Abstract:

Detachment folds are widely distributed in the leading edge of Kuqa fold and thrust belt (KFTB). Although the Cenozoic sedimentary strata involved into the structural deformation and their underlying basal detachment (Paleogene salt layer) are the same, the structural characteristics and the evolution of detachment folds are quite different. In this paper, on the basis of the field observations and the interpretations of highquality seismic and well data, we demonstrate the initial salt layer thickness and discuss their relations to the deformation styles and the evolution of detachment folds, taking the examples from the Nanka and Misikantage anticlines. The results of our analysis indicate that the initial underlying salt layer of Nanka anticline is about 0.10.5 km in thickness, generally of 0.10.3 km, which is in contrast to the Misikantage anticline with thickness of as much as ~ 1.0 km. Meanwhile, the alongstrike variations in the structural styles have been observed both in the Nanka and Misikantage anticlines. The Nanka anticline is a lowamplitude detachment fold, whose eastern segment is buried under the ground surface and characterized by the folding deformation. However, in its western segment the fold with relative higher average upliftrate is consequently exposed on the surface associated with both folding and thrusting, as evidenced by thrust faults developed at the core of anticline. In contrast, the Misikantage anticline is a largescale detachment fold, which can be divided into three segments of different geometry. The northvergence of the anticline in the eastern segment are different from the southvergence of the anticline in the central and western segments. Meanwhile, thick salt is mainly accumulated underlying the north limb and at the core of anticline in the eastern segment, which is in contrast to the salt accumulated at the core of anticline, thinning towards both limbs and even forming the salt welds in the central segment, and also obviously different from the boxfold with underlying salt layer of at least 1 km in thickness in the western segment. In summary, seven distinct deformation styles of detachment folds in the leading edge of KFTB have been demonstrated, and, according to our analysis, the deformation style of detachment fold was strongly affected by the salt layer thickness, compressive shortening and salt flow. Among them, the salt layer thickness was dominative in controlling the deformation style of detachment fold. The results of this study concerning about the structural characteristics and evolution of detachment folds and their controlling factors might be helpful for the investigations of the detachment folds in other fold and thrust belts, especially within the regions without highquality seismic data or poor quality due to intense strutural deformation.

Key words: salt layer thickness, salt flow, detachment fold, structural characteristics and evolution, Kuqa fold and thrust belt