Structural wedges, which consist of closely packed thrust sheets in fold-and-thrust belts and accretionary wedges, are a key focus of research due to their widespread occurrence worldwide. Examples include the Alpine-Himalayan system (Caméra et al., 2017; Cifelli et al., 2016; Qayyum et al., 2015; Yin, 2006), The South Pyrenean Central Salient (Ford et al., 2022; Lacombe et al., 2022; Muñoz et al., 2013; Sussman et al., 2004), The Zagros fold-and-thrust belt (Le Garzic et al., 2019; McQuarrie, 2004; Sherkati et al., 2006), and the Keping fold and thrust belt (Zhang et al., 2019). The geometry and dynamics of structural wedges play a crucial role in various geological processes such as hydrocarbon accumulation (Fan et al., 2020; Fan et al., 2024; Liu et al., 2019; Ryan et al., 2017), tectonic evolution (He et al., 2023b; Hubert-Ferrari et al., 2007; Zhou and Zhou, 2022), and earthquake assessment (Barchi et al., 2021; Lu et al., 2016). Studies have shown that the evolution of structural wedges is controlled by many factors [e.g., detachment rheology (Pla et al., 2019; Ruh et al., 2012), surface process (Konstantinovskaya and Malavieille, 2011; Wang et al., 2022), mechanical stratigraphy (Dean et al., 2013), and inherited structures (Wu et al., 2014; Yang et al., 2024)]. This paper mainly focuses on the thickness variation of the deformed strata, which is also a primary parameter controlling the structural evolution in natural structures (Jiao et al., 2021; Santolaria et al., 2022; Santolaria et al., 2024).

Using analog modeling and numerical simulations, the influence of thickness variation above the detachment has been tested in different model set-ups, including the sharp transition, gradual transition, and other complex models (Calassou et al., 1993; Jiao et al., 2021; Santolaria et al., 2022; Santolaria et al., 2024; Sun et al., 2016). Previous studies suggest that the increased thickness of the overburden units can affect the deformation propagation with larger structural intervals, less thrust faults, and longer deformation front (Marshak and Wilkerson, 1992; Soto et al., 2002).

However, previous studies mainly focus on the structural wedges with sedimentary covers deforming along a basal detachment, and the buried structural wedge developed in multiple detachments system has rarely been discussed. The buried structural wedges mainly develop in duplex structures. The deformation is separated by lower and upper detachments, and a structural wedge develops between them (Baby et al., 1992; Banks and Warburton, 1986; Couzens-Schultz et al., 2003; Mitra, 1986). The buried structural wedge deforms along the lower detachment. The buried structural wedge exhibits quite different features in geometry and kinematics from the systems with single detachment. More intense deformation is formed by imbricate thrust faults (Couzens-Schultz et al., 2003; Li et al., 2017).

In this study, numerical simulations are used to test the influence of strata thickness on the deformation of the buried structural wedges, using the Kuqa fold-and-thrust belt as an example. Two detachments control the deformation of the Kuqa fold-and-thrust belt, with a salt layer decoupling the deformation above and below it as the upper detachment. (Izquierdo-Llavall et al., 2018; Pla et al., 2019; Qi et al., 2023; Tang et al., 2004a; Wang et al., 2017; Wang et al., 2011). A long-propagated fold belt develops above the upper detachment, and a buried structural wedge consisting of imbricate thrusts forms above the lower detachment.

The buried structural wedge shows varied geometries with the strata thickness increases from west to east along the strike. Based on the seismic interpretation, the structural variation along the strike has been described. Then, a numerical simulation program was designed according to the stratigraphy of the Kuqa fold-and-thrust belts. The setup contains two detachments, and two model suites were carried out with and without syn-tectonic sedimentations. Each model suite includes three models, with different intermediate layer thicknesses between two detachments.

By comparing the model results with the structural features of the Kuqa depression, this study highlights the control of Mesozoic strata thickness on structural variations along the strike in the Kuqa fold-and-thrust belt. Besides, the simulations also provide insights into the structural evolution of fold-and-thrust belts containing two detachments. This is not only helpful to understanding the geometry and kinematics of multiple detachment systems, but also useful to the structural interpretation and hydrocarbon exploration in such structures.

The Kuqa fold-and-thrust, located in the northern Tarim basin, developed a Cenozoic multiple detachments system (Figure 1). The Cenozoic and Mesozoic successions are involved in the Cenozoic deformation, which mainly consists of clastic sediments, including conglomerate, sandstone, and mudstone (Neng et al., 2018; Pla et al., 2019; Qi et al., 2023) (Figure 2).

Previous studies suggest that the deformation of the Kuqa fold-and-thrust belt is controlled by two detachment systems (Chen et al., 2004; Gao et al., 2020; Li et al., 2021; Li et al., 2014; Tang et al., 2004b; Wang et al., 2011; Yu et al., 2008). The salt layer located at the lower part of Cenozoic stratigraphy acts as the upper detachment and the base of Mesozoic acts as the basal detachment (He et al., 2023a; Long et al., 2021; Wang et al., 2023; Wang et al., 2017; Yang et al., 2024). The salt layer decouples the deformation in the units above and below it. A board deformation zone consists of gentle detached folds developed in the suprasalt units with the salt layer as the upper detachment, and a tight deformation zone containing imbricate thrusts forms in the subsalt units. Two salt layers develop in this region, the Paleogene Kumugeliemu salt layer in the western zone and the Neogene Jidike salt layer in the eastern zone (Adeoti and Webb, 2022; Li et al., 2021; Li et al., 2012; Neng et al., 2018; Wang et al., 2011; Wu et al., 2014; Yang et al., 2024). The Kumugeliemu salt layer is thicker and broader than the Jidike salt layer (Pla et al., 2019; Wang et al., 2023).

The geological map shows two strong deformation belts in the suprasalt units, the Kelasu structural belt and the Qiulitage structural belt. However, the deformation in the subsalt units occurs in the Kelasu structural belt (Figure 1). The subsalt units show varied structural features along the strike with similar deformation backgrounds. From the western zone to the central zone, the deformation front moves toward the foreland and then withdraws slightly from the central zone to the eastern zone. The number of faults decreases from the west to east in general.

The Kelasu structural belt is located in the western of the Kuqa depression. Structural geometries and kinematics show apparent differences along the strike. Three seismic profiles are interpreted to show the structural features (Figure 3). The seismic-well correlation is used to determine the tops of strata, which is introduced in Wang et al. (2023). The interpreted structural model of the Kelasu structural belt is mainly controlled by two detachments, which is consistent with previous studies (He et al., 2023a; Wang et al., 2023; Wang et al., 2017; Wang et al., 2020). The thick Kumugeliemu salt layer acts as the upper detachment, and the shales at the bottom of Mesozoic strata as the basal detachment. Considering the salt layer could form salt structures during the deformation, the structural units are divided into four parts: suprasalt units, salt layer, subsalt structural wedge, and basement units.

The difference in the suprasalt unit is mainly expressed as the number of structures and sedimentation records. The geological map of the Kuqa depression shows a varied number of folds along the trike in the Kelasu structural belt (Figure 1A). No fold develops in the western zone, while one thrust-related fold occurs toward the central zone. Two folds form in the eastern zone, but the distance between the two folds increases from the west to the east. In the cross-sections, suprasalt units also show different features (Figure 3). The sedimentation of the western zone shows a changed depocenter from the north to south, and then the Quaternary units cover the previous units uniformly. The central and eastern zones both develop a detached fold with the salt layer as the detachment. The long interval between the two folds forms a board syncline between them in the eastern zone.

The salt layer shows similar features in all three cross sections. The salt layer acts as the upper detachment layer decoupling the deformation above and below it. Some salts accumulate in the core of the anticline and salt welding develops above the anticlines of subsalt units. The subsalt units mainly consist of thrust sheets and the main difference is the number and scale of thrust sheets. Generally, the interval and scale increase from the west to the east. Specifically, the interval of thrusts is about 2 km in the western zone, 3–4 km in the central zone, and over 6 km in the eastern zone.

According to the seismic profiles, the thickness of the subsalt Mesozoic units increases from the basin to the orogenic. The eastern zone develops the least thrusts in the subsalt units. The clear reflection shows that the thickness of the subsalt unit increases from 4 km to 5 km, with the location moving 10 km toward the orogenic belt (Figure 3C). Besides, the thickness of the subsalt units also increases along the strike. The seismic profile along the strike shows that the thickness of subsalt Mesozoic units increases from about 0.5 km in the western zone to 2 km in the central zone, and then to 2.5 km in the eastern zone (Figure 4).

The discrete element method (DEM) is a particle-based numerical method that simplifies natural structure into an assemblage of balls. The balls move to follow Newton’s equations under the compression or extension of boundaries. The DEM method has been detailly introduced by Morgan (2015) and has been widely used to investigate the deformation of fold-and-thrust belts in previous studies (Morgan, 2015; Wang et al., 2022; Zhang et al., 2013). The Version of DEM used in this study is ZDEM (Li et al., 2021), which is developed according to the RICEBAL (Morgan, 2015) and the open-source code TRUBAL (Cundall and Strack, 1979).

The models consisted of balls with radii of 60 m and 80 m. The bulk mechanical properties of the numerical material are determined according to the interaction of particles. The models in this paper contain two types of materials, the clastic sediments and the salt layer. The clastic sediments are represented by the bonded balls, and the particle properties of the balls are based on the natural data and previous studies (Table 1) (Morgan and Bangs, 2017). The salt layer is mechanically weak and flows like a fluid (Hudec and Jackson, 2007). The unbonded particles are used to represent it, as previous studies demonstrate similar ductile behaviors during deformation when the friction coefficient is zero (Dean et al., 2015; Maxwell, 2009).

Common particle properties: shear modulus 2.9E09 Pa, Poisson’s ratio 0.2, Time step 0.05 s, Wall velocity 1 m/s, Local damping coefficient 0.4, Gravitational acceleration 9.8 m/s ².

Based on the seismic interpretation and numerical simulation, we conclude as follows:

(1) The influence of thickness on the buried structural wedge shows similar features with and without syn-tectonic sedimentation. The increased thickness leads to enlarged intervals of thrusts, larger thrust anticlines, and fewer thrusts in both cases.