“Eyed folds” in Precambrian marbles from southeastern Rajasthan, India

Abstract

“Eyed folds” — acute domes and basins having elliptical cross sections and subparallel hinge lines at the two ends of the ellipses — on major and minor scales are common in the Precambrian marbles in part of southeastern Rajasthan, India. Closely associated with these structures are tight to isoclinal folds with rectilinear axes that are subparallel to the hinge lines of the “eyes.” These structures belong to the first phase of deformation. A later cleavage cuts across them, and minor second-generation folds occur on the limbs of the earlier structures. Where the “eyed” structures are bent by later folds, a boomerang-shaped pattern results. The axes of the earlier and later folds are everywhere mutually parallel to subparallel, but their attitudes vary from one part of the area to another. It is suggested that the “eyes” and “boomerangs” of this area were not formed by the superposition of later shear folds on earlier folds having rectilinear axes, but, rather, the “eyed folds” were formed by strong flattening during later deformation of earlier folds having original plunge culminations and depressions. Buckling accompanying this later compression was responsible for the formation of the boomerangs. The strong compressive strain rotated all the linear elements toward the X-axis of the strain ellipsoid, and this explains the parallelism of the early and the late linear directions.