Geometry and Kinematics of Bump-Type Fault-Bend Folding

The quantitative characterization of faulting and folding in sedimentary basins is crucial for understanding the relationship between architecture and kinematics at upper crustal levels. A kinematic model of classic fault-bend folding provides a quantitative relationship between fold geometry and fault movement, making the growth strata with upward-narrowing kink-bands a complete, decipherable record of deformation. When folding is produced by material being transported along a bumped detachment fault (an original flat detachment layer folded), the geometry of the growth strata becomes complicated; thus, its quantitative relationship with detachment displacement is unknown. This folding model, here called “bump-type fault-bend folding”, is well exemplified by the Shenghe 2 structural belt in the West Kunlun Cenozoic foreland thrust system. Seismic interpretation indicates that the growth strata in this case are characterized by lateral thickness fluctuations, which indicate folding of the detachment layer. Combined with kinematic forward simulation and structural trend analysis, we noted that the migrating distances of the lateral thickness fluctuation zones in various growth sequences reflect their experienced detachment displacements. The application of these findings to the Shenghe 2 structural belt reveals an early Miocene folding event of the basal Cenozoic detachment, four subsequent detachment slip increments, and clockwise transport of the Cenozoic thrust sheet. The quantitative kinematic model of bump-type fault-bend folding and the method presented here may have widespread applications in dating the folding of detachment layers and reconstructing the slip history and orientation of thrust sheets.