Thrust initiation and propagation during shortening of a 2-layer model lithosphere

Analogue modelling was used to investigate thrust initiation and propagation in the lithosphere. The model comprised a top layer of light sand (model brittle crust) overlying a layer of heavy silicone putty (model ductile lower crust), both resting upon higher density and lower viscosity natural honey (model ductile mantle). The experiments show the following. (1) Thrust initiation may occur in two main modes: fold-first or fault-first. They seem to be the result of presence or absence of an initial perturbation close to the piston, respectively. The fault-first mode eventually goes into a fold-first mode after a certain amount of deformation. (2) In both modes, thrusts localized around the inflection points of both fore and back-limbs of buckles as a result of tectonic (horizontal), gravitational (vertical) and elastic forces. (3) Thrust propagation in the fault-first mode occurred by thrust wedge thickening and downward flexure due to isostatic compensation, which led to forward formation of bulges and subsequent amplification by buckling. Continued shortening increased the stresses that reached the brittle yield of sand and new thrusts originated. (4) Bulging ahead of thrusts tended to develop into asymmetric buckles with the shorter limb (back-limb) steeper towards the piston. (5) As a whole, thrust initiation and propagation seem to depend mostly on how stresses evolve and propagate through the model and how material strength varies with progressive deformation. The main stresses at play are born at the moving piston (model horizontal tectonic force), from thickening and isostatic compensation in the model (gravitational forces), and from elastic resistance to buckling.