BENEFITS OF USING ITERATIVE RESTORATION AND FORWARD MODELING FOR BALANCED SECTION CONSTRUCTION: AN EXAMPLE FROM THE SUN RIVER CANYON, MONTANA

Despite a century of method development and refinement, creating balanced cross-section interpretations in areas of poor data remains a difficult and often subjective endeavor. The Sun River Canyon of Montana represents a classic area for applying such techniques, comprising excellent near-surface exposure of a series of imbricate thrusts with limited subsurface control. We have analyzed over 40 interpretations of a single transect, hand-drawn by professional geologists following several days of local and regional orientation, all using identical surface constraints and tied to good-quality seismic data at the leading edge. Observation of the construction process suggests an overall “top-down” approach – surface bedding and fault orientations are projected to arbitrary depths, where they begin to merge with assumed regional subhorizontal bedding and detachment orientations. The majority of the hand-drawn sections are not restorable: compared to a viable interpretation, the fault trajectories tend to be too steep in the trailing imbricates, and too shallow in the leading sheets. In contrast, a “bottom-up” construction methodology provides an efficient and reproducible technique for creating balanced interpretations, albeit generally requiring the use of computer-based restoration, fault prediction, and forward modeling algorithms for practical application. Key elements comprise: (1) Defining, or assuming, with as much control as possible, the orientation and depth of the basal detachment and leading thrust structure. (2) This youngest thrust hanging wall is restored, along with as much of the next trailing-edge fault geometry as is known. (3) The restored trailing fault is extrapolated as necessary using the shape of a standard restored fault trajectory through the lithotectonic package. (4) This restored fault is then forward modeled to the deformed state based on the footwall geometry. (5) The next higher/older hanging wall block is then generated with an appropriate analytical/algorithmic forward model (e.g., fault bend, propagation, detachment, or trishear), constrained by the observed data. (6) Continuation of the restoration/forward modeling process, accounting for complexities such as out-of-sequence faulting, generates a replicable, balanced, viable interpretation.