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Thursday, May 15, 2014

May 15. Devonian Gondwana



I gave a general description of the distribution of lands and seas during the Devonian back on May 7. There’s a map on the blog for that episode that probably is an easier way to grasp that than from my words. But I want to say a little more about Gondwana, the supercontinent. 

Laurentia or North America, and Baltica (Europe) and Siberia were all getting pretty close to becoming a second supercontinent, but Gondwana had been pretty much one big continent for tens of millions of years. But it wasn’t just sitting there.


The heart of Gondwana was today’s Africa and South America, which is called West Gondwana, plus Arabia, India, Antarctica, and Australia making up East Gondwana. Australia projected away from the continent as a wide peninsula. At the start of the Devonian, the south pole was somewhere in southern Brazil or perhaps in adjacent Namibia, but there doesn’t seem to have been much if any glaciation there, at least not until near the end of the Devonian. One problem is that continental environments, including glacial areas, are typically less well preserved in the rock record than marine environments, simply because the terrestrial environments usually cover smaller areas and receive fewer sediments. There are exceptions, of course, and both the Old Red Sandstone and the Catskill Delta are examples of terrestrial environments that are indeed well preserved in the record.

Gondwana was evidently pretty unstable, at least along its northern margin. That’s where the long microcontinent called Avalonia, including parts of what are now Britain and Newfoundland and Nova Scotia, rifted away. Other relatively small continental fragments appear to have been rifting away from what is now North Africa, and they continue to do so. They’ve become much of southern Europe, including Iberia and Italy.

It’s not at all clear what was happening with the margin of Gondwana that contained what are now Arabia, India, and Australia, but it seems that various blocks that are now in Central Asia, Tibet, and China, were probably breaking off of Gondwana in Devonian time and starting a relatively independent motion. A good modern analogy for this would be Madagascar today – it has rifted away from the east coast of Africa and is moving independently.

While all those pieces were rifting away from Gondwana on its northern and northeastern margin, the entire continent was rotating, pretty much in a clockwise direction. That meant that South America, at the other end of Gondwana from Australia, was moving to the north and northwest. Northern South America and the adjacent part of Africa – northwest Africa today, as well as what’s now Florida – were all approaching North America. I’ve said it before and I’ll say it again: Gondwana was coming. The ocean between North America and Gondwana was closing, and a big crunch was on its way.

—Richard I. Gibson

Reference:
Nice map of early Devonian Gondwana


Map above based on public domain map by Peter Bøckman

2 comments:

  1. Hi!
    I just had a question...
    What is the country adjoining India and Antarctica in your map? And Where is Philippines and Indonesia located?

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    Replies
    1. Hello - that space between the east side of India and Antarctica would probably be 'error' or unexplained; or you might turn Antarctica a bit clockwise to accommodate some of that gap. Philippines and Indonesia do contain small continental blocks, but much of those island chains did not yet exist in the Devonian - they are island arcs that only formed millions of years later. The more sizable continental blocks, such as Borneo, may (or may not) have good constraints as to their positions this long ago. Also, while most of the big continental blocks were indeed attached to each other, as the map shows, there were other small continental blocks that were elsewhere. I hope this helps, thanks for the question. - Dick Gibson

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