In order to study the kinematics and mechanism of the fold-and-thrust belts, writers designed and finished three types of sandbox experiments. Each model included two detachments in different depths. And writers calculated the displacement fields in the model sections in the processes of the experiments using particle image velocimetry (PIV) technique, to get a better understanding of the kinematics. The results indicate that the deep detachment dominated the structure and evolution of the fold-and-thrust belt, and locally shallow structure was controlled by the upper detachment. This conclusion was demonstrated by comparing the modeling results with the natural example in the south segment of the fold-and-thrust belt in Longmen Mountains. PIV analysis suggests that, in the primary stage, deformation was characterized by parallel shortening of the sand layers. Deformation above and below the ductile layer is quite different. Deformation upon the ductile layer propagated faster and farther. After more horizontal compression, thrust began to generate and the rupture would happen. Once the fault initiated, the velocity in the foot wall decreased to zero, and deformation was localized on the fault plane, the hanging wall thrusted up synchronously.%本文设计完成了三种双滑脱层模型的物理模拟实验,并运用粒子成像测速( PIV,Particle Image Velocimetry)技术计算出实验过程中各阶段模型剖面上的速度场分布,进而对褶皱冲断带的运动学过程和变形机制进行讨论.实验结果表明,双滑脱层模型中,基底滑脱层控制了整体的构造样式,浅部滑脱层决定局部的浅层构造.笔者等将实验结果与龙门山褶皱冲断带南段双滑脱体系构造进行了比较,验证了这一结论.PIV分析显示,逆冲断层的产生经历一个平行层缩短的变形过程.该过程在塑性层上,下具有明显差异,塑性层上的变形传递得更快更远.当缩短进行到一定阶段,断层开始发育,发生初始破裂,断层下盘的变形消失,应变集中在断面上,断层上盘沿断面同步逆冲.
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