New 40 Ar/ 39 Ar results from drill-hole cuttings of basaltic and basaltic andesite flows from the Guaje well field of the Pajarito Plateau along the western part of the Española Basin in north-central New Mexico yielded Middle Miocene ages (11.5–13.2 Ma). The volcanic eruptions were closely associated with intense faulting, subsidence, and sedimentation, and the results provide age constraints for the volcanic and tectonic processes along the western margin of the Española Basin. The Middle Miocene volcanic rocks are interbedded within the Santa Fe Group, which is divided into the Hernandez and Vallito Members of the Chamita Formation and the Chama–El Rito Member of the Tesuque Formation, in descending stratigraphic order. New and published geochemical results from the Guaje well field and from other surface and subsurface mafic and intermediate lava flows within the Pajarito Plateau suggest that the volcanic rocks erupted from different magmatic sources and centers close to the Pajarito fault zone. Multiple pulses of volcanic eruptions mostly confined to the hanging wall of the Pajarito fault zone, which represents the current western boundary fault of the Española Basin, suggest that the Pajarito fault system has been sporadically reactivated several times, beginning at least in the Middle Miocene and continuing to the Plio-Pleistocene. Moreover, the volcanic, tectonic, and sedimentary records in the Pajarito Plateau suggest that there is no evidence for eastward migration of tectonic and volcanic activities from the Cañada de Cochiti fault zone in the southern part of the Jemez Mountains to the Pajarito fault zone during the early Pliocene (4–5 Ma).

The structural geometry of transfer and accommodation zones that relay strain between extensional domains in rifted crust has been addressed in many studies over the past 30 years. However, details of the kinematics of deformation and related stress changes within these zones have received relatively little attention. In this study we conduct the first-ever systematic, multi-basin fault-slip measurement campaign within the late Cenozoic Rio Grande rift of northern New Mexico to address the mechanisms and causes of extensional strain transfer associated with a broad accommodation zone. Numerous (562) kinematic measurements were collected at fault exposures within and adjacent to the NE-trending Santo Domingo Basin accommodation zone, or relay, which structurally links the N-trending, right-stepping en echelon Albuquerque and Española rift basins. The following observations are made based on these fault measurements and paleostresses computed from them. (1) Compared to the typical northerly striking normal to normal-oblique faults in the rift 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 dextral strike-slip components 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 reduced stress tensors suggest that far-field, ~E-W–trending σ 3 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 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 coevally with the oblique rift segment mainly due to mechanical interactions of large faults propagating toward each other from the adjoining basins as the rift narrowed. Our results suggest that negligible bulk strike-slip displacement has been accommodated along the north-trending rift during much of its development, but uncertainties in the maximum ages of fault slip do not allow us to fully evaluate and discriminate between earlier models that invoked northward or southward rotation and translation of the Colorado Plateau during early (Miocene) rifting.