INSIGHTS INTO FOLD GROWTH USING FOLD-RELATED JOINT PATTERNS

Along-strike fold propagation has proven elusive to document. However, if a fold grows laterally, early-formed fold-related joints may differ significantly in orientation from joints that form later. We present joint orientation data from Sheep Mountain Anticline, Wyoming, USA, that show two sedimentary units in which joint orientation differs from proximal units in the same structural position. Unlike joints in nearby units, these joints do not match the stresses predicted for the present fold curvature, suggesting changes in fold shape during fold growth. Calculations of the fracture resistance of the anomalous beds confirm that they are less resistant to fracturing than the other units in the study, and therefore would have fractured earlier in fold growth history. We present a plate-bending model that incorporates lateral fold growth to illustrate potential joint patterns for early and late stages of folding. The joint strike predictions for early stages of folding match the orientations in our anomalous beds. The combined analysis of fracture pattern and mechanical stratigraphy provides a new way to investigate fold and underlying fault evolution.