Tuesday, November 19, 2019

Kumaon Lesser Himalaya- Lessons In Mountain Building

Mountain belts like the Himalaya and the Alps formed when continental crust was squeezed, deformed and uplifted during the collision of two continental plates. The Himalaya, which is the deformed edge of the Indian continental plate is made up of different terrains. The Tethyan Himalaya is the northernmost terrain whose northern edge meets the Asian continental plate. The Greater Himalaya and the Lesser Himalaya are the two terrains successively to the south of the Tethyan Himalaya.

The Tethyan Himalaya are made up of rocks of Cambrian to Eocene in age  (542 -50 million years old) and have suffered the least burial and metamorphism. The Greater Himalaya are rocks which were buried as deep as 20-25 kilometers, suffering the highest degree of metamorphism and even partial melting. They range in age from the Neoproterozoic to the Ordovician (1000 -450 million years old). The Lesser Himalaya rocks were subjected to an intermediate level of burial and metamorphism. They span the Paleoproterozoic to Neoproterozoic in age (1840 -800 million years old).

The Siwalik ranges that occur to the south of the Lesser Himalaya are made up of sediments that were derived from the erosion of the rising Greater and Lesser Himalaya. They range in age from about 12 million to 0.5 million years.

So, exactly what happens when the crust gets caught up in such a continent-continent collision?

A recent paper by Subhadip Mandal and colleagues in Lithosphere explains the structural architecture and mechanisms of crustal deformation of the Kumaon fold and thrust belt. They propose a resolution of some long standing problems in Kumaon geology, namely the interrelationships between the different fault systems and exposed terrains.

See the map which shows part of the Kumaon and Gharwal Himalaya. Before India Asia collision, the crust between the two orange lines would have been ~575 kilometers wider! ....Squeeeze!

How did the crust get 'shortened' during India- Asia collision? In the schematic that I have drawn below, shortening of a particular length of crust takes place by folding it or by breaking it up into blocks and stacking them. The Himalaya have formed by a combination of such folding and thrust stacking.

Off course, the Himalaya is not one big fold, nor is it a stack of blocks forming a tower like the way I've drawn it. Rather, think of inclined books on a shelf. The books are inclined towards the right, or north. There are 4 books. From right to left they are the Tethyan Himalaya, Greater Himalaya, Lesser Himalaya and the Siwaliks. 

A shelf with books inclined to the right will grow by shelving from left to right. The Himalaya have grown in exactly the opposite manner. 

Imagine four books lying flat forming a chain on a bookshelf.  The rightmost book (Tethyan Himalaya) made contact with Asia and was thrust up. Then the book to its left was thrust up (Greater Himalaya), then the Lesser Himalaya and finally the Siwaliks. Deformation moved from right to left, or in the real world, from north to south.

But enough of abstraction! In the real world.. A  has been crumpled up to form B.  A in the figure below is the original disposition of rock units of the Indian plate. B shows those units as they are today,  folded and faulted after the collisional process. The Greater Himalaya is the topmost pink layer. The Lesser Himalaya layers are shown in green, blue and orange. The Siwalik ranges are fawn colored. The Tethyan Himalaya not shown in this figure. 

Source: Subhadip Mandal et.al. 2019

The structure looks like a mangled disordered heap of strata. But there is an order to this apparent chaos. As in our book analogy, these units were deformed in a sequence. The cross sections below shows the sequence of deformation and how the structural architecture evolved.

Source: Subhadip Mandal et.al. 2019

The collision of the Indian and Asian plates has been timed to around 55-50 million years ago. The first significant topography formed with the uplift of the Tethyan Himalaya between 45-35 million years ago.  The rocks that became the Greater Himalaya were buried the deepest during continental collision. They were uplifted between 23-16 million years ago. This crustal block was moved along a giant fault system known as the Main Central Thrust. In the two figures above you can see the Greater Himalaya as the pink layer overlying the Lesser Himalaya.

The Lesser Himalaya rocks which show imprints of a shallower buried state were lifted up between 16 - 4 million years ago. This rise of the rocks of  the Lesser Himalaya took place in two broad phases. In the first phase, the oldest rocks of the Lesser Himalaya were thrust up along another big fault system known as the Ramgarh-Munsiyari Thrust. In the figure, these oldest Lesser Himalaya rocks are the thick green layers immediately below the pink Greater Himalaya. Subsequently, more and more of the younger Lesser Himalaya strata got caught up in the deforming pile of rocks. Slices of the younger Lesser Himalaya were moved along thrust faults and stacked in a southerly growing fold and thrust belt.   

Initially, the Greater Himalaya and the oldest Lesser Himalaya were placed atop the younger Lesser Himalaya along very low angle faults in a manner similar to the stacked blocks I showed in the beginning of the post.  Later, the growth of the younger Lesser Himalaya lifted, tilted at steeper angles, and folded the overlying Greater Himalaya and the older Lesser Himalaya thrust sheets in a series of broad domes (anticlines) and troughs (synclines).  

The Greater Himalaya and the oldest Lesser Himalaya domes were more susceptible to erosion. As a result, these domes were removed over time, leaving behind synclinal remnants known as klippen. Isolated outcrops of Greater Himalaya and the oldest Lesser Himalaya rocks sit atop younger Lesser Himalaya at many places along the Lesser Himalaya belt. For example, the town of Almora in Uttarakhand is on a klippen of Greater Himalaya rocks. Further to the east the small town of Askot is on a klippen of the oldest Lesser Himalaya.
And finally, sediments which were being deposited in a southerly moat in front of the rising Greater and Lesser Himalaya rose to become the Siwaliks beginning around 1-0.5 million years ago. 

Mandal and colleagues work clarifies to a great extent the structure of Kumaon Himalaya and the mechanism of how fold and thrust mountain belts are constructed. I have simplified the story here. The paper has more nuanced details of the methods and techniques used to reconstruct a long and complicated process.

When you travel next across the Kumaon region, think of inclined books (thrust sheets) and their sequential uplift.



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