Geoscientist Online

Joe McCall has difficulty with one of the central assumptions of plate tectonics - the creation and destruction of global oceans - or panthalassas...

Geoscientist Online 27 January 2010

1. Introduction

Pangaea and Panthalassa were terms coined long before Plate Tectonics appeared.

Logically, they should be terms which originated with Wegener, for the existence of a supercontinent and its complimentary superocean in the geological past presupposes that continents move about, but these terms actually seem to have originated earlier with Suess. Some years ago the author reviewed Special Publication No 37 of the Geological Society – Gondwana and Tethys, Audley Charles and Hallam (1988) and expressed an intuitive misgiving about the introduction of Panthalassa into Plate Tectonics (the reference to this review has unfortunately been lost). His interest in the topic was further aroused from a major task which he undertook, editing ‘The Geology of Hydrocarbon and Mineral resources associated with Post-Middle Jurassic Sequences in the Oceans and on the Continents’ (Levin et al. 1993: editor English Language Version, GJ.H.McCall); this covered the last ~200 Myr of Earth history. With the years, his doubts have increased. Was there ever this vast complimentary ocean to the single and smaller-in-extent accumulation of continents that was Pangaea? Here, he emphasises our ignorance about, and the problems raised by, Panthalassa.

The assembly of the continents into one mass during a hundred million years from the Permian into the mid-Jurassic is well-established, the evidence being largely based on Palaeomagnetism. Pangaea would seem to be a reality, though there may still be questioning voices. Two published reconstructions are shown in Figures 1 and 2. Since the early Jurassic (~200 million years ago), this assemblage has split apart, with the formation of the oceans that we have today. This could be explained by global expansion [advanced by Hilgenberg (1965); Carey (1976); and Scalera (2003a)], but the difficulties are elegantly expressed by Van Andel (1994)]:

“…the expanding Earth hypothesis, stubbornly defended by S. Warren Carey of Tasmania. Take a small Earth, equipped 250 million years ago with a single supercontinent, and make it expand in the Mesozoic. As it swells ,the supercontinent is torn and the pieces scatter wide and far. At first sight this makes sense, it dos not require subduction, nor can it account for the compression of the crust that builds mountains, but small changes in the model might accommodate the points ,nor can it account for the well-documented drift of the continents that assembled them to join the supercontinent in the first place.”

Unfortunately, there is no physical evidence at all that the Earth has expanded, indeed it is generally believed that the planet has maintained its size through the 4 billion or so years of recorded geological history. If so, a complimentary superocean must have co-existed with Pangaea, and as Pangaea accreted this must have been formed, to keep the circumference more or less the same.

There is another reason for needing Panthalassa: while Pangaea existed the seas had to go somewhere? There is a continuing record of marine sedimentation and thriving marine life through geological time, including through the age of Pangaea. One might resort to a far-fetched idea of Pangaea entirely surrounded by shallow shelf seas underlain by continental crust (no deep ocean), or Pangaea itself covered entirely by shallow seas, but neither is surely consistent with the geological record, and the first requires an enormous addition of continental crust that is totally absurd. No, the existence of Pangaea requires a complimentary Panthalassa.

3. Our ignorance about Panthalassa compared with Pangaea

It is part of the Plate Tectonic paradigm that ocean crust is lost: we have preserved for our study only the oceans of the present cycle of Pangaea break-up, but not any older oceans. This is not strictly true, because we have the ophiolites, which are relics of deep ocean crust and marginal basins that have been preserved by obduction onto continental edges and incorporation into orogenic fold systems such as the Alps or the Makran, but it is part of the Plate Tectonic paradigm that most of the ocean crust is taken down by subduction, the thin ocean crust becoming denser as it cools and passing down to the mantle depths under the overriding plate.

That ocean crust is lost seems to be supported by statistics of terrestrial impact craters (McCall 2009): 170 are known, 25 were on marine targets, but all but one of these was onto shallow shelf seas: only one, the Pliocene Eltanin structure, in the southern ocean, is known to have been produced by a fall into deep ocean of a mesosiderite or howardite meteorite, which, remarkably is preserved as fine specks in highly disturbed sediments. Various other explanations have been given for this deficiency in the deep oceans, but the obvious explanation is that ocean crust is lost: all of it prior to the present cycle of Pangaea break-up, and much formed since, partly or largely by subduction. This loss means that we cannot study the past history of ocean crust in the way that we can of continental crust and shelf seas.

Plate tectonists tend to concentrate on subduction, but a critical fact is that many present ocean margins are passive, including both sides of the Atlantic ocean, one side of the Indian Ocean and all the margins of Antarctica. It appears to be part of the paradigm that these margins will eventually become subductive to destroy the ocean crust, but this may not be true. We cannot look into the future, and computer-generated predictions cannot be verified.

This is a subject which geotectonists seem to have avoided. It is reasonable to suppose that in the present Pangaea break-up cycle, which commenced in the Mesozoic, we have reached the stage when the new Panthalassa could well be forming. If Panthalassa is a reality, and there is going to be a new one, Novopanthalassa or Pangea Ultima, in the geological future, the most obvious answer to me is that the Atlantic Ocean will continue to spread and will form the nucleus of the new Panthalassa, which will probably also incorporate the Southern Ocean and part of the Indian Ocean. But this is only a guess, we simply do not know what will happen.

As to how the Panthalassa , which is believed to have existed from the Permian to the mid-Jurassic, originated, this is obscure: there may be literature on this subject, but it is not known to the author (who is not a specialist geotectonist, and is writing from a much broader viewpoint). This is not the really the big problem: the big problem is how you get rid of Panthalassa once you have it in place.

5. Pangaea and Panthalassa in place

Nield (2007, illustration facing p 128) presents a splendid colour image of Novopangaea’, the proposed supercontinent of the future, derived from Dr Roger Livermore. I wonder why the bottom side has a destructive plate margin? Also, there are a lot of constructive plate boundaries within Panthalassa, but surely spreading from such boundaries has to go two ways. Can Panthalassa have a destructive margin without there being any complimentary other side to the Wilson cycle of ocean spreading? You can have a plate with compression on either side as is required by the internal constructiuve plate boundaries –Africa is such a plate – but in any case, since Pangaea retained its dimensions for 100 million years, the corollary is surely that Panthalassa must have been formed completely by the time Pangaea was completed, and no significant change in its dimensions could have occurred for 100 million years, otherwise the terrestrial globe would have expanded?

Nield (2007) says that “the great ‘C’ of Pangaea drifted slowly northwards, coming to straddle the Equator more symmetrically….” This drift, presumably evidenced from Palaeomagnetism, is critical, if there can be no two-sided Wilson cycle operating at the time (such a cycle requires requires two continental masses). If Pangaea moved bodily in this way, bodily, it would seem to establish that continental plates are moved by an undertow effect of the convection current: one proposed mechanism of ridge-push was too weak anyway, and slab-pull is an absurdity as a sole mechanism,, because plates move, as in the case of the Atlantic, without the existence of any lateral subduction. But this is a digression.

We surely really know nothing of the internal configuration of Panthalassa, assuming it existed at all, and Livermore’s diagram for Novopangaea does not seem to be a credible model in several respects.

A final small point: I published a brief comment about research on Pangaea’s climate and dunes (McCall 2008: Rowe et al, 2007). Surely, it is obvious that one cannot extrapolate present day global climatic controls to Pangaea and Panthalassa, which would have possessed unique climate controls.

If the terrestrial globe retains its size and does not expand, then Panthalassa, if it existed during the age of Pangaea, must have started to disappear ~200 million years ago. However, there does not seem to be a vestige of this process in the geological record. Assuming that it was destroyed by subduction, it would take millions and millions of years for such an immense ocean to be subducted. There are other ways it could disappear – obduction as ophiolite or accretion of prisms adding to the continent as is splendidly evident in the case of the Makran (McCall 2002), but these do not seem to be adequate to be the supply the main mechanism of Panthalassa disappearance. Furthermore, there is the problem of how you initiate the subduction processes outside the conglomeration of continents that is Pangaea, with only a single continental mass?

What we have today is well illustrated by Condie (1976) in the fold-in map at the end of his book on Plate Tectonics (which strangely does not seem to mention Panthalassa at all!): all the present oceans were formed by continental break-up inside Pangaea from ~200 Ma onwards. Statements like that of Van Amdel (1994):

“When the Cenozoic began, there were small Atlantic and Indian Oceans and a huge remnant of the superocean Panthalassa, now the Pacific…….”

...seem to be totally unsupported by the evidence. The fact is that we now have no remnant of Panthalassa in our present oceans, and there seems to be no evidence at all of the existence of the diminishing Panthalassa through the geological record of the last ~200 million years (Table 1). Surely this great ocean would during its decline have severely influenced global climate and also palaeontology? Is there any evidence of this?

7. The search for possible explanations

1. Pangaea and Panthalassa are not entirely evidenced by Palaeomagnetism, there is climatic and palaeontological supporting evidence, so the first and most obvious answer, that palaeomagnetism is a faulty tool must surely be rejected.
2. We could well be at the stage in the cycle at which Paleothalassa has entirely been consumed and a Novopanthalassa is forming. However, it beggars belief that such a vast ocean could have slowly disappeared, mainly by subduction, through the last 200 million years, without being strikingly apparent in the geological record, which it is not.
3. The truth seems to be that the geological history in the last two hundred million years has been entirely concerned with the break-up of Pangaea, with processes internal to Pangaea.
4. There is a strong temptation therefore to believe that Pangaea covered the entire globe during the age of Pangaea (100 million years)(see Figure 3).
5. However, the palaeomagnetic and other evidence relating to Pangaea surely renders this idea preposterous?

The author reaches the state where he has severe doubts whether Panthalassa ever existed, yet rational thinking requires it, unless one has a shrinking and expanding globe (see 8, below).

8. Problems of repetition

The discussion above has been restricted to the present cycle, but geotectonists have recognised or invented a whole gaggle of pre-existing supercontinents: Ur, Arctica, Nena, Nuna or Columbia, Vaalbara, Kenorland, Rodinia, Pannotia: and have predicted 250 million years ahead, Novopangaea or Pangea Ultima (Nield 2007). Each one must have an accompanying Panthalassa, and each must have the complication of the totally not understood prolonged disappearance of its complementary superocean. This problem does not seem to have been given much, if any, thought?  Is this not a case of  “We are blinded by what we think we know……”?  [from Carey (Scalera 2003b)].

In fact, one cannot accept simply the expanding Earth explanation for the anomaly, if one believes in these supercontinents: one has, in stead, to invoke a concertina world, expanding and contracting as the continents cluster and separate!

9. Conclusion

The author has had a remarkably varied and interesting career in Geology lasting nearly 60 years on all continents but South America and Antarctica. During his retirement years, he has been able to think deeply about and review certain subjects such as the history of meteoritics, terrestrial impact and the Ediacara fauna: here, he believes that he highlights a major conundrum in the Plate Tectonics Paradigm.

He simply does not know the answer to the conundrum, but is convinced there is a largely overlooked difficulty in the Plate Tectonics Paradigm, where Panthalassa is concerned. The fundamental constraints which have perhaps been underscored are the need to keep the circumference of the globe reasonably constant and the amount of time needed to get rid of Panthalassa. The first constraint can be discounted if one believes in an expanding and contracting planet, but the changes required are on an immense scale if one dispenses with Panthalassa altogether, and need to be repeated again and again; the time constraint is inherent in the very concept of Panthalassa, and cannot in any way be overcome?

References

• Audley-Charles, M.G., Hallam, A., 1988. Gondwana & Tethys. Geological Soeciety of London Special Publication 3, Oxford University Press;, 317 pp.
• Carey, S.W.,1975. The Expanding Earth. Elsevier, Amsterdam, 488 pp.
• Condie, K.C., 1976. Plate Tectonics and Crustal Evolution. Pergamon Press, New York, Toronto, Oxford, Sydney, Braunschweig, Paris, 288 pp.
• Hallam, A., 2000. Pangaea. In: Hancock, P.L., Skinner, B.J. (Eds.), The Oxford Companion to the Earth, Oxford University Press, 789-791.
• Hilgenberg, O.C., 1965. Die paläogeographie der expandierenden Erde vom Karbon bis zum Tertiär nach paläomagnetschen Messungun. Geologis. Rundschau, 55, 878-924.
• Levin, L.E., Gramberg, I.S., Isaev, E.N.; McCall, G.J.H. (English language editor), 1993. Geology of Hydrocarbon and Mineral Resources associated with Post-Middle Jurassic Sequences in the Oceans and on the Continents. VNIIZARUBEZHGOEOLOGIA information systems department, Moscow, 704 pp.
• McCall, G.J.H., 2002. A summary of the geology of the Iranian Makran. In: Clift, P.D., Kroon, D., Gaedecke, C.. Craig, J. (Eds.), The Tectonic and Climatic Evolution of the Arabian Sea Region. Geological Society of London, Special Publication 195, 147-204.
• McCall, G.J.H. 2008. Trouble with Pangaea. Geoscientist, 18 (6), 6.
• McCall, G.J.H. 2009. Half a century of progress in research on terrestrial impact structures: a review. Earth Science Reviews, 92(3-4), 99-116..
• Nield, T. 2007. Supercontinent. Granta, London, 288 pp.
• Scalera, G. 2003a. The expanding Earth: a sound idea for the new millennium. In; Scalera,G., Jacob, K-H. (Eds.), Why expanding Earth? – a book in honour of O.G.Hilgenberg. INGV, Rome, 181-232.
• Scalera, G. 2003b. Samuel Warren Carey – a commemorative memoir. Ibid. 85-95.
• Van Andel, Tj. H. 1994. New views on an old planet. Cambridge University Press, 439 pp.

G.J.H.McCall
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