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.
Bibliographical notePublication title: "Folding and Fracturing of Rocks: 50 Years of Research since the Seminal Text Book of J. G. Ramsay"
Thanks to Yuki Totake for field assistance and for providing images used in the virtual outcrop construction. Midland Valley Exploration is thanked for academic use of the Move 2016 software. We thank the two anonymous reviewers whose careful work and constructive comments improved the manuscript.
This study was carried out as part of a University of Aberdeen PhD supported by the NERC Centre for Doctoral Training in Oil & Gas (grant No. NE/M00578X/1 awarded to A.J. Cawood).