Wednesday, March 8, 2017

Papers: Tectonics And Physical Volcanology Of Deccan Traps

There are plenty of research papers on the geochemistry of the Deccan Basalts. But nature lovers and trekkers like me come face to face not with chemistry but with the physical forms of lava and the structural elements of the volcanic pile.

I found this list of papers most useful. They have helped me sort out my confusions regarding lava morphology and taught me something about the structural fabric of the western margin of the Deccan Volcanic Province.

1) Near N–S paleo‑extension in the western Deccan region, India: Does it link strike‑slip tectonics with India–Seychelles rifting? - Achyuta Ayan Misra Gourab Bhattacharya, Soumyajit Mukherjee, Narayan Bose

This is a structural analysis of the fracture systems that cut across the western margin of the Deccan province.  The area of study is the coastal plains, about 100 km north and south of Mumbai. The Indian western margin is a rifted margin i.e. it formed by the breakup of India with Madagascar (88 million years ago) and then Seychelles (64 million years ago). This type of margin is formed by tensional forces splitting apart continents and so you would expect normal faults, wherein blocks of crust have moved down along inclined fault planes.  Except here, the researchers find evidence of strike slip movement along sub-vertical fault planes. This means crustal blocks slid past each other. This implies oblique rifting with components of both extension and transverse movement between India and Seychelles. There are some really revealing field photos of this transverse (strike slip) movements.

2) Geology of the Elephanta Island fault zone, western Indian rifted margin, and its significance for understanding the Panvel flexure- Hrishikesh Samant, Ashwin Pundalik, Joseph D’souza, Hetu Sheth, Keegan Carmo, LoboKyle D’souza, Vanit Patel

Wait a minute. There are normal faults with downthrown blocks in this region too. And from the famous Elephanta Island. The fault planes dip eastwards producing easterly downthrows. That means the easterly crustal block has moved down. Again, some good field photos of fault planes and slickensides ( fault surfaces which get a polished striated appearance due to the frictional movement of rocks). These faults with easterly downthrows are found all along the west coast.  There is one near the proposed site of the nuclear power plant at Jaitapur in southern Maharashtra, which shows signs of intermittent movement over the past fifty thousand years. So, there is a very practical reason for understanding these faults.

3) Deccan Plateau Uplift: insights from parts of Western Uplands, Maharashtra, India- Vivek. S Kale, Gauri Dole, Devdutta Upasani and Shilpa Patil Pillai

This is a study of part of the Deccan plateau. I visited this region a few weeks back.  Very useful information of the various fracture systems that cut across the stacks of lava and their significance in terms of recent (Quaternary) crustal movements and controls on the drainage systems. Well thought out block diagrams illustrate the authors ideas very clearly.

4) Pahoehoe–a'a transitions in the lava flow fields of the western Deccan Traps, India-implications for emplacement dynamics, flood basalt architecture and volcanic stratigraphy-  Raymond A. Duraiswami, Purva Gadpallu, Tahira N. Shaikh, Neha Cardin

Good explanations of the morphology of basalt lava flows.  I really liked the sketches showing the internal structure of lava flows and the emplacement of pahoehoe lava fields with its transformation into transitional and a'a type lavas. Very useful guide for my next outing into the Deccan basalts!

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