Improving Fold and Thrust Belt Imaging, Wyoming Thrust Belt, Wyoming—A Case Study

The structural geologic interpretation of reflection seismic data is affected by conceptual uncertainty, particularly in challenging onshore settings. This uncertainty can be significantly reduced by the integration of cross-section restoration and balancing techniques into the seismic interpretation workflow. Moreover, these techniques define a solid and comprehensive basis, grounding the interpretation and allowing a closer investigation of the deformation history that led to the interpreted structures. These benefits are demonstrated on the basis of a case study from the eastern Jura Mountains in northern Switzerland. This mountain range was formed by a thin-skinned foreland fold-and-thrust belt with a multiphase prethrusting tectonic history. Despite significant seismic acquisition and processing efforts, seismic imaging of the strongly deformed parts of the belt widely remains ambiguous. We have developed a detailed systematic interpretation workflow that is exemplified here for a single seismic profile across the Jura Main Thrust. Classical cross-section balancing techniques of equal bed lengths and areas were applied to validate and reinterpret the given seismic interpretation. Our results suggest that most of the observed structures resulted from thin-skinned deformation along a basal décollement in Lower Triassic evaporites, which is generally inferred for the Jura Mountains. Nevertheless, secondary detachment levels in above lying strata have to be considered as well. The stepwise restoration of the analyzed cross section points toward different styles of thin-skinned deformation and possibly several episodes of earlier basement-rooted faulting events, which are indicated by subtle stratigraphic thickness changes. In summary, our workflow allowed us to significantly improve the original seismic interpretation, highlight specific deformation styles, and illuminate possible prethrusting deformation events that would otherwise be easily overlooked.

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

Geosciences ◽

10.3390/geosciences11070276 ◽

2021 ◽

Vol 11 (7) ◽

pp. 276

Author(s):

Fabrizio Cocco ◽

Antonio Funedda

Keyword(s):

Ductile Deformation ◽

Mechanical Anisotropy ◽

Thrust Belt ◽

External Zone ◽

Layer Cake ◽

Thrust Belts ◽

Fold And Thrust Belt ◽

Vertical Variations ◽

Stratigraphic Succession

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.

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