Tuesday, April 30, 2019

Eastern Ghats- The New Kid On The Block

We who live in the Deccan Volcanic Province in and near about the Western Ghats generally look down upon the Eastern Ghats. Call them the poor man's mountains. Point out that the Eastern ranges have a more gentle topographic profile than the Western ranges. We smirk at the lack of spectacular escarpments, narrow gorges and the mesas and pinnacles.

But, when it comes to geology, the Eastern Ghats more than holds its own. In fact, it has a much more complicated and interesting geologic history than the Western Ghats, at least the Deccan Volcanic part of the Western Ghats.

The Deccan Volcanic part of the Western Ghats is an elevated plateau which formed by the piling up of lava 66 million years ago and which since has been dissected by rivers, forming gorges, narrow valleys, and high relief. The edge of this plateau is the Western Ghat escarpment. The Eastern Ghats on the other hand is an ancient orogenic belt which formed by the collision between crustal blocks, resulting in the formation of fold mountains.

The map below shows the broad geology of the Eastern Ghat with the inset showing its location within the Indian continent.


Source: Relative Chronology in High-Grade Crystalline Terrain of the Eastern Ghats, India: New Insights: Samarendra Bhattacharya, Rajib Kar, Amit Kumar Saw, Prasanta Das 2011.

The Eastern Ghats is a Late Archean to Proterozoic age crustal block that has evolved through long and multiple episodes of magmatism, metamorphism and deformation.  It contains rocks ranging in age from 2. 9 billion years to 900 million years old. The rocks have some of the coolest names in petrology; charnockites and enderbites, khondalites, anorthosites and syenites along with granitic rocks and  sedimentary rocks like quartzites. Charnockites (and enderbites) and khondalites are granulite grade metamorphic rocks, i.e. they formed at very high temperatures of around 900-950 deg C by transformation of older igneous and sedimentary rocks respectively. Anorthosite is an igneous rock made up almost entirely of plagioclase feldspar. Syenite is also an igneous rock containing potassium and sodium rich feldspars with no or little quartz.

The interesting part is that the Eastern Ghat block was not part of India when these rocks formed. It may have been an independent block in the Archean (more than 2. 5 billion years ago), but at some point it became part of a larger block that is now the Antarctic continent. This region then underwent magmatism around 1.7-1.6 billion years ago, an episode of granulite metamorphism around 1.6 billion years ago in its southern regions, followed by sedimentary basin formation around 1.3 to 1.2 billion years ago. These sediments were then buried, intruded by magmas like syenites,  and subjected to another episode of granulite grade metamorphism around 1.2 to 1 billion years ago. This last episode of metamorphism and deformation was a result of continental movements and collisions related to the formation of the Rodinia Supercontinent.

When did the Eastern Ghats become part of India? Geologists have timed that event to around 500 million years ago, part of the assembly of Gondwanaland.

How did they figure that out? When the Eastern Ghat terrain collided with India in the Bastar region, it caused the Baster region crust to be buried to great depths resulting in the partial melting of that crust. Radiogenic dating of minerals titanite and zircon, which formed in these new melts, give an age of around 500 million years to this melting event.

I love it when these big ideas are depicted in simple and clean diagrams. Below is a graphic that shows the separation of the Eastern Ghat terrain from its conjugate Antarctica block called the Rayner Complex.


Source: Eastern Ghats Province (India)–Rayner Complex (Antarctica) accretion: Timing the event- Pritam Nasipuri, F. Corfu, and A. Bhattacharya 2018

Two scenarios are shown. The upper panel shows a composite Eastern Ghat Province-Rayner Complex colliding with the Greater Indian landmass around 500 million years ago, followed by a breaking away of the Rayner Complex. The lower panel shows that the Eastern Ghat Province had broken away from the Rayner Complex by 800 million years ago. It then collided with India around 500 million years ago.

The Indian continent was put together by the collision and welding of several smaller continental blocks, namely Dharwar, Aravalli, Bundelkhand, Bastar and Singbhum. This assembly took place between 2 billion and 1 billion years ago.

The Eastern Ghat block was the last to join India. As recent work suggests, as late as 500 million years ago.

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