Two Oligocene conglomeratic units, one primarily nonvolcaniclastic and the other volcaniclastic, are preserved on the west side of the Jemez Mountains beneath the 14 Ma to 40 ka lavas and tuffs of the Jemez Mountains volcanic field. Thickness changes in these conglomeratic units across major normal fault zones, particularly in the southwestern Jemez Mountains, suggest that the western margin of the Rio Grande rift was active in this area during Oligocene time. Furthermore, soft-sediment deformation and stratal thickening in the overlying Abiquiu Formation adjacent to the western boundary faults are indicative of syndepositional normal-fault activity during late Oligocene–early Miocene time. The primarily nonvolcaniclastic Oligocene conglomerate, which was derived from erosion of Proterozoic basement-cored Laramide highlands, is exposed in the northwestern Jemez Mountains, southern Tusas Mountains, and northern Sierra Nacimiento. This conglomerate, formerly called, in part, the lower member of the Abiquiu Formation, is herein assigned to the Ritito Conglomerate in the Jemez Mountains and Sierra Nacimiento. The clast content of the Ritito Conglomerate varies systematically from northeast to southwest, ranging from Proterozoic basement clasts with a few Cenozoic volcanic pebbles, to purely Proterozoic clasts, to a mix of Proterozoic basement and Paleozoic limestone clasts. Paleocurrent directions indicate flow mainly to the south. A stratigraphically equivalent volcaniclastic conglomerate is present along the Jemez fault zone in the southwestern Jemez Mountains. Here, thickness variations, paleocurrent indicators, and grain-size trends suggest north-directed flow, opposite that of the Ritito Conglomerate, implying the existence of a previously unrecognized Oligocene volcanic center buried beneath the northern Albuquerque Basin. We propose the name Gilman Conglomerate for this deposit. The distinct clast composition and restricted geographic nature of each conglomerate suggests the presence of two separate fluvial systems, one flowing south and the other flowing north, separated by a west-striking topographic barrier in the vicinity of Fenton Hill and the East Fork Jemez River in the western Jemez Mountains during Oligocene time. In contrast, the Upper Oligocene–Lower Miocene Abiquiu Formation overtopped this barrier and was deposited as far south as the southern Jemez Mountains. The Abiquiu Formation, which is derived mainly from the Latir volcanic field, commonly contains clasts of dacite lava and Amalia Tuff in the northern and southeastern Jemez Mountains, but conglomerates are rare in the southwestern Jemez Mountains.

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.

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 used tephrochronology for upper Neogene deposits in the Española Basin and the adjoining Jemez Mountains volcanic field in the Rio Grande rift, northern New Mexico, to correlate key tephra strata in the study area, identify the sources for many of these tephra, and refine the maximum age of an important stratigraphic unit. Electron-microprobe analyses on volcanic glass separated from 146 pumice-fall, ash-fall, and ash-flow tephra units and layers show that they are mainly rhyolites and dacites. Jemez Mountains tephra units range in age from Miocene to Quaternary. From oldest to youngest these are: (1) the Canovas Canyon Rhyolite and the Paliza Canyon Formation of the lower Keres Group (ca. <12.4–7.4 Ma); (2) the Peralta Tuff Member of the Bearhead Rhyolite of the upper Keres Group (ca. 6.96–6.76 Ma); (3) Puye Formation tephra layers (ca. 5.3–1.75 Ma); (4) the informal San Diego Canyon ignimbrites (ca. 1.87–1.84 Ma); (5) the Otowi Member of the Bandelier Tuff, including the basal Guaje Pumice Bed (both ca. 1.68–1.61 Ma); (6) the Cerro Toledo Rhyolite (ca. 1.59–1.22 Ma); (7) the Tshirege Member of the Bandelier Tuff, including the basal Tsankawi Pumice Bed (both ca. 1.25–1.21 Ma); and (8) the El Cajete Member of the Valles Rhyolite (ca. 60–50 ka). The Paliza Canyon volcaniclastic rocks are chemically variable; they range in composition from dacite to dacitic andesite and differ in chemical composition from the younger units. The Bearhead Rhyolite is highly evolved and can be readily distinguished from the younger units. Tuffs in the Puye Formation are dacitic rather than rhyolitic in composition, and their glasses contain significantly higher Fe, Ca, Mg, and Ti, and lower contents of Si, Na, and K. We conclude that the Puye is entirely younger than the Bearhead Rhyolite and that its minimum age is ca. 1.75 Ma. The San Diego Canyon ignimbrites can be distinguished from all members of the overlying Bandelier Tuff on the basis of Fe and Ca. The Cerro Toledo tephra layers are readily distinguishable from the overlying and underlying units of the Bandelier Tuff primarily by lower Fe and Ca contents. The Tshirege and Otowi Members of the Bandelier Tuff are difficult to distinguish from each other on the basis of electron-microprobe analysis of the volcanic glass; the Tshirege Member contains on average more Fe than the Otowi Member. Tephra layers in the Española Basin that correlate to the Lava Creek B ash bed (ca. 640 ka) and the Nomlaki Tuff (Member of the Tuscan and Tehama Formations, ca. 3.3 Ma) indicate how far tephra from these eruptions traveled (the Yellowstone caldera of northwestern Wyoming and the southern Cascade Range of northern California, respectively). Tephra layers of Miocene age (16–10 Ma) sampled from the Tesuque Formation of the Santa Fe Group in the Española Basin correlate to sources associated with the southern Nevada volcanic field (Timber Mountain, Black Mountain, and Oasis Valley calderas) and the Snake River Plain–Yellowstone hot spot track in Idaho and northwestern Wyoming. Correlations of these tephra layers across the Santa Clara fault provide timelines through various stratigraphic sections despite differences in stratigraphy and lithology. We use tephra correlations to constrain the age of the base of the Ojo Caliente Sandstone Member of the Tesuque Formation to 13.5–13.3 Ma.