Mechanical Influence of Inherited Folds in Thrust Development: A Case Study from the Variscan Fold-and-Thrust Belt in SW Sardinia (Italy)

Fold-and-thrust belts have a high variability of structural styles, whose investigation provides continuous updates of the predictive models that try to better approximate the geometries recognized in the field. The majority of studies are focused on the geometry and development of folds and thrust surfaces and the amount of displacement, taking into account the role played by the involved stratigraphic succession assumed as a layer cake. We present a case study from the external zone of the Variscan fold-and-thrust belt in SW Sardinia, where it was possible to investigate the lateral and vertical variations of the mechanical properties of the involved succession, how they related to previous folding, control thrust geometry, and kinematics. In this case, the superposition of two fold systems acted as a buttress that induced extensive back-thrusting. We found that there is a close connection between the attitude of the bedding and the geometry of back thrust surfaces, shear strength during thrust propagation, and variation in the shortening amount, depending on which part of the folds were cut across. The folding-related mechanical anisotropy also seems to have induced a ductile deformation in the footwall of back-thrusts. Although the case study considers the development of back-thrust, the relations between thrust and not-layer cake geometries could also be applied to fore-thrust development.

Automated summary

This case study of the Variscan fold-and-thrust belt in SW Sardinia focuses on the lateral and vertical changes in mechanical properties of the stratigraphic succession, and how they relate to previous folding, thrust geometry, and kinematics. The investigation also highlights the close connection between bedding attitudes, back thrust surfaces geometry, shear strength during thrust propagation, and variation in shortening amount in different parts of folds. The study suggests that folding-related mechanical anisotropy may induce ductile deformation in the footwall of back-thrusts, providing insights for fore-thrust development as well.