The influence of a weak upper ductile detachment on the Longmen Shan fold-and-thrust belt (eastern margin of the Tibetan Plateau): Insights from sandbox experiments.

• Weak shallow detachment in the Longmen Shan belt can lead the deformation into the foreland. • Duplex detachment has controls on the out-of-sequence style of the Longmen Shan belt. • The current structural pattern of the Longmen Shan belt is produced by upper crustal thrusting. The topic of the Longmen Shan fold-and-thrust belt has been vigorously debated due to the steep topography in contrast with the low convergence rate and earthquake hazards. The Longmen Shan fold-and-thrust belt is characterized by two main detachments: a strong lower detachment at 15–17 km depth in the hinterland and a weak upper detachment at ~7 km depth in the Sichuan Basin. Nevertheless, how the two detachments control deformation in the Longmen Shan remains unclear. In this study, we focus on the mechanical strengths of the two detachments in the study area and design three analog models to investigate the kinematics and mechanisms of the Longmen Shan fold-and-thrust belt. All three model experiments have the same strong lower detachments at the basement, but different upper detachments. The results indicate that for Model 1 with no upper detachment and Model 2 with a strong frictional upper detachment, the strain and deformation only concentrate near the mobile backwall, and particle image velocimetry analysis reveals that both models deform in the forward in-sequence style. However, for Model 3 with a weak ductile upper detachment, the strain and deformation propagate into the foreland, and the model deforms in the out-of-sequence style consistent with the Longmen Shan fold-and-thrust belt. The model results indicate that the spatial relation of strong lower detachment and weak upper detachment may be one of the important factors producing the current structural pattern and the out-of-sequence style of the Longmen Shan fold-and-thrust belt. [ABSTRACT FROM AUTHOR]