Microstructures and evolution of folds in SC-mylonites from Dudatoli-Almora crystallines of Garhwal Himalaya |
| |
Authors: | V K Gairola R A Singh |
| |
Institution: | (1) Department of Geology, Banaras Hindu University, 221005 Varanasi, India |
| |
Abstract: | 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 of flaggy to schistose quartzites, granite gneisses and garnetiferous mica schist members
in an ascending order. In different localities. different members of the Dudatoli-Almora crystallines are exposed along the
thrust plane. Southwest of Adbadri fine-grained mylonitized schistose quartzites of Dudatoli-AImora crystallines are in contact
with the underlying metabasites of the Garhwal Group. The mylonitized schistose quartzites consist of alternating thick (1
to 2m) quartzite and thin (10 to 20cm) micaceous quartzite bands. The micaceous quartzites can be further differentiated into
alternating quartz-rich (0-5 to 2.0 cm 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 alternating 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 lithological layering. The evolution of these folds has been envisaged in three phases of deformation on the basis
of field evidence, fold geometry and microstructures.
During the first phase buckle folds (F
1) developed 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 ofF
1 folds were folded (F
2). During the third phase of deformation which coincided with thrusting, the rocks were sheared, mylonitized and developed
microstructures exhibiting dynamic recrystallization by the processes of subgrain rotation, and continual and discontinuai
grain boundary migration. This phase 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 ofF
2 folds. |
| |
Keywords: | SC-mylonites folds dynamic recrystallization Dudatoli-AImora crystallines Garhwal Himalaya |
本文献已被 SpringerLink 等数据库收录! |
|