We document kinematics and rheological behaviour of brittle shears (~50 cm wide) postdating solid-state tectonic fabric in the Salem–Attur (SASZ) and Bhavani (BSZ) shear zone that constitute a Paleoproterozoic (~2500 Ma) suture juxtaposing disparate granulite blocks in south India. We constrain brittle deformation mechanisms from established relationship between changing orientation of deflected strain marker (quartz vein) and foliation within the shear band with respect to their orientation outside the shear band. Quartz c-axis orientation in charnockite (host lithology) and phyllonite (reworked charnockite) from the SASZ show presence of mixed basal 〈a〉 (low-T) and prism 〈a〉 (high-T) slip, and single basal 〈a〉 slip mechanism, respectively. This suggests considerable cooling of the granulite block prior to the onset of brittle shearing. Distribution of strain parameters – effective shear strain (Γ), shear strain (γ), stretch K2 along intermediate strain axis Y – from margin to the centre of the shear band, show peaked distribution with a single maximum at the shear zone centre. This implies rheological-weakening/strain-softening induced localizing shear zone character. Kinematically heterogeneous strain distribution during brittle shearing varies from transpression dominated for the BSZ to transpression-to-transtension switchover for the SASZ. Demonstrably, contrasting cooling-exhumation, hitherto unexplored, characterizes post-accretionary tectonics along the paleo-suture zone.
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AC acknowledges the financial support from Pondicherry University and DST-SERB, Grant-in-Aid No. SERB/F/5928/2018-2019, for carrying out geological fieldwork, sample preparation and analysis. Ritabrata is acknowledged for his support during EBSD analyses. The authors are grateful to the reviewers for their critical suggestions and constructive comments. Prof Saibal Gupta is thanked for editorial handling.
Supplementary materials pertaining to this article are available on the Journal of Earth Science Website (http://www.ias.ac.in/Journals/Journal_of_Earth_System_Science).
Communicated by Saibal Gupta
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Ananth, C., Bhadra, S. & Goswami, A. Contrasting kinematics of brittle-shears within the Salem–Attur and Bhavani shear zone, south India: Tectonic implications. J Earth Syst Sci 129, 62 (2020). https://doi.org/10.1007/s12040-019-1331-2
- Strain marker
- EBSD analysis
- Salem–Attur shear zone
- Southern Granulite Terrain