QUANTITATIVELY MODELING ANGULAR UNCONFORMITIES IN FAULT-RELATED FOLDS

Structures formed by fault-related folding often show internal angular unconformities. These unconformities result from variations in the rates of fault slip and syntectonic sedimentation, regional shifts in base level, and structural inversions. However, deciphering the stratigraphic and kinematic implications of these unconformities can be extremely difficult. In particular, regional erosion events complicate structural and stratigraphic analysis because footwall or hanging wall strata may be partially or entirely eroded. Models of fault-related folds are often used to correlate horizons across faults and investigate fault shape and kinematics. However, these models generally do not allow folded surfaces to cross-cut one another as observed in naturally occurring structures. This is because established kinematic forward models assume that stratigraphic ages increase with depth in both the hangingwall and footwall, precluding the erosion of older stratigraphy by younger surfaces. We present a new method for interactively creating forward models of fault-related folds. This method parameterizes model surfaces by age and allows distal footwall and hangingwall burial histories to be arbitrarily complex. The predicted folding of surfaces depends on fault shape, shear angles, and horizon slip, according to established kinematic theories (specifically inclined shear fault bend folding and trishear fault propagation folding). Where younger surfaces intersect older surfaces, the younger surfaces erode and truncate older surfaces.

Using this method, we can model a variety of extensional, contractional, and inversion structures with internal unconformities that match seismic observations from world-wide basins. Even where there is significant missing section, the combined fold and unconformity geometries often constrain the tectonic and stratigraphic history surprisingly well. By interactively adjusting model parameters to match fold and unconformity geometries we estimate the relative timing and size of regional base level shifts, the shape of eroded fault trajectories, and the relative timing and amount of structural inversion.