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.

Geologic mapping, age determinations, and geochemistry of rocks exposed in the Abiquiu area of the Abiquiu embayment of the Rio Grande rift, north-central New Mexico, provide data to determine fault-slip and incision rates. Vertical-slip rates for faults in the area range from 16 m/m.y. to 42 m/m.y., and generally appear to decrease from the eastern edge of the Colorado Plateau to the Abiquiu embayment. Incision rates calculated for the period ca. 10 to ca. 3 Ma indicate rapid incision with rates that range from 139 m/m.y. on the eastern edge of the Colorado Plateau to 41 m/m.y. on the western part of the Abiquiu embayment. The Abiquiu area is located along the margin of the Colorado Plateau–Rio Grande rift and lies within the Abiquiu embayment, a shallow, early extensional basin of the Rio Grande rift. Cenozoic rocks include the Eocene El Rito Formation, Oligocene Ritito Conglomerate, Oligocene–Miocene Abiquiu Formation, and Miocene Chama–El Rito and Ojo Caliente Sandstone Members of the Tesuque Formation (Santa Fe Group). Volcanic rocks include the Lobato Basalt (Miocene; ca. 15–8 Ma), El Alto Basalt (Pliocene; ca. 3 Ma), and dacite of the Tschicoma Formation (Pliocene; ca. 2 Ma). Quaternary deposits consist of inset axial and side-stream deposits of the ancestral Rio Chama (Pleistocene in age), landslide and pediment alluvium and colluvium, and Holocene main and side-stream channel and floodplain deposits of the modern Rio Chama. The predominant faults are Tertiary normal high-angle faults that displace rocks basinward. A low-angle fault, referred to as the Abiquiu fault, locally separates an upper plate composed of the transitional zone of the Ojo Caliente Sandstone and Chama–El Rito Members from a lower plate consisting of the Abiquiu Formation or the Ritito Conglomerate. The upper plate is distended into blocks that range from about 0.1 km to 3.5 km long that may represent a larger sheet that has been broken up and partly eroded. Geochronology ( 40 Ar/ 39 Ar) from fifteen volcanic and intrusive rocks resolves discrete volcanic episodes in the Abiquiu area: (1) emplacement of Early and Late Miocene basaltic dikes at 20 Ma and ca. 10 Ma; (2) extensive Late Miocene–age lava flows at 9.5 Ma, 7.9 Ma, and 5.6 Ma; and (3) extensive basaltic eruptions during the early Pliocene at 2.9 Ma and 2.4 Ma. Clasts of biotite- and hornblende-rich trachyandesites and trachydacites from the base of the Abiquiu Formation are dated at ca. 27 Ma, possibly derived from the Latir volcanic field. The most-mafic magmas are interpreted to be generated from a similar lithospheric mantle during rifting, but variations in composition are correlated with partial melting at different depths, which is correlated with thinning of the crust due to extensional processes.

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.

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.

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.

Mima-type soil mounds result from the repeated outward tunneling of Geomyid pocket gophers from nest and food storage centers and the resultant backward displacement of soil and its accumulation near such centers. These mounds are widespread in alpine and montane grassland habitats in the western United States. Their abundance in highland areas is confirmed by Google Earth surveys, by published studies and other sources, and by personal fieldwork of the author and colleagues. Highland areas in Canada and Mexico were also surveyed by Google Earth, but Mima-type mounds have not yet been found in these locations. Almost all alpine mound sites surveyed are on ridge tops or south-facing slopes, with many best expressed just above timberline. In some northern locations alpine and montane mounds appear to have formed since the Pleistocene. The presence of mounds only on moraines of Illinoian age at one Wyoming site suggests that mounds there, and thus perhaps in more southern locations, may have begun forming much earlier.

The Jurassic Todilto Formation of NW New Mexico and SW Colorado, USA, has utility as an analog of Martian flood evaporites. The Todilto Formation is a concentrically and vertically zoned carbonate (calcite with minor late dolomite) to sulfate (gypsum) evaporite deposit that developed over a short time span (10 4 –10 5 yr) after rapid flooding of the vast dune field of the Entrada Formation. Within the limits of the very different hydrogeologic environments of Mars and Earth, the Todilto setting of short-lived brine evolution in a largely eolian environment, with terminal formation of a salt hydrate common to both planets (gypsum), provides a useful field area for descriptive and petrogenetic studies of evaporite evolution and interaction with a porous, sandy substrate. The Todilto Formation has an added feature of interest in its association with bituminous materials that have likely microbial precursors, providing a brine-microorganism association that may represent a potential setting for primitive life as might be found on Mars.

Mountain ranges in the southern Rocky Mountains, first uplifted during the early Cenozoic Laramide orogeny, have followed separate landscape evolutionary pathways in the late Cenozoic. We present a model that reconstructs the post-Laramide tectonic and geomorphic history of Sierra Nacimiento and the Taos Range, two nearly adjacent rift-flank ranges in north-central New Mexico that serve to illustrate the various processes shaping landscapes across the southern Rocky Mountains. The Sierra Nacimiento landscape reflects the exhumation of hard Precambrian rocks from beneath a softer Phanerozoic sedimentary cover. The exhumation is continuous, but not steady, being driven by distal base-level fall. Downstream diverging river terraces in the Jemez River valley on the eastern flank of Sierra Nacimiento and late Pliocene to Holocene fluvial deposits on the western Sierra Nacimiento piedmont document the base-level fall. The timing and contemporary rates of incision from these river systems suggest that exhumation is being propagated from south to north as knickzones work their way headward from the Rio Grande. In contrast, the Taos Range landscape reflects alternating active stream incision and aggradation astride, and throttled by, an active range-front normal fault. The distinction between the exhumation-dominated and tectonic-dominated mountain front is best quantified by analyses of first-order stream gradients and a watershed metric we call the drainage basin volume to drainage basin area ratio ( R va ). Gradients of first-order streams in the exhumation-dominated Sierra Nacimiento have a mode of 6.8 degrees, significantly less than the 17.7 degrees obtained from a comparable data set of Taos Range first-order streams. The distinct stream gradient and R va populations hint at an important change in the processes shaping hillslopes and low-order channels, which is supported by the lack of slope-clearing landslides in the Sierra Nacimiento landscape and the presence of such landslides in the Taos Range. Analogue and numeric models find that steep, rugged, faceted topography associated with tectonically active mountain fronts like the Taos Range can only be produced and maintained by creep and landslides where the sediment flux scales as a power law with respect to average hillslope or low-order channel gradient. Here, the fingerprint of active tectonics is recorded by both high R va values and steep modal channel gradients. By comparison, the Sierra Nacimiento landscape is shaped primarily by creep where the sediment flux has a linear relationship to average hillslope and low-order channel gradient. In this situation, the signatures of distal base-level fall are low R va values and relatively gentle modal channel gradients.