Inverted fold hinge: An end member of hinge rotation by superposed buckle folding in the Precambrian terrain of western India

Plunge reversal by hinge rotation due to superposed buckle folding ends when one of the hinge segments rotates to a reclined (sensu stricto) geometry. Rotation beyond this limit (called the critical stage of hinge rotation) inverts the rotating hinge segment from an antiform to a synform or vice versa. In the Sawar-Bajta area in the Precambrian terrain of western India, a segment of a synformal early fold has been inverted to an antiform, producing an antiform-synform fold pair with a common hinge and a common axial surface. Later compression folded the limbs and axial surface of the early synform, and rotated its two adjacent hinge segments in opposite directions. Unequal rotation inverted one of the segments and bowed the common hinge upward to produce a mega sheath fold with 46° hinge angle. The sheath folded basement-cover rock contact delineates an eye-fold with more than 30 km long perimeter on map profile. The inverted segment rotated more than 17° beyond the critical position to what is probably the end member of hinge rotation. The interlimb angle of the inverted segment increased, indicating progressive opening up of the fold, which inhibits further rotation beyond the limit set by the end member.