Application of SEM-EBSD to regional scale shear zone analysis: A case study of the Bhavani Shear Zone, South India

Abstract

The Bhavani Shear Zone (BSZ), part of the Palghat-Cauvery shear system of southern India, comprises a number of subparallel shear zones ranging in width from a few centimetres to tens of metres. These shear zones are characterised by progressive mylonitisation and shear fabrics corresponding to increasing strain, as documented by grain size reduction, flattening, elongation and dimensional preferred orientation of constituent minerals (quartz, hornblende, biotite, plagioclase and orthoclase). However, SEM-EBSD measured crystal preferred orientations (CPO) of these minerals are more complex and indicate interchange of maximum (X) and intermediate (Y) tectonic axes during different phases of deformation, with the minimum axis (Z) remaining approximately constant. The tectonic evolution of the BSZ and the disposition of the surrounding Archaean and Pan-African granulites are interpreted therefore in terms of polyphase deformation involving both dip-slip and strike-slip movement events in a multiply reactivated setting. SEM-EBSD CPO analysis provides clearer indication of potentially conflicting movement directions that are otherwise poorly constrained by shear markers and shear sense indicators.

Role of microfabrics and magnetic fabrics in the tectonic evolution of the Achankovil shear-zone, South India

“…The LPO could be described as a (100)-fibre texture based on the analogy with the olivine fabric data (Ben Ismail and Mainprice, 1998) and hence may also indicate slip on (100)[010]. Rigid body rotation has been considered to play a vital role in the LPO development of hornblende and biotite (Diaz Aspiroz et al, 2007;Prasannakumar and Lloyd, 2010). Grains are rotated towards the XY plane of the finite strain ellipsoid during deformation (Passchier and Trouw, 2005), such that the amphibole (100) and biotite (001) planes become parallel to the macroscopic foliation.…”