Planar, non-planar and refolded sheath folds in the Phulad Shear Zone, Rajasthan, India

Abstract

Progressive ductile shearing in the Phulad Shear Zone of Rajasthan, India has produced a complex history of folding, with development of planar, non-planar and refolded sheath folds. There are three generations of reclined folds, F 1, F 2 and F 3, with a striping lineation ( L 1) parallel to the hinge lines of F 1. The planar sheath folds of F 1 have long subparallel hinge lines at the flanks joining up in hairpin curves at relatively small apices. L 1 swerves harmoniously with the curving of F 1 hinge line. There is a strong down-dip mineral lineation parallel to the striping lineation in most places, but intersecting it at apices of first generation sheath folds. Both the striping and the mineral lineation are deformed in U-patterns over the hinges of reclined F 2 and F 3. Folding of axial surfaces and hinge lines of earlier reclined folds by later folds was accompanied by very large stretching and led to the development of non-planar sheaths. The reclined folds of all the three generations were deformed by a group of subhorizontal folds. Each generation of fold initially grew with the hinge line at a very low angle with the Y-axis of bulk non-coaxial strain and was subsequently rotated towards the down-dip direction of maximum stretching. The patterns of deformed lineations indicate that the stretching along the X-direction was extremely large, much in excess of 6000 percent.