MICROSTRUCTURES AND EVOLUTION OF FOLDS IN SC-MYLONITES FROM DUDATOLI-ALMORA CRYSTALLINES OF GARHWAL HIMALAYA

In the Lesser Garhwal Himalaya, the North Almora Thrust separates the overlying medium-grade Dudatoli-Almora crystallines of Precambrian age from the unmetamorphosed to partly metamorphosed rocks of the Garhwal Group of Late Precambrian age. The crystalline nappe sheet consists o f flaggy to schistose quartzites, granite gneisses and garnetiferous m ica schist members in tin ascending order. In different localities. di fferent members of the Dudatoli-Almora crystallines are exposed along the thrust plane. Southwest of Adbadri fine-grained mylonitized schist ose quartzites of Dudatoli-Almora crystallines are in contact with the underlying metabasites of the Garhwal Group. The mylonitized schistos e quartzites consist of alternating thick (1 to 2 m) quartzite and thi n (10 to 20 cm) micaceous quartzite bands. The micaceous quartzites ca n be further differentiated into alternating quartz-rich (0.5 to 2.0 c m thick) and mica-rich (0.2 to 1.0 cm thick) layers. In the quartzites the C-surfaces are parallel to the S-surfaces defined by the alternat ing quartz-rich and mica-rich layers. Further, the S-surfaces exhibit almost similar folds with multiple wavelengths where the axial planes are nearly parallel and enveloping surfaces are oblique to the litholo gical layering. The evolution of these folds has been envisaged in thr ee phases of deformation on the basis of field evidence, fold geometry and microstructures. During the first phase buckle folds (F-1) develo ped in thin micaceous quartzite layers. whereas thick quartzite bands underwent only layer parallel shortening. During the second phase the stress orientation changed and the limbs of F-1 folds were folded (F-2 ). During the third phase of deformation which coincided with thrustin g, the rocks were sheared, mylonitized and developed microstructures e xhibiting dynamic recrystallization by the processes of subgrain rotat ion, and continual and discontinual grain boundary migration. This pha se was also responsible for the development of C-surfaces parallel to the lithological layering. Further, in the folded micaceous quartzite layers shearing resulted in the development of C-surfaces parallel to the axial planes of F-2 folds.