COMBINING TRISHEAR, FLEXURAL SLIP AND 3D STRAIN INTO FAULT-PROPAGATION FOLD MODELING

Multiple kinematic mechanisms and 3D slip partitioning in fault-propagation folds require a full evaluation of all possible mechanisms and variables when using modeling to quantify the folding process. For instance, calculating the dip of a fault underlying a fold is a useful application of fold modeling but the results can vary radically depending on the choice of the fold mechanism. Trishear modeling can match a fold shape and calculate the dip of an underlying fault as long as folding occurs by penetrative shear, with units both thinning and thickening within the trishear zone. While this assumption may be valid near the fault tip, bed-parallel flexural slip at higher structural levels or in well-layered sedimentary strata commonly preserves bed thickness and length. Flexural-slip fold modeling can also allow the dip of the underlying fault to be calculated by simply breaking the folded surface in the forelimb and unfolding the limbs individually, revealing the fault-equivalent of the fold. In the case of thrust-related folds, the application of trishear fold methods to flexural-slip folds can result in an underestimation of the fault dip and visa versa. Another problem is that good solutions may not be unique. For instance, identically curved fold surfaces can be replicated with trishear modeling by varying a fault’s slip-to-propagation ratio and/or by changing the amount of heterogeneous trishear.

The addition of 3D oblique slip and axis-parallel stretching makes fault-propagation folds even more difficult to interpret because slip components are typically partitioned unequally over folds. Near synclinal axes, fold crowding results in axis-normal shortening, mostly on thrust faults. Axis-parallel slip components are accommodated by strike-slip and oblique-slip faults in the upper forelimbs and anticlinal crests of folds. At higher levels, outer arc extension during buckling of beam-like sedimentary packages can combine with oblique slip to form normal faults that transect anticlinal axes. An additional component of axis-parallel elongation commonly occurs along curved anticlinal axes due to 3D balancing constraints. In summary, fault-propagation folds require careful, 3D analysis to accurately understand their tectonic significance and any associated petroleum reservoir heterogeneity.