A study of mylonites from parts of the Salem-Attur shear zone (Tamil Nadu) and its tectonic implications

Abstract

The E-W running Salem-Attur shear zone demarcates the tectonic boundary between Archaean Dharwar Craton in the north and Proterozoic Southern granulite terrane in the south. This study reveals that the shear zone is a low angle thrust. The thrust zone is around 10 m thick and it merges with the main shear zone along the strike. The thrust is developed on charnockite near Odyarpatti, which is retrograded into schists. Further, it is marked by gently dipping mylonitic foliation and subhorizontal lineation. The S-C fabric, mantled porphyroclasts and intragranular faults indicate northeasterly slip along the thrust. Recumbent shear folds SF 1 are developed within the thrust zone. The thrust has been folded by late stage F 2 fold which has brought variation in the orientation of the mylonitic foliation from subhorizontal to vertical attitude; the mylonitic lineations have been rotated to subvertical orientation also. Additionally, the F 2 crenulations and shear cleavages and intersection lineations are superimposed on the mylonitic fabric. Thrusting along the Salem-Attur shear zone is probably the cause for upliftment of the charnockites to the upper crust. Post-upliftment stage has witnessed brittle deformation in the form of development of shear fractures in NNE-SSW and E-W directions. Pseudotachylites are emplaced along these fractures.

Insights into the crustal architecture from the analysis of gravity and magnetic data across Salem-Attur Shear Zone (SASZ), Southern Granulite Terrane (SGT), India: an evidence of accretional tectonics

“…1) is a discrete tectonic block that evolved through Neoarchean arc magmatism in a convergent margin setting (Samuel et al, 2014). The SASZ, which separates the Shevaroy Block (SB) and Namakkal Block (NkB, Li et al, 2018), is interpreted as strike-slip shear zone (Drury and Holt, 1980;Chetty et al, 2003) with basal decollement and along which PCSS is thrusted over NGB (Biswal et al, 2010, Behera et al, 2019. The NkB is bordered by the SASZ at the north, the Cauvery-Bhavani Shear Zone (CBSZ) at the west and the Palghat Cauvery Shear Zone (PCSZ), at the south (Fig.…”