Kinematic models of fault-related folds predict a direct relationship between fault angle and fold shape and imply a specific sequence of deformation. Some of the characteristics of these models are: 1. footwall rocks are undeformed; 2. thrust ramps cut upsection from a lower detachment; 3. fault displacement is either constant (fault-bend folds) or decreases uniformly upsection (fault-propagation folds); 4. fold hinges migrate. Analyses of outcrop-scale structures reveal fault-fold configurations that depart from those depicted by the models.
We examined several folds in Silurian rocks from the Valley and Ridge province (Maryland, Virginia and West Virginia) of the Appalachian Mountains. Some of the deformation patterns recognized are: 1. thrust ramps may lose displacement upsection and downsection; 2. ramps do not necessarily join a lower detachment (flat); 3. footwall rocks are folded (syncline); 4. displacement decreases from competent to incompetent beds. Deformation styles are influenced by the character of the sedimentary rocks (rock types, relative thickness, order of units), an observation that is not readily incorporated into kinematic models.
Some structures are similar to those recognized in fold-and-thrust belts (e.g. duplexes) but the mesoscopic deformation patterns are interpreted to suggest an alternative mode of formation that does not require the presence of either a lower or an upper detachment horizon.
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