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The Kuqa Fold-and-Thrust System: Geometry of a Deformed Multilayered System Involving Coal and Salt Décollements


The 3D geometry of fold-and-thrust systems is strongly controlled by both the rheology and lateral extent of weak décollements interlayered in the deformed sequences. These décollements trigger the nucleation of structures at their boundaries independently of their orientation with regard to the shortening direction. The Kuqa basin, in NW China, is a Cenozoic, oil-bearing fold-and-thrust system located in the southern foreland of the Tian Shan Range. The basin hosts a continental infill made of up to 12 km of Permian to Holocene sequences that unconformably overlie Paleozoic basement. From a structural point of view, it consists of a central syncline bounded to the South by a NE-SW-striking antiform that is detached on Paleogene evaporites and displays an arcuate shape in map view (the Qiulitage structure). To the North of the central syncline, Mesozoic and Cenozoic units are deformed by a south-vergent thrust system partly rooted in Triassic-Jurassic units (the Kelasu structure). Its southern boundary is linear, E-W-striking and oblique to the frontal Qiulitage structure. This obliquity between the Kelasu and Qiulitage structures generates significant along-strike variations in the geometry of the Kuqa basin. Field and seismic data indicate that this along-strike structural variation is due to the areal distribution of two main décollements: a lower one that crops out to the North and consists of thin coal layers, and an upper one, located in the central and southern part of the basin and mainly made up of salt. The different rheology of these décollements triggers different structural styles: thrusts with hanging-wall fault-bend folds are dominant above the coal, whereas tight detachment folds, diapirs and salt nappes developed where the décollement is represented by salt. Where both décollements overlap, a duplex system formed with a floor thrust rooted in Triassic-Jurassic units and a roof thrust along the Paleogene salt. We carried out sand-box models with two interlayered polymer décollements simulating different mechanical properties and displaying an areal distribution that is similar to that inferred for the Kuqa basin. Experiments yield results that are consistent with the deformation pattern observed in nature. This combined structural and analogue modeling study contributes to the understanding of the previously poorly characterized structure of the Mesozoic units underlying the salt in the Kuqa basin where the main oil reservoirs are found.