WHITE, ROBERT S., Dept. Earth Sciences, University of Cambridge
Abstract: Influence of the Iceland mantle plume on the North Atlantic region
The Iceland mantle plume has dominated the geological evolution of the North Atlantic region since it first impinged on the area at about 62 Ma. I discuss four main effects of the mantle plume: rapid production of huge volumes of igneous melt as the lithosphere rifted; lateral flow of melt as dykes, sills, underplate and surface flows, thus affecting areas up to hundreds of kilometers away; regional uplift with accompanying erosion and modification of the sedimentation, and subsequent subsidence of the northwest European continental margin and hinterland; pulsing of the plume on timescales of a few million years causing episodes of uplift and subsidence.
Early Tertiary Melt Production
Interaction between the newly arrived Iceland plume with the thinning lithosphere beneath the nascent Atlantic rift generated massive volumes of melt as the mantle decompressed under the rift. Some 1-2 million km 3 of extrusive basalts and several times that volume of underplated, or lower crustal intruded igneous melt, was produced in only a few million years at most. It was generated along a 2000-km long section of the North Atlantic rift, and is now found on the continental margins on both sides of the Atlantic (Fig. 1).
The temperature of the plume can be estimated from the volume of melt and from its geochemistry, and in particular the rare earth element concentrations. Initially the Iceland plume spread rapidly across an area more than 2000 km across, probably as a thin sheet beneath the lithosphere. Following the onset of seafloor spreading, the temperature in the distal areas of the plume dropped from an estimated 200°C to only 50°C or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism. Only above the narrow central core of the plume some 100 km across did the mantle temperature remain high, producing the 30-40 km thick crust of the Greenland-Iceland-Faroe Ridge and the island of Iceland itself.
Lateral Melt Flow
Not all the melt produced in the early Tertiary rifting was extruded as basalts. Probably even more was intruded into the lower crust or underplated beneath it. Nor was all the melt trapped in the rift zone. Where sediments lay adjacent to the main region of melt production, as for example in the Faroe-Shetland Basin, basaltic lavas flowed more than 150 km on the surface (Fig. 2).
Lava also flowed many hundreds of kilometers in dykes, thus transporting melt large distances away from the rift zones where it was generated and into the surrounding hinterland. In the North Atlantic region,Tertiary dykes now exposed at the surface extend across Britain and into the North Sea: there are likely to be many more in the mid-crust. The dykes deliver melt long distances laterally and then allow some of it to rise to the surface in areas well removed from where it was first generated.
Uplift and Subsidence
The Iceland plume exerted a major control on the uplift and subsidence pattern in the continental regions adjacent to the North Atlantic rift, leading to episodes of considerable.erosion and of marked changes of sediments accumulated in the basins. It did so in two main ways.
First, transient uplift was caused by the arrival beneath the lithosphere of the abnormally hot, and therefore less dense, mantle of the plume. Maximum uplift of 2000 meters or more occurred close to the plume center, but even hundreds of kilometers away in the North Sea, the uplift reached hundreds of meters. This uplift was only transient, since as the region moved away from the plume, or the mantle temperature decreased back to normal, the cause of the uplift was removed.
The second main cause of uplift was the addition of new igneous material to the crust. The uplift from this crustal thickening is permanent. Near the Atlantic rift zone the uplift was sufficient to initially negate the subsidence caused by lithospheric stretching and subsidence, causing even greatly rifted areas to remain at or above sea level until post-rift thermal cooling of the underlying mantle allowed subsidence (Fig. 3).
Unsteady Plume Flow
The Iceland plume itself is likely to have varied in both temperature and in flow rate throughout its history. In particular, it was probably hotter and extended as a sheet over a greater area when it first initiated at about 62 Ma than it did subsequently in its mature stage, as at present. This caused transient effects of far-flung uplift, of huge melt production and of rifting in the late Paleocene. Subsequently the plume flow has varied on a time-scale of a few million years, best seen by the V-shaped ridges of thickened crust formed in the oceanic crust of the present North Atlantic. This has caused cyclic episodes of uplift and subsidence, and modification of the oceanographic currents as barriers have been elevated and then subsided again in response to variations in the mantle plume temperature of the order of 30°C.
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