Influence of syntectonic sedimentation on kinematic evolution of fold‐and‐thrust belts with lateral changes in shallow décollement properties and basement inherited structures: insights from analogue modeling

Structural deformation of fold-and-thrust belts is influenced by the properties of décollements (number, rheology, thickness, etc.), the presence of inherited structures in the basement as well as the amount of syntectonic sedimentation, among others. Although the effect of each of these parameters has been well constrained with a series of numerical and experimental works in the literature, few sandbox models comprehensively consider all these parameters together, and particularly investigate the effect of their lateral variation. In this context, we carried out several 3-D sandbox models to investigate the effect of increasing syntectonic sedimentation rate on kinematic evolution of fold‐and‐thrust systems which contain a basal brittle detachment layer and a shallow detachment layer that changed from a brittle to a viscous domain along the mountain strike. The influence of different basement width structures, affecting the kinematics and geometry of the interbedded viscous décollement, has been also tested.
Results indicate that the rate of syntectonic sedimentation exerts a first-order control on the kinematic evolution of fold‐and‐thrust belts since increasing syntectonic sedimentation rate stops (in the brittle domain) or delays (in the viscous domain) the propagation of deformation towards the foreland. Moreover, syntectonic sedimentation prohibits the propagation of deformation in the deep décollement level due to the modification of the taper angle. Structural evolution of the transfer zone in between the brittle and viscous domain is also affected since if becomes narrower and more orthogonal to the mountain front at higher sedimentation rates. Specifically, in the brittle domain, the fault dip angle increases with the increase in syn-sedimentation rate and its cross-sectional geometry becomes straighter. In the viscous domain, syntectonic sedimentation affects the partitioning of deformation with development of long-lived and complex 3-D salt structures near the hinterland (such as squeezed diapirs, salt welds and salt tongue), whereas frontal structure becomes more cylindrical. Toward the hinterland, syntectonic sedimentation increases backthrust activity, which becomes increasingly different between the brittle and viscous domain. For instance, the increase in backthrust displacement in the ductile domain is greater than the one in the brittle domain. About the basement high, our study reveals that it has a strong controlling effect on the viscous domain, dominating the development of structural belt on the top of the basement high and promoting the propagation of deformation front to the pinch-out of the salt layer. Besides, syntectonic sedimentation simplifies the structural style between the basement high and the hinterland. It strengthens the structural influence of the transfer zone, which localizes into a single strike-slip transfer fault which increases the frontal fault displacement.
Our experimental results are compared with structures in the Wushi-Kuqa fold-and-thrust belts in Southern Tianshan (Central Asia) and help better understanding interaction between syntectonic sedimentation, décollement properties and basement configuration.