Critical Slope Angle For The Developing Of The In-sequence Imbricate In Foreland Thrust Belt:Evidence From Sandbox Modelling

Thrust nappe structure of the orogenic belt is the result of the progressive deformation of the thrust fault, and it can be divided into the piggy-back propagation and the overstep propagation, according to defferent expansion modes. In recent years, research about the thrust nappe structure has connect the hinterland imbricate thrust system and the foreland thrust fold belt. The foreland thrust fold belt is like a wedge, and its development and evolution process mainly obey the critical Coulomb wedge model, while the orogenic process is always accompanied by certain denudation, which is tending to reduce the degree of the wedge vertex. When the vertex angle exceeds the critical angle, the erosion will reduce the vertex degree to reach the critical angle, and if the erosion rate decreases, the thrust wedge expand forwardly, for example, the formation of Jura type folds, whereas if the denudation rate increases, the thrust wedge will stop expanding forwardly, instead, upwardly, so the critical angle of the thrust wedge is the key to control the development of the foreland fold and thrust structure. Due to the later reformation and other factors, information about the critical angle of the thrust wedge is difficult to get, and it will make it more difficult to start the research. In view of this situation, and on the basis of previous work about the Longmen Shan thrust belt, a series of sandbox geophysical modeling experiment has been designed to simulate the orogenic process, making the slope angle and the shortening rate as two variables. So the experiment is separated to two parts, the first part of which is designed to make the shortening rate lower than 0-33%, and the second part is to make it between 0-33% and 0-50%. In each part of the experiment, using the slope angle as the single variabl,26 experiments were designed due to 13 different slope angles, and doubled each experiment. To see whether the shortening rate influent the critical or not,12 experiments were designed when the shortening rate was 0-33%-0-50% due to 6 different slope angles, and also doubled each experiment. During the experiments, using a plank to scrape the sand to simulate the natural orogenic process of denudation, and in the corresponding single experiment process, always maintain the same set slope angle. Throughout the experiments, a camera and a particle image velocimetry (PIV) were used to complete the record. After each experiment, the sand body was sliced for further observation and analysis of the experimental results. By observing the process of the formation of each fault and the deformation of the foreland thrust system, the effects of the slope and the shortening rate to the process of the orogenic deformation were analysed.Results show that the shortening rate and the slope angle effect the time and style of the foreland thrust belt deformation as followes:(1) there were two critical slope angles of special significance, which were the slope angle that the foreland area began deformation and the slope angle that the foreland area began piggy-back propagation respectively. The shortening rate researchded 33%, the critical slope angle of the forland area beginning deformation is 16 degree, whereas the critical slope angle of the foreland area beginning piggy-back propagation is 19 degree respectively. While the shortening rate reaches 50%, the critical slope angles changed to 15 degree and 18 degree instead. (2) according to the two critical slope angles, the deformation process of the orogenic belt can be divided into 3 stages. Stage 1 means Stage of no deformation in the foreland area, and Stage 2 means Stage of having deformantion in the foreland area, whereas Stage 3 means Stage of intense deformation in the foreland area. The 3 stages were a little different due to different shortening rate. When the shortening rate was<0～33%, Stage 1 was<16 degree, and Stage 2 was 16-18 degree, whereas Stage 3 was>18 degree respectively. When the shortening rate was 0-33%-0-50%, Stage 1 was<15 degree, and Stage 2 was 15-17 degree, whereas Stage 3 was>17 degree respectivity.The experiments also show that the hinterland belt consistently demonstrated a model of delamination forms, which was that layers above the silica gel layer showed the formation of thrust faults, whereas the layers below the silica gel layer became basal imbricate piggy-back propagation, while in the foreland area, thrust fault deformation could be formed as backthrust fault and pop up structure. With the increase of the slope angle and the raise of the shortening rate, two pop up structures, or piggy-back progagation formed in the foreland area. As the sandbox geophysical modeling experiments show, the formation of the faults in foreland area was due to lack of space for the formation of the basal imbricate thrust system in the hinterland. With the increase of the slope angle in the hinterland belt, overburden pressure became larger, resulting in the interruption of the formation of the basal imbricate thrust system, and the beginning of the deformation in the foreland area. With the increase of the slope angle in the foreland belt, the space of the hinterland belt was then released. The process of the deformation of the thrust belt in the foreland belt and the imbricate thrust system in the hinterland belt developed repeatedle and alternately.