Effect of mechanical stratigraphy on the structure and kinematics of salt-bearing fold-and-thrust belts: Insights from scaled analogue experiments with optical strain monitoring (digital image correlation)

Abstract

2D contractional scaled analogue experiments with composite materials including silica-sand and mica-flakes for overburden and silicone for salt analogue are used to investigate effects of mechanical stratigraphy on the structural evolution and kinematics of salt-detached fold-thrust-belts. Specific parameters tested are mechanical stratigraphy of the overburden and thickness variation of the basal silicone layer. The silicone-detached models in general are characterized by low-taper thrust wedge geometries and non-systematic vergence of folds and thrusts. Strain localization in the undeformed layer occurs as an in-sequence foreland breaking sequence. Strain is nucleating as detachment folds including thrust-bounded and concentric folds. Increased shortening develops break-thrusts in fold limbs. In-sequence frontal thrust interacts with out-of-sequence reactivation of older thrusts in the internal thrust wedge. Syn-kinematic silicone mobilization causes diapirism, allochthonous sheets and source-fed thrust. The specific distribution of discordant and allochthonous silicone structures vary with the mechanical stratigraphy. The impact of the mica-interlayer in the overburden sequence is strain-dependent. It strengthens the undeformed sand-pack compared to initial thrusting while active thrusts with mica-flakes in shear zones are weaker and active for longer than in homogeneous sand-pack. The longevity of thrusts correlates with the transfer of silicone to external domains and hanging-walls of thrusts. The silicone thickness controls the strain nucleation modes whether thrusting-dominated or folding-dominated predating main-thrusting stages. It also governs silicone supply and flow regimes with thick silicone source layers being readily remobilized to source-fed thrust and inflate silicone massifs in the foreland.

Insights from the modelling results are that the formation of large-transport source-fed thrusts such as Quele Thrust (China) and Chazuta Thrust (Peru) observed in salt-bearing FTB's is possibly attributed to salt detachment thickness and anisotropic overburden resulting from mechanically layered stratigraphy.