Development of crystal lattice preferred orientation and seismic properties in Bhavani shear zone, Southern India.

Shear markers and shear sense indicators from the Bhavani shear zone (BSZ), a member of the south Indian Proterozoic shear system, separating the northern Archaean and southern Pan African granulite terranes, suggest multiphase reactivation. There is a progressive increase in mylonitisation corresponding to a progression in strain. The LS fabrics indicate near vertical principal flattening plane trending ENE-WSW, but with subvertical as well as subhorizontal stretching lineations implying a possible reactivation history with opposing movement vectors. Whilst the SEM-EBSD derived LPO of amphibole, biotite and feldspars suggest plastic deformation through the activation of intracrystalline slip systems, diffusional creep accommodated deformation can be inferred from the quartz LPO. All LPO suggest modifications in the fabric due to both retrograde and prograde reactivation during the tectono-thermal history of the BSZ over a period of nearly 2.0Ga. Petrofabric-derived seismic properties for the BSZ suggest that it exhibits a considerable seismic anisotropy, which reflects the petrofabrics of hornblende and biotite. Both LPO and seismic property observations imply conflicting tectonic X and Y directions, indicating possible superposition of contrasting X and Y vectors during different phases of the shear zone reactivation.