ABSTRACT
Fault-bend folds form as a geometric consequence of the mismatch in shape of hangingwall and footwall during displacement at a ramp in a thrust fault. Geometric and kinematic models of such folds are straight-limbed and sharp hinged. Most existing physical analog models and numerical models of fault-bend folds produce structures lacking angularity and with broad hinge zones. Natural fault-bend folds show much variation in shape, from angular to broad-hinged. Mechanical anisotropy is an important property of layered rocks that strongly influences rock deformation and likely is an important factor in determining the shape of fault-bend folds. We investigate this by simulating the deformation of layered rocks involved in thrust faulting at a ramp by the flow of layered, anisotropic viscous materials. In our study, the constitutive equations for orthotropic anisotropy are incorporated into a two-dimensional finite element code in incompressible viscous fluids. The degree of anisotropy is given by normal viscosity/shear viscosity (A = N/Q > 1). We take the footwall of the thrust as being nearly rigid, and represent the hangingwall as a layered anisotropic viscous medium. The fault is represented by a very ‘soft’ isotropic viscous layer. Displacement on the fault is produced by a constant velocity boundary condition at the ends of the hangingwall block.
Fault-bend folds of similar amplitude and dip are produced near the thrust in both isotropic (A = 1) and anisotropic (A = 20) cases. The ramp anticline becomes broader as displacement is increased. The fold dies out into the hangingwall rapidly for an isotropic layer, but propagates well into the hangingwall in the anisotropic case. In addition, the ramp anticline in the isotropic layer is broad and lacks a sharp hinge point, whereas in the anisotropic models it is sharp-hinged, becoming flat-topped and double-hinged as fault displacement increases. Anisotropy results in folds very similar to those of the geometric kinematic models and some natural folds.
