Abstract 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.

中文翻译：

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弱上部韧性拆离对龙门山褶皱冲断带（青藏高原东缘）的影响：沙箱实验的启示

摘要 龙门山褶皱冲断带地势陡峭、收敛速度慢、地震灾害多，因此一直备受争议。龙门山褶皱冲断带具有两个主要拆离特征：腹地15-17 km深度的强下部拆离和四川盆地约7 km深度的弱上部拆离。然而，这两个拆解如何控制龙门山的变形仍不清楚。在本研究中，我们重点研究了研究区两个拆解的力学强度，设计了三个模拟模型来研究龙门山褶皱冲断带的运动学和机制。所有三个模型实验在地下室都具有相同的强下部支离，但上部支离不同。结果表明，对于没有上部脱离的模型 1 和具有强摩擦上部脱离的模型 2，应变和变形仅集中在移动后壁附近，粒子图像测速分析显示两种模型均以正向顺序变形。然而，对于弱韧性上部脱离的模型3，应变和变形传播到前陆，模型变形为与龙门山褶皱冲断带一致的无序变形。模型结果表明，强下拆离与弱上拆离的空间关系可能是造成龙门山褶皱冲断带现今构造格局和失序样式的重要因素之一。应变和变形仅集中在移动后壁附近，粒子图像测速分析显示两种模型均以正向顺序变形。然而，对于弱韧性上部脱离的模型3，应变和变形传播到前陆，模型变形为与龙门山褶皱冲断带一致的无序变形。模型结果表明，强下拆离与弱上拆离的空间关系可能是造成龙门山褶皱冲断带现今构造格局和失序样式的重要因素之一。应变和变形仅集中在移动后壁附近，粒子图像测速分析显示两种模型均以正向顺序变形。然而，对于弱韧性上部脱离的模型3，应变和变形传播到前陆，模型变形为与龙门山褶皱冲断带一致的无序变形。模型结果表明，强下拆离与弱上拆离的空间关系可能是造成龙门山褶皱冲断带现今构造格局和失序样式的重要因素之一。对于弱韧性上部脱离的模型 3，应变和变形传播到前陆，模型变形为与龙门山褶皱冲断带一致的无序变形。模型结果表明，强下拆离与弱上拆离的空间关系可能是造成龙门山褶皱冲断带现今构造格局和失序样式的重要因素之一。对于弱韧性上部脱离的模型 3，应变和变形传播到前陆，模型变形为与龙门山褶皱冲断带一致的无序变形。模型结果表明，强下拆离与弱上拆离的空间关系可能是造成龙门山褶皱冲断带现今构造格局和失序样式的重要因素之一。