MECHANICAL BASIS OF FAULT-BEND FOLDING

We establish a mechanical basis for fault-bend folding using a series of models developed with the discrete element method (DEM). The DEM models employ an aggregate of circular, frictional disks that incorporate bonding at particle contacts to represent the numerical stratigraphy. A vertical wall moving at a fixed velocity drives displacement of the hangingwall section along a pre-defined fault and leads to the development of an emergent fold as the hangingwall passes across the fault bend. We utilize this setup to study the mechanics of fault-bend folding with varying mechanical strength, stratigraphic layering, and fault geometries. Anticlinal and synclinal fault-bend cases produce well-defined fold limbs that generally reproduce the primary characteristics of kinematic fault-bend fold models. In the presence of mechanical layering, folds develop primarily by flexural slip; whereas, in the absence of layering, folds develop by shear along discrete faults that are generally parallel to axial surfaces. The pre-growth strata of synclinal fault-bend models accord very closely with the kinematic theory over a wide range of fault and fold geometries. However, the anticlinal fault-bend folds exhibit behavior that is distinct from kinematic predictions. Specifically, we find that the anticlinal folds maintain a linear relationship between fold shape (γ) and cutoff angle (θ) for a given fault bend (φ). This behavior occurs over a wide range of model properties, both with and without mechanical layering, and is clearly distinct from the two modes of anticlinal fault-bend folding and the non-linear relationship between fold shape (γ) and cutoff angle (θ) for a given fault bend (φ) that are prescribed by the kinematic theory. These observations lead us to define a new, simple relationship between fold and fault shape in anticlinal fault-bend folds that effectively describes several natural structures imaged in seismic data. Finally, we show that the limits of fault-bend folding behavior in our models are defined by a transition to fault-propagation folding. This transition is generally governed by boundary conditions, with fault geometries, material properties, and fault friction playing secondary roles.