Progressive development of s-type flanking folds in simple shear

Abstract

Flanking structures are deflections of planar or linear fabric elements in a rock alongside a crosscutting element (CE), e.g. a vein or fault. This study provides new results from analogue experiments, which test and extend recent numerical models of flanking structures. A linear viscous matrix material (PDMS) was deformed in a ring shear rig that allows continuous observation to large values of shear strain. Rotational behaviour, offset and deflection of marker lines around a predefined, lubricated CE were monitored for different initial orientations of the fault with respect to the shear zone boundary, and the results were compared with numerical results and natural examples. At high initial angles to the shear zone boundary (>135°), a structure previously described as an ‘s-type flanking fold’ develops. During progressive deformation, an initially straight marker line passing through the centre of the CE is offset in a sense synthetic with the bulk sense of shear and shows a shortening displacement across the CE. Simultaneously, this central marker line is deflected and forms symmetrical folds, which are convex in the direction of shear along the CE (i.e. normal drag). Both offset and deflection of the marker lines decrease towards the tips of the fault. Natural examples of s-type flanking folds, directly comparable with the model results, are more common than is generally appreciated.