Analysis of large scale superposed folding in a migmatite terrain

Abstract

In the "hook syncline" of central Rajasthan, India, the large metasedimentary bands of early Precambrian age, considered to form synclinal cores in the older basement gneisses, represent extensively migmatized palaeosomes. The curious, hook-shaped double closures in the map pattern of these palaeosomes are due to a superposition of tight upright folds with NS-striking axial planes on isoclinal reclined folds plunging westward, a feature duplicated in numerous examples in hand specimen and outcrop. The later folds, which vary in style from open in the southeast to isoclinal in the northwest, have caused a wide scattering in the orientation of the axes and axial planes of the first set of folds. Although the later folds always trend NS, they range in plunge from subhorizontal to vertical, depending on their location in the limbs and hinges of the early folds. Flexure modified by flattening and flow seems to have been the dominant mechanism in the evolution of the folds. Migmatization, which has transgressed the stratigraphic levels, is broadly synkinematic with the first deformation. The style and orientation of the folds of the two systems are mutually incompatible in any single deformation plan. The increase in the intensity of the second deformation westward near the contact of the younger Precambrian (Proterozoic) Delhi System of rocks suggests that it is connected with the Delhi orogeny, whereas the first folding on an EW trend is linked with an earlier orogeny, hitherto unrecognized in Rajasthan.