Geology and Himalaya's Asymmetric Growth

Subduction: Subduction - Wikipedia, the free encyclopedia is the name for the movement of one continental body under the other. Nature is not symmetrical. The forces at work are too immense to describe.

Thank you very much for your inputs.

One of the most important thing which happened, is monsoon rains returned back to India after hitting Himalyas. So the land on north of Himalyas turned into a desert. Then the rivers , starting from Himalyas made India /Pakistan/Bangladesh fertile. Brhmaputra ( Jamuna of Bangladesh) started from north of Himalyas. Both Ganges and Brhmaputra are the foremost deltaic river in the world, so the flats lands of Bengal ( East and West) got created over millions of years.

Another important thing is deccan plateu. I gather this got created by active volcanos (before or after Himlayas happened )

Lynlynch,

Subduction is often though of as simply one plate sliding under another causing lift and passages for magma.

With the Indian subcontinent this is different. Not only is there a subduction and lifting there is also horizontal compression that distorts the continental masses. It is refered to as an active fold belt. The Himalyas are three mountain ranges laying next to each other. Westward is Karakoram, Hindu-Kush and Pamirs with the Tien Shan range to the North. I have traveled along the western part of the Tien Shan Range from Tashkent to Samarqand. To the west the plates are pivoting producing curved mountain ranges.

You can see examples of this north of Italy, the Rocky Mountains in the North America and the Andes in South America. Such forces produce a lot of heat and soften rock.

In the Eastern US you can see long low ridges and valleys caused by compression. People live there are sometimes called "ridge runners."

Jon

Hi,

subduction is one principle to generate mountains,

but subduction is not sufficient to explain Tibetan plateau.

See any other mountain that is generated by subduction.

In most examples there are on both sides vast and deep sedimentary basins at low level.

As one plate is bent up, the other plate bent down and the total is slowly attaining a new floating equilibrium on beneath highly viscous magma there are two troughs formed that fill with sediments and often entrap gas or oil.

With Himalaya the situation at the Tibet side is different as Tibet seems to be surrounded by colliding plates. (?) If the Tibetan plate is very thick it will neither bend nor crack. But if not supported from minimum two sides it would tilt.

RHABE

It seems to me that subduction is the best answer, although I admit that most of what I know about such things comes from Discovery and PBS television. The Himalaya range is the result of the Indian sub-continent crashing into Asia, and is therefore composed mostly of continental rocks. But the reason India crashed into Asia is that it is riding on a northward moving slab of oceanic crust. The oceanic crust is being subducted under the Asian crust. While oceanic crust is heavier than continental rock, it is lighter than the mantle, so it provides additional buoyancy, lifting the Tibetan plateau. Something similar is happening in the western US, where the Pacific plate dives under the North American plate. The Sierras are the mountains forced up by the collision, but the great basin area between the Sierras and the Rockies is also forced upwards several thousand feet by the additional buoyancy from the oceanic crust.

Most text book pictures show that at these margins the oceanic crust is forced under the continental plate and then down into the mantel at a steep angle where it is remelted. A band of volcanoes occurs maybe 100 miles inland where the bits of islands and sediments riding on the ocean crust melt and bob back toward the surface. It seems however that sometimes the ocean crust can slip a thousand miles or so under the continent, uplifting a large plateau.

This slipping one continental crust beneath the other would make sense as buoyancy will dictate a high elevation as in Tibet.

So the typical European mountains: Alps and Pyrenees are different? Both crash one continental plate against another one. But no plateau!

Maybe also the very high elevation is protecting Tibet from being eroded down to ordinary mountain like peaks and valleys by low precipitation.

RHABE

Actually, if I remember my TV education correctly, the Alps are quite different in that they contain rock not only from Italy crashing into Europe, but also a lot of African rock and Mediterranean sea floor rock on top of the European crust. The presence of sea floor rock high in the Alps at least suggests that the expected subduction is not occurring, so no plateau behind the Alps.

If, as Duckinthepond suggests, the crust itself under Tibet is unusually thick, then it may well be that the Tibetan plateau is not the result of India crashing into Asia, but was perhaps an existing feature at the time of the collision.

There is a northerly progression from low angle thrust faulting (a significant horizontal stress component) in the Himalayan region through a region of strike-slip faulting to a region of tension in northern China and eastern Siberia. The collision of the two plates involved in the formation of the Himalaya Mountains are of crustal origin and therefore of same relative density. This adds to some interesting and bloody tall mountains. Typical oceanic/continental subduction is not an active process of formation here.

While many theories exist regards the formation of the Himalayan Mountains, a general consensus exists...

The Himalayan orogeny consists of a Hinterland Crust with a Peripheral Bulge preceded by a Foreland Basin adjacent the Thrust Belt. In typical fashion of orogenic deformation, a Plateau exists, a region of thickened Continetal Crust. The Plateau is then preceded by the Hinterland Crust.

Your "Plato" is in fact a typical aspect of the collision between two continental plates and an understanding of the dynamics (complex) between the Foreland and Hinterland crust must first be achieved to understand what you describe as "asymetric growth".

Numerous complications add some fun, such as "Ramps", "Flats" and "Imbricate Ramps"...and I will stuffed if I could find any reference on the WWW for all I mentioned...without requiring a paid subscription. I did try

...so it's off to the local University if you want to know what the hell is going on

Thank you, everyone for there contributions, and sorry for bit delayed response from my side.I have just lodged in after a outside visit and I am really pleased to see so many contributions.

I am studying every post and will get back to individual soon.

Thank you once again,

Regards

From somebody else' analysis....(unfortunately I don't know who should get the credit) quote:.....

......Because the crust of the Tibetan Plateau is twice its normal thickness, this mass of lightweight rock sits several kilometers higher than average through simple buoyancy and other mechanisms.

Remember that the granitic rocks of the continents retain uranium and potassium, "incompatible" heat-producing radioactive elements that don't mix in the mantle beneath. Thus the thick crust of the Tibetan Plateau is unusually hot. This heat expands the rocks and helps the plateau float even higher.

Another result is that the plateau is rather flat. The deeper crust appears to be so hot and soft that it flows easily, leaving the surface above it level. There's evidence of a lot of outright melting inside the crust, which is unusual because high pressure tends to prevent rocks from melting

I have been pondering if I should present this reply or not. It is not my intent here to prove an individual is either correct or incorrect regards a certain matter. My intent in this reply is just to present the current understanding of plate techtonics and its relevance to the formation of the Himalayan Orogeny.

1. The Indo-Asian collision has seen 1400 km of south-north crustal shortening absorbed by the Himalayan orogen.
2. Excessive crustal shortening has built the Cenozoic Tibetan plateau, that began in the Eocene (50–40 Ma) in the south Himalaya and extends 1000–1400 km to the north.
3. Paleozoic and Mesozoic mechanics have impressed an important modal formation with respect to strain processes during the Cenozoic.
4. The Deccan Flood Basalts were formed at 65Ma.. Flood basalts occured on a continental scale and typically in excess of a kilometre in thickness...but occurred before significant techtonic activity that formed the Himalayan mountains and subsequent hinterland/foreland morphology.
5. S-type granites formed by crustal melting are abundant in exposed areas of orogenic granites in the Himalaya. The P/T graphs, phase diagrams and AFM deltas all support the notion that crustal material under high pressure easily melts, it is a usual occurence and the depth (pressure) at which melting of crustal rocks occurs is significantly reduced in the presence of water.
6. The presence of Uranium etc will over time induce melting within a dormant crust but the mechanism of melting in the Himalayan orogeny is a direct result of techtonic formation and not radioactive decay. Also, the fact that a crust is "hot" will not cause the hot crust to float higher relative to a cooler crust of same composition and induce mountain formation topologies.
   
7. I draw your attention to the following links I have since discovered, so that people may learn a bit or two about geology and some of the terms used... LinkLink
8. Alps and Pyrenees...they are techtonically different modes of formation relative to that of the Himalayans.
9. The crust under the Himalayans are thick relative to adjacent continental plates and extend in parts into the Moho. The weight of the overlying crust actually pushes the crust into the Moho. Buoyancy clearly is not the cause of crust "sitting" several kilometres higher than normal.
10. Heat will expand rocks but does not on a geological scale create mountains., cause them to float higher or flatten continents. Techtonic activity creates morphologies such as plateau, mountains on a continental scale.
    

...I will provide more details and further information with diagrams should I feel the need.

Thanks for all the info, it is very helpful. I do have a question about #9. You say that 'The crust under the Himalayans are thick relative to adjacent continental plates'.

I would have thought that the Himalayas were the crust, thickened by compression. Are you saying that these mountains are a 'foreign body' pushed on top of a pre-existing area of crust that was already unusually thick before the mountains formed? Maybe my lack of proper geological vocabulary is limiting my understanding.

Then you say that 'The weight of the overlying crust actually pushes the crust into the Moho. Buoyancy clearly is not the cause of crust "sitting" several kilometres higher than normal.' Maybe there is a point here that I am missing, but if the weight of these mountains is pushing the crust into the Moho (the underlying mantel), doesn't the denser mantel push back? Isn't this buoyancy? Or are you refering to the post that suggested that radio active heating is causing the crust to expand (slightly) making it more buoyant than it would be if it was cooler?

"Himalayas were the crust, thickened by compression"

I don't believe I have inferred otherwise. To do so would be plain crazy....I ain't that crazy

The Himalaya orogeny is indeed made of primarily crustal derived materials.

"these mountains are a 'foreign body' pushed on top of a pre-existing area of crust that was already unusually thick before the mountains formed?"

UMMM, certainly am not inferring the mountains we see on the surface are a "foreign body". Neither am I suggesting (although others may have suggested that) the crust was "unusually thick" prior to techtonic deformation.

"Isn't this buoyancy?"

Okay, here is the low down in simple terms...The mantle is made up of denser material than oceanic crust. The continental crust is made up of lighter material than oceanic crust. Activity of two continental plates has pushed the merging two continental plates together, resulting in partial intrusion in places into the mantle and also into the troposphere. Continental and oceanic crust will "float" above the mantle. Collision of two continetal plates in the case of the Himalayan mountains has formed the various types of techtonic distinct formations between the foreland and hinterland as I mentioned earlier. Part of that distinct orogenic formation is the plateau in question.

The fact that the crustal material is buoyant relative to the mantle and oceanic crust does not give rise to the Himalayan Mountain formation. It is the collision of two plates that formed what is in question.

"Or are you refering to the post that suggested that radio active heating is causing the crust to expand (slightly) making it more buoyant than it would be if it was cooler?"

...yes I am, very much so. Radioactive decay and the resultant heat does produce crustal melts, this melt will either assimilate into the mantle or be extruded to the surface. Those that do not do either solidify as granites. The extent of bouyancy inferred by the heating of the crust will not produce the geomorphological structures we see today.

I would also like to clarify a point made by another member, "Another result is that the plateau is rather flat. The deeper crust appears to be so hot and soft that it flows easily, leaving the surface above it level."

...that type of orogenic activity ceased billions of years ago, where the crustal material was relatively plastic and ductile at the time of their "formation".

Perhaps now a drawing is in order to help describe various morphologies between foreland and hinterland, that explains the existance of the plateau and other morphologies in question...do remember, I have only attempted to show the general consensus to the topology we see, "while many theories exist regards the formation of the Himalayan Mountains...". How we get to the general consensus is a topic in continual debate and change.

image taken from,

Kearey, P. & Vine, F.. (1990) Global Techtonics, 200.

I think the photo you have used here is from my area.. Maharashtra India, all basalt region