Did you know that millions of years ago, all the land on this earth was part of a single huge supercontinent? Look carefully at the world map and you will notice that different continents fit together like a jigsaw puzzle. Now you know why.
When the supercontinent 'broke up' – again, millions of years ago – different continents went to different parts of the world, finally reaching where they are today. This process of slow 'drift' of continents had several interesting effects: animals and plants moved around with them, for example.
India had a particularly interesting time. An idea that’s been knocking about for a while is that India (along with Madagascar, Sri Lanka, Australia and Antarctica together called eastern Gondwana) broke off (a very long time ago) from Africa and South America (western Gondwana). After the continents in eastern Gondwana separated and drifted to different parts, the Indian landmass rifted from Madagascar about 90 million years ago and floated across the Indian Ocean to join the Asian landmass (Eurasia continent).
A critical concept for understanding supercontinent geography is ‘suture zones’. The term is borrowed from medicine where sutures are used to join two parts of skin, bone, etc. leaving a line of stitches. These ‘stitches’ seen throughout the earth’s crust in various locations represent areas of juxtaposition of landmasses. However, unlike the body, the landmasses that come together need not be, and are usually not, of the same age, structure and characteristics. Thus suture zones serve as a means of pinpointing the origins of various lands; in fact they are the reason that it was first thought that India stemmed from Africa; specifically sharing history with the island Madagascar. Even though the concept was first put forward in 1979, it is still a matter of debate owing to variations in the experimental findings. One such inconsistency is that they were formed in different eras. This complicates understanding the geographical evolution.
The Mercara suture zone in western peninsular India joins the Dharwar and Coorg blocks of lands. In a recent paper, more evidence for the India-Madagascar correlation has been put forward by linking the Mercara suture zone in southern India with the Bestimisaraka suture zone in Madagascar. The primary clue for this correlation is given by the structural lineaments extracted from various satellite images and digital elevation models of India and Madagascar. Images were digitally processed and different image enhancement techniques have been carried out to extract the structural information.
The research was carried out by subjecting samples of rocks from the lands joined by the suture zone to various analyses. One such technique is to study mineral composition for the presence of compounds like metal oxides (Eg: aluminium oxide and iron oxide). These samples were also tested for the presence of elements like calcium and manganese. The result was that the samples taken from either side of the suture had different elemental compositions. Another concept used in the study was to study the pressure and temperature (P-T) conditions under which the rocks (ergo the suture) were formed. This is done by studying the mineral reactions in thin sections using special microscopes and analysing the mineral compositions. These results stimulate the P-T conditions needed for rock formation and study the mineral compositions. The timing of suturing can be determined by various radiometric dating methods. This is done by radioactive dating of minerals like zircon, monazite within the suture. The time of suturing is an indication of the amalgamation of the landmasses and it falls in line with the proposed hypothesis of the Dhawar block joining with the Coorg block.
Furthermore, the P-T conditions also indicate that the areas around the sutures were subjected to tectonic movements akin to two landmasses coming together and then being sutured together. This final clue helps piece together the puzzle in better understanding our country’s geographical history.
Author information:This study involved people from Indian Institute of Science, University of Adelaide and Curtin University, Australia, University of Tsukuba and Okayama University of Science, Japan, China University of Geosciences, University of Johannesburg, South Africa, and the University of Leicester, UK.
The lead author, Ishwar-Kumar and the senior author, K Sanjeev are from the Indian Institute of Science.
About the publication:The paper, Mesoproterozoic suturing of Archean crustal blocks in western peninsular India: Implications for India–Madagascar correlations', appeared in Lithos.
Link to the paper: http://dx.doi.org/10.1016/j.lithos.2016.01.016