Strata-dependent thrust localisation in multilayers

In most idealised models of thrust fault formation thrusts form along a weaker basal detachment and then propagate upwards through strata, producing a hang-wall anticline and developing in a piggy-back sequence. This idealised thrust fault geometry commonly fails to match actual outcrop structures. Conversely, in a stiff layer thrusting model, the thrust originates and localises in mechanically competent beams as ramps and the thrust tip propagates both up and down slowly to create a linked fault system. This model has had few tests at outcrop level and the lack of field examples has hindered adoption of this model. 

In the UK, thrust structures in the Old Red Sandstone are ideal test outcrops as they contain a multilayer stratigraphy of competent sandstone beds encased in cleaved mudrocks. Field and photogrammetric mapping has been applied to the St Brides Haven outcrop on the west coast of Pembrokeshire in SW Wales, the 10m high outcrop exposes an open fold pair cut by thrusts typical of Variscan deformation. The outcrop has an abrupt rheological change within the multilayer with a 1m thick sandstone bed within mudstones. Thrust faults are confined to the strong sandstone beam, with a fault spacing along the competent beam, with cleavage development in the mudstone above and below. The structural style is controlled by variations in the multi-layer rheology, and fault localisation in the competent sandstone beams appears to be balanced by distributed deformation in the form of cleavage development in the mudstones. The outcrop matches key features of the stiff layer thrusting model.

Observations from this outcrop show that the observed thrust fault geometry evolved from soft to hard linked across the multilayer. Drag faults have developed in both the hanging and footwalls, showing thrusts initiate in the stiff layers. Cleavage developed in the mudstones and along rheological boundaries the cleavage and thrust trajectories connect. The outcrop matches key features of the stiff layer thrusting model. This work contributes to a series of studies on fold-thrust outcrops to expand the range of widely used models and avoid bias in interpretation of fold-thrust belts when basing work on theoretical models.