Forward and Inverse Modeling of Fault-Bend Folding

Connors, Christopher D.¹, Simon Levy², Amanda Hughes² (1) Washington and Lee University, Lexington, VA (2) Washington and Lee University, Lexington,

We present a flexible forward modeling approach for fault-bend folding based on a velocity description of deformation. Flexural slip and inclined shear deformation associated with displacement over bends in faults are unified into one transformation. For fault shapes in which the fundamental fault-bend fold equations are valid, hanging wall fault-bend folds are produced that conserve cross-sectional area, layer thickness, and line length. If the active axial surface orientation is independently defined from fault shape, local thickening or thinning within a thrust sheet occurs while still conserving cross-sectional area. Our fault-bend folding inversion approach searches for the solution that best honors the existing data by comparing how closely the observed input data is to that derived from the forward modeling output. The inversion modeling makes use of multi-objective optimization performed by a genetic algorithm to evolve a population of increasingly better solutions from an initial population constrained by the observed data. Because a full forward model is produced for each comparison, the inversion provides predictions for areas in the section where data is lacking and suggests alternatives in poorly constrained areas, while honoring specified boundary conditions such as conservation of area, line length or layer thickness. Through multiple inversions, probabilistic uncertainty of the solutions can be obtained. Alternatively inversions can be run with feedback from an interpreter with multiple inversions guiding refinement of the interpretation. Thus this inversion method is an objective generalization of classical structural balancing for areas that have been deformed by fault-bend folding.