Physical models and natural examples of fold interference
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Fold interference is used to identify areas of polyphase deformation and to evaluate their deformation history. In this study, dynamically scaled physical models were shortened in two orthogonal directions in a centrifuge to test the effects of material properties and deformation history on the style of fold interference. Models shortened sequentially demonstrate that rheological contrast strongly controlled the interference style. Models with low competence contrast layering had circular to elliptical interference patterns. In addition to folding, the models accommodated strain by significant layer-parallel shortening. Models with high competence contrast layering had folded early hingelines and axial surfaces. In plan view, the second-generation folds were lobate-cuspate to box-style, and the axial traces of the box folds formed conjugate pairs. Models shortened coevally had distinct structural styles from the sequentially shortened models, as illustrated by serial Computed Tomographic (CT) X-ray scans. High competence contrast models were dominated by irregular elliptical to crescent map-shapes in the center of the models. Near the model edges, folds had straight hingelines, parallel to the boundaries. Coevally shortened models faulted and fractured less commonly than did the sequentially shortened models. Although some of the structures resembled superposed folds, the regionally inconsistent fold orientations and overprinting relationships, as well as the predominance of dome-and-basin over crescent-style interference differentiated coeval from sequential fold interference. For both deformation histories, gravitational body forces effectively damped the vertical amplification of folds, thereby accentuating the change in fold style with depth. Comparison of results from these models with regional-scale natural examples demonstrates that buckle-fold interference occurs in a wide range of rock types and tectonic settings. Comparison of the sequentially shortened, high competence contrast models with the Narragansett Basin, Rhode Island, suggests an alternative tectonic model for its third phase (D3) of Alleghenian deformation. According to previous tectonic models, D 3 sinistral shear produced both E-W trending folds and sinistral kinematic indicators. I propose that the same features could be caused by N-S shortening, which is locally partitioned into a NE-trending megakink band.