Rodinia muddles

In the early 1990s, Ian Dalziel, Eldridge Moores and Paul Hoffman speculated on the former existence of a supercontinent comparable with Pangaea, between about 1100 and 750 Ma.  The name Rodinia, from the Russian for Motherland, seemed appropriate.  They based sketchy reconstructions on the way in which orogens formed almost globally between 1300 and 1000 Ma could be fitted together by shuffling older crustal fragments, along with evidence from sediments in North America, and Antarctica that the supercontinent began to disassemble around 800 Ma.  A great  conundrum of later Neoproterozoic times seemed to be partly resolved by what might have happened when Rodinia broke apart and its fragments drifted across the globe.  This was the event that welded together the southern supercontinent of Gondwana between 800 to 500 Ma ago, forming the web of orogens known colloquially as the Pan African and Brazilide belts of Africa and South America.  Palaeomagnetic pole positions for the 1200-750 Ma period, from the supposed components of Rodinia, were an obvious test of Rodinia’s former existence and its gross structure.  As they appeared the palaeomagnetic data seemed to confirm the early ideas that were based on Wegener’s method of linking now far-separated orogens to reassemble his Carboniferous Pangaea supercontinent.  A reasonable consensus existed by the early years of the 21st century.  One of the main contributors of palaeopole data for Rodinia reconstruction has been Trond Torsvik of the Geological Survey of Norway, so it is noteworthy that he has cast the first shadows of doubt on what seemed to be an elegant general solution to more than half a billion years of global tectonics (Torsvic, T.H. 2003.  The Rodinia jigsaw puzzle.  Science, v. 300, p. 1379-1381).

The problem that Torsvik recognises is that superficially convincing geological jigsaw fits are coming into increasing conflict with better evidence for the palaeolatitudes of different segments.  This is compounded by a lack of palaeomagnetic data for some of the 13 major continental segments that had formed earlier in Precambrian times.  The central element in the original Rodinia model was the way that India, Antarctica and Australia’s 1300-1000 Ma orogens fitted in what appeared to be a rational reconstruction of East Gondwana.  The first fly in the ointment is that revision of Australia’s palaeolatitude seems to make its fit with India impossible.  Likewise the position of the geologically fitted Congo and Kalahari cratons, that now make up West Africa, is less certain.  Amazonia is also not “behaving” as expected, and Baltica may have been rotated by 180 degrees relative to its former orientation in the old Rodinia model.  As well as varying quality of palaeomagnetic data, and its lack from crucial components such as Siberia and North China, their dates vary so much that it is impossible to allow for large-scale readjustments through the lifetime of the putative supercontinent.  Torsvik figures a “worst case” scenario, in which the whole Rodinia concept becomes merely continents that were near one another and separated by a variety of active rifts; something of a dog’s breakfast that should spur more dating, palaeomagnetism and tectonic research on the orogens that first suggested a grand unification.  That is, if the main proponents do not become so profoundly depressed that they simply give up!

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