Explanation of Raised Plateaux Inland from Ocean Margins

[exchemist]

There is a recent paper that's been in the news a bit, from Thomas Gernon, a Prof. at Southampton,  et al, which accounts for why there are uplifted plateaux several hundred km from the coast in a number of places, e.g. S Africa, Brazil, India. https://www.nature.com/articles/s41586-024-07717-1

It's quite technical, but as I understand it, they propose, via modelling, that after rifting to form a new ocean, an unstable vortex of convection is set up in the asthenosphere due to something called Rayleigh-Taylor instability (a thing in fluid dynamics, apparently). This vortex moves away from the mid-ocean ridge under the continent and progressively delaminates the "keel" of lithosphere at the base of the tectonic plate.  

I gather the rigid upper mantle which forms the lower part of the lithosphere is actually denser than the asthenosphere it sits above (it is rigid because it has cooled, which also makes it denser), so tearing (or melting?) off this "keel" actually causes the lithosphere to tend to rise, isostatically. The delaminated material, plus the heat in the ascending side of the vortex can cause a breakthrough of volcanism, creating the Kimberlite pipes that are a curious feature in some of these places.  The vortex makes its way under the continent over a period of several tens of millions of years, so these features start to arise quite a long time after the ocean opening event.

I don't pretend I know a great deal about this subject but I thought it was interesting. I had not realised that our understanding of the lithosphere has moved on so much, and that tectonic plates are now thought to comprise not just crust but a substantial chunk of cooling, solid mantle beneath as well. As it cools, the density increases and eventually the whole thing becomes unstable, due to "floating" on asthenosphere that is less dense than it has become - which is a driver for subduction. 

   

[studiot]

31 minutes ago, exchemist said:

There is a recent paper that's been in the news a bit, from Thomas Gernon, a Prof. at Southampton,  et al, which accounts for why there are uplifted plateaux several hundred km from the coast in a number of places, e.g. S Africa, Brazil, India. https://www.nature.com/articles/s41586-024-07717-1

It's quite technical, but as I understand it, they propose, via modelling, that after rifting to form a new ocean, an unstable vortex of convection is set up in the asthenosphere due to something called Rayleigh-Taylor instability (a thing in fluid dynamics, apparently). This vortex moves away from the mid-ocean ridge under the continent and progressively delaminates the "keel" of lithosphere at the base of the tectonic plate.  

I gather the rigid upper mantle which forms the lower part of the lithosphere is actually denser than the asthenosphere it sits above (it is rigid because it has cooled, which also makes it denser), so tearing (or melting?) off this "keel" actually causes the lithosphere to tend to rise, isostatically. The delaminated material, plus the heat in the ascending side of the vortex can cause a breakthrough of volcanism, creating the Kimberlite pipes that are a curious feature in some of these places.  The vortex makes its way under the continent over a period of several tens of millions of years, so these features start to arise quite a long time after the ocean opening event.

I don't pretend I know a great deal about this subject but I thought it was interesting. I had not realised that our understanding of the lithosphere has moved on so much, and that tectonic plates are now thought to comprise not just crust but a substantial chunk of cooling, solid mantle beneath as well. As it cools, the density increases and eventually the whole thing becomes unstable, due to "floating" on asthenosphere that is less dense than it has become - which is a driver for subduction. 

   

Interesting, thanks, +1

 

I will comment further once I've read it properly.

Edited August 11 by studiot

[exchemist]

2 hours ago, studiot said:

Interesting, thanks, +1

 

I will comment further once I've read it properly.

It's quite a dense read, and I got stuck for a while because I had not realised the lithosphere is now thought to include rigid mantle that, when it gets old at least, is more dense than the asthenosphere it sits on. There are some good diagrams - and even some videos which I have not watched yet.

One report I read suggested this is quite a radical and important piece of work.  I have not worked out how this Rayleigh-Taylor instability arises. I looked that up and it seems to be when you have  2 liquid phases with the higher density one above the lower density one. But the "keel" that is peeled off is solid, so I don't quite get it. Unless it gets melted and then is unstable w.r.t. the asthenosphere underneath. I wonder if that's it.

Edited August 11 by exchemist

[studiot]

On 8/11/2024 at 9:27 PM, exchemist said:

It's quite a dense read, and I got stuck for a while because I had not realised the lithosphere is now thought to include rigid mantle that, when it gets old at least, is more dense than the asthenosphere it sits on. There are some good diagrams - and even some videos which I have not watched yet.

One report I read suggested this is quite a radical and important piece of work.  I have not worked out how this Rayleigh-Taylor instability arises. I looked that up and it seems to be when you have  2 liquid phases with the higher density one above the lower density one. But the "keel" that is peeled off is solid, so I don't quite get it. Unless it gets melted and then is unstable w.r.t. the asthenosphere underneath. I wonder if that's it.

Isn't it just!  and full of techie words with special meanings.

 

So first thoughts.

I have not experience of the escarpments the authors are describing. The onlymega  one I have direct personal experience of is the Arabian peninsula escarpment.
That is definitely a rift valley escarpment, still active today. It is estimated at 30 Myears old.

The continental plates have shifted a long way inthe over 200Myears they are talking about, and oceans have opened and closed.
So their comment that these escarpments occur at continental edges and are one sided (unlike a rift vally)  may have occurred due to plate displacement over the timescale.

One thing the new theory does not explain is the lack of deltas at the continental margins, building up new sedimentary land as England was built up on the edge of the Scottish peninsula of the old red continent. There are many small escarpments from the brecon Beacons to the Kentish Weald.

 

I'm not saying I think it is wrong, just that there is room in our world for many different mechanisms and routes to escarpments and flat topped raised areas.
 

Edited August 12 by studiot

[exchemist]

1 minute ago, studiot said:

Isn't it just!  and full of techie words with special meanings.

 

So first thoughts.

I have not experience of the escarpments the authors are describing. The onlymega  one I have direct personal experience of is the Arabian peninsula escarpment.
That is definitely a rift valley escarpment, still active today. It is estimated at 30 Myears old.

The continental plates have shifted a long way inthe over 200Myears they are talking about, and oceans have opened and closed.
So their comment that these escarpments occur at continental edges and are one sided (unlike a rift vally)  may have occurred due to plate displacement over the timescale.

One thing the new theory does not explain is the lack of deltas at the continental margins, building up new sedimentary land as England was built up on the edge of the Scottish peninsula of the old red continent. There are many small escarpments from the brecon Beacons to the Kentish Weald.
 

This is not about escarpments adjacent to a rift. It is about elevated plateaux that lie far inland from an opening ocean. If you look at the diagrams it may become clearer. 

[studiot]

27 minutes ago, exchemist said:

This is not about escarpments adjacent to a rift. It is about elevated plateaux that lie far inland from an opening ocean. If you look at the diagrams it may become clearer. 

The word rift or rifting occurs 5 times in the abstract alone!

[exchemist]

9 minutes ago, studiot said:

The word rift or rifting occurs 5 times in the abstract alone!

Yes it isn’t that easy to follow, but the diagrams in Fig 6 help a lot. Take a look at those. 

[studiot]

Here is another viewpoint, but covering the whole globe.

 

I note the Indian and african examples the raised upland is 'protected' from eroding rainfall by the escarpment mountains.

 

https://orca.cardiff.ac.uk/id/eprint/56857/1/Blenkinsop%2C Moore - 2013 - Tectonic Geomorphology of Passive Margins and Continental Hinterlands.pdf

[studiot]

On 8/12/2024 at 9:55 PM, exchemist said:

Yes it isn’t that easy to follow, but the diagrams in Fig 6 help a lot. Take a look at those. 

 

On 8/11/2024 at 9:27 PM, exchemist said:

One report I read suggested this is quite a radical and important piece of work.  I have not worked out how this Rayleigh-Taylor instability arises. I looked that up and it seems to be when you have  2 liquid phases with the higher density one above the lower density one. But the "keel" that is peeled off is solid, so I don't quite get it. Unless it gets melted and then is unstable w.r.t. the asthenosphere underneath. I wonder if that's it.

 

Thank you once again for pointing to a most interesting and thought provoking investigation.

 

The idea of continental delamination but rising hot magma was originally proposed by P Bird in 1979 as an explanation of the Labrador trough.

Here is a reference to  this

http://peterbird.name/publications/1979_delamination/1979_delamination.htm

 

This was extended in 1981 by Dimroth as in this diagram.

As discussion (references later) has since caused this approach to be discarded as no one has been able to make the necessary heat flows add up.

See the discussion also in the scan.

 

Further explanations are also introduced and discussed.

The scan is from my copy of

Global Tectonics by Kearey and Vine  2n ed

I note that there is now a 3rd ed with nearly 200 extra pages.

A (free) pdf is available here

https://www.geokniga.org/bookfiles/geokniga-global-tectonics.pdf

warning it is 66MB

 

The conventional explanation, based on the Wilson cycle of breakup of continental crust (cratons) is greatly expand and firmly entrenched in the £rd ed.

But it really is an excellent text as it examines the conditions of tectonic process as applied to the real earth geology and geography, including the mineralogical and rheological environments.

 

A final picture towards our next exchange from Wikipedia.

The real earth examples in the Nature article all relate to the breakup of the last supercontinent, Panges.

So we need to go back about 200Mya and start there, because the examples did not all occur at the same time but over a period of about 200Mya to 50 Mya.

[exchemist]

5 hours ago, studiot said:

 

 

Thank you once again for pointing to a most interesting and thought provoking investigation.

 

The idea of continental delamination but rising hot magma was originally proposed by P Bird in 1979 as an explanation of the Labrador trough.

Here is a reference to  this

http://peterbird.name/publications/1979_delamination/1979_delamination.htm

 

This was extended in 1981 by Dimroth as in this diagram.

As discussion (references later) has since caused this approach to be discarded as no one has been able to make the necessary heat flows add up.

See the discussion also in the scan.

 

Further explanations are also introduced and discussed.

The scan is from my copy of

Global Tectonics by Kearey and Vine  2n ed

I note that there is now a 3rd ed with nearly 200 extra pages.

A (free) pdf is available here

https://www.geokniga.org/bookfiles/geokniga-global-tectonics.pdf

warning it is 66MB

 

The conventional explanation, based on the Wilson cycle of breakup of continental crust (cratons) is greatly expand and firmly entrenched in the £rd ed.

But it really is an excellent text as it examines the conditions of tectonic process as applied to the real earth geology and geography, including the mineralogical and rheological environments.

 

A final picture towards our next exchange from Wikipedia.

The real earth examples in the Nature article all relate to the breakup of the last supercontinent, Panges.

So we need to go back about 200Mya and start there, because the examples did not all occur at the same time but over a period of about 200Mya to 50 Mya.

That’s interesting. So the basic idea is not entirely new, then. I suspect the novelty is the idea of this rolling vortex propagating away from the spreading centre due to this Raleigh-Taylor instability that they have modelled. Perhaps that takes care of the heat flow issue you refer to.

[studiot]

12 hours ago, exchemist said:

That’s interesting. So the basic idea is not entirely new, then. I suspect the novelty is the idea of this rolling vortex propagating away from the spreading centre due to this Raleigh-Taylor instability that they have modelled. Perhaps that takes care of the heat flow issue you refer to.

That's a lot of energy. The edge mountain ranges/escarpments are 3km high and tens of thousands of km long, normally associated with the sort of energy of whole plate movement/interaction.

It may be that this energy could be supplied continuously over a long period, by the 'vortex' of roiling magma, as figs 2 and 6 imply.

The question arises.

The original craton material predates this activity and was stable for hundreds of millions of years. The material of the elevated plateau remains unchanged by process.

The edge margin mountains should be heavily metamorphosed versions of this original material, not for instance solidified magma alone.

A purely erosional hypothesis would mean that the edge material should be original craton material.

 

I have been trying to determine the rocks in both places and wil let you know what I dig up.

 

It is a great pity that no one else seems interested in this, the only truly scientific thread around.

 

 

[sethoflagos]

On 8/16/2024 at 11:01 AM, studiot said:

It is a great pity that no one else seems interested in this, the only truly scientific thread around.

Hey! Don't jump to conclusions!

Just to throw something into the mix, how about mantle delamination at constructive plate margins.

https://www.nature.com/articles/s41561-024-01473-7#:~

This model for creation of the Tibetan plateau has been kicking around for decades, but along with the OP it reinforce the role of delamination as a global phenomenon. So uplands aren't necessarily evidence of large scale granite emplacement.

[Mordred]

On 8/16/2024 at 4:01 AM, studiot said:

 

It is a great pity that no one else seems interested in this, the only truly scientific thread around.

Oh I have been following the thread just haven't anything to contribute at the moment lol. 

[studiot]

A word of warning.

I am not happly with the author's use of the word 'delamination', although I will accept it is better than the geologist's  standard use of the word.

 

Quote

The Penguin Dictionary of Geology 2nd ed 2001

Philip Kearey

Delamination

The process of CRUST DETACHMENT from the MANTLE during continental collision or when an upper crustal layer detaches from a lower layer by FLAKE tECTONICS

 

I do wish experts in one discipline would refrain from pinching a word already well established in other disciplines and redefining it.

[exchemist]

36 minutes ago, studiot said:

A word of warning.

I am not happly with the author's use of the word 'delamination', although I will accept it is better than the geologist's  standard use of the word.

 

 

I do wish experts in one discipline would refrain from pinching a word already well established in other disciplines and redefining it.

Like astronomers' perversion of the term "metals", you mean? 😁

[studiot]

23 minutes ago, exchemist said:

Like astronomers' perversion of the term "metals", you mean? 😁

Luckily for me I don't know that one, but probably.

Do tell

[studiot]

2 hours ago, exchemist said:

Like astronomers' perversion of the term "metals", you mean? 😁

 

Quote

In astronomy, “metals” refers to all the elements on the periodic table other than hydrogen and helium. That's because the lightest two elements were formed in the earliest moments of the Big Bang and make up the vast majority of atomic matter in the cosmos.8 Jan 2024

 

 

gosh I never heard this before.

Thanks. +1

 

And there was I ready to argue that hydrogen can also be considered a metal !

[TheVat]

On 8/16/2024 at 4:01 AM, studiot said:

It is a great pity that no one else seems interested in this, the only truly scientific thread around.

Following this with great interest.  No intention of delamination from this thread anytime soon.

[exchemist]

On 8/17/2024 at 12:54 PM, sethoflagos said:

Hey! Don't jump to conclusions!

Just to throw something into the mix, how about mantle delamination at constructive plate margins.

https://www.nature.com/articles/s41561-024-01473-7#:~

This model for creation of the Tibetan plateau has been kicking around for decades, but along with the OP it reinforce the role of delamination as a global phenomenon. So uplands aren't necessarily evidence of large scale granite emplacement.

Interesting. Maybe it is this Rayleigh-Taylor fluid dynamic modelling that is the really new contribution, then, i.e. the mechanism that drives the progressive delamination that others had already proposed. 

4 hours ago, studiot said:

 

 

 

gosh I never heard this before.

Thanks. +1

 

And there was I ready to argue that hydrogen can also be considered a metal !

As you imagine, a chemist finds this misuse particularly ridiculous.

By the way, re hydrogen, I understand hydrogen is predicted to have a metallic structure when compressed sufficiently. However I don't think anyone has succeeded in forcing it into a metallic state so far, though there was I think a (strongly disputed) claim to have done so a year or two ago.  

[sethoflagos]

17 hours ago, exchemist said:

Interesting. Maybe it is this Rayleigh-Taylor fluid dynamic modelling that is the really new contribution, then, i.e. the mechanism that drives the progressive delamination that others had already proposed.

That's my reading too. Must say that I'd be more swayed by better hard corroborating field evidence to support the thermomechanical simulation, though I appreciate the logistical difficulty of getting the material evidence to the earth's surface in the case of divergent plates; the most relevant materials are heading in the wrong direction. Also a bit concerned by the dismissal without much explanation of late palaeozoic, early mesozoic structures that didn't fit the pattern as well as the later mesozoic to early cenozoic structures the study focused on.    

[sethoflagos]

On 8/11/2024 at 6:48 PM, exchemist said:

There is a recent paper that's been in the news a bit, from Thomas Gernon, a Prof. at Southampton,  et al, which accounts for why there are uplifted plateaux several hundred km from the coast in a number of places, e.g. S Africa, Brazil, India. https://www.nature.com/articles/s41586-024-07717-1

You might be interested in a further related article in Nature:  'Multistage lithospheric drips control active basin formation within an uplifting orogenic plateau'. PDF here.

This analysis differs from the OP article in that the upper mantle delamination seems to be driven purely by isostatic imbalance and does not require a subduction induced Raleigh-Taylor current to shear it free. Again, it focuses on relatively modern fold zones, but is potentially more general in its global applicability.  

For a mental picture, Anton Petrov has put together quite a good Youtube video overview of it at: