Modification of preexisting structures by interlayer slip during fold evolution: a multistage model from outcrop-scale structural analysis

Introduction Interlayer slip is a key mechanism during fold evolution; however, its effects on preexisting or contemporaneous secondary structures (e.g., cleavage and fractures) remain poorly constrained. This study investigates a well-exposed outcrop in the southern Yidun Arc, northwestern Yunnan, to decipher the modification of fold-related structures by interlayer slip and the resulting tectonic sequence. Methods We combined detailed field-based structural analysis with stereographic projections to systematically measure and analyze the orientations and geometries of bedding, cleavage, interlayer small folds, and joint sets. Kinematic indicators and stress orientations were reconstructed from the collected structural data. Results Axial planar cleavage, formed during the late-stage of major folding, is systematically deflected adjacent to bedding interfaces by subsequent interlayer slip, providing a distinct kinematic indicator for syn-folding ductile deformation. Interlayer small folds, sharing a hinge line with the major fold, are direct products of ductile interlayer slip. Two joint sets (J1 and J2) are developed, with orientations of 235°∠70° and 35°∠75°, respectively. Their Mode I (extension) geometries indicate that the minimum principal stress (σ 3 ) was oriented approximately NE-SW during J1 formation and NW-SE during J2 formation. These joints represent independent brittle fracturing events post-dating folding. Discussion Integrating geometry, kinematics, and deformation mechanisms, we propose a three-stage evolutionary model: Stage I involves ductile contraction and syn-folding flow, forming the major fold; Stage II features small folds and late-stage cleavage development, where ongoing slip modifies earlier cleavage; Stage III is characterized by exhumation and post-folding brittle fracturing, forming transecting joint sets. This work not only clarifies the intrafolding sequence of structures but also provides a critical outcrop-scale example and methodology for unraveling polyphase tectonic overprinting.