Curious journey of the Indian subcontinent

Around 500 million years ago, during the late Ediacaran period, our planet only housed one supercontinent called, Gondwanaland. This continent eventually split up into landmasses we now recognise as Africa, Australia, South America, Antarctica, the Indian subcontinent and the Arabian Peninsula. The Indian subcontinent then slowly drifted north as an isolated land mass until it reached its current position around 55 million years ago.

The subsequent collision of the isolated landmass with the Eurasian subcontinent led to the formation of the spectacular Himalayan-Tibetan system. The strongest evidence for this comes from paleo-magnetic records. Flood basalts deposited on continental rift and sedimentary packets are known to preserve information about the paleo-magnetic pole, the magnetic pole of the basalt during its formation millions of years ago.

And on studying these basalts, the latitudinal position of the Indian subcontinent during its transit could be determined. Although the migration of the Indian plate is widely accepted based on geophysical, biological and geochemical signatures, large discrepancies still exist in its exact position during the transit and the rate of migration. A collaborative effort from researchers from around the globe was required to identify the exact path taken by the Indian subcontinent.

Now, a team of researchers from the Centre for Earth Sciences and Divecha Centre for Climate Change at Indian Institute of Science, Bengaluru (IISc), Geological Studies Unit at Indian Statistical Institute, Kolkata, Department of Earth and Planetary Sciences at Tokyo Institute of Technology, Japan and the Department of Earth and Environmental Sciences at the University of Michigan, USA, have come together to track the migration of the Indian plate.

Answers in soil carbonates

The focus of the study was to use temperature and oxygen isotope of carbonates deposited in successive layers of sediments as a geochemical indicator of the latitudinal position of the landmass. According to Prosenjit Ghosh, an associate professor at the Centre for Atmospheric and Oceanic Sciences, Bengaluru, “Our group has carried out a study to identify the sensitivity of isotopic composition of soil carbonates to ascertain position of Indian plate through time. The novel geochemical tool known as a clumped isotope palaeothermometer is used for this purpose.”

The Satpura Range found in central India has an especially well-preserved record of soil carbonates. The range rises in the eastern state of Gujarat running along the borders of Maharashtra and Madhya Pradesh and ends in Chhattisgarh. The range also houses some of the major rivers of India including the Narmada, Tapti, Godavari and Mahanadi.

Samples of soil carbonates deposited in sequences of sediments from the present day Narmada river valley were collected and analysed. These carbonates are known to capture the isotopic signature of ancient soil water. And since these carbonates were precipitated during the Permian to Cretaceous periods, which coincides with the movement of the Indian subcontinent to the north, they could be used to determine the latitudinal position of the subcontinent.

“The analytical results yielded information about temperature and meteoric water composition at the time of carbonate formation for the time period between 270 and 65 million years back. The well-preserved geological archives served as book chapters. On investigation, we were able to trace out the air temperature and oxygen isotope compositions for the period between 270 million years and about 68 million years ago,” explains Prosenjit. And since the variation of the isotopic composition of rainwater is related to the latitudinal position, the team was, with some uncertainty, able to track the path taken by the Indian plate.

Apart from finding the path of the subcontinent, the team also made other unintended discoveries. For instance, at studying the results, the team was able to verify that global scale hydrological cycles, which is the movement of water as it evaporates and then rains down, was not different during the climates of the late Paleozoic and Mesozoic periods. Based on the results, the team was also able to estimate the rate of migration of the subcontinent. The researchers estimate the rate of migration to be around 51mm/year between 269 million years and 243 million years ago and subsequently dropping to 10mm/year during the period between 243 million years ago and 135 million years ago.

The new method not only performs as a new proxy for determining the latitudinal positions of plates but can also be used to find other features. “Our finding also implies that the global-scale hydrological cycle was not fundamentally different during these time scales. The new approach also revealed that the rate of movement of the Indian plate varied considerably during the entire journey,” explains Prosenjit.

Currently, we can track the movement of the tectonic plates with great accuracy using global positioning systems (GPS). But decoding the past movements of these plates has been a challenging task. The current study aims to enable researchers to not only track these past movements but also analyse other features of the earth from times gone by.

(The author is with Gubbi Labs, a Bengaluru-based research collective)