Upside–down erosion by mantle plumes
Earth’s relief is in continuous change; mountains are eroded by wind and water, the valleys, seas and oceans are filling up with the scoured sediments, and the continents get thinner or thicker during these processes. But this is not all! Deep down, sometimes hundred of kilometres from the surface, much slower forces at work carving the continents upside down. New work published in Nature Communications reveals just that for Africa!
An international team led by scientists from the Dublin Institute for Advanced Studies (DIAS) in Ireland, in collaboration with researchers from Canada (Geological Survey of Canada) and Norway (University of Oslo, CEED; including CEED Director Prof. Carmen Gaina) has discovered that large portions of the tectonic plate, or lithosphere, beneath Africa have disappeared over the past 200 million years. Their findings were published in the journal Nature Communications earlier this month.
The team employed seismic tomography – a technique that uses seismic waves propagating from earthquakes around the world – and obtained a detailed three-dimensional model of the lithosphere beneath Africa as it is today. They then used plate tectonics and knowledge about diamonds occurrence through time to get a glimpse into the structure of the lithosphere in the past. “The volume of the missing tectonic plate material is as much as 100 million cubic kilometres, over 10 times the volume of the Tibetan Plateau”, says lead author, DIAS PhD student Nicolas Celli.
Although the African continent is over 2.5 billion years ago and very stable, vigorous mantle plumes started to carve its deeper part while the continent was separating from the North and South Americas, a process that started about 200 million years ago.
Seismic tomography revealed thin cratonic regions in South Africa, Angola, and Tanzania, where it used to be thick only 30 to 200 million years ago, according to diamonds. By reconstructing the position of South America and Africa and the modelled trajectory of mantle plumes underneath cratonic Africa, DIAS and CEED scientists realised that the catastrophic thinning of the lithosphere was caused by the impact of the hot mantle upwellings. Other cratonic regions in the world, like North China and Wyoming cratons have been thinned due to hydrous melts rising from subducting slabs, but western and central Africa were far from any shallow slabs at the time of lithospheric erosion.
The open access Nature Communications paper is available at: https://www.nature.com/articles/s41467-019-13871-2.
Reference: Nicolas Luca Celli, Sergei Lebedev, Andrew J. Schaeffer & Carmen Gaina (2020). African cratonic lithosphere carved by mantle plumes Nature Communications v11