Folds in multilayered rocks of Proterozoic age, Rajasthan, India

Abstract

First phase folds F 1 developed in polydeformed Ajabgarh Group rocks of Proterozoic age are studied using various geometrical methods of analysis for compatibility of homogeneous strain in both class 1-3 pairs by correlating t α /α plots with existing curves for competent layers and matching t α /α plots with the flattening curves for the incompetent layers. F 1 folds were initiated by the process of buckling but underwent [(λ 2 /λ 1) = 0.2 to 0.7] for competent layers and R-values of 1.1 to 5 for incompetent layers. The varying flattening is also revealed by the geometry of folds. The apparent buckle shortening of folds which ranges between 49 and 67 per cent with a majority of the folds having shortening values between 50% and 55% (exclusive of layer parallel strain) and inverse thickness method strain up to 50%. Besides flattening, the fold geometry was also modified by the pressure solution. This is borne by the presence of dark seams rich in phyllosilicates and disseminated carbonaceous material offsetting limbs of buckled quartz veins in slates.