Multi-layers: Do incompetent horizons always act as thrust flats?

During thrusting incompetent weak layers are often interpreted as acting as thrust flats, creating décollements on which material is laterally translated. Thrust flat interpretations create specific thrust geometries with implications for lateral shortening and partitioning of strain within the rock volume. The pervasive nature of thrust flat interpretations in thrust belts therefore has significant impact on predicted shortening and strain. However, thrust interpretations, even in multilayers, do not always require thrust flats. Here, we show that strain evidenced by folds and faults in a multi-layer sequence from the Swiss Jura cannot be explained by thrust flats in incompetent horizons. We found, through line length balancing seismic sections in a 3D seismic cube, that the strain is partitioned into discrete zones above basement faults. The strain cannot be located in these positions simply through lateral displacement on a thrust flat or flats, rather it is explained by vertically distributed shortening above these basement structures. Multiple interpretations of the same seismic sections show that uncertainty in seismic interpretation creates a range in predicted shortening that is greater than the differences in shortening with depth. Indicative of no lateral movement of material on incompetent horizons within the stratigraphy, more the stratigraphy has shortened as a whole. Line length balancing in blocks along individual seismic profiles indicates deformation partitioned within and between sections, creating a complex picture of thrust evolution and strain partitioning at least in the latter stages of thrusting creating the geometries seen. The results have implications for the interpretation and understanding of the evolution of the Jura fold-thrust belt and for thrust-belts globally. • We challenge the oft-assumed narrative that ‘weak’-layers act as thrust-flats during contractional deformation. • Line-length restorations show that contractional shortening is partitioned in vertical blocks above basement features. • Interpretational uncertainty spans the range of shortening values calculated vertically through the stratigraphy. • Line-length balancing suggests shortening is vertically kinematically linked. • Contractional structures mapped above basement faults suggest no translation on thrust-flats towards the foreland. • C s ontractional shortening changes along strike.