The Deccan Traps, India (Part 1): The story of its genesis

Mugdha Chimote (India)

Introduction

Sandwiched between the Arabian Sea to the west and the vast Indian subcontinent to the east, the Western Ghats, a haven for trekkers and travellers, are a 1,600km long range of mountains along western edge of Deccan Plateau. Also known as “Great Escarpments of India”, the range extends from Gujarat in the north to Kanyakumari in the south.

The Ghats traverse the states of Maharashtra, Karnataka, Kerala, and Tamil Nadu. They comprise of more than 39 wildlife sanctuaries, reserve forests and national parks. The Western Ghats were declared as one of the eight hottest ecological hotspots in the world in the year 1988, as the area is home to nearly 325 globally threatened floral, amphibian, fish, bird and faunal species. Famous hill stations such as Mahabaleshwar, Panchgani, Munnar, Wayanad, Coorg and Ooty are among some of the perfect weekend getaways and popular tourist attractions here. Thanks to the rich ecological reserve and tourist attractions, Western Ghats were awarded the title of UNESCO World Heritage Site in 2012. The Western Ghats however cover just a small portion of Deccan Traps (see also box: Kaas Pathar below).

Deccan is an anglicised word derived from the Sanskrit word “Dakshin”, meaning south (the region is located in the southern part of Indian subcontinent). “Traps” mean step-like features. Thus, the Deccan Traps are step-like volcanic features found mainly in Southern India. The laterally extensive basaltic lava flows of the Deccan Volcanic Province (DVP) occur between latitudes 16° to 24°N and longitudes 70° to 77°E. The lava flows spread over western, central, south western, north western and southern parts of the Indian peninsula, covering a total exposed area of 500,000km². It is estimated that the total exposure prior to erosion (including the concealed area beneath the Arabian Sea) is of the order of 15 million square kilometres. The approximate thickness of the traps is about 3,000m.

Flood Basalts are thick successions of basaltic flows erupted on continental regions over short time spans (about one to three million years), such as the Columbia flood basalts, the Siberian traps and the Deccan Traps of India, among others. Today, the DVP holds the title of being the most extensively studied yet poorly understood Continental Flood Basalt (CFB). For decades, geologists all over the world have been trying to decipher the mystery of DVP emplacement. Here is a quick read on theories of the genesis of the Deccan Traps, their inadequacies and possible alternative models.

The mantle plume model for the emplacement of the DVP

Eastern Gondwana witnessed two prominent volcanic episodes during the Cretaceous period: the Bengal-Sylhet-Rajmahal flood basalt volcanism at 118 to 115Ma during the Early Cretaceous; and the Deccan Volcanism at 67 to 60Ma during the Late Cretaceous. Of these, the DVP is the largest and most outstanding of the Phanerozoic volcanisms of India. It is believed that the process of supercontinent disintegration was triggered by mantle plumes and hotspots, which in turn led to the formation of Large Igneous Provinces (LIPs) and CFBs. The DVP is believed to have formed during the breakup of Gondwana as part of Seychelles-India separation event. Due to the interaction of northward drifting Indian plate with the Reunion mantle plume, the DVP manifested mainly in the western parts of Indian subcontinent.

As the Indian plate migrated northward from Antarctica to Asia, it rode over the Reunion hotspot, located at 21°S latitude near Mauritius (with a diameter of approximately 2,000km). The separation of the Seychelles bank from western India led to the formation of the Arabian Sea. Currently, this mantle plume model, which was postulated by Mahoney in 1988, is the most widely accepted theory for the formation of the DVP.

The following steps summarise how Mahoney’s model is hypothesised to work:

1. A plume with a large, bulbous head followed by a narrow tail was the site of considerable melting and entrainment of surrounding mantle, which led to an initial outburst of high thermal and volcanic activity when the plume first reached the surface.
2. The DVP originated from the head of the Reunion plume during its early eruptive phase at the end of Cretaceous and recorded its first establishment on the Indian subcontinent. The extremely high magmatic outpour during this period suggests that the hotspot activity was highly vigorous. Its weak remnant at Reunion Island in the Indian Ocean is currently fed by a narrow tail rising from core-mantle boundary.
3. Extensions of the Deccan volcanism are today found in the Arabian Sea and further west and southwest up to the Chagos-Maldive-Lakshadweep-Mascarene-Carlsberg hotspot volcanic chain. The oceanic track defined by this ridge-rift system represents the post Deccan interaction of plume with newly formed oceanic lithosphere.

With advancements in geophysical, geomagnetic and geochemical studies in the last three decades, a lot of research has gone into understanding the processes of the formation of the DVP. Based on these datasets, researchers like Hetu Sheth have pointed out a few gaps in the “classic” mantle plume model of DVP:

• There is little petrographic evidence that the Deccan source was abnormally hot, and the short (about 1.0 to 0.5Ma) duration of eruption contradicts the recent Ar-Ar age data that suggests that the total duration was around at least eight million years.
• There is little geophysical data from the Deccan traps available in support of the plume model and weaken its credibility altogether.

Alternative theories for DVP formation

Following Sheth’s scepticism regarding the classic mantle plume model, many alternative theories were provided. One such was proposed by Sethna in 2016. He put forward the possibility that an interplay of intersecting rift zones may be responsible for emplacement of the DVP and it may have evolved in three stages:

Stage 1: The Indian subcontinent moved over the Reunion plume in the zone of the Narmada fault, near the Amba Dongar area, when the Satpura rift was activated to initiate the beginning of volcanic eruptions that mainly filled up the rift zone of the Narmada-Tapi valley.

Stage 2: This stage represented major outpours of basaltic flows from south of Tapi Valley, around Nashik-Igatpuri, to form a major part of a shield volcano. Eighty percent of the DVP was emplaced during this stage. Stages 1 and 2 took place in the late Cretaceous to Early Tertiary period, between 67 to 65Ma.

Stage 3: This stage represented rifting of the DVP along the present day, west coast of India with the formation of the Panvel flexure, due to subsidence that occurred because of stretching and thinning of crust and the consequent intrusion of underplating basalt. Stage 3 took place between 65 to 60Ma.

Palaeomagnetic studies have provided convincing evidences for this rifting theory. However, for it to be accepted as an established model, it will require geochemical, geophysical, and geomorphological backing.

Did Deccan volcanism together withthe Chicxulub impact trigger the K/T boundary mass extinction?

Geology is a science where, every now and then, new theories are proposed, which may not have compelling evidences to prove them but not enough to refute them. One such theory is the relation of CFBs with mass extinctions.

The K/T boundary mass extinction has been correlated with the Chicxulub asteroid impact at about 65Ma. Evidence of similar such impacts have been discovered, correlating with times of other extinction events, but there is considerable debate as to whether they actually caused the extinction on their own. Interestingly, Deccan volcanism was active even before the Chicxulub event. It is thus quite likely that the asteroid impact was not the main or only cause of the DVP but may have acted as the triggering factor for outpour of huge amounts of magma to the surface by increasing the permeability of the mantle underneath.

Table 1 compares the ages of major LIPs with the estimated ages of major mass extinctions.

Flood Basalt Episode	Age (Mya)	Stratigraphic boundary	Age (Mya)
Columbia River	16	Early/Mid Miocene	16.4
Ethiopia	31	Early/Mid Oligocene	30
North Atlantic	57	Palaeocene/Eocene	54.8
Deccan	66	Cretaceous/Tertiary	65
Madagascar	88	Cenomanian/Turorian	93.5
Rajmahal	116	Aptian/Albian	112.2
Etendeka	132	Jurassic/Cretaceous	142
Antarctica	176	Aalenian/Bajocian	176.5
Karoo	183	Early/Middle Jurassic	180.1
Newark	201	Triassic/Jurassic	205.7
Siberia	249	Permian/Triassic	248.2

The Chicxulub impact triggered volcanic events throughout the globe. Researchers from UC Berkeley claim that the asteroid impact and DVP eruption are “uncomfortably close” events. They even argue with the original notion that the dinosaur extinction at K/T boundary was a result of asteroid impact.

The missing piece of puzzle between flood basalts and mass extinctions can be found by studying the environmental impacts of flood basalts. Many gases, such as CO₂and SO₂ and sulphuric acid, with dissolved sulphur are released during basaltic volcanism. The environmental impacts of these gaseous releases include acid rain, injection of poisonous gases into stratosphere, and changes in oceanic chemistry, oxygenation and circulation.

This story, correlating the DVP with mass extinction and asteroid impact, is quite interesting. In fact, it has been suggested that for certain mass extinctions to occur, both massive volcanism and asteroid impacts are necessary. However, until further researches provide compelling evidences, it remains as a mere plausible theory.

Kaas Pathar

Kaas Pathar or Kaas Plateau (the Plateau of Flowers; Fig. 5), located near Mahabaleshwar in Maharashtra is a Reserved Forest (a part of the same UNESCO World Heritage Sites) situated in Sahayadri sub-cluster of the Western Ghats. It is a biodiversity hotspot for seasonal wild flowers and endemic species of butterflies. The plateau has more than 850 species of flowering plants, with 39 unique to Kaas region.

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This is the first of four articles on the Deccan Traps of India. The others consist of:
The Deccan Traps, India (Part 1): The story of its genesis
The Deccan Traps (Part 2): Stratigraphy and geomorphology
The Deccan Traps, India (Part 3): Evidence of recent tectonic activities in Deccan basalts
The Deccan Traps, India (Part 4): Quaternary sediments of the Godavari River basin, Maharashtra

About the author

Mugdha Chimote is a budding Geologist and an avid trekker. As part of her master’s thesis, she carried out extensive research and published manuscripts on Quaternary Geological studies around Godavari River in India. The research received special commendation from senior geologists in India as it refuted decade’s old beliefs regarding neo-tectonics in Deccan Traps.

In the academic and professional capacity, she has carried out numerous geological excursions in India in the states of Maharashtra, Karnataka, and Rajasthan. For the past four years, she has been volunteering with an NGO that aims towards promoting geosciences to common man and creating awareness about the subject, especially in the young minds. If you do not find her wandering not-so-touristy regions with a geological hammer in one hand, she is probably out capturing the mountains.

References

Devey C. W., Lightfoot P. C. (1986). “Volcanological and tectonic control of stratigraphy and structure in the western Deccan traps.” Bulletin of Volcanology, vol. 48, pp. 195-207.

Ranjan S., Tiwary A., Pandey D. “The Deccan Volcanic Province: Thoughts about its genesis.” www.MantlePlumes.org (accessed: October 2020).

Sethna S. F. (2004). “Probable Evolution and Growth of the Deccan Volcanic Province, India.” Indian Journal of Geochemistry, vol. 19, pp. 9-18.

Sheth H. C. “The Deccan Beyond Plume Hypothesis”, www.MantlePlumes.org (accessed: October 2020).