Structural evolution and strain pattern in sheared gneissic-graniticcomplexes and felsic volcanics of Babina-Prithvipur area, Bundelkhand Craton, central India

The sheared Bundelkhand gneissic complex of Meso to Neoarcheanage (3.4-3.1Ga) and mylonitisedmetasedimentary-felsic volcanics and granitoids of Precambrianto Palaeoproterozoic age are recognizedas important rock units in the study area. The older TTG and streaky gneisses having inclusions of mafic, ultramafic, amphibolites and quartzite were affected by three phases of folding under three regional compressive tectonic episodes (D1-D3). An E-W sub-vertical brittle-ductile crustal shear zone(200-500 meter) transecting gneissic, volcanic and granitic terrains, evolved in syntectonicD4 phase is traced. The linear quartz reefs were emplaced along NE-SW shear zones in last tectonic (D5) phase and were followed by last magmatic intrusion of doleritic dykes. In the present work the attempts were made to understand the implications of microstructures and strain indicators in evolution of shear zones transecting the older crustal blocks. The shear indicators (mylonitic foliation, lineation, asymmetrical and rotated porphyroclasts) observed on meso and micro scale are found dominant in all mylonites of crustal sheaer zones. The mantled porphyroclasts (σa and σb) occurring in protomylonite zone exhibit dominant sinistral sense of shear vergence. The presence of undulose extinction, deformation lamellae in quartz and feldspar grains reveal the dominance of crystal plastic and strain softening processes in evolution of crustal shear zones (under low to medium temperature conditions). The overprinting of brittle deformation on ductile phase is evidently supported by presence of micro faults, extensional fractures and pull apart structures in few grains of quartz and feldspar. The higher values of two (Rs=3.4) and three dimensional strain (∑s=0.95) estimated in few specimens are indicative of dominance of high shear strain in central parts of shear zones. Contrary to this, the low values of strain quantified in few specimens are suggestive of presence of low shear strain in marginal parts of shear zones. The affinity of k- values towards flattening field implies that the flattening strain was dominant parallel to Y-axis and is suggestive of volume loss.