Hourglass Structure: An Evidence of Buckle Folding

The Nimbahera shale beds show an hourglass map pattern due to juxtaposition of an anticline and a syncline along a common NNE striking, upright axial plane. Several possible hypotheses, such as superposed folding, sheath folding, variation in shortening, variation in orientation of pre-Existing S-Surfaces and coalescence of fold waves are tested for explaining the development of the hourglass structure. Evidence from geometrical characteristics of the folds, associated mesoscopic scale structures and the published experimental results suggest that the hourglass structure has developed due to the linkage of the two out-Of-Phase fold waves that propagated towards each other during the folding of the Nimbahera shale beds by buckling.