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Size distribution and roundness of clasts within pseudotachylytes of the Gangavalli Shear Zone, Salem, Tamil Nadu: An insight into its origin and tectonic significance

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Gangavalli (Brittle) Shear Zone (Fault) near Attur, Tamil Nadu exposes nearly 50 km long and 1–3 km wide NNE–SSW trending linear belt of cataclasites and pseudotachylyte produced on charnockites of the Southern Granulite Terrane. Pseudotachylytes, as well as the country rock, bear the evidence of conjugate strike slip shearing along NNE–SSW and NW–SE directions, suggesting an N–S compression. The Gangavalli Shear Zone represents the NNE–SSW fault of the conjugate system along which a right lateral shear has produced seismic slip motion giving rise to cataclasites and pseudotachylytes. Pseudotachylytes occur as veins of varying width extending from hairline fracture fills to tens of meters in length. They carry quartz as well as feldspar clasts with sizes of few mm in diameter; the clast sizes show a modified Power law distribution with finer ones (<1000 \({\upmu }\)m\(^{2})\) deviating from linearity. The shape of the clasts shows a high degree of roundness (>0.4) due to thermal decrepitation. In a large instance, devitrification has occurred producing albitic microlites that suggest the temperature of the pseudotachylyte melt was >1000\(^{\circ }\hbox {C}\). Thus, pseudotachylyte veins act as a proxy to understand the genetic process involved in the evolution of the shear zone and its tectonic settings.

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Authors are greatly thankful to the Head of the Department of Earth Sciences, Indian Institute of Technology, Bombay for the permission and funding to carry put fieldwork in and around Gangavalli area, Salem, Tamil Nadu. They also convey their gratitude to B Sivalingam (JRF at IIT Bombay) and Sridharan, Thiyaga Ranjan and Madan Raj (Students of Department of Geology, Govt. Arts College, Salem, Tamil Nadu) for their extensive support and help during the field visit. Critical reviews by Dr. Surajit Mishra, BGRL, Govt. of India and the other anonymous reviewer helped in improving the quality of the manuscript.

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Correspondence to Bhuban Mohan Behera.

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Behera, B.M., Thirukumaran, V., Soni, A. et al. Size distribution and roundness of clasts within pseudotachylytes of the Gangavalli Shear Zone, Salem, Tamil Nadu: An insight into its origin and tectonic significance. J Earth Syst Sci 126, 46 (2017).

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