Abstract: The tri-shear fault propagation fold is an important type of structural pattern and trap of fold-thrust belts.Its genetic mechanism can be explored by studying the characteristics of structural deformation geometry and strain distribution patterns.Based on improved tri-shear velocity distribution model which meets the requirements of divergence-free velocity field and strain compatibility,the instantaneous strain rate within the tri-shear zone was worked out,and in turn its strain difference distribution was analyzed.The forward modeling of the tri-shear fault propagation folds was performed with MATLAB software to obtain the geometry and cumulative strain distribution of folds under the influence of various para-meters.Subsequently,the possible fault propagation modes were discussed according to the strain distribution.The results show that(1) the predicted strain concentrates directly in the region near the fault tip and serious deformation occurs in the adjacent areas of the fault tip,causing the forward propagation of the tip,so the strain attenuates sharply with increa-sing distance from the tip,and rocks fold accordingly;(2) The parameters affecting the strain distribution of tri-shear fault propagation folds are the apical angle of the tri-shear model and the ratio of P/S.The width of the fault propagation fold is directly controlled by the apical angle:with the decrease of the apical angle,the deformation zone becomes tighter and the strain grows stronger; while the P/S ratio affects the extent of strata involved in deformation and the cumulative strain time.When the ratio is high,the fault propagates fast and there is no significant internal deformation within the tri-shear zone,but when the ratio is low,beds have sufficient time to accumulate strain and deform before being cut by fault and result in pronounced synclines on the downthrown side of the fault and bed thickening.

Key words: tri-shear, strain compatibility, strain distribution, strain concentration, fault, structural style, fold-thrust belt