Duplexes as slip transfer zones: Application to the Himalayan fold-thrust belt

摘要

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). 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, accommodating regional bed-parallel shortening and transferring slip from a lower stratigraphic horizon to a higher one, thereby providing a mechanical advantage to the emplacement of large thrust sheets. However, faulting is not the only means of layer parallel shortening (LPS) and displacement transfer within duplexes; LPS cleavage and detachment folds are also common and can play the same role as imbricate faults. Based on some selected examples, we recognize three types of duplexes: (1) fault duplex, (2) cleavage duplex, and (3) fold duplex. The style of duplex that forms is likely a function of mechanical stratigraphy and deformation conditions. The relationships among the three basic duplex styles can be 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 cleavage shortening, can also form. Hybrid duplexes plot along the side of the triangle as fault-fold, fold-cleavage, and fault-cleavage duplexes, while those combining all three mechanisms plot in the middle. If the partitioning of shortening by the different mechanisms can be quantified the ternary diagram provides a means of representing the contributions of the three mechanisms in shortening and slip transfer.Along much of the Himalayan fold-thrust belt a significant portion of the Lesser Himalayan shortening is taken up by the Lesser Himalayan Duplex, a first-order fault duplex, with subsidiary cleavage and fold development. The geometry of and shortening within the duplex varies significantly along the length of the Himalaya.