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In a "passive-roof duplex" (Banks and Warburton, 1986), shortening is accommodated above the roof thrust by a backthrust sense of displacement. Such a duplex formed during Cenozoic time in the northeastern Brooks Range of Alaska. The floor thrust of the duplex lies at depth in a heterogeneous assemblage of rocks deformed in pre-Mississippian time. The roof thrust lies near the base of the overlying Mississippian and younger cover sequence, in the Mississippian Kayak Shale. Horses were displaced to the north. Shortening above the roof thrust was accommodated mainly by detachment folds that range from open box folds to tight chevron folds, and display no consistent sense of asymmetry. The dominant unit in these folds, thick carbonates of the Mississippian-Pennsylvanian Lisburne Group, immediately overlies the Kayak Shale detachment horizon, which is thickened in the cores of anticlines. Few thrust faults cut the roof sequence, and these are mostly minor out-of-syncline faults where the detachment folds are tightest. The passive-roof duplex and the detachment folds in its roof both disappear northward due to thinning and depositional discontinuity of the Kayak Shale. Sub-Mississippian and unconfonnably overlying Mississippian and younger rocks deformed together as a single structural unit where the detachment horizon is absent. Shortening in both the duplex and its roof must have been accommodated entirely south of this northward termination of the roof thrust, requiring a backthrust sense of displacement on the roof thrust. South of the passive-roof duplex, the sequence above the Kayak Shale was shortened more and displays a different style of deformation. Folds are tight and asymmetrical, consistently displaying steep to overturned short limbs to the north. The limbs of the folds commonly are cut by southdipping thrust faults. A thrust-emplaced roof is preserved locally above these folds, indicating that they constitute a duplex of faulttruncated folds. Formation of the passive-roof duplex and the symmetrical detachment folds in its roof may have been facilitated by relatively low shortening and the effectiveness of the Kayak Shale as a detachment horizon. Strongly asymmetrical, thrust-faulted folds formed instead of symmetrical detachment folds where shortening was greater and significant forward slip occurred on the detachment horizon, where a structural roof was present and depositional overburden was greater, and where folds beneath the detachment horizon were smaller. The relative importance of these various factors in controlling the structural geometry remains uncertain. Detachment folds above a passive-roof duplex, northeastern Bmks Range, Alaska (W.K. Wallace)

Detachment folds above a passive-roof duplex: an example from the northeastern Brooks Range, Alaska