Exposure of folded rocks at the Earth's surface provides the opportunity to better understand deformation mechanisms and make predictions about structural architectures in the subsurface. This collection includes outcrops from Pembrokeshire (SW Wales), the French Subalpine Chains, The Sawtooth Range (NW Montana) and meso-scale fold structures from a number of localities around the world.
Deformed and folded lithologies at the localities below include carbonates, clastic multilayer sequences and low-grade metamorphic rocks. Fold characteristics are influenced by rock composition, layer thicknesses and competence contrasts, among other factors. Explore the virtual outcrops below and look at the variety in folding geometries and structural style. Note also the interaction between folding and thrusting and the role of small-scale structures in accommodating deformation.
Stackpole syncline in Pembrokeshire is a classic outcrop which exposes Visean limestones in an upright structure.
The fold is cylindrical and plunges moderately to the ENE. On the seaward side of the structure, the hinge zone is clearly exposed, with cleavage visible. Close inspection of the hinge zone reveals cleavage diffraction, meso-scale deformation structures and bed-thickness changes.
This outcrop was sketched by Lady Murchison for Roderick Murchison's Silurian System of 1839 and appeared on the cover of the first issue of The Journal of Structural Geology (1979). In 2017 Stackpole was used as a test site for comparing LiDAR and photogrammetry for use in structural analysis (Cawood et al., 2017; https://doi.org/10.1016/j.jsg.2017.04.004)
Swift Reservoir Anticline in The Sawtooth Range, NW Montana exposes Mississippian carbonates of the Madison Group in a fault-related fold structure.
This virtual outcrop is of the anticline forelimb, exposed here just below the reservoir spillway. Rotate the virtual outcrop and notice how the fold has been used as part of the spillway.
The fold is asymmetric - the forelimb dips steeply to sub-vertical towards the ENE; the fold backlimb, in contrast, dips shallowly (~26 degrees) to WSW. This asymmetry is likely due to a thrust fault at depth.