Coupled passive extension and compression on salt‐based passive margins analyzed by physical models

Abstract

This study analyzes the kinematics of thin-skinned, gravitational extension and coupled passive contraction affecting the salt cover on passive margins. Analogue models were designed to simulate the sliding and/or spreading of low viscosity horizons lying beneath frictional covers. No syn-kinematic sedimentation was reproduced, with the scope of analyzing the factors controlling the geometry and kinematics of active faulting. Our models, after deformation, are partitioned into domains whose cross-sectional lengths are independent of the areal extent of the basal viscous layer. However the crosssectional length, its geometry, the distribution of deformed domains as well as the total length and the rate of extension are directly controlled by the margin's slope, the thickness of viscous d ecollement layers and its overlying frictional cover and the relative ratio of their respective thicknesses. This ratio also controls the fault throw and the grounding of the cover, forming welding surfaces. Grounding tends to stop extension and limit the cross length of the deformed domains. Due to the lack of sincinematic sediment supply, upslope extension results mainly in the form of symmetrical grabens that are concave basinward in the map view. When rafts form, their crosssectional length commonly increases downslope.Compression downslope takes place at the frontal termination of the viscous d ecollement, due to the increased basal friction. Compression is expressed by allocthonous tongues in the central part of the model, whereas folds and thrusts form at the lateral termination, enhanced by the lateral friction. These structures evolve as a breakback propagation sequence. Although analogue models are simplified replicas of natural systems, the results obtained by our models provide valuable insights concerning the evolution of these geodynamic systems.