Duplexes are formed through continued thrusting along a floor thrust with successive collapse of thrust ramps. Antiformal stacks are defined as systems of totally overlapping thrust horses that are characterized by a coincident trailing branch line. Antiformal stacks result when the forward motion of a forward-breaking thrust sequence is interrupted or completely blocked from regular forward development of a foreland-vergent duplex system. Antiformal stacks commonly occur in the cores of mountain chains, mainly in continent-continent or arc-continent collision zones, where the subducted plate acts as an obstacle, forcing faulting upward.
The Lewis thrust forms a 450 km long fault with thrusting of Precambrian limestone over the top of Cretaceous shale (taking place 160-145 Ma). This Alberta-Montana thrust sheet has a duplex structure that exhibits the geometries of both a hinterland-dipping duplex and an antiformal stack, and that contains inclined and stacked thrust horses that are bounded by the main fault traces.
The Lewis thrust surface is a low-angle thrust fault with ramp-flat geometry, indicating that the thrust moved horizontally, stepping upwards through stratigraphic layers. The higher thrust faults in the Lewis duplex are folded over lower faults ramps and their associated horses, indicating that slip on the higher thrusts occurred first, as the formation of thrusts progressed downward and toward the foreland.
◙ subduction zone magmas ◙
[link: images: animations: animation of duplex formation; panoramas: Tarndale, New Zealand, Wairau Quicktime panorama (along the fault zone); Death Valley Quicktime Panorama; Interactive 360 degree view of the San Andreas Fault at Wallace Creek; Earth Revealed, courtesy of Anneberg Media, requires Windows media Player; horse: horse wedged between two faults; formations: remarkable internal imbrication or "duplexing" of a single layer (turbidite sequence somewhere in Middle East, from AAPG Bulletin); extensional duplex; duplex in late Paleozoic limestone, Crows Nest Pass, southern Canadian Cordillera; viewing a duplex reverse fault structure, Redwall Fault in Sinclair Canyon, Kootenays, Rockies; beautifully developed duplex structure in glacial lakebeds at Skardu, Karakoram-Himalaya; outcrop scale extensional duplex, Triassic rocks, coast of Chile; thrust fault duplex, near Albany, NY, within the Hudson Valley fold and thrust belt; isoclinal upright anticline (lower part of the Lønstrup Klint Formation in the frontal part of the Stortorn Section) with right limb that constitutes an imbricate duplex formed by connecting thrust-fault splays (white dot-and-dash lines). The fold is interpreted as a hanging-wall anticline developed during fault propagation and successive imbricate stacking; complex duplexing at least three imbricates have been sheared off from and shoved under the continuation of the layer above, in sandstone layer in Tertiary turbidites of Olympic accretionary prism, Washington; imbricated slump structures, imbricated slump structures in sandstone beds and closer view;duplex in sedimentary sequence deformed under partially consolidated conditions, Pliocene tuffaceous mudstones and sandstones, south of Tokyo Japan; duplex in Albany limestone, VT; geomorphic disruption in response to the Hope Fault oblique strike-slip duplex in the Lottery-Mt Lyford area, North Canterbury, NZ; Valley and Ridge, duplex, Pellissippi Parkway; diagrams: fault propagation fold duplex; footwall imbrication; duplex-unit model for fault-bend folding of duplex segments, wp; duplex thrust interpretation of the effect of the downdip step in the plate interface, Hikurangi Subduction Interface, NZ; Gunnison thrust, duplex structure of fault-bend-fold, central Utah; webpages: Thrust Faults; DeGray Spillway, I, II, III]
image of imbricate thrusting in the Himalayas courtesy of USGS