Monday, March 19, 2018

How Old Are The Aravalli Mountains Of Rajasthan?

By the age of a mountain range I mean the time since the formation of significant topography. I don't mean the age of rocks making up the mountains. There are plenty of instances where terrains made up of rocks of a particular age have been rejuvenated and uplifted by earth movements later in time. The most spectacular example in India are the Himalaya. The oldest rocks in the Himalaya are dated to about 1.8 billion years. These, along with rocks ranging in age from more than a billion years to about 50 million years, have up thrust up during mountain building that began about 25 million years ago.

There are other less well known examples from India. The Bababudan hills in Karnataka are made up of rocks as old as 3.5 billion years. The topography though is much younger. This area is a southern extension of the Deccan plateau which has been rejuvenated during the Cenozoic. Another example are the massifs of the Nilgiri Hills in the Western Ghats. These massifs reach about 7500 feet ASL. They are made up of charnockite, a high grade metamorphic rock. This terrain was metamorphosed about 2.5 billion years ago and then again around 550 million years ago. It too has been uplifted in the more recent Cenozoic.

So, how old are the Aravalli mountains?

In a recent article in LiveMint on the ecology, geology and archaeological significance of the Aravalli mountains of Rajasthan, Ananda Banerjee writes-

"These ancient rocks are part of the oldest mountain range in the world—the Aravalli range, or “The Ridge”, as it has been more commonly known in Delhi from the days of British rule".

He quotes author Pranay Lal " “It took two billion years (from a point in time between 3.2 billion years to 1.2 billion years ago) of shoving and pushing of tectonic plates and magma outpourings to create these ancient fold mountains

This statement does not really inform us about the sequence of geological events that took place. Did the Aravalli  mountain building really take two billion years? Was it really initiated 3.2 billion years ago?

The Aravalli fold mountains are made up of layers of sediments interlayered with volcanic rocks. The deposition of these volcano-sedimentary successions took place on the sea floor. This entire pile has been subdivided into the Aravalli Super Group and the Delhi Super Group corresponding to two distinct cycles of sedimentation and orogeny.

The satellite image below shows the folded ridges of the Aravalli mountains west of the city of Udaipur


The geologic story begins, as Lal pointed out, about 3.3. to 3.2 billion years ago (1). At this time prolific 'granitic' magmatism was creating new crust. These magmas go under the name TTG, for tonalites, trondhjemites, and granodiorites. Sediments and interlayered basalt volcanic layers were deposited in contemporaneous basins. Geologists think that the tectonic setting for TTG magmatism would have been similar to a convergent plate margin, where one plate subducts or slides underneath another plate. The setting for volcanic-sedimentary deposits may have been a rifted oceanic basin.

The schematic below shows the evolution of cratons during the Archaean. Early continental nuclei were separated by oceanic basins.


Source: P.A. Cawood, C.J. Hawkesworth, and B. Dhuime - The continental record and the generation of continental crust

 These different terrains slowly sutured and welded together to form larger continental fragments. In this process the TTG  and the volcano-sedimentary deposits got metamorphosed and deformed. The result was a complicated terrain with slices of granite gneiss (metamorphosed TTG) interleaved with low to medium grade metamorphic rocks like chlorite and amphibolite schists (metamorphosed  basalt and other volcanic rocks and sediments).

The cross section below summarizes the complicated structure of the granite-greenstone belts of Rajasthan.


D.B. Guha 2008 - Tectonostratigraphy and Crustal Evolution of the Archaean Greenstone-Granulite Belt of Rajasthan

This granite-greenstone terrain (due to the presence of green colored minerals like chlorite and amphiboles) is called the Banded Gneiss Complex. It is made up of two sub terrains named the Sandamata Complex and the Mangalwar Complex. The formation of the Banded Gneiss Complex was completed by about 2.5 billion years ago when profuse granitic magmatism ended. Geologists call this craton stabilization. Initially, this terrain may have had topography. Hill ranges may have stood out in this area about 2.5 billion years ago. However, this was followed by a long period of erosion wherein the terrain was peneplained. Evidence for deep weathering of this terrain comes from paleosols (soils) which mantle parts of the Banded Gneiss Complex.

This was followed by the sagging of the granite-greenstone crust and the formation of new sedimentary basins.

The Banded Gneiss Complex forms the basement on which the Aravalli Supergroup sediments were deposited. These older rocks therefore were the sea floor at that time. No fold mountain ranges existed in this region around 2 billion years ago.

Galena (Lead Sulphide) which occurs in volcanic rocks interlayered in the lower part of the Aravalli Super Group has been dated to about 2 billion years. This is taken as roughly the start of Aravalli sedimentation. This basin lasted for about 200 million years. Granites intruding the Aravalli Supergroup have been dated to about 1. 85 billion years. These are syn-orogenic granites which form when continental fragments collide, and the deeply buried crust partially melts to generate granitic magma. Geologists think that the tectonic event responsible for this was the collision of the Aravalli craton and the Bundelkhand craton. The Aravalli orogeny and fold belt formation is thus about 1. 8 billion years old.

At this time there would have been a fold mountain range made up of crumpled up Aravalli Supergroup rocks. Subsequently, beginning around 1.7 billion years ago, another basin developed in the north and west of the older Aravalli basin. In this basin were deposited sediments and volcanic material that make up the Delhi Supergroup of rocks. Among these rocks are the resistant quartzites that make up the Delhi Ridge. The Delhi basin closed and the rocks folded and  uplifted by about 1 billion years ago. The tectonic event responsible for the Delhi orogeny is thought to be the collision between the Aravalli-Bundelkhand craton and the Marwar craton to the west. The contact between the two is the Western Margin Fault along which the Phulad Ophiolite rocks lay sandwiched. These are remnants of the oceanic crust that existed between the two continental blocks.

The Aravalli fold belt, made up of the Aravalli Supergroup and the Delhi Supergroup formed over an extended time period in two phases, the first one about 1.8 billion and the second about 1 billion years ago.

The map below shows the different geologic terrains of the Rajasthan craton


Source: Joseph Meert et.al. 2010 - Precambrian crustal evolution of Peninsular India: A 3.0 billion year odyssey

Does that mean we can say that the maximum age of the Aravalli mountains is about 1.8 billion years?

This is an intriguing question and it depends on what might seem a rather esoteric question. What is the nature of the contact between the Aravalli Supergroup rocks and the younger Delhi Supergroup rocks? The Aravalli and Delhi rocks have a sheared and faulted contact. This means that the two terrains have been moved along faults from their original positions and juxtaposed against each other. But some work suggests that their original relationship was different. Field relations and inferred contrasting folding histories (2, 3)  implies an angular unconformity between the two. That means that Aravalli Supergroup rocks were folded earlier and then over a time span of 100 million years or so, erosion wore down the Aravalli Supergroup fold mountains to a plain. The crust then sagged, and the Aravalli rocks along with the Banded Gneiss Complex became the basin floor upon which the Delhi Supergroup sediments were deposited.

If this scenario is true, then the Rajasthan fold mountain topography formed during the younger Delhi Supergroup orogeny, that is about 1 billion years ago. The rolling hills and the gentle stream gradients suggest that erosion has been wearing the mountains down and there have not been significant earth movements affecting this part of the crust since.

How does this compare with other ancient mountain ranges. The Barbeton Greenstone Belt, also known as the Makhaonjwa Mountains, on the border of South Africa and Swaziland are thought to be the oldest mountain range in the world. They are made up of 3.5 to 3.2 billion year old rocks. On the web there are top 10/9 lists of the oldest mountain ranges in the world, which include the Hammersley Range in Western Australia (3.4 billion)  and the Waterburg Mountains in South Africa (2.7 billion) (strangely they exclude the Aravallis!) But has the topography existed since the claimed age or has an old peneplain been rejuvenated in more recent times?

That is the billion year(s) question that must be asked when evaluating any "my oldest mountains are older than your oldest mountains" claim.

4 comments:

  1. Many thanks for this post Suvrat! Most interesting. Needed to ask, so 1. where are the oldest rocks in the Himalaya that you speak of, and what are they exactly? and 2. and when you say the Western Ghats have risen in the recent cenozoic, when more precisely. Between when and when? Many thanks,

    theo

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    1. In Kumaon and Gharwal, oldest rocks of the Berinag Formation of Lesser Himalaya have been dated to 1800 my. Slices of a schistose gneiss occur below the Berinag thrust sheets. This gneiss known as Ramgarh Gneiss is dated to ~1850 my and is considered the crystalline basement up on which Lesser Himalaya sediments were deposited. Generally the oldest rocks of the Lesser Himalaya sequence are about 1800 my in western Himalaya too.

      Reg. Western Ghats; there has been episodic uplift through the Cenozoic, first in the Paleocene and later in the Miocene . see these posts - http://suvratk.blogspot.in/2009/02/why-tea-grows-at-higher-elevations-than.html

      http://suvratk.blogspot.in/2009/01/panchgani-tableland-informs-about.html

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  2. A layman's query. Did the events happen before the Gondwana landmass impacted the pre-Asian landmass or after?

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    1. Niraj- Much before. These events occurred between 2 billion and 1 billion years ago. The Gondwana supercontinent assembled around 550-600 million years ago. It began breaking up by 180 million years ago. One piece of it (India) then collided with Asia beginning around 50- 55 million years ago.

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