INSIGHTS INTO MECHANICAL EVOLUTION OF RELAY ZONES IN EXTENSIONAL TECTONIC SETTINGS USING THREE DIMENSIONAL FINITE ELEMENT MODELING

Relay zones in extensional tectonic settings transfer displacement between adjacent en echelon normal fault segments. With increasing extension, the interaction between the faults results in a combined rotational and distortional strain field in which relay zones evolve. Existing models of relay zones do not address the three dimensional evolution of this complex strain field. Using a commercial finite element package ABAQUS, we have developed a three dimensional nonlinear numerical model to analyze the influence of (1) fault separation, (2) fault overlap/gap,(3) relative orientation of faults and, (4) material properties on the evolution of strain paths and fault interactions at various structural positions within the relay zone. The model uses simple fault geometries, isotropic material properties, displacement based boundary conditions and models the faults as frictional sliding surfaces. The model demonstrates that even under orthogonal extension and an overall plane strain deformation, the relay zone evolves in a three dimensional strain field along a non-coaxial strain path. The results suggest that the model derived maximum principal extension directions deviate from the regional extension direction within the relay zones. This deviation increases with a decreasing separation to overlap ratio between the normal faults. For a given value of the separation/overlap ratio, maximum deviation occurs at a structural level where the tip lines of the normal faults are closest. In addition, the sense of deflection (clockwise or counterclockwise) of the model derived principal strain and stress vectors from the imposed regional extension direction remains consistent at different structural positions in a synthetic relay zone. However in the case of an antithetic relay zone, this deflection sense changes in a systematic manner with the structural elevation. In general, in an antithetic relay zone, the sense of deflection of the principal strain and stress vectors changes across the structural elevation at which the tip lines of the normal faults are closest.