Abstract It has long been recognized that the structures that accommodate shortening within fold-and-thrust belts exhibit a wide variety of styles that reflect the mechanical behaviour of the stratigraphic units that are being deformed. The ability to characterize these different structural styles, and to understand the factors that control their variability, is essential to many applications, including petroleum geology, earthquake hazard assessment and regional geological studies. The relative contributions of different aspects of the mechanical stratigraphy and boundary conditions to determining fault-related folding style are investigated through the use of the discrete-element modelling (DEM) method in this study. Modelling emergent contractional structures within a shortening wedge with this method demonstrates that (a) The major different styles of shortening structures can all be reproduced under different mechanical circumstances within the range of realistic mechanical conditions, and (b) Different aspects of the mechanics of the deforming rock units (for example, peak strength, strain weakening, layer strength anisotropy) exert various degrees of control on the styles of structures that emerge from the models as shortening progresses. These analyses inform our understanding of the relative importance of these different factors in determining the style of structures which accommodate shortening in different fold-and-thrust belt systems.

Abstract: The initiation, growth and interaction of faults within an extensional rift is an inherently four-dimensional process where connectivity with time and depth are difficult to constrain. A 3D discrete element model is employed that represents the crust as a two-layered brittle–ductile system in which faults nucleate, propagate and interact in response to local heterogeneities and resulting stresses. Faults nucleate in conjugate sets throughout the model brittle crust; they grow through a combination of tip propagation and interaction of co-linear segments to form larger normal faults. Segment linkage occurs by merging of adjacent fault segments located along strike, downdip or oblique to one another. Finally, deformation localizes onto the largest faults. Displacement distribution on faults is highly variable with marked along-strike and temporal variations in displacement rates. Displacement maxima continuously migrate as smaller fault segments interact and link to form the final fault plane. As a result, displacement maxima associated with fault nucleation sites are not coincident with the location of the maximum finite displacement on a fault where segment linkage overprints the record. The observed style of fault growth is consistent with the isolated growth model in the earliest stages which then gives way to a coherent (constant-length) fault growth model at greater strains.

Abstract: The growth of normal faults in mechanically layered sequences is numerically modelled using three-dimensional Distinct Element Method (DEM) models, in which rock comprises an assemblage of bonded spherical particles. Faulting is induced by movement on a pre-defined normal fault at the model base whilst a constant confining pressure is maintained by applying forces to particles lying at the model top. The structure of the modelled fault zones and its dependency on confining pressure, sequence (net:gross) and fault obliquity are assessed using various new techniques that allow (a) visualization of faulted horizons, (b) quantification of throw partitioning and (c) determination of the fault zone throw beyond which theoretical juxtaposition sealing occurs along the entire zone length. The results indicate that fault zones become better localized with increasing throw and confinement. The mechanical stratigraphy has a profound impact on fault zone structure and localization: both low and high net:gross sequences lead to wide and relatively poorly localized faults. Fault strands developing above oblique-slip normal faults form, on average, normal to the greatest infinitesimal stretching direction in transtensional zones. The model results are consistent with field observations and results from physical experiments.