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.

Tuesday, May 17, 2016

Plate Tectonics And Coral Reef Distribution And Diversity

Plate tectonics drive tropical reef biodiversity dynamics- Fabien Leprieur, Patrice Descombes, Theo Gaboriau, Peter F. Cowman, Valeriano Parravicini, Michel Kulbicki, Carlos J. Melian, Charles N. de Santana, Christian Heine, David Mouillot, David R. Bellwood & Loıc Pellissier

Interesting work!

Tropical coral reefs require sunlit shallow sea water to thrive. This is provided by coastlines and continental shelf areas. Plate motions has shifted continents. The geography of shallow sea habitats in which corals thrive has accordingly shifted. This study reconstructs this dynamic over the past 140 million years of the breakup of Gondwanaland and the movement of continents since. It is summarized beautifully in this graphic.

Source: Leprieur et al 2016

The optimum locations of coral habitats moved as the configuration of the Tethyan ocean changed. The fossil coral record shows that coral diversity was maximum in the Western Tethys in the Eocene (55 -33 ma), then shifted to the Arabian Peninsula and Western Indian Ocean during the Late Eocene - Oligocene (37-15 ma). Finally as the Tethys Ocean closed due to the India Asia collision, coral biodiversity hotsposts shifted since the middle Miocene to the Indo- Australian Archipelago.

There is a further step. The study models species diversification based on the distribution of estimated paleo-bathymetry and the residence time of habitat (both controlled by plate tectonics). The reasoning is that the frequency of evolution of new species (biodiversity) depends on the distribution of optimal habitat and how long these habitats remain in place. Large long lasting shallow continental shelves will offer an opportunity for populations of ancestral species to disperse over broad areas and diverge into new species. Tectonically complex areas like western Tethys (in the Eocene) and Indo-Australian Archipelogo in Pliocene-Quaternary have sea floor topography and shelf configurations which fragments habitats based on changes in water depth, current and wave strength and direction and nutrient availability. Populations get split and isolated in such regions leading to genetic divergence and speciation. This is mimicked well by the simulations.  Over time, rich diversity can evolve in such long lasting optimas by the process of dispersal and isolation. The locus of evolution of biodiversity can then shift as plate motions move ideal habitats across the globe. New species can arise from parapatric speciation (adjacent to the range of the ancestor) or sympatric speciation (within the range of the ancestor). The study finds that for either modes of speciation, their simulation matches the observed distribution of fossil and extant coral diversity.

The graphic below captures the comparison between fossil coral diversity and simulated coral diversity. Eocene, Miocene and Quaternary observed diversity (left side)  matches the model results (right side).

Source: Leprieur et al 2016

The scientists caution that local fluctuations like sea water temperature and acidity and ecological complexity will also play a role in evolution of  diversity and call for further validation of their work. But overall, its a nice demonstration of the role of plate tectonics in controlling coral habitats and diversity.

Thursday, May 12, 2016

Papers- Precambrian Crustal Evolution Of Peninsular India

Over the last year or so I've collected quite a few papers on the subject of Precambrian crustal evolution and sedimentary basins of Peninsular India. I'm sharing the list with links. Many of them are available open access from various outlets.

I got my teeth cut in field geology and carbonate sedimentology during my M.S. thesis research. This involved mapping a small area of the Mesoproterozic Cuddapah basin in South India.  Peninsular India is a vast repository of these Precambrian rocks. These papers provide very interesting perspectives on various aspects of their geology. Perhaps the big leap in the past decade or so has been -finally!- the increased availability of accurate geochronology that has made it possible for geologists to start piecing together their complex polyphase history.

1) Precambrian Crustal Evolution of Peninsular India: A 3 billion year odyssey ;  Joseph G. Meert,, Manoj K. Pandit, Vimal R. Pradhan, Jonathan Banks, Robert Sirianni, Misty Stroud,Brittany Newstead, Jennifer Gifford

2) Proterozoic orogenic belts and rifting of Indian cratons: Geophysical constraints ; D.C. Mishraa, M. Ravi Kumar

3) The Archean and Proterozoic History of Peninsular India: Tectonic Framework for Precambrian Sedimentary Basins in India; Joseph G. Meert and Manoj K. Pandit

4) Precambrian Basins of India: Stratigraphic and Tectonic Context: Rajat Majumdar and Patrick G. Eriksson eds.- Collection of Paper from the Lyell Collection Geological Society of London Memoirs : Behind Paywall

5) An overview of the Palaeoproterozoic geology of Peninsular India,and key stratigraphic and tectonic issues: Dilip Saha and Rajat Mazumdar

6) Morphodiversity, complexity and macroevolution: Revealed by the megascopic life of the Palaeo-Neoproterozoic Vindhyan Supergroup, India: Poornima Shrivastava

7) Stratigraphy and correlation of the Neoproterozoic deposits of central and western India: An overview: S. Kumar

8) The Central India Tectonic Zone: Geophysical perspective on continental amalgamation along a Mesoproterozoic suture:  K. Naganjaneyulu and M. Santosh

Happy reading!

Sunday, May 8, 2016

Review- Human Dispersals Out Of Africa

Human Dispersal Out of Africa: A Lasting Debate -Saioa López, Lucy van Dorp and Garrett Hellenthal

Here is a good review of the fossil, archaeological and genetic data that has spawned various theories of how anatomically modern humans dispersed out of Africa and colonized the world.

The graphic below summarizes the dispersal scenarios-

Source: Lopez et al. 2016

One big lacunae is the lack of a Pleistocene skeletal record in the Indian subcontinent.

Tuesday, April 26, 2016

Maniraptoran Dinosaurs Show No Decline In Disparity Before Mass Extinction

Its hard to unravel and unpack complex phenomenon like patterns of faunal turnover during mass  extinctions. The methods chosen, the materials (fossils) available for study and the granularity of the study influences the results.

My last post was about a modeling study that concluded that for 40 million years before the mass extinction,  extinction rates exceeded the evolution of new species in many dinosaur lineages. Dinosaur diversity was declining towards the end of  the Cretaceous. Only a few herbivorous dinosaurs showed an increase in diversity.

Now a different study by Derek W. Larson, Caleb M. Brown , David C. Evans published in Current Biology focuses on just one sub group  of dinosaurs and comes to a different conclusion. This is an analysis of over 3000 teeth  of bird-like maniraptoran dinosaurs from 18 stratigraphic units in Western N. America which shows that there was no decline in disparity in different maniraptoran lineages for the last 18 million years before the mass extinction. The maniraptors were largely omnivorous but may have included specialized carnivorous and as well as herbivorous species.

Disparity is a measure of the range of variation of morphology, in this case teeth shape. This in turn is something of a proxy for the range of ecologic niches being exploited.

The authors remark-

Overall, these inferred ecomorphological patterns indicate that toothed maniraptorans, including toothed birds, were ecologically diverse and stable leading up to the end-Cretaceous mass extinction, at least in North America.

An interesting angle explored  in the paper is why toothed bird-like maniraptors and some lineages of toothed birds died out, while their physiologically and morphologically close relatives, the ancestors of modern birds, the Neornithes,  survived the mass extinction.

In the fallout of the bolide impact that marks the end of the Cretaceous, terrestrial food webs that relied on photosynthesis would have collapsed. However, seed banks derived from plants, including relatively abundant angiosperms, could have been a common, nutrient-rich resource that would have persisted among the detritus, which itself has been suggested as a key food source related to species survival across the boundary. With clearing of vegetation, either during a global firestorm or widespread burning of dead ground cover, exposed seeds in these areas worldwide could have been exploited by granivores. This pattern is observed in modern fire succession communities, where granivorous birds are the first avians to re-occupy disturbed habitats due to food resource accessibility. A persistent seed bank, which can remain viable for more than 50 years in modern temperate forests, most likely outlasted the catastrophic ecosystem collapse caused by a bolide impact and associated infrared thermal pulse, acid rain, darkness, and impact winter. Toothed maniraptorans, with their feeding systems adapted to faunivorous or potentially folivorous diets, would have been restricted to food chains dependent on living plant matter and unable to access these seed banks. Therefore, dietary specialization toward granivory in some lineages of crown group birds may have been one of the key factors in their survival through the end-Cretaceous mass extinction.

Although very similar to maniraptors, the Neornithes had evolved a unique dietary specialization. The presence of a beak, which served as a crushing apparatus, may have helped these ancestors of modern birds exploit seeds as a food source.