Thrust system growth by segment linkage in sedimentary multilayers

This presentation challenges the conventional theory that thrust systems necessarily grow by the upward propagation of tip-lines of faults away from a basal detachment. The model underpins mechanical considerations of thrust belt dynamics, explanations of thrust-related folding and commensurate forecasting of distributed deformation in the surrounding strata. Yet there have been few tests of the paradigm at the cross-section scale – not least because the quality of seismic imagery in most foreland thrust belts is insufficient to resolve structural geometries. Rather, the widespread application of the upward propagation (conventional) theory results from a variety of cognitive biases, especially restrictive citation of published literature.

Here we present interpretations of exceptional 3D seismic imagery from the eastern, lateral termination of the Jura fold-thrust belt of Switzerland. The area (the Baden-Irchel-Herdern Zone, referred to here as the BIHZ - broadly overlying the eponymous basement lineament) shows relatively little horizontal shortening and therefore preserves structures that would otherwise be overprinted by more extensive thrusting.  Contractional structures are developed in a multilayer of Mesozoic strata that comprise competent carbonates and incompetent mudrocks and marlstones. Interpretations based on 2D seismic profiles display thrusts sweeping through the Mesozoic strata as simple, continuous ramps, splaying from a basal detachment in Triassic evaporites. However, the higher-quality 3D seismic imagery does not support this type of interpretation. Within the BIHZ, statal reflectors are offset by small-offset (<50m) thrust segments. These do not map out as continuous fault surfaces, either in depth or in map-view. Some thrusts dip forelandward, others towards the hinterland. Collectively, these thrusts are layer-confined and, in plan-view, have highly sinuous forms. These geometries are indicative of having formed by linkage of originally distinct fault segments. Collectively this zone of segmented/low-displacement thrusting represents a “bead” of distributed deformation, restricted to the BIHZ. Presumably, if the area then evolved with greater contraction, part of the thrust array will become fully-linked and develop as a continuous structure – perhaps as elsewhere along the Jura arc. Those fault segments not incorporated into the now-continuous thrust would remain, forming a broad halo of distributed faulting in the surrounding rock. In this spatial context, the swathe of distributed deformation might be designated as a “damage zone” with respect to the main thrust but in fact, the distributed faulting would have no causative relationship to this main thrust.

Our interpretation of thrust system evolution derives from the “ramps first” model of Eisenstadt & DePaor (1987) – a structural concept that has received substantially less attention that the “conventional theory”. We do not wish to imply that there is a single mechanism by which thrusts nucleate and grow – the “conventional” theory may well apply in some situations. So too, may others. We do however emphasise the importance of considering a range of different behaviours and their resultant structural geometries when interpreting thrust systems. Some of these may yet to be described!