INTERPRETING THE GEOLOGIC MAP EXPRESSION OF CONTRACTIONAL FAULT-RELATED FOLD TERMINATIONS: LATERAL/OBLIQUE RAMPS VERSUS DISPLACEMENT GRADIENTS

Abstract

Fault-related folds in contractional settings do not extend indefinitely, but rather commonly terminate as plunging anticlines near the tips of the genetically-related fault. Geologists often attribute the formation of such terminations to loss of slip on the underlying fault (i.e. a displacement gradient), to changes in fault geometry in which the fault laterally changes stratigraphic position along strike (i.e. a lateral/oblique ramp), or to some combination of these mechanisms. Discerning between these formative mechanisms solely through the interpretation of the geologic map expression of the termination can be difficult because of the subjective nature of the criteria used (e.g. lateral/oblique ramps often are interpreted at terminations where folds plunge at 'steep' angles, where hanging wall cutoff lines trend at 'high' angles to fault strike, where stratigraphic contacts trend at 'high' angles to fault strike, etc.).We created pseudo-three-dimensional model terminations of individual fault-related folds using both displacement gradients and lateral/oblique ramps to determine if unique characteristics in their map expressions exist. We show that map patterns of folds produced by a displacement gradient along a thrust fault of constant geometry are similar to map patterns of folds produced by constant slip on a thrust fault with a lateral/oblique ramp. Specifically, our modeling results suggest that (1) simple fault-bend folds that plunge less than 20° and simple fault-propagation folds that plunge less than 50° at their terminations can be created by both displacement gradients and lateral/oblique ramps; angles greater than these values suggest that a lateral/oblique ramp may be involved in forming the termination, (2) hanging wall cutoff lines at terminations that trend at angles less than about 35° to fault strike for simple fault-bend and fault-propagation folds also may be produced by both displacement gradients and lateral/oblique ramps; higher angles likely indicate the presence of a lateral/oblique ramp at the termination, (3) the angle at which stratigraphic contacts trend to the fault strike cannot be used to uniquely identify displacement gradients or lateral/oblique ramps for simple fault-related folds, (4) stratigraphic separation diagrams can indicate the presence of ramps in the thrust sheet, but do not uniquely differentiate between frontal ramps with displacement gradients and lateral/oblique ramps, and (5) actual changes in fault orientation (after topographic influences have been taken into account), by definition, indicate a lateral/oblique ramp. In reality, most natural fault-related fold terminations probably share components of both displacement gradients and lateral/oblique ramps, with each structure possessing contributions from each mechanism.