Origin of oblique structures controlled by pre-tectonic thickness variations in frictional and salt-bearing fold-and-thrust belts: Insights from analogue modelling

This work includes, first, a synthesis of experimental analogue modelling programs assessing the influence of along-strike thickness variations in the development of oblique structures in fold-and-thrust belts. As a second part, we present a new experimental program testing, systematically, the influence of tapered decollement-cover thrust wedges. Previous experiments show that frictional, brittle models feature numerous, short-wavelength structures where the cover is thinner. Deformation fronts are totally or locally oblique to the backstop when the cover thinning is progressive or sharp, respectively. Low (<30) or higher structural obliquities occur when thickness variations are progressive or sharp, respectively. The addition of a basal or intermediate decollement, commonly entails the transference of deformation towards its external pinch-out. Consequently salients occur and transfer zones and oblique structures form over decollement boundaries. Their location and orientation strongly depend on the pre-compressional shape of the decollements. Furthermore, as demonstrated by our modelling results, tapered brittle covers over viscous decollements result in asymmetric thrust wedges formed by structures that end or change their vergence laterally, resulting in obliquities generally <30. Two natural case studies, showing strong (South Pyrenean Central Salient) or moderately oblique (Keping Shan fold-and-thrust belt) structures are revisited and compared to the described analogue models.