Monday, May 30, 2016

Map: Thickness Of The Crust

A ten km contour interval? Well yes, if  you are mapping the thickness of the earth's crust!

This map brings out beautifully the distribution of the two distinct types of crust on earth. Crust making up the continents is granitic to andesitic in composition, buoyant and is old. Crust making up the ocean basins is mafic in composition, gravitationally unstable (it is heavier and it subducts) and is young.

The 30 km contour outlines roughly the continental crust:

I got this from - The Continental Record and the Generation of Continental Crust (open access)

How does the earth look in terms of its topography? There is a bimodal distribution of the surface elevations on earth. This is a consequence of the contrasting chemical-mechanical properties of the continental and oceanic crust. The figure below brings out the distribution.

Why is there some continent  below sea level? Plate tectonic configuration can be such that at different coastlines continents could be in the act of converging with an oceanic plate. Or, having long broken away from another continent, they posses a passive or divergent margin. At such passive margins, continental crust does not end at the present day coastline, but extends further out until the edge of the continental shelf where there is a sudden deepening of the sea floor. That is roughly where the ancient continent broke up. As it drifted away, new oceanic crust formed between its conjugate continent on the opposite side. Sea level rise after the last glaciation has flooded continents, thereby submerging portions of these passive margin low gradient shelves.

Source: The Continental Record and the Generation of Continental Crust.

Sometimes, a single map or a graph can bring out a fundamental truth about the making of the earth.

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 reefs, lagoons and sparkling shell sand beaches, is Holocene in age (past 12 thousand  years). These coral communities rest on earlier Pleistocene reefs. So, the history of reefs and atolls is a Quaternary Period phenomenon going back several hundred thousand years. Periodic polar ice cap growth and melting drove sea level fluctuations, resulting in  episodic shallow seas and vertical coral growth and reef building. Below these Pleistocene and Holocene corals lie earlier Cenozoic carbonate sediments (Source 1, 2 ) . These sediments were deposited in a subtidal (deeper water) marine setting.  We are not sure whether sea level drops during deposition of these earlier carbonate sequences exposed the sea floor. It is possible that during this long Cenozoic history, there may have been episodic exposure of the carbonate platform. 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.

And what lies below? All this Eocene to Pleistocene  (56 million to 2.5 million years) sediment sequence has been deposited on top of a Palaeocene-Eocene  (66 million to 56 million years) volcanic basement. This basement is the northern part of the Chagos-Laccadive ridge, formed when the Indian plate rode over a hot area of the mantle known as the Reunion hotspot. Below the lava is 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 the islands are made up of marine sediment and oceanic crust of a subducting slab (oceanic Indian plate) which got scraped off and plastered on to the overriding plate (oceanic South East Asian plate).

A tectonic cross section of the Andaman subduction complex is shown below.

 Source:  Mud volcanoes show gas hydrates potential in India's Andaman Islands-  Vignesh Ayyadurai et. al. 2015

Sediment and volcanic material and mafic igneous oceanic crust making up the 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 ophiolite blocks 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. As tectonic plates continue to push against each other, these younger sequences in turn are moved upwards along thrust faults and become exposed above sea level. Certainly, by Pliocene times (5 million years to 2.5 million years ago), there would have been a large enough island chain.

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 (craton- earliest formed pieces of continental crust going back more than 3 billion years ago) 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 (2.5 billion to 1 billion years ago). 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:  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. In the above figure the Dharwar and the Bhandara Cratons form a South Indian crustal block, while the Bundelkhand and Aravalli Fold Belt form the North Indian crustal block.  The Aravalli mountains terminate north of the Central Indian Tectonic Zone (shown by roughly east-west trending fault lines). This is the 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 Srivastava

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.