THREE DIMENSIONAL FINITE ELEMENT MODELING TO UNDERSTAND THE ROLE OF FAULT INTERACTION IN THE EVOLUTION OF RELAY ZONE STRUCTURES: IMPLICATIONS FOR THE CENOZOIC RIO GRANDE RIFT

The Rio Grande Rift (RGR) of north-central New Mexico, USA comprises a series of right stepping N-S normal faults. During the Cenozoic, the E-W extension was predominantly accommodated along the eastern margin of the RGR defined by the N-S Pajarito and Taos faults and the ENE Embudo fault. The Embudo fault is confined to the relay zone defined by the Pajarito and Taos faults. Oblique structures such as the Embudo fault have been traditionally attributed to rotation of far-field stresses and or oblique extension. Such interpretations presume various episodes of plane strain deformation in different orientations and do not take into account the role of fault interaction in the evolution of relay zone structures. Using a commercial finite element analysis program MARC we have developed a nonlinear model to understand the interaction between a pair of normal faults and its influence on the structural evolution of the relay zone. The model uses simple fault geometries, isotropic material properties and models the faults as frictional sliding surfaces and existing throw profiles for the Pajarito fault have been used to impose displacement based boundary conditions for the model. It demonstrates that even under orthogonal extension and an overall plane strain deformation, the relay zone may evolve in a complex manner. The model results suggest that the displacement of the material particles in the relay zone deviates from the regional transport direction and that the relay zone may undergo a non-coaxial deformation. The deviation from the transport plane and the extent of non-coaxial deformation depend on the fault spacing, fault overlap/gap, the relative orientations of the bounding faults and also on the structural elevation within the relay zone. The model-derived minimum compressive stress vectors within the relay zone are oblique to the regional extension direction throughout the deformation. This suggests that oblique structures (e.g. the Embudo fault) may develop to accommodate the regional extension in the same episode of deformation.