About 200 million years ago, the Earth had only one continent called Pangea. Gradually this vast landmass disintegrated into smaller continents and today’s tectonic plates as the supercontinent split apart and new oceans formed.
“When Pangea split, the African and South American continents moved away from each other, and the Atlantic ocean was born. Our research shows new details about exactly how this process of continental rifting worked”, says Professor Trond H. Torsvik from the University of Oslo’s Department of Geosciences.
The research team set out to explore the evolution of the South Atlantic and how the African and South American plates split during the Cretaceous Period. They have published their findings in the prestigious journal Nature Communications.
“We show how Earth’s mantle – the rocky shell beneath Earth´s crust – started to deform when the supercontinent Pangea started to rift. Hot material from deep inside the Earth rose and created domes beneath South America and Africa. It was the heat from the mantle plumes that caused the rifting of Pangea and split the continents of Africa and South America”, says Assistant Professor Reidun Myklebust from the University of Oslo.
The project is called SPLIT AFRICA and was funded by the Research Council of Norway. The team consisted of geologists and geophysicists from the University of Oslo, the Norwegian Polar Institute, the Geological Survey of Norway (Norges Geologiske Undersøkelse), and several universities in Brazil and the UK.
Uncovering the forces at play deep beneath the Earth’s surface
Pangea assembled during the late Paleozoic Era (about 335–300 million years ago) and began to break up some 175 million years ago. When the split of the supercontinent started, the African and South American plates moved away from each other, and the Atlantic Ocean began to form.
The opening process lasted for about 130 million years and involved extensive crustal deformation and magmatism. The research team used seismic tomography – a technique similar to a medical CT-scan, but using seismic waves instead of X-rays – to image the present structure of Earth beneath South America and Africa and gain insight into conditions and processes that occurred during rifting.
The images provide a detailed view of deep Earth structures beneath South America and Africa. They reveal the deep roots of the African and South American continental lithosphere, the thickness and nature of the crust, the depth and topography of the Moho (the boundary between the crust and the mantle), the structure and properties of the upper mantle, and the extent to which the mantle has been replaced by hot upwelling material from deep in the Earth.
“We found significant differences between the South American and African sides of the South Atlantic. The continental crust beneath South America is much thicker than under Africa, and we can see that a larger portion of the mantle has been replaced by hot upwelling material beneath South America”, says researcher Anne-Marie Weidle, who works at the University of Oslo and performed all seismic imaging for this study.
How Earth´s structure controls continental rifting
The structure of Earth´s upper mantle contains important clues to the processes that controlled the continental rifting. The lithosphere is the outermost rigid part of Earth, and it behaves elastically on short time scales. However, on geological time scales, the lithosphere can deform and flow like a viscous fluid because of the high temperatures in Earth’s interior.
The scientists compared their observations of the deep Earth structure with results from numerical models that simulate the process of continental break-up. These simulations show that the thickness and temperature structure of the lithosphere play an important role in localizing the deformation, and the models indicate that the hot upwelling mantle preferentially localized deformation at weaker zones within the continental lithosphere.
The weak zone in South America that localized deformation is still visible today as the Paraná Basin. This sedimentary basin formed after continental breakup and is an important region for energy exploration.
“The structures and processes that we find can be seen as a natural laboratory that is helping us to better understand continental breakup in general, which has implications for understanding the formation and breakup of other continents and ocean basins”, says Torsvik.
