NEW INSIGHTS INTO THE EVOLUTION OF THE RIO GRANDE RIFT OF NORTH-CENTRAL NEW MEXICO FROM THREE DIMENSIONAL FINITE ELEMENT MODELING

The Rio Grande Rift of southwestern Unites States comprises a series of right stepping en echelon basins bounded by north striking normal faults. During the late Cenozoic, the E-W extension was predominantly accommodated along the N-S Pajarito and Taos faults and the intervening ENE Embudo fault. Opposing displacement gradients on the Pajarito and Taos faults, opposing polarities of the Espanola and San Luis basins and the observed attendant rotations in the syn-rift basin fill classify the stepover between the faults as an Antithetic relay zone (the Embudo Relay zone, ERZ).

Numerical modeling helps in gaining insight into the evolution of the distortional and rotational components of the complex strain field in the ERZ. Using a commercial finite element package ABAQUS, we have developed a three dimensional nonlinear numerical model to understand the evolution of the strain field within the ERZ as a function of the total imposed extension. Frictional sliding surfaces embedded in an elastic-plastic medium act as faults in the analysis. Extension estimates for the Pajarito fault, based on existing throw profiles, have been used to impose displacement based boundary conditions on the model. The model deformation accurately reproduces the first order geometric features of the ERZ. Model results suggest that the relay zone rock volume evolves in a complex three dimensional strain field, whereas outside the relay zone the rocks undergo a simple overall plane strain deformation. The model maximum extension vectors within the relay zone initiate oblique to the imposed regional extension direction and rotate towards the regional transport direction with increasing extension. Within the ERZ, the occurrence of (1) northeasterly striking Miocene faults suggestive of NW-SE extension, (2) northerly striking Neogene faults suggestive of E-W extension and (3) preliminary field observations on the orientations of fracture networks, are all consistent with the near surface variations in the three dimensional strain field predicted by the model. The model results suggest that the various oblique structures observed in the ERZ may have formed during the same episode of orthogonal extension that gave rise to the Pajarito and Taos faults.