A catalogue of extensional and contractional structures generated along gravity-driven detachments

Although detachments form an integral component of gravity-driven downslope movement, their largely bed-parallel nature can make them difficult to identify in both seismic and outcrop studies. Sediments above bed-parallel detachments (BPDs) associated with intra-stratal deformation are carried passively downslope and therefore represent a form of mass transport deposit (MTD). Using outcrop examples of MTDs created in late-Pleistocene lacustrine sediments around the Dead Sea, we present a catalogue of exceptional extensional and contractional structures generated along BPDs. Detachments may form individual structures that locally ramp to higher structural levels, creating extensional ramps in the upslope direction and contractional ramps further downslope. Alternatively, detachments may comprise multiple strands that kinematically interact with one another via ‘soft-linkage’, or are geometrically coupled via connecting faults to create a ‘hard-linkage’. Lower detachments involving multiple slip surfaces interact with one another to create localised extensional and contractional duplexes, whilst collectively forming the base of the slide sheet. Upper detachments may consist of several strands, or alternatively form ‘soft-linked detachments’ where deformation is distributed across units with no discrete slip surface. Thrusts above detachments frequently follow a ‘piggyback’ sequence with younger shallower thrusts displaying less overall shortening forming downslope of older steeper imbricates. This suggests that they are formed during downslope migration of the duplex rather than upslope propagation of compressive strain during ‘locking up’ of gravity-driven deformation. The fields of extensional and contractional strain may expand and change location during evolution of the slide, leading to normal faults and thrust faults locally overprinting one another. In addition, thrust and normal faults may operate coevally, or locally reactivate one another during positive and negative inversion of individual structures. Transfer of BPD displacement to different levels is achieved by local ramping across older normal and thrust faults, resulting in significant changes to the stratigraphic position of BPDs, together with notable variations in heave across fault zones. Movement on the BPDs is facilitated by high fluid pressures that create sediment injections along the slide planes, especially where normal faults intersect the lower detachment. Where multiple BPDs are formed along the lower detachment, sediment injections cut the upper detachment, suggesting that deformation and movement was longer-lived along the lowermost BPD in a system. Overall, the observation that all measured BPDs (N > 170) consistently display a top-towards the basin sense of displacement suggests that they were sequentially developed through the sediment pile during numerous slope failures, rather than forming synchronously in a single major event that could lead to local apparent reversals in shear sense across adjacent BPDs.