable mechanism to e xplain these observations. However, in order to reconcile the idea of an ul trafast plume with the observed plate velocities, a physical mechanism is n eeded for inducing a separation of timescales between the plume speed and t he surrounding mantle circulation. Thermal convection with a non-Newtonian temperature and depth-dependent rheology provides such a mechanism wherein extremely fast plumes ascending at velocities between one to tens of meters per year can be produced in an otherwise slowly convecting mantle moving a t cm/yr. This transport mechanism is capable of bringing up very hot matter from the transition zone to the lithosphere. The fast upward velocities le ad to the production of high viscous heating rates surrounding the plume up on impinging the lithosphere. It is thus possible to explain the near simul taneous onset of magmatism in West Greenland, SE Greenland and the British Isles in the early Tertiary by a fast moving mantle plume spreading out hor izontally with a velocity of around 0.5 m/yr. For resolving numerically the thermal-mechanical state of these strongly time-dependent mantle flows, ex tremely high spatial resolution, on the kilometer scale, is required. Final ly we suggest the possibility that the source of the fast upper-mantle plum e under Iceland may be rooted in the lower mantle. This is consistent with the recent findings by seismic tomography of a deep mantle plume under Icel and. (C) 1999 Elsevier Science B.V. All rights reserved.