Folds above angular fault bends: mechanical constraints for backlimb trishear kinematic models



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Texas A&M University


The backlimb trishear velocity field is compared to that of mechanical models of fault-bend folds in an incompressible anisotropic viscous media to determine the relationship between the magnitude and orientation of mechanical anisotropy and the kinematic parameters of the trishear model. The trishear model can describe the velocity field of the mechanical model, at least to first order approximation for some cases. We find that the apical angle, asymmetry angle and overall geometry of the hanging-wall syncline above the ramp depend on the magnitude and orientation of the planar anisotropy inherent in stratigraphic sequences. The asymmetry of trishear zone in the backlimb region mimics that of the planar anisotropy. In general, as the magnitude and inclination of the anisotropy increase, the trishear apical angle decreases. The trishear parameters that describe physical models of fault-bend folds with different magnitudes of anisotropy also show a decrease in apical angle with an increase in magnitude of anisotropy. Yet the apical angles of the backlimb of physical models generally are less than these predicted by the mechanical model for the same magnitude of anisotropy. In addition, the physical models display significantly more negative asymmetry than predicted by the mechanical model. The results of this study may be used to determine the conditions under which the trishear model is an acceptable approximation to natural formation and help guide the selection of trishear parameters for subsurface structural interpretations in fault-fold terrains.