Ramps first – Interpreting thrust nucleation in multilayers

Models are key for geoscientists working in subsurface fold thrust belts, who want to interpret complex geometries. However, models based on a few landmark outcrop studies dominate interpretation. In these models thrust faults form first as flats along weaker beds and propagate upwards, producing a "hard linked", fully connected thrust fault structure. The Eisenstadt and De Paor (1987) model challenges the conventional thrust flat-first, reflecting field observations which show that fold thrust outcrops vary remarkably from each other, with a variety of geometric, linkage, and stratigraphic behaviours. Here we investigate an outcrop of thrusted sediments at St Brides Haven, Pembrokeshire. Structural observations of the outcrop show an imbricated stack, where isolated thrusts have developed within and localised along sandstone layers. The outcrop provides an example of the alternative Eisenstadt and De Paor model of ramps first. But here deformation in the encasing 'soft' mudstone layers is accommodate by homogeneous shortening. We suggest that the prevalence of "hard linked" thrust models is a bias towards conventional models and that promotion of a greater variety of fold thrust structures, geometries and evolution styles is needed to ensure a broader range of interpretations and evolutionary understanding that better reflects reality.