Abstract
Abstract Classic fold-thrust structures within Carboniferous-age strata at Broadhaven, SW Wales are well-known for their excellent preservation of Variscan deformation. These sites have been important for conceptual model generation of the link between faulting and folding, and are often cited as exemplars of fault-propagation folds following work by Williams & Chapman. Here we employ the virtual outcrop method to digitally map and measure, in detail, the classic Den’s Door outcrop. 3D reconstruction of the site by digital photogrammetry allows us to extract high-density structural measurements, reassess the existing model of structural development for the outcrop, and re-evaluate the link between faulting and folding. We find that digital mapping highlights greater variability in fault displacement and bed thicknesses than previously documented. Fracture analysis shows that fracture intensity is closely linked to structural position and bed-thickness variability, and fracture orientations record the existence of discrete mechanical boundaries through the structure. These results record complex patterns of strain distribution and multi-phase deformation. Evidence for temporal and spatial variability in strain distribution suggests that multiple kinematic and non-kinematic models of deformation are required to faithfully describe even this apparently simple structure. This calls into question the applicability of end-member models of fault-related folding, particularly for multilayered stratigraphy.
Highlights
Distance from reference pointOf particular note are differences in interpreted bed thicknesses: the graphic interpretation of Williams & Chapman (1983) (Fig. 4a) suggests that significant thickening of units occurs right up to the thrust surface in the footwall syncline, with no apparent influence of axial-trace positions
Assessment of the relationships between faults, folds and fractures in deformed rock is important for understanding deformation processes and the kinematic evolution of structure (e.g. Ramsay 1967; Ramsay & Huber 1987)
These kinematic models – fault-bend fold, fault-propagation fold, detachment fold and trishear (Fig. 1) as outlined in Shaw et al (2005) – provide a classification structure for natural examples by which they can be understood. They are useful in ensuring that foldthrust interpretations are geometrically valid (e.g. Suppe 1983), and have been used to relate deformation history to fold- and fault-related fractures (e.g. Bellahsen et al 2006; Ghosh & Mitra 2009; Watkins et al 2015). These kinematic models of fold-thrust evolution rely on a series of assumptions: a basal detachment or thrust flat; folding that occurs as a consequence of faulting; and concentric folding
Summary
Of particular note are differences in interpreted bed thicknesses: the graphic interpretation of Williams & Chapman (1983) (Fig. 4a) suggests that significant thickening of units occurs right up to the thrust surface in the footwall syncline, with no apparent influence of axial-trace positions Our data suggest this is not the case (Fig. 10); a difference in interpretation due to virtual access provided by the virtual outcrop method. Thickness measurements (Fig. 10) record a degree of complexity, with greater variability in the hanging wall to the thrust (Fig. 11), higher in the stratigraphy This hanging-wall thickness variation is likely to imply that subsequent to initial folding, deformation by fault propagation was restricted to the hanging wall of the structure. The nature of the ductile bead is influenced internally by mechanical heterogeneity, arising from multilayered strata and pre-thrust deformation
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