The Palghat (Palakkad) Gap is the most prominent mountain pass in southern India, historically serving as a trade route for early merchants from Rome and Arabia to the eastern part of India. The east-west orientation of the geomorphic features, including that of the Bharathapuzha river, generally aligns with the trend of this feature. This gap is a significant influencer of weather patterns in southern India, as it provides a conduit for the passage of moisture-laden southwest monsoon winds into Tamil Nadu and the Deccan, and it moderates the summer season in Tamil Nadu by redirecting hot winds from there to eastern Kerala.

Stretching from Coimbatore in the state of Tamil Nadu to Palghat in Kerala, this physiographic break across the Western Ghats, with a width of 24-30 km at an elevation of 140 m, is part of a highly deformed segment of the South Indian crust – most likely a product of ancient tectonic processes, where crustal plates interacted, sutured, or slid past each other. Like the ongoing interactions along the northern boundary of the Indian plate with the Eurasian plate raising the Himalayas, such plate interactions have occurred in the geological past. In many studies, the Palghat Gap is described as a remnant marker of plate interactions between 2.5 and one billion years ago or slightly later. The debate continues whether this event occurred during the Late Archean or Late Proterozoic time intervals. Although some researchers have suggested that an asteroid impact, approximately 1.5 billion years old, created a crater around a hundred kilometres wide as a possible reason for the origin of the Gap, this notion has not yet gained significant traction.

Tectonic legacy

The rock records demonstrate that differential horizontal movements of continental and oceanic plates, which shaped the Earth’s surface, became a reality at least three billion years ago, thanks to the Earth’s internal “heat engine” that drives these processes. When the continental crusts converge, the compression allows land to be accreted at their interface to form mountains. India, once a collage of crustal blocks, preserves the geological evidence of the crustal-scale suture zones where these blocks merge. The studies provide further evidence that north-south-verging compressional tectonics have resulted in the closure of an ocean. This closure is attributed to the descent of the Southern Indian block, which carried the oceanic crust beneath the much older Dharwar craton, around 2.5 billion years ago, along a major continental-scale fault zone known as the Palghat-Cauvery Suture Zone (PCSZ) in the geological literature. The Palghat Gap is situated within this suture zone.

Another theory interprets the PCSZ as a remnant of a Late Neoproterozoic continental suture (1 billion to 540 million years ago) that led to the closure of the then-existing Mozambique Ocean during the final phase of the merger of the eastern and western Gondwana continental blocks, forming a supercontinent. This supercontinent occupied a vast area extending from near the South Pole to near the equator, representing the largest expanse of continental land that ever existed from 800 million years to 650 million years ago. The Indian fragment, part of this super-continental mass, was fused with Madagascar. These merger processes resulted in a massive mountain chain — an event referred to in the literature as the East African Orogeny. Involved in the closure of an ocean and the uplift of a mountain, some geologists believe that the Palghat-Cauvery shear zone was part of a significant transcontinental suture within the Proterozoic plate configuration, tracing back to Madagascar.

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The gap morphology

How did the Palghat Gap with its peculiar morphology of a ‘break’ within a mountainous area come about? The geophysical studies reveal that the crustal thickness under the Palghat region is 36 to 38 km, compared with over 50 km beneath the Nilgiris and Palani-Cardamom hills. Decoupling processes beneath the Palghat Gap must have caused this crustal mass depletion. The compressed and warped continental crust around the Palghat Gap, initially four to five kilometres thicker than that of the Dharwar craton, would have buckled and fractured along the zone of sudden variation in crustal thickness, leading to land subsidence and drop in elevation. The erosion by ancient rivers must have chiselled the morphology further. Even today, erosion is active as the Gap hosts the west-flowing Bharathapuzha and east-flowing Cauvery rivers.

The southern Indian crustal parts are also termed as Southern Granulitic Terrain (SGT) by geologists because much of the area is now covered with “granulite rock”. These are lower crustal rocks metamorphosed under intense heat and pressure. These low crustal level rocks are isostatically exhumed from depths in response to the continued erosion of the upper crust. The processes related to isostasy are less talked about, although they cause major landform changes in many parts of the globe. Some parts of the Gap also have mineable quantities of marble derived from limestone. Most limestones, primarily composed of calcium carbonate formed from the accumulation of marine organisms. The presence of limestone provides a valid clue to the presence of an ocean in the region during Late Archean/Proterozoic times and its eventual closure.

Low-moderate seismicity has been detected within the Palghat Gap. A magnitude 4.3 earthquake on December 2, 1994 near Wadakkancheri in Thrissur district, Kerala, was located within the confines of this feature. A few smaller tremors have been reported previously from the source region of the 1994 earthquake. Historical records show the occurrence of earthquakes near Coimbatore, on the northern extremity of the gap. The magnitude of the event that occurred on February 8, 1900, near the Palakkad-Coimbatore border was close to 5.5. Thus, regional seismicity can be attributed to active structures and enhanced stress concentration within the Palghat Gap.

Gap as a faunal boundary line

Early explorers used to wonder how geologically formed barriers could divide habitats. The answer emerged from the work of the British naturalist Alfred Russel Wallace, a contemporary of Charles Darwin, who delineated a faunal boundary in the 1850s, separating the biogeographic realms of Asia and Australia. Even during the Pleistocene, the last major glaciation period that lasted from 2.58 million to 11,700 years ago, when sea levels were as much as 120 metres lower, the landmasses of Asia and Australia along with the neighbouring island arcs were separated by deep-sea channels.

Apart from ancient sea level variations, the complex biogeography of Australasia is also a result of the region’s tectonic evolution over the past 50 million years, characterised by the merger and separation of several microplates. The Wallace Line was a unique example of discovery to support the notion that subtle geographic changes can generate significant species endemism in the animal world. Russel utilised these observations when he independently wrote about natural selection and the theory of evolution in the 1800s, separate from Charles Darwin. Globally, geographic barriers are an important promoter of species endemicity, and such areas can become laboratories of evolution. Geological processes play a crucial role in forming geographical barriers.

The Palghat Gap has a long tectonic history exhibiting certain milder characteristics of a biogeographical boundary. For instance, the genetic analysis of Nilgiri tahr populations in the Western Ghats reveals the presence of two diverged groups to the north and south of the Palghat Gap. The differing paleoclimatic conditions, critical during the Pleistocene ice age, may have resulted in population diversification. Similar studies of the elephant populations in the region have also shown genetic differentiation between the south and north of the Palghat Gap, which must have acted as a barrier to gene flow. A similar divergence of montane bird populations has been observed in the Western Ghats, with the Palghat Gap acting as a major biogeographic barrier. This study also suggests that enhanced divergence during the Pleistocene Glacial Period was characterised by intense fluctuations between dry and wet periods, impacting wet evergreen habitats. Hidden in its rock folds, the Palghat Gap holds back its origin story, challenging the geologists to unravel its age-old mysteries.

The author is an adjunct professor at the National Institute of Advanced Studies, Bengaluru, and the director of the Consortium for Sustainable Development, Connecticut, U.S.A.

Citation:

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Banner image: A train passes through the Palghat gap. Image by Robtheequalizer via Wikimedia Commons (CC BY-SA 4.0).

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Earth ScienceWestern GhatsKerala

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