Nature and evolution of subhorizontal Crenulation cleavage in the type Aravalli rocks around Udaipur, Rajasthan

Abstract

A set of crenulation cleavage subparallel to the axial planes of small- and intermediate-scale recumbent folds (representing the third phase of deformation, F₃) has developed quite extensively in the early Precambrian (Aravalli) phyllitic rocks and metagreywacke units around Udaipur in southern Rajasthan (western India). These are unevenly spaced planar discontinuities which separate thin slices of plicated rocks (microlithons) of variable thickness. The crenulation cleavage planes deviate from being parallel to the axial planes of fold in passing from one bed to another of different competency, within a single bed of gradational lithologic character (e.g. metagreywacke with graded beds), and at the contact of folds in competent units. Generally, crenulation cleavage appears as mica-chlorite-rich domains bounded by surfaces parallel to the axial planes of microfolds. The domains develop across the sharply bent limbs of asymmetric microfolds. The other morphological forms include those denned by parallelism of layer silicates and trails of magnetite or graphite along axial planes and straight limbs of tight chevron o folds, and those appearing as clean-cut faults. The crenulation cleavage planes indicate directions of maximum finite extension; and upto 43 per cent shortening has been noted in the folds in quartz veins which are oriented normal to the cleavage planes in phyllite. The shortening of the rock column initially leads to the formation of symmetrical open folds (F₃) which, with further tightening accompanied by layer-parallel shear, become asymmetrical. Removal of quartz, leading to the development of mica-chlorite-rich domains across the sharply bent shorter limbs of these asymmetric folds, starts when these limbs make an angle of less than 40° with the direction of maximum finite extension. The shorter limbs ultimately become flattened parallel to the length of the domains of concentrated layer silicates. Metamorphism ushers in late and outlasts the deformation. The subhorizontal crenulation cleavage planes, parallel to the axial planes of recumbent folds, are believed to have developed because of flattening in response to excessive vertical stresses resulting from the weight of steeply dipping layered complex.