Three-dimensional Structural Geometry Of Fault-related Folds

Much of progress has been made in establishing the 2D geometric and kinematic models of fault-related-folds and the current research of fault-related-folds focus on the transition from 2D to 3D.2D models of fault-related-folds have been extended to 3D using two kinds of formulations:a pseudo-3D formulation and a true-3D formulation. But both of the two kinds of formulations have their own limitations. The real sense of 3D fault-related-folds models remain at the exploratory stage. Studying the three-dimensional structural geometry of fault-related folds is important in the research of implementing the transition of fault-related-folds from 2D to 3D.The pseudo-3D models of fault-related-folds are constructed by using Trishear2D, Gocad and Trishear3D programs to discuss the influence of the main factors on the 3D geometry of fault-related-folds. Then, we will apply the results of the research to nature examples. The results indicate that the main factors which influence the 3D geometry of fault-related-folds including ramp angle, trishear angle, P/S, displacement gradients, lateral propagation of fault, lateral/oblique ramps, oblique thrusting and so on. Ramp angles influence the uplift of folds and dip angles of the tip folds. Trishear angles influence the deformation of fold fore limbs, which become steep when the trishear angles are small. When P/S is small, the fore limbs become steep and layers thicken. When P/S is big, the fore limbs become low-angle and there is no layer thickens in fore limbs. Displacement gradients influence the uplift of folds and dip angles of the tip folds. Displacement gradients and lateral/oblique ramps are the main causes of the termination of nature folds. Other factors such as rock mechanics properties, lithology distribution and exist of multi faults can also have influence on the 3D geometry of fault-related-folds.The results of analyzing fault lateral propagation indicate that the part of lateral propagation produces a concave displacement profile, while the other part leads to a linear displacement profile under the assumption that the original displacement profile is linear. So we can use the displacement profile to estimate the length of the original part and the lateral propagation part of a fault.The results of studying oblique-thrust fault-related-folds indicate that there are two main distinctions between oblique-thrust fault-related-folds and dip-thrust fault-related-folds. One distinction is that the line linking peak point and midpoint of back limb in the contour map or the line linking core point and midpoint of back limb in the horizontal slice of dip-thrust fault-related-folds is perpendicular to fault strike while the similar line is not perpendicular to fault strike in oblique-thrust fault-related-folds and the line is parallel to slip vector. The other distinction is that the line connecting every peak point of all bedding planes on the cross-sections parallel to fault strike is vertical in dip-thrust fault-related-folds while the line would be oblique to horizontal plane if oblique thrusting exists. The two distinctions can be used to identify whether oblique thrusting exists or not and estimate the slip directions of fault-related-folds.Qiongxi, Daxingxi and Sumatou anticline covered by 3D seismic data in the southwest of Sichuan basin are chosen as nature examples for the research. This research indicates that oblique thrusting exists in Qiongxi and Daxingxi anticlines and the intersection angles between slip direction and fault strike are both about 70°. The slip vectors of Qiongxi anticline and Daxingxi anticline are about NE79° and NE77° respectively, which is consistent with the stress field of Longmen Shan since the late Pliocene and the most compressive stress direction inferred from coseismic slip of the Wenchuan earthquake. The original fault length of Qiongxi anticline is about 3260m and the lateral propagation part is about 8500m. Sumatou anticline is a dip-thrust fault-related-fold. The slip vectors of Sumatou are about ES450, which is consistent with the stress field of Longmen Shan before the late Pliocene and the thrust direction of Longquan Shan.