Monday, May 23, 2016

Which Are Older? Lakshadweep Islands Or Andaman Nicobar Islands?

A friend asked me this question:

Which formed first, Andamans or Lakshadweep?

My answer was-

Lakshadweep islands as a system of living coral atolls etc is a Holocene system (past 12 thousand  years). These coral communities rest on earlier Pleistocene reefs. So the history of reefs and atolls is probably a Quaternary phenomenon going back several hundred thousand years when ice age driven sea level fluctuations resulted in shallow seas and vertical coral growth. Below these Pleistocene and Holocene corals lie earlier Cenozoic carbonate sediments (Source 1, 2 ) . These sediments were deposited in a subtidal marine setting.  We are not sure whether sea level drops during deposition of these earlier carbonate sequence exposed the ridge. It is possible that during this long Cenozoic history, there may have been episodic exposure of the carbonate platform and small sand shoals may have appeared as islands above sea surface. However,  I haven't come across studies showing presence of earlier (pre-Pleistocene) reef ecosystems that could have formed the kind of coral atoll island system of the Quaternary.

There is a several hundred meters of Eocene to Pleistocene sediment sequence deposited on top of a Palaeocene-Eocene volcanic basement. This volcanic basement forms the northern part of the Chagos-Laccadive ridge which is a volcanic ridge formed in the late Cretaceous early Paleocene. Below the lava is most likely Indian Precambrian continental crust. The foundation of the Chagos-Laccadive ridge is therefore a rifted sliver of continental crust separated from the west coast shelf margin during India's separation from Africa.

The map below summarizes the setting of the Chagos-Laccadive ridge with respect to the Indian shelf margin. 


Source: Deepwater West Coast India - Pre-Basalt and Other Mesozoic Petroleum Plays: Glyn Roberts et al. 2010

Regarding Andamans.. This island chain are the central part of the Burma-Sunda-Java subduction complex in which an accretionary prism and deep sea turbidite deposits are exposed. This means it is made up of marine sediment and oceanic crust of a subducting slab (oceanic Indian plate) which got scraped off and accreted on to the overriding plate (oceanic South East Asian plate). Sediment and volcanic material and mafic igneous oceanic crust making up Andaman chain may have started appearing above sea level from Eocene times.  Eocene sediment of the Mithakari Group contains detritus derived from earlier Late Cretaceous -Early Eocene ophiolites (slices of oceanic crust). This indicates that slices of ophiolites were thrust up and were exposed above sea level and were being eroded.  Such accretionary prism settings and forearc basins are cannibalistic, in that, the older deposits are emplaced above sea level and become a source of sediment for younger sequences.  Certainly by Pliocene (5 million years to 2.5 million years ago) there would have been a large enough island chain.

So I guess to the best of my knowledge the answer is that the Andamans are older.

One misconception I have encountered regarding Lakshadweep is that the Chagos-Laccadive ridge is a southerly extension of the Aravalli mountain chain.

This is not correct.

As I mentioned above, the basement of the ridge is likely Precambrain continental crust  which rifted apart from the southerly west coast margin of India. So, the continental crust making up the ridge would have been part of the Southern Granulite Terrain and western Dharwar craton of south India. The Aravalli craton and the Southern Granulite Terrain / Dharwar craton were two distinct cratonic blocks which collided and sutured by early -mid Proterozoic times. The Chagos-Laccadive ridge is oriented NNW-SSE parallel to the Indian west coast shelf margin and the Dharwar structural trends.  Post rifting, as the Indian western margin moved over the Renunion hot spot, volcanism covered this basement with lava enhancing the ridge structure. The Chagos-Laccadive-Maldive ridge is a hotspot trail which marks the movement of the Indian plate above the Reunion hotspot.

One can imagine extending in an arcuate line the Aravalli mountain trend south to connect with the Chagos-Laccadive ridge.


Source: from - The Central India Tectonic Zone: A geophysical perspective on continental amalgamation along a Mesoproterozoic suture-  K. Naganjaneyulu and M. Santosh 2010

But these were two different pieces of continental crust in the Archean (Fig). The Aravalli mountains terminate north of the Central Indian Tectonic Zone which is a suture zone between the North Indian and South Indian crustal blocks.

9 comments:

  1. Great article Suvrat. May I Ask which is the oldest river? Indus, ganges or brahmaputra? How do the ages of south indian rivers compare with north indian rivers?

    Thank you, Mohan

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  2. Hi Mohan- thanks...

    Paleo- Ganges, Brahmaputra would have originated as India collided with Asia in early Miocene as a southerly drainage evolved. Tectonic changes may have modified their courses but you can think of these big drainage systems as evolving by Miocene times. see my post on this : http://suvratk.blogspot.in/2016/03/sedimentation-patterns-bay-of-bengal.html

    Indus may be the oldest of the three, since there is evidence that a paleo Indus was debouching sediment in the Katawaz delta in Pakistan (an earlier version of the Indus delta) by late Eocene times. see this : http://webapp1.dlib.indiana.edu/virtual_disk_library/index.cgi/2870166/FID3366/PDF/172.PDF

    South Indian rivers are older, delivering sediment to the eastern Indian margin by Cretaceous times.

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  3. Thanks Suvrat. Is it also true that the western and eastern ghats are much older than the Himalayas and Vindhyas? Have you written a blog on western ghat geology? Also,are the mountains of Sri Lanka a continuation of the Indian ghats?

    Mohan

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    Replies
    1. reg your question on Western Ghat geology blog - these two on the uplift history-

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

      http://suvratk.blogspot.in/2009/02/why-tea-grows-at-higher-elevations-than.html

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  4. As topographic features the Vindhyans are likely the oldest (although there may be some Negene rejuvenation), then the Eastern Ghats (Cretaceous-Eocene?), then the Western Ghats (beginning Eocene) and then the Himalaya (prominent topography beginning Miocene). although some workers say the Western Ghats show rejuvenation in the Neogene, so it has a Miocene and younger uplift component also.

    Sri Lanka has high grade metamorphic terrains which show a similarity with the Southern Granulite province of India. I am not sure if their topography development has anything to do with the Indian ghats.. future post? :)

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  5. Wow what an informative post. I never thought at such levels. Thanks for sharing :)

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  6. thanks for reading Archana :)

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  7. nice article Suvrat, very informative

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