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

Fabrizio Cocco,Antonio Funedda

doi:10.3390/geosciences11070276

Abstract

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.

Highlights

Fold-and-thrust belts have been extensively studied since the second half of the last century to describe the structural style and define the mechanism of development [1,2,3,4,5,6,7,8,9,10,11]
We present geological and structural data and the reconstruction of the geometry, kinematics, and displacement of the back-thrusts that unravel what relationships exist between the structural style of the fold-and-thrust belt and the structural inheritance
This seemingly simple mechanical stratigraphy is structurally complicated by the occurrence of the Sardic and Variscan antiforms and synforms

Summary

Introduction

Fold-and-thrust belts have been extensively studied since the second half of the last century to describe the structural style and define the mechanism of development [1,2,3,4,5,6,7,8,9,10,11]. Differences may arise because of different geodynamic settings where the fold-and-thrust belts develop, namely, collisional [37], convergent [38], or gravity-driven [39]; depending on which part of the crust is involved, if it is thick- [40] or thin-skinned tectonics [41]; according to the rheology of the involved rocks [42,43], mechanical stratigraphy [44], the depth of the detachment [45], and the deformation rates [46]; and based on the occurrence of salt tectonics [47,48], syn-orogenic erosion and deposition [49,50] and pre-existing structures [51,52,53]. The spatial variation’s role of mechanical properties of layer-cake stratigraphy in influencing fault and fold development and linkage has been highlighted [55], but open questions remain about the influence of mechanical anisotropy related to former structures, except for the role of pre-existing normal faults [54] or fault-controlled thrust ramps [56], and how the inherited structures can be detected [34]

Results

Discussion

Conclusion