Quantifying and modeling curved thrust fault-propagation folds at different scales

Abstract

Classical models of fault-related folds, which invoke planar faults, are inappropriately applied in many instances, as curved faults are commonly observed in nature. The application of classical planar fault models for such folds hinders the assessment of the geometric relationship between these features and the quantification of crustal shortening during faulting and folding. Our study, for the first time, attempts to establish fundamental geometric relationships for curved thrust fault-propagation folds by examining the geometries and kinematics of three contractional structures at different scales. By relating the deformation of backlimb to the underlying curved thrust with an integrated algorithm incorporating forelimb trishear and backlimb shear, we successfully reproduced most of the fold geometry with a smoothly curving backlimb and an abrupt forelimb. We discovered that the deformation of folded backlimb is a combination of limb rotation and kink-band migration. This unique deformation mechanism means that curved faults do not experience greater slips (less than 2%) compared to planar faults of similar dips. Although the introduction of minor curvature in the fault trajectory cannot have a significant impact on shortening, our results emphasize that the choice of fault geometry is crucial for the deformation of folded backlimb and landform surface. We then combine our new work with previous studies and find that the arc angle distributions of curved thrust fault-propagation folds at different scales are in the range of ∼30°–∼60°, which is important for reasonable constraints on the geometry of deep faults in the absence of detailed seismic controls. Finally, our study further suggests that introducing area-depth strain analysis prior to the modeling analysis could minimize the assigned uncertainty in interpretation and test the validity of kinematic models in determining balanced structures.