FAULT KINEMATICS AND PALEOSTRESSES OF AN ACCOMMODATION ZONE LINKING BASINS OF THE MIDDLE RIO GRANDE RIFT, NEW MEXICO

The structural geometry of transfer and accommodation zones that relay strain between extensional domains in rifted crust has been addressed in previous studies. However, details of the kinematics of deformation and related stress changes within these zones have received little attention. We investigated the kinematics and mechanisms of strain accommodation and transfer within a broad, complex zone within the late Cenozoic Rio Grande rift of northern New Mexico; 562 fault-slip measurements were collected within and surrounding the NE-trending Santo Domingo basin that structurally links the right-echelon, N-trending Albuquerque and Española rift basins. The following observations are made based on these fault measurements and best-fit reduced paleostress tensors computed from them: (1) Compared to the typical northerly striking normal to normal-oblique faults in the basins to the north and south, normal-oblique faults are broadly distributed within two merging NE-trending zones on the northwest and southeast sides of the Santo Domingo basin. (2) Faults in these zones have greater dispersion of rake values and fault strikes, greater components of dextral strike slip over a wide northerly strike range, and small-to-moderate clockwise deflections of their tips. (3) Relative-age relations among fault surfaces and slickenlines used to compute paleostress tensors suggest that far-field, ~E-W-trending σ₃ stress trajectories were perturbed 45° to 90° clockwise into NW to N trends within the Santo Domingo zones. (4) Fault-stratigraphic age relations constrain the stress perturbations to the later stages of rifting, possibly as late as 2.7–1.1 Ma. Our fault observations and previous paleomagnetic evidence of post-2.7-Ma counterclockwise vertical-axis rotations are consistent with increased bulk sinistral-normal oblique shear along the Santo Domingo rift segment in Pliocene and later time. Regional geologic evidence suggests that the width of active rift faulting narrowed and became increasingly confined to the Santo Domingo basin and axial parts of the adjoining basins beginning in the late Miocene. We infer that the Santo Domingo clockwise stress perturbations developed due to mechanical interactions of fault tips propagating towards each other from the adjoining basins as the active rift narrowed.