TWO-DIMENSIONAL RESTORATION OF COMPLEX FAULT-RELATED FOLDING BY INTEGRATING TRISHEAR WITH OTHER FOLDING ALGORITHMS

Fault-related folding occurs by multiple mechanisms whose collaboration complicates the restoration of naturally occurring structures. The structural evolution of fault-propagation folds, where the folding of beds close to the fault is not parallel, can be particularly complex.

A new forward and backward modelling implementation of trishear, where fault slip is distributed in a triangular shear zone in front of the fault tip, is similar to the Hardy & Ford (1997) method in that it utilizes linear relationships between velocity vectors across the trishear zone.

This new implementation defines a velocity vector for each node in the section allowing it to be equally applied to a digital cross-section or a three-dimensional digital model. The trishear deformation is driven by five parameters: fault slip, the fault propagation-to-slip ratio, the trishear zone angle, the amount of slip per deformation step, and the orientation of the trishear zone to the fault. Heterogeneous trishear, where the amount of shear increases toward the center of the shear zone, is modelled by varying the zone angle during deformation.

Node-based trishear deformation allows a wide range of both extensional and compressional fault propagation folds to be sequentially restored, especially when it is combined with other kinematic deformation methods such as fault-bend folding, fault-parallel flow, block rotation or inclined shear. Once an acceptable restoration is achieved, forward modelling can be used to predict a fold’s internal strain and the resulting fracture geometries.

Analysis of compressional monoclinal structures from the Rocky Mountains of the western U.S.A. shows that heterogeneous trishear is dominant at deeper levels in the structures where faults transition into folds. Differences in stratal dip across faults can be explained by basement block rotations. At higher levels, flexural slip predominates, and the fold can be restored by line-length balancing techniques.