FOLD DUPLEXES

In fold-thrust belts, fold duplexes play the same role as fault and cleavage duplexes, tranfering displacement upward from one detachment level to another. However, the nature of the roof and floor thrusts may differ among the various types of duplexes. Fault and cleavage duplexes have roof and floor thrusts that are discrete faults or thin fault zones. Fold duplexes also possess a clearly defined floor detachment; but in many cases the roof is a zone of multiple detachments, resulting from the changing fold geometry within the duplex as it develops.

The geometry of folds within some fold duplexes suggest that they grow in a sequential-synchronous fashion. Near the leading edge of a fold duplex, the folds are more open, of lower amplitude, and widely spaced; toward the trailing edge, folds have higher amplitudes, are tighter, and often isoclinal. This geometric variation indicates that folds develop in a hinterland-to-foreland sequence, new folds initiating while existing ones continue to grow. As folds tighten and adjacent anticlines converge, the strata in the intervening synclines are squeezed out of the synclines, producing a roof detachment. Thus, a roof detachment may not materialize until synclines are sufficiently tightened. Due to spatial and temporal changes in the geometry of folds within the duplex, roof detachments form at different structural levels as the duplex evolves. This kinematic history is suggested by the presence of multiple roof detachments.

Fold shortening within the duplex must be balanced by displacement of the roof sequence, internal deformation within the roof, or a combination of the two. As with fault and cleavage duplexes, this internal deformation or displacement may occur above active or passive roofs. In an active-roof duplex, the roof sequence is translated relative to the floor thrust’s footwall. In a passive-roof duplex, the deformed core of the duplex underthrusts the roof’s hanging wall, which remains fixed relative to the floor’s footwall. Active-roof fold duplexes usually occur near the trailing edge of thrust sheets, the roof fault usually being a dominant thrust. Passive-roof fold duplexes frequently occur near the leading edge of a thrust sheet and may possess multiple roof detachments. Examples are provided from the US Rocky Mountain thrust belts.