Analysis of Deformation Characteristics and Controlling Factors of the Upper Combined Structure: A Case Study of the Structural Physics Simulation Experiment in the Zhongqiu-Dongqiu Area of Kuqa Depression

Technical Summary: Analysis of Deformation Characteristics and Controlling Factors of the Upper Combined Structure in the Kuqa Depression

Problem Statement
Research regarding the supra-salt tectonic models in the Zhongqiu-Dongqiu area of the Kuqa Depression has remained relatively superficial, particularly concerning the deformation modes and mechanisms controlled by a "double salt lake" (double salt layer) system. While previous studies have established that supra-salt structures in the Dongqiu area are dominated by fractured anticlines and those in the Zhongqiu area by detachment thrusts, the specific genetic mechanisms linking the geometry of salt structures to upper assemblage deformation remain unclear. Key gaps include the weak understanding of the link between supra-salt structures and salt-structure geometry, insufficient analysis of double salt-layer effects, and a lack of explanation for structural transitions in zonal areas. The study aims to resolve these issues by systematically analyzing deformation patterns and the controlling roles of gypsum-salt rock properties, paleo-uplifts, and syndeposition.

Methodology
The study employs a multi-faceted approach combining structural geometry, kinematics, and dynamics with physical simulation:

1. Field and Seismic Analysis: The authors analyzed three typical seismic framework sections (L1228, L1660, L2560) across the Zhongqiu-Dongqiu area. These sections represent distinct geological settings: a single gypsum-rich salt layer (E1-2km), a double salt layer superposition zone, and a single salt-rich layer (N1j).
2. Physical Simulation Experiment: A sandbox model was constructed to simulate the deformation of a double salt layer system over a paleo-uplift.
   • Materials: The model utilized quartz sand for basement and overlying strata, and silica gel mixed with sand to simulate the mechanical properties of the gypsum-salt layers. The E1-2km layer was simulated with a 20% sand-doped silica gel mixture (representing higher toughness/gypsum content), while the N1j layer used pure 700,000 molecular weight silica gel (representing higher plasticity/salt content).
   • Setup: The model featured a conical wedge geometry where the two salt layers interdigitated. A paleo-uplift was simulated with varying angles (9° in the west, 7° in the east).
   • Process: The model was subjected to lateral compression at a constant rate (0.03 mm/s) until a 45% shrinkage was achieved. Deformation was monitored using side cameras and a top PIV (Particle Image Velocimetry) system.
3. Quantitative Analysis: Five specific slices (a–e) from the experimental model were selected for detailed structural analysis, measuring parameters such as thrust wedge angles, extension distances, and structural amplitudes.

Key Results
The study reveals that the thickness and distribution range of gypsum-salt rock layers are the core elements controlling supra-salt structural deformation. The results highlight a spatial evolution from west to east:

• Zonal Distribution: The study area transitions from a gypsum-rich salt area (E1-2km dominant) to a gypsum-rich coordination conversion zone, a double salt lake superposition zone, a salt-rich coordination conversion zone, and finally a salt-rich gypsum salt area (N1j dominant).
• Deformation Styles:
  • Gypsum-Rich Areas (Single E1-2km): Characterized by high-angle thrust faults and large-scale faulted anticlines. The high toughness of the gypsum-rich layer leads to vertical separation and bedding detachment.
  • Double Salt Layer Area: The synergistic action of the two layers creates complex structures. The E1-2km acts as a deep decoupling layer, while the N1j forms shallow salt anticlines and nappes. This zone exhibits the longest forward extension of thrust wedges, the smallest wedge angles, and the largest structural amplitudes. Faults often bifurcate at the interface of the two salt layers, forming fault-controlled salt arc triangle belts.
  • Salt-Rich Areas (Single N1j): Dominated by imbricate thrust structures and low-angle fault-bend folds. The high plasticity of the salt-rich layer facilitates large-scale plastic flow, leading to salt pillows and salt uplifts.
• Control Factors:
  • Salt Properties: The plasticity of the salt layer directly dictates the structural style. Strong plasticity favors imbricate structures, while poor plasticity favors high-angle faults.
  • Paleo-Uplift Angle: A steeper uplift angle (9°) correlates with higher upward thrust angles, shorter extension distances, and high-angle anticlines. A gentler angle (7°) allows for longer thrust wedge extensions and the development of imbricate fans.
  • Syndeposition: Syn-tectonic sedimentation results in thickness variations (thicker in synclines/depressions, thinner in anticlines), which further influences stress distribution and fault propagation.

Key Contributions

• Mechanism Elucidation: The paper clarifies the "double salt superposition" to "single salt distribution" law and explains how the differential flow of two salt layers intensifies longitudinal zonation and non-integration in the supra-salt structure.
• Trap Classification: The study categorizes the dominant structural traps in the upper assemblage into five types based on the controlling salt layer: reverse thrust anticline, fault-slip anticline, slope-flat fault-bend anticline, imbricate anticline, and anticline triangle traps.
• Experimental Validation: The physical simulation successfully reproduced the transition from high-angle structures to multi-overlapping imbricate tiles, validating the hypothesis that salt layer properties and thickness differences control the evolution from small punch-up traps to large imbricate anticline traps.

Significance
The paper claims to provide a theoretical basis for the exploration and deployment of upper combination structural traps in the Kuqa Depression. By demonstrating that the properties of gypsum-salt layers, the synergistic effects of double salt layers, and paleo-uplift angles are the three major controlling factors, the study offers a refined framework for understanding the spatial distribution of traps. Specifically, it aids in predicting the transition from fractured anticlines to imbricate structures, which is critical for hydrocarbon exploration in complex salt-related settings. The findings suggest that in double salt-layer areas, the interaction between layers creates unique trap geometries (such as salt arc triangles) that differ significantly from single-layer models, thereby improving the accuracy of structural modeling in the region.