## 4 Kinematic Model  
### Model Concept and Setup  
Since seismic interpretation cannot accurately constrain the dip of the peripheral fault, kinematic modeling was performed to better determine the fault's dip angle. Kinematic models establish a relationship between fold geometry, fault shape, and displacement. They offer a systematic framework for constructing the evolution of geological structures that fits the law of conservation of volume [31]. Trishear is a kinematic model for fault propagation folds, an alternative to the parallel kink model. It resembles both the geometry and the finite strain field of fault propagation folds. Trishear, similar to other kinematic models, relies solely on idealized velocity distributions, but it has proven mechanically reasonable in most cases [32]. Therefore, it is a powerful tool for reproducing the geometry of fault propagation folds, predicting the distribution and orientation of fractures within the folds, and estimating the nucleation point of underlying blind thrusts [32].  
In trishear, folds develop progressively in a triangular zone of distributed deformation that opens away from the propagating fault tip (Figure 4). At the top of the trishear zone, slip vectors are equal to that of the hanging wall. They are parallel and equal in magnitude to the main fault. At the base of the trishear zone, the footwall is fixed, and the slip is zero. The movement of rock material within the triangular zone ensures the preservation of the cross-sectional area during deformation. To conserve the area, the triangular zone must be symmetrical with respect to the fault [33]. The propagation of the fault tip and migration of the triangular zone through the rock material is explicitly specified in the model by the P/S ratio, which represents the ratio of fault propagation to fault slip.  
There are six parameters in trishear kinematic models including the ramp angle, trishear apical angle, displacement, P/S ratio, and X and Y positions of the fault tip line (Figure 4) [33]. In the study area, the trishear apical angle and final X and Y positions of the fault tip line can be generally constrained by the seismic interpretation. Therefore, the model aims to obtain the displacement, ramp angle, and P/S ratio of the peripheral fault. Based on these parameters, the kinematic modeling of the fault can be built.  
The left (northwestern) limb of sill 1 in Figure 3a is selected for the mode as the fault and the deformation of the host rock are clearly imaged. The result of seismic interpretation after time-depth conversion is shown in Figure 5a. The fault displacement measured at the disconformity between the Middle and Upper Devonian strata is about 52 m (Figure 5a). According to the deformation of the strata above the fault, the trishear zone bounded by deformation axes is outlined (Figure 5a). The apical angle of the trishear zone is about  
Figure 4: Basic geometry of the trishear kinematic model redrawn from [32]. Velocity vectors within the hanging wall, shear zone, and sectors of equal velocity vector are illustrated schematically.  
40\({}^{\circ}\), and the tipline position is also constrained (Table 1). Based on these results, the initial model for the trishear kinematic is built (Figure 5b). Note that the initial X and Y positions of the fault tip line in Figure 5b are derived from the final trishear kinematic model.