Active Flexural‐Slip Faulting: Controls Exerted by Stratigraphy, Geometry, and Fold Kinematics

Abstract Flexural slip plays an important role in accommodating fold growth, and its topographic expression, flexural‐slip fault (FSF) scarps, may be one of the most commonly occurring secondary structures in areas dominated by active thrusts and folds. Where FSF scarps are present and what factors control their occurrence, however, are typically poorly known. Through an investigation of clearly expressed FSF scarps, well‐preserved fluvial terraces, and well‐exposed bedrock at eight sites in the Pamir‐Tian Shan convergent zone and Kuche fold belt, NW China, we summarize the most favorable conditions for active flexural‐slip faulting. Our study yields six key results. First, flexural slip operates commonly in well‐layered beds, although uncommonly can occur in massive, poorly layered beds as well. Second, in well‐layered beds, the slip surface is commonly located either (a) close to the contact of competent and incompetent beds or (b) within thin incompetent beds. Third, FSF scarps are always found overlying steep beds with dips of ~30–100°. Fourth, slip surfaces are typically spaced between ~10 and 440 m but can reach up to ~600 m. Fifth, FSF scarps at most sites can be observed far away from the hinge‐migrated fold scarps, suggesting that compared to hinge migration, limb rotation is generally required to accumulate flexural slip and produce associated topographic scarps. Finally, a higher regional convergent rate seems to facilitate the creation of FSF scarps more often than lower rates, whereas well‐preserved, old terraces capped by thin deposits are more likely to record FSF scarps than unevenly preserved, young terraces with thick sedimentary caps.