Plunging fault propagation folds: a case study from the Bhutan Himalayas

Abstract

In the mapped area of the Lesser Himalayas of eastern Bhutan, an antiform-synform fold pair above the propagating tip of a splay ramp emerging from the Shumar floor thrust represents frontal fault propagation folds. Contrary to what the available graphic models suggest, the folds are plunging and are noncoaxial with the causative thrust fault (in the sense that the fold axes are oblique to the ramp-flat hinge). These features, which cannot be explained by post-thrusting tilting of initially coaxial and horizontal fault propagation folds, point to the development of the folds in a sequence which was laterally arched in the form of a culmination prior to folding. Axes of the frontal fault propagation folds growing in such a folded sequence will be curvilinear and parallel to the line of intersection of the axial surfaces with the curviplanar layers. The observed 25° plunge of the fault propagation folds reflects an apparent dip of the left lateral wall of the culmination. The available models also cannot adequately explain progressive growth of fault propagation folds in culmination zones or folded sequences. Application of the model in such curviplanar sequences leads to a locked situation in which the fault propagation folds cannot grow further. Moreover, as the tip lines of faults are curvilinear (ideally elliptical, as the model of fault growth from a nucleus suggests), the fault propagation folds growing parallel to the propagating tip line will be plane noncylindrical, even in a horizontal sequence. While in culmination zones the range of plunge variation of the folds will be controlled by the culmination geometry, in horizontal sequences it will increase with increase in dip of the causative fault and decrease in aspect ratio (ratio of the axes -a/b) of the tip line ellipse. Coaxiality is not diagnostic of fault propagation folds and will be observed only at the central part of the faults, where the axes will be parallel to the ramp-flat hinge.