Clast size analysis of pseudotachylite co-existing with mylonite: Constraints on evolution of Mahanadi Shear Zone, Eastern Ghats Mobile Belt, India

The Mahanadi Shear Zone (MSZ) in the Eastern Ghats Mobile Belt (EGMB) separates the southern segment of EGMB with dominant NE-SW structural trend from its northern segment with dominant E-W trend. The MSZ is a high-temperature ductile shear zone in which mylonite-ultramylonite occurs intimately associated with pseudotachylite. The association of two components of contrasting origins (ductile and brittle deformations) is possibly attributed to contemporaneous evolution or multiple stages of evolution. Therefore, field and petrographic studies including clast-size analysis of three samples from different localities across the MSZ have been carried out. Field observations suggest that pseudotachylite veins often cut across the mylonitic fabric. In addition, there is variation in orientation of gneissic and mylonitic fabrics in the clasts within the pseudotachylite zones. The pseudotachylites occasionally exhibit crude flow structures and rims around clasts. These features clearly indicate that the mylonitic fabric in the MSZ predates brittle deformation, and the pseudotachylites have not suffered any subsequent deformation after their formation. Clast-size analysis in all three cases follows the Power Law and demonstrates linear distribution. Lower value for fractal dimension (D = 0.51-0.69) and involvement of large range of size distribution of clasts in the studied pseudotachylite samples are attributed to brittle deformation at high strain rate, independent of the ductile deformation event. The present study suggests evolution of the MSZ in two stages, i.e., early development of mylonite-ultramylonite by ductile deformation at deeper level followed by pseudotachylite development at later stage by brittle deformation at a shallower level, thereby overprinting of brittle deformation over the ductile deformation.