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).

We describe the structure of the eastern Española Basin and use stratigraphic and stratal attitude data to interpret its tectonic development. This area consists of a west-dipping half graben in the northern Rio Grande rift that includes several intrabasinal grabens, faults, and folds. The Embudo–Santa Clara–Pajarito fault system, a collection of northeast- and north-striking faults in the center of the Española Basin, defines the western boundary of the half graben and was active throughout rifting. Throw rates near the middle of the fault system (i.e., the Santa Clara and north Pajarito faults) and associated hanging-wall tilt rates progressively increased during the middle Miocene. East of Española, hanging-wall tilt rates decreased after 10–12 Ma, coinciding with increased throw rates on the Cañada del Almagre fault. This fault may have temporarily shunted slip from the north Pajarito fault during ca. 8–11 Ma, resulting in lower strain rates on the Santa Clara fault. East of the Embudo–Santa Clara–Pajarito fault system, deformation of the southern Barrancos monocline and the Cañada Ancha graben peaked during the early–middle Miocene and effectively ceased by the late Pliocene. The north-striking Gabeldon faulted monocline lies at the base of the Sangre de Cristo Mountains, where stratal dip relations indicate late Oligocene and Miocene tilting. Shifting of strain toward the Embudo–Santa Clara–Pajarito fault system culminated during the late Pliocene–Quaternary. Collectively, our data suggest that extensional tectonism in the eastern Española Basin increased in the early Miocene and probably peaked between 14–15 Ma and 9–10 Ma, preceding and partly accompanying major volcanism, and decreased in the Plio-Pleistocene.

The late Cenozoic extension in the Rio Grande rift of north-central New Mexico was predominantly accommodated by the north-south–trending Pajarito and Sangre de Cristo normal faults and the intervening east-northeast–striking predominantly strike-slip Embudo fault. Using this segment of the rift as our primary example, we have analyzed a series of three-dimensional nonlinear elastic-plastic finite-element models to assess the role of mechanical interactions between pairs of en echelon rift-scale listric normal faults in the evolution of intervening relay zones. The model results demonstrate that under orthogonal extension and an overall plane-strain deformation, relay zones may evolve in a three-dimensional strain field and along non-coaxial strain paths. The extent of non-plane strain and non-coaxial deformation depends on the fault overlap to spacing ratio, the relative orientations of the bounding faults, and the structural position 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. Within the Rio Grande rift of north-central New Mexico, the occurrence of northerly striking Neogene faults suggestive of east-west extension in the Española and the San Luis Basins, geographic variations in the vertical-axis rotations from paleomagnetic studies, and secondary fault patterns are consistent with the near-surface variations in the strain field predicted by the model. The model suggests that interaction between the Pajarito and the Sangre de Cristo faults may have played a major role in the evolution of this segment of the rift.