Structural style in thin-skinned thrust belts is controlled substantially by the stratigraphic succession and relative thicknesses of lithotectonic units (stiff layers and weak layers). In contrast to the relatively predictable geometry of fault-related folds (fault-bend folds, fault-propagation folds, and detachment folds), ductile deformation of a stratigraphically thick weak layer generates a ductile duplex that elevates and distorts the overlying stiff layer. A ductile duplex is herein termed a "mushwad" (Malleable, Unctuous SHale, Weak-layer Accretion in a Ductile duplex). In the Appalachian thrust belt in Alabama, mushwads nucleated in a thick, shale-dominated succession of weak rocks in association with basement faults beneath the regional decollement. The stiff-layer roof is deformed by folds and faults and is elevated by tectonic accretion of weak rocks in a mushwad. Shortening within the mushwad may drive translation of the stiff layer from the roof over the foreland. Lateral terminations range from plunge of the stiff-layer roof over a laterally thinning mushwad to a transverse fault. The examples in the Appalachian thrust belt suggest a style of structure that may be more common than presently recognized. The structural geometry of a mushwad, as well as the potential for concentration of fractures in the distorted stiff layer, suggests important applications in petroleum exploration and reservoir development.