Forming condition, position and evolution of back thrusts in the sandbox analogue modelling

Foreland fold-and-thrust belt (FAT), as the dynamic junction between hinterland and foreland, adjusts the evolution of orogeny. Back thrusts (BT) also play a crucial role in the evolution processes of FAT, such as settling deformations. 16 published sandbox analogue modelling, which have different setting background on material and thickness of detachment, number of detachment layer, velocity and direction of baffle, and deformation stages, were collected and re-explained to clarify the structural style, forming and progressive processes of back thrusts which may provide the clues on active fault discussion. We find that: (1) it is easier for a back thrust to initiate from the root part of a fore thrust (FT). If the elder BT move with FT, the new-born BT will form on the previous position of the elder BT, which corresponding to the new foot-wall of the elder BT. On the contrary, when an elder BT does not move with the hanging wall of FT, there will be no enough space in the foot wall, thus the new-born BT will form on the hanging-wall of the elder BT. (2) Space is an important factor during BT group progressing. Obvious space barrier makes the BT group progress out-of-sequencely, while abundant progressive space provide an environment of in-sequencely propagation. (3) A strong detachment with enough thickness provides an opportunity to transfer the deformation forwardly and preferentially, until the deformation comes across barrier. (4) Detachment can de-couple the deformation above and below it as a soft barrier, make the deformations form in different neighbor structural level alternatively.