#metacraton

The core of Northern Africa

I found this image while searching yesterday and thought it was a particularly insightful view into the assembly of one of the continents today. Africa as we know it today is mostly surrounded by spreading centers. It isn’t colliding with anything, it isn’t running into other continents, and there is no active subduction beneath Africa. However, it wasn’t always that way, and this simple image tells a portion of that story, at least for the northern ½ of the continent.

Africa was part of the supercontinent Gondwanaland along with India, Australia, Antarctica, and South America. Each of those continents we recognize today was itself an almagamation, created from much more ancient building blocks.

These building blocks are what geoscientists call “Cratons”. They are slices of ancient continental crust, many of them established more than 2 billion years ago. In-between them are the active zones or “mobile belts”, the areas that are deformed when the cratons collide.

2 of the main cratons shown in this image can be traced all the way back to the Archaean, before the rise of oxygen in Earth’s atmosphere. The oldest rocks exposed in the West African craton are 3.5 billion years old and the oldest rocks in the Congo craton are 2.9 billion years old. These cratons started off as something like modern day island arcs, and grew outwards as more things collided with them. Each of them shows more than a billion years of volcanism and metamorphism, but both of them had been established as blocks of somewhat stable, continent-like crust by about 2 billion years ago.

The Saharan block is called a Metacraton because it is thought to have been active at its core much more recently than the other cratons. Much of that area is covered by the Sahara Desert and the thick Nubian Sandstone and by igneous rocks that are about 750-550 million years old. This area could be a recently assembled craton or it could have been a piece of older crust where the assembly of Gondwanaland caused volcanism that overprinted many of the older rocks.

In-between these 3 main blocks are the mobile belts. Sometimes a thin sliver that broke off of a different continent gets trapped in these areas by running into the edge of a growing craton – the Tuareg block and the Oubanguides Belt are examples of these as both contain Precambrian and Archaean rocks but they are stuck in the middle of provinces that were deformed much more recently.

The areas in-between these blocks were deformed and overprinted during what is called the Pan-African Orogeny, from about 750-550 million years ago. These cratons collided at that time to make up the supercontinent Gondwanaland, which then lasted for 300 million years.

There are other cratons in Africa and in ancient Gondwanaland that aren’t shown on this image – just happens to be the one I found. The Kaapvaal craton in South Africa, for example, contains rocks that are at least 3.7 billion years old, some of the oldest continental rocks on Earth.

-JBB

Image credit: http://bit.ly/2wFF4TA