A new Gondwanan reconstruction
Over 150 million years ago, the continents of South America, Africa, Australia, and Antarctica, along with other slices of land such as the Indian subcontinent, were joined together in a single giant landmass known as Gondwana. Since that time, that landmass has progressively been torn apart by the opening of new rift zones and creation of new ocean, as is happening today at the Red Sea rift.
The fit between Africa and South America is so good that it was recognized over a century ago by Alfred Wegener when he first proposed the concept that plates could migrate across the surface of the Earth.
If you’ve ever tried to fit together puzzle pieces shaped like the African and South American coasts, you’ve probably found that they come very close to fitting together, but there are some indentations that just don’t match up well. If they were puzzle pieces the shapes wouldn’t be quite right, there’s just a little more complexity.
When continents rift apart, they don’t just break cleanly along lines. Large rift zones form, the crust thins, faults propagate in multiple directions, rock units are stretched out and broken, and new rocks can be created by volcanism or sedimentary deposition. Consequently, the boundaries of continents never match up perfectly. If you’re trying to piece South America and Africa back together, this isn’t a big deal; the continents fit together with an accuracy of a few kilometers anyway and the match is hard to miss.
But, for the other landmasses that made up Gondwana, these small mismatches are much more important. At some point, Australia, India, and Antarctica were all hooked together, but as you see in this image, the coastlines don’t have the type of puzzle-piece matching shape that South America and Africa do. These coastlines are pretty smooth, and given the distortion that happens when continents rift apart, there can be tens to hundreds of kilometers of variation in fitting these continents together.
That’s exactly the case right now; in scientific publications there are a variety of reconstructions for how Gondwana fit together. This lack of agreement on where these continents joined leads to other issues in understanding the geology. If the map can’t reconstructed accurately, the dates the continents rifted apart can’t be determined accurately, leading to errors in understanding how species evolved once continents became isolated and in deciphering why the supercontinent broke up in the first place.
This image is a reconstruction from a just-published model putting India, Australia, and Antarctica together again based on newer data. There are a variety of techniques to do these types of reconstructions, and so in this piece I’m not going to endorse theirs as the correct answer; I’ll let peer review figure that out.
To reconstruct the plate positions, these authors tried to use geologic features common to the different continents. This can be tricky as well, because units can be folded, they can be continuous over hundreds of kilometers, and glaciers make everything more complicated.
These authors used a series of faults and basins that developed as rifting began as their tie-points. It looks like a good setup and it’s been done well, now we’ll wait to see how it holds up as new data is collected.
In fact, one of the dirty little secrets of geology is that this type of work will probably become much more accurate in the future…because information with bearing on how Australia and Antarctica fit together is buried under the East Antarctic Ice Sheet. It probably won’t happen in my lifetime, but with the rate CO2 is going up in the atmosphere, it won’t be long before someone gets to map these rocks and really put together the history of this part of the world.
-JBB
Image credit/Press report: http://phys.org/news/2013-07-reveals-ancient-jigsaw-puzzle-supercontinent.html
Study (subscription required): http://www.sciencedirect.com/science/article/pii/S1342937X1300213X