Geoscientist Online

Fossil drainage system reveals rapid ancient uplift and subsidence strongly suggesting that convection currents in the mantle were responsible. Monique Tsang reports.

Geoscientist Online 3 November 2011

Geologists have known for a long time about a mysterious landscape buried under two kilometres of ocean sediments west of the Shetland and Orkney Islands. They suspected that an upwelling mantle plume might have pushed up the seafloor, allowing the landscape to be carved, but they just didn't know for sure. Now, a detailed study¹ has revealed an intricate river drainage system. It provides for the first time compelling evidence linking mantle plume activity to the landscape's formation.

Image : Perspective view of buried landscape.

Using seismic reflection and software developed by co-author Gareth Roberts, Ross Hartley (Bullard Laboratories, University of Cambridge) found that short pulses from the Iceland Plume may have raised the seafloor above sea level 55-57 million years ago, allowing rivers to carve channels on the exposed surface. When the plume subsided, the seafloor became submerged again and sediments preserved the drainage system in fine detail.

It all happened very quickly. “The landscape was elevated by almost a kilometre, and then subsided again by a kilometre in about two million years” says Hartley, lead author of the study, who is currently completing his PhD. At the time the landscape was about 600km away from the Iceland Plume’s centre. These were its early days of the plume, before it had even begun to build today's Iceland.

Drainage networks can also be used to trace the uplift histories of continents, but this tends to be limited to more recent geologic times due to erosion removing the information about older uplift. “In the Faroe-Shetland Basin the fact that the area became submarine and was re-buried under further sediments and thus preserved is what allows us to calculate an uplift history further back in time,” he says.

“Convection can actually push the surface up and down very rapidly as we've seen, and that should be happening everywhere” Hartley told Geoscientist Online. The team is convinced that mantle convection was the main factor. The magnitude of the uplift suggests the area's exposure to the surface was not due to sea-level change, and its transience rules out tectonic forces. The uniform rock composition of the area also rules out any other factors that might have affected topography.


Image : Black square indicates study area against a reconstructed palaeogeography of the North Atlantic Ocean during the Late Palaeocene.

Philip Allen, from the Department of Earth Science & Engineering at Imperial College London, who was not involved in the study, believes² the research is significant for several reasons. Vertical movement of the surface has the potential to tell geologists a lot about how the Earth's interior moves, and that this can happen geologically very quickly - within the space of just one or two million years. “This is quick. It offers the possibility that we might in the future be able to recognize these ups and downs better in the stratigraphic record and therefore gain an idea of the pulsing of the mantle in the past” Allen says.

PETM

The landscape formed at the same time as the Earth underwent rapid and dramatic global warming, a period known as the Palaeocene-Eocene Thermal Maximum (PETM). Changes in seafloor elevation might have triggered this global climatic event. “Uplift and subsidence of the seafloor may have caused the opening and closing of barriers and gateways for ocean currents – which means that a small local effect can have global ramifications” says Allen. “More exotically, the melting of methane ice … over an area 1000km in diameter might release sufficient quantities of carbon into the atmosphere to affect global conditions.”

Further reading:

1. Ross A Hartley, Gareth G Roberts, Nicky White and Chris Richardson, August 2011: Transient convective uplift of an ancient buried landscape, Nature Geoscience v 4 no 8.
2. Philip A Allen, August 2011: Surface impact of mantle processes Nature Geoscience v 4 no 8.