| Rock deformation in the fold-and-thrust belts of mountain ranges involves complex interactions of faulting and folding. The geometry and fabric of natural fault-related folds are analysed to document the accommodation of folding on meso- and microscopic scales, and to constrain the deformation histories of the folds sampled. Fold geometries and fabrics are compared to analogue models to better understand deformation processes.; An outcrop-scale fault-related fold in a carbonate unit in the Wyoming fold-and-thrust belt displays a hanging wall backthrust, veins, dissolution seams, calcite twins, fractures and microfaults. Their orientations vary with position in the structure. The distribution and orientation of structures around the fold, and the fold geometry, are indicative of early buckling adjacent to a steep ramp, followed by slip over a gentler, modified, ramp geometry.; Three hand-sample-scale folds were taken from the limbs of the Wills Mountain Anticline in western Maryland, in the central Appalachian Valley and Ridge. Structural fabrics differ for each fold, indicating rheological contributions to and local structural control of fabric development. Two of the samples show asymmetric vein distributions consistent with asymmetric flexural folding. Veins, stylolites, and intracrystalline deformation in the third fold are suggestive of (symmetric) tangential longitudinal strain followed by horizontal flattening.; Experiments with layered butter and saturated quartz sand produce structures analogous to those seen in natural folds. Backthrusts develop parallel to the strike of each portion of the ramp, initiate at the ramp base, and are transported up ramp as displacement progresses, thickening the hanging wall. Extensional fractures develop in the outer arc of the fold crest. In addition, lateral extensional and contractional structures develop where the fault ramp dramatically changes orientation. Thus, complex three-dimensional suites of structures can be attributed to along-strike variations in fault geometry in a single deformation event. We infer that contractional and extensional features in natural fault-related folds form in a similar manner. |
