Upper mantle conditions in the North Atlantic Ocean and their implications for continental break-up
Creators: name: Zalai, Zsófia
Publisher: name: Imperial College London
Year: 2021

description: Mantle conditions during the opening of the North Atlantic Ocean and specifically the presence or otherwise of a deep mantle plume have been much debated. Current models fall into two groups: plume impingement and plate-driven models. The Plume model associates the arrival of the Icelandic plume with continental break-up of the North Atlantic and the observed excess magmatism is associated with passive upwelling and elevated mantle potential temperatures. However, the Plate model associates this excess magmatism with increased mantle fertility due to inherited lithospheric structure, small-scale convection induced by sub-lithospheric topography and/or extensive warming of the mantle beneath a supercontinent. I examine the spatial and temporal variation of upper mantle conditions at the time of continental break-up using an inventory of 42 published seismic refraction velocity-depth profiles acquired between the Charlie Gibbs and the East Greenland Fracture Zones. I make use of the Hc-Vp method to estimate mantle potential temperature and the ratio of active to passive upwelling by extracting igneous crustal thickness, Hc, and its mean p-wave velocity, Vp. Finally, I compare the spatial and temporal patterns obtained to those predicted by previously proposed models of mantle conditions around the time of break-up. My results show more than 300°C variation in mantle potential temperature across the study area. The hottest areas are the Northeast Greenland margin and the Greenland-Iceland-Faroes Ridge, while the coldest are located near the extinct spreading centre, Aegir Ridge and offshore the Hatton Bank. Mantle potential temperatures are generally high shortly after break-up or at the time of break-up and they decrease with time, reaching steady-state ~10 Ma after break-up with temperatures around normal mantle temperatures of 1300°C. A few places are characterised by active upwelling including the Greenland-Iceland-Faroes Ridge and the Voring Spur. Elsewhere passive upwelling dominates. I model the observed temperature anomaly with an axisymmetric synthetic thermal anomaly to determine whether the observed pattern can be reproduced by a circular mantle plume. Parameters of the thermal anomaly, a varying peak temperature amplitude between 1350 and 1600C, wavelength (σ = 400 km), were chosen to best match observations. I apply a grid search method to locate the centre of the anomaly for every 5 million years from the plate reconstructed positions of mantle potential temperature observations. These locations reveal that the Iceland plume traversed Greenland in 15 million years, being located beneath West Greenland at 55 Ma and arriving at the Northeast Greenland margin by 40 Ma. The fluctuation in plume centre temperature based on the inversion results indicates the effect of continental insulation, but that alone is unable to account for the more than 250°C excess mantle potential temperature. Hence both a mantle plume and continental insulation could have played a role in the break-up of the North Atlantic Ocean.