The principles and assumptions are described in Xiao and Suppe (1992, Origin of Rollover, AAPG Bulletin, V. 76, No. 4). Xiao and Suppe showed that the shapes of rollover folds are controlled by a number of variables, including:
- The shape of the fault
- The history of sedimentation rate relative to fault slip
- The total slip after each bed is deposited.
- The direction of relative rock motion in hangingwall collapse
- Compaction.
StructureSolver lets you model all of these variables with the exception of compaction.
Structural Modeling Basics
They showed that there are two different types of deformation axes associated with extensional fault bends.
- Antithetic kink axes originate at concave-upwards bends and are responsible for the rollover that takes place towards the master fault.
- Synthetic kink axes originate at convex-upward fault bends and are responsible for the misnamed "normal drag" in which beds are deformed so that they dip away from the master fault.
A combination of deformation across antithetic and synthetic shear surfaces produces the more complex structures that we commonly see in nature.
Another important insight in Xiao and Suppe's work was the recognition of growth axial surfaces, whose inclination depends on the relationship between fault slip rate and sediment growth rate.
Xiao and Suppe's approach to modeling 2-D structural deformation above extensional faults has been shown to be highly accurate in many common and important situations. If faults have both convex and concave bends, the approach is significantly more accurate than modeling approaches that assume a uniform deformation angle within the fault hangingwall. Xiao and Suppe (1992) illustrate numerous examples where models match reality very impressively. StructureSolver users have also found this to be the case.
You do not need to explicitly specify fault slip rates and sedimentation rates to construct a StructureSolver model. Instead you specify reference depths for footwall and hangingwall beds in an easy graphical fashion by grabbing and moving reference points on either footwall or hangingwall structural surfaces. The footwall reference depth for a bed is equivalent to the depth of its footwall cutoff on the fault, assuming that the footwall strata are horizontal. The hangingwall reference depth for a bed is its depth at a distance far enough away from the fault that it is beyond the most distal kink axis.
If the cross-section you are analyzing is not parallel to the principal displacement direction, then you will get erroneous results. Likewise you should carefully consider whether there have been multiple phases and directions of strain.
If there are layers of mobile salt or shale in your section, the model will not account for these. However, StructureSolver modeling can sometimes be used to establish constraints on the timing and extent of movement of salt or mobile shale, by comparing the differences between the model and reality.
Although compaction changes the fault shape and the relationship between rollover and fault shape, Xiao and Suppe concluded that "under certain common conditions, the history of compaction can be neglected if the folding is modeled in the compacted state". These conditions are primarily that compaction factors change relatively smoothly in horizontal and vertical directions. This is not the case when there is significant overpressure in the footwall of faults.