Kinematic and mechanical evolution of relay zones in normal faulted terranes: Integrating field studies in the Rio Grande rift of north-central New Mexico and three dimensional finite element modeling

The echelon arrangement of normal faults is a fundamental feature of extensional tectonic settings. The interaction between adjacent fault segments is accomplished by displacement transfer through the intervening rock volume referred to as a relay zone. Understanding the evolution of the complex three dimensional deformation in relay zones will give useful insights into the growth of normal faults. In this study, field observations from the Rio Grande rift (RGR) of north-central New Mexico are combined with three dimensional finite element modeling of normal fault interactions to understand the evolution of complex strain and stress fields in relay zones.;The Rio Grande Rift of north-central New Mexico offers all the key elements necessary to develop a numerical model, constrained by field-data, to understand the role of interaction between first order faults in the evolution of regionally extended terranes. Three dimensional elastic-plastic finite element models suggest that relay zones bounded by rectangular faults evolve in a three dimensional strain field, along a non-coaxial strain path. The maximum total extensional strains (S1) are initially deflected from the regional extension direction and rotate towards the regional extension direction with increasing slip on the faults. Therefore, it is dangerous to make traditional assumptions of plane strain deformation within relay zones. The model results suggest that the interaction between the Pajarito fault (PF) and the Sangre de Cristo fault (SCF) in the RGR of north-central New Mexico may have played a major role in the evolution of this segment of the rift.;Generalized three dimensional finite element models of interactions between normal faults bounded by elliptical tiplines suggests that synthetic, antithetic convergent and antithetic divergent relay zones evolve along non-coaxial strain paths. The model results suggest that the fault overlap to spacing ratio, relative orientations of the adjacent faults, coefficient of friction (mu) on the faults and fault tipline shape all exert significant control on the evolution of the strains and stresses in relay zones. The shape of the fault tipline might influence the possible locations of linkage between adjacent normal faults. For relay zones that occur in layered rocks, mechanical stratigraphy plays a major role in the orientations of maximum stretching (S1) vectors in different layers. The refraction of the S1 vectors across adjacent layers is dependent on whether the contact between them is bonded or allowed to slip and on the competence contrast between the adjacent layers. Structural studies of the km scale Hernandez relay zone in the RGR, and of the regional step-over between the PF and SCF have been used to verify the finite element models and to gain additional insights from the models. While field observations can give information about the finite strains and deformed geometry, well correlated numerical models can be used to understand the evolution of particle trajectories and stress vectors and hence provide a mechanical rationale for structures observed in relay zones.