KINEMATIC AND MECHANICAL MODELS OF COMPLEX DEFORMATIONS: TOWARD A COUPLED SOLUTION

In a basin modeling sense, complex deformations include structural styles involving large lateral translations of material relative to the vertical component of translation. Primary examples include large listric normal fault systems, thin-skinned thrust belts and allochthonous salt bodies. These translations and associated deformations (folding, faulting, volume / area changes, etc.) have traditionally been addressed through kinematic analyses that geometrically relate resultant fold shapes to causative fault shapes assuming geometric compliance and simple assumptions of material behavior (i.e., deformation mechanisms) cast as trigonometric relations. Well known examples include fault-bend folding, fault-propagation folding, and trishear. Although widely applied and remarkably successful, kinematic analysis is totally dependent on an accurate rendering of the major structural features (faults and fold limbs / dip domains). Kinematic analyses do not consider the specific rheologies of the materials involved. Mechanical (numerical) models on the other hand are critically dependent upon the assumed rheology and the intrinsic material properties, which are strongly coupled to the environment of deformation (confining pressure, temperature, pore pressure, strain rate, etc.). The modeling of complex structures is further complicated by the fact that material properties are not constant but evolve through time and space depending on the ambient environment and strain state. Hence, deformation and the resultant structural style are strongly coupled to transient environmental parameters, particularly pore pressure. This and other types of coupling are generally neglected in kinematic analyses that invoke constant properties and geometric relations (rules) throughout the evolution of the structures being restored or forward modelled. Forward numerical models can be used to investigate sensitivities of specific parameters and their sensitivity to changes in the environment (i.e., the coupling). These concepts and principals are illustrated in small-scale (sandbox) and large-scale (tens of km) numerical forward models of deep-water fold-thrust belt structures, offshore Nigeria.