Abstract

Structural style in thin-skinned thrust belts is controlled substan tially by the stratigraphic succession and relative thicknesses of lith otectonic units (stiff layers and weak layers). In contrast to the rela tively 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 suc cession 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 termina tions range from plunge of the stiff-layer roof over a laterally thin ning mushwad to a transverse fault. The examples in the Appala chian 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 petro leum exploration and reservoir development.

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