DUPLEXES AS SLIP-TRANSFER ZONES: AN UPDATED SYNTHESIS

Duplex fault zones and imbricate fans, originally termed schuppen, Schuppenstruktur, or imbricate structure, were first described in the late 1800s, from the Alps, NW Highlands (Scotland) and Montana Rockies (Lewis thrust sheet) . Little was written about duplexes until the 1950s, when Canadian petroleum explorationists discovered extensive gas reserves in large-scale duplexes of the Alberta foothills. As originally defined, duplexes contained shingled imbricate slices bounded above and below by through-going thrusts. These duplex fault zones commonly occur along the base of major regional thrust sheets, or within the body of such sheets, transferring slip from a lower stratigraphic horizon to a higher one. By means of this displacement transfer, duplexes provide a mechanical advantage to the emplacement of large thrust sheets. However, faulting is not the only means of tectonic shortening and displacement transfer within duplexes; folds and layer parallel shortening (LPS) are also common and can play the same role as imbricate faults. Folds may be either detachment (lift-off) or buckle folds, and LPS can occur by either tectonic compaction in clastic rocks, or formation of pressure-solution cleavage in carbonate-rich rocks. Therefore, based on some selected examples, we recognize three types of duplexes: 1) fault duplex, 2) fold duplex, and 3) LPS duplex . These three types of duplexes span brittle to ductile behavior, and the styles of duplexes that form are a function of the mechanical stratigraphy. The relationships among the three basic duplex styles can be conveniently plotted on a ternary diagram, each duplex type constituting an apex. Given the complexities of most sedimentary sequences, hybrid duplexes, with a mix of fault, fold and layer-parallel shortening, can also form. Hybrid duplexes plot along the side of the triangle as fault-fold, fold-LPS, and fault-LPS duplexes. Hybrid duplexes combining all three mechanisms would plot in the middle of the diagram. The ternary diagram provides a means of representing different duplex types with the attendant implication of variations in lithotectonic stratigraphy.