Deformation patterns and structural vergence in brittle–ductile thrust wedges: An additional analogue modelling perspective

Scaled physical models of brittle–ductile thrust wedges investigated the causes leading to the development of the various structural styles observed in fold-and-thrust belts and accretionary prisms. This study focused on some potential controlling factors, such as (1) relative strength between the brittle overburden and the viscous décollement layer (brittle–ductile coupling), (2) the effect of very low basal friction, and (3) pre-shortening rheological stratigraphy, including décollement strength. Type 1 models had a single basal décollement, whereas in Type 2 models the décollement was embedded in brittle material simulating an upper roof sequence and a lower floor sequence. Other models investigated the effects produced by an horizon of mechanical weakness at the base of the viscous décollement (Type 1a), and the role of décollement offset (Type 3). The experiments suggest the occurrence of a genetic relationship between the absolute values of shear stresses and the development of dominating hinterland- or foreland-verging thrust faults, or dually-verging thrusts. By controlling the orientation of the σ1 axis, basal friction and décollement offset is found to effectively influence the deformation pattern and structural vergence, and could explain the development of dominating backthrusts in Cascadian-type margins. The decoupling of a frontal monocline via the upper décollement (Type 2 models) produced passive-roof duplex structures, termed “composite-roof duplex” when the frontal monocline coexists with outward fold propagation.