Volcanic clasts incorporated in the lower portion of the Tertiary Santa Fe Group sedimentary rocks of the Culebra graben, San Luis Basin, Colorado, provide constraints on the timing of regional tectonic events by provenance determination. Based on currently exposed volcanic terrains, possible clast sources include Spanish Peaks and Mount Mestas to the east, the San Juan volcanic field to the west, and the Thirtynine Mile volcanic field, a remnant of the Central Colorado volcanic field, to the north and east of the San Luis Basin. Provenance was determined by a variety of geochemical, mineral chemical, and geochronologic data. Large porphyritic Santa Fe Group volcanic clasts are potassic with a wide compositional range from potassic trachybasalt to rhyolite. The whole-rock chemistry of the Culebra graben clasts is similar to that of the Thirtynine Mile and San Juan volcanic fields. Culebra graben amphibole and biotite chemistry is generally consistent with that of rocks of the San Juan volcanic field, but not with Spanish Peaks samples. Trace-element data of Culebra graben volcanic clasts overlap with those of the San Juan and Thirtynine Mile volcanic fields, but differ from those of the Mount Mestas. Thermobarometric calculations using mineral chemistry suggest that many Culebra graben rocks underwent a three-stage crystallization history: ~1120 °C at 7–10 kbar, ~1100 °C at 2.3–4.6 kbar, and hornblende formation ~800 °C at 3 kbar. Within the Culebra graben clasts, zircon rim U-Pb geochronologic systematics as well as amphibole and biotite 40 Ar/ 39 Ar plateau data yield ages ranging from 36 to 29 Ma. These ages are consistent with ages of the Thirtynine Mile volcanic field (36–27 Ma) and the Conejos Formation of the San Juan volcanic field (35–29 Ma), but predate Spanish Peaks (ca. 27–21 Ma) and Mount Mestas (ca. 25 Ma). Based on these data, Spanish Peaks and Mount Mestas are excluded as potential source areas for the Santa Fe Group volcanic clasts in the Culebra graben. The San Juan volcanic field is also an unlikely source due to the distance from the depositional site, the inconsistent paleo-current directions, and the pressure-temperature conditions of the rocks. The most likely scenario is that the Central Colorado volcanic field originally extended proximal to the current location of the Culebra graben and local delivery of volcanic clasts was from the north and northeast prior to the uplift of the Culebra Range and Sangre de Cristo Mountains.

The Santa Fe Group is a succession of Neogene sandstone, siltstone, conglomerate, and intercalated volcanics that fills the basins of the central Rio Grande rift. Data from twelve deep wells and recently released seismic profiles in the Albuquerque rift basin document a dramatic basinward thickening of the Santa Fe Group across the major basinal faults. Abbreviated thicknesses of less than 1,220 m (4,000 ft) occur on the structurally higher, outer benches, but the section thickens to more than 4,270 m (14,000 ft) in the basin center, on the downthrown sides of the basin’s seismically defined master normal faults. Thickness of the Santa Fe mimics the basin’s structural asymmetry. In the north, it forms an eastward-thickening clastic wedge in response to eastward tilting of the north part of the basin toward the westward-flattening master faults (Rio Grande, Sandia, and San Francisco-Placitas faults). In the south, it thickens westward toward the Santa Fe-Coyote master faults along the southwestern margin of the basin. As a result, depositional rates of the Santa Fe were rapid on the structurally deep, master-fault sides of the basin in the northeast and southwest and relatively slow on the basin’s hinge sides in the northwest and southeast. At the northeast-trending Tijeras accommodation zone, which divides the north and south halves of the basin, the section thickens abruptly to the north in the basin center, indicating a major dip-slip as well as probable strike-slip component of motion between the two subbasins. Much of the thickening in the Santa Fe Group seems to occur in the upper Miocene part of the section, possibly implying a phase of accelerated crustal extension at that time.

Interpretation of gravity, aeromagnetic, and magnetotelluric (MT) data reveals patterns of rifting, rift-sediment thicknesses, distribution of pre-rift volcanic and sedimentary rocks, and distribution of syn-rift volcanic rocks in the central San Luis Basin, one of the northernmost major basins that make up the Rio Grande rift. Rift-sediment thicknesses for the central San Luis Basin determined from a three-dimensional gravity inversion indicate that syn-rift Santa Fe Group sediments have a maximum thickness of ~2 km in the Sanchez graben near the eastern margin of the basin along the central Sangre de Cristo fault zone, and reach nearly 1 km within the Monte Vista graben near the western basin margin along the San Juan Mountains. In between, Santa Fe Group thickness is negligible under the San Luis Hills and estimated to reach ~1.1 km under the Costilla Plains (although no independent thickness constraints exist, and a range of thicknesses of 600 m to 2 km is geophysically reasonable). From combined geophysical and geologic considerations, pre-rift, dominantly sedimentary rocks appear to increase in thickness from none in the Sanchez graben on the east to perhaps 800 m under the San Luis Hills on the west. The pre-rift rocks are most likely early Tertiary in age, but the presence of Mesozoic and Paleozoic sedimentary rocks cannot be ruled out. Geophysical data provide new evidence that an isolated exposure of Proterozoic rocks on San Pedro Mesa is rooted in the Precambrian basement. This narrow, north-south–trending basement high has ~2 km of positive relief with respect to the base of the Sanchez graben, and separates the graben from the structural depression beneath the Costilla Plains. A structural high composed of pre-rift rocks, long inferred to extend from under the San Luis Hills to the Taos Plateau, is confirmed and found to be denser than previously believed, with little or no overlying Santa Fe Group sediments. Major faults in the study area are delineated by geophysical data and models; these faults include significant vertical offsets (≥1 km) of Precambrian rocks along the central and southern zones of the Sangre de Cristo fault system. Other faults with similarly large offsets of the Santa Fe Group include a fault bounding the western margin of San Pedro Mesa, and other faults that bound the Monte Vista graben in an area previously assumed to be a simple hinge zone at the western edge of the San Luis Basin. A major north-south–trending structure with expression in gravity and MT data occurs at the boundary between the Costilla Plains and the San Luis Hills structural high. Although it has been interpreted as a down-to-the-east normal fault or fault zone, our modeling suggests that it also is likely related to pre-rift tectonics. Aeromagnetic anomalies over much of the area are interpreted to mainly reflect variations of remanent magnetic polarity and burial depth of the 5.3–3.7 Ma Servilleta Basalt of the Taos Plateau volcanic field. Magnetic-source depth estimates are interpreted to indicate patterns of subsidence following eruption of the basalt, with maximum subsidence in the Sanchez graben.

Two- and three-dimensional electrical resistivity models derived from the magnetotelluric method were interpreted to provide more accurate hydrogeologic parameters for the Albuquerque and Española Basins. Analysis and interpretation of the resistivity models are aided by regional borehole resistivity data. Examination of the magnetotelluric response of hypothetical stratigraphic cases using resistivity characterizations from the borehole data elucidates two scenarios where the magnetotelluric method provides the strongest constraints. In the first scenario, the magnetotelluric method constrains the thickness of extensive volcanic cover, the underlying thickness of coarser-grained facies of buried Santa Fe Group sediments, and the depth to Precambrian basement or overlying Pennsylvanian limestones. In the second scenario, in the absence of volcanic cover, the magnetotelluric method constrains the thickness of coarser-grained facies of buried Santa Fe Group sediments and the depth to Precambrian basement or overlying Pennsylvanian limestones. Magnetotelluric surveys provide additional constraints on the relative positions of basement rocks and the thicknesses of Paleozoic, Mesozoic, and Tertiary sedimentary rocks in the region of the Albuquerque and Española Basins. The northern extent of a basement high beneath the Cerros del Rio volcanic field is delineated. Our results also reveal that the largest offset of the Hubbell Spring fault zone is located 5 km west of the exposed scarp. By correlating our resistivity models with surface geology and the deeper stratigraphic horizons using deep well log data, we are able to identify which of the resistivity variations in the upper 2 km belong to the upper Santa Fe Group sediments

Analysis of borehole and reflection seismic data from the Alamosa basin (northern San Luis Basin, Rio Grande rift) reveals tectonic development in response to three Tertiary events, each with an associated package of rocks distinguished by mineralogy and petrology. Eocene redbeds of the Blanco Basin Formation (0 to 696 m thick) are micaceous, sandy mudstone and coarse arkosic sandstone units containing lithic pebbles derived from granitic basement rock. They were deposited in a late Laramide basin formed during wrench-fault-related segmentation of the early Laramide San Luis-Brazos uplift. The western half of the younger, rift-related Alamosa basin is superposed over this late Laramide basin. Initiation of Oligocene volcanism is marked by andesitic lava flows and volcaniclastic rocks of the Conejos Formation (0 to 2,300 m thick), also limited in extent to the western half of the Alamosa basin. Ash-flow tuffs (380 to 580 m thick) correlative to 29 to 27 Ma tuffs of the San Juan volcanic field cap the Conejos Formation in the western half of the basin and rest directly on denuded Precambrian basement in the eastern half of the basin. These tuffs exist in deep wells across the Alamosa basin and together represent a basinwide time marker. Extension related to the Rio Grande rift resulted in eastward-tilting of the entire basin area following emplacement of the ash-flow tuffs. Filling the resulting half graben is the upper Oligocene-middle Pleistocene Santa Fe Group (as much as 5.6 km thick) composed of variegated mudstones and coarse lithic sandstones and conglomerates. Lithic fragments in the Santa Fe Group represent two sources: variable-composition volcanic rocks from the San Juan volcanic field to the west (majority) and plutonic-metamorphic-sedimentary, basement-derived rocks from the Sangre de Cristo Range to the east (minority). An angular unconformity within the Santa Fe Group documents strong early tilting due to movement on the Sangre de Cristo fault zone during an early phase of rifting (late Oligocene-early Miocene). The rift-related geometry of the crust beneath the Alamosa basin is that of two east-tilted crustal blocks creating two second-order half grabens within the basin.

Detailed stratal tilt information for radioisotopically dated Tertiary rocks in the Lemitar Mountains of the central Rio Grande rift allows discrimination of two episodes of extension in the late Oligocene (largely 28.6 to 27.4 Ma) and middle to late Miocene (16 to 10 Ma). Rapid deformation in the late Oligocene (at least 25% extension) produced narrow, wedge-shaped accumulations of ash-flow tuffs and mafic lavas within half grabens defined by closely spaced (2 to 5 km) planar-rotational normal faults. Volcanic accumulation rates during this episode were sufficient to bury the developing fault-block topography, resulting in preservation of only minor amounts of intercalated sedimentary rocks. Following a period of weak extension in the early Miocene, rates of extensional strain again began to increase in the early middle Miocene (about 16 Ma) and continued to increase until about 10 Ma, when a more moderate rate ensued. During the time interval of 16 to 10 Ma, the Lemitar Mountains area was extended about 30% along widely spaced (5 to 15 km) planar-normal faults. Increased stratal-tilt rates in the middle Miocene caused the development of accommodation space to locally exceed sediment supply on the down-thrown side of a major intrabasinal fault (Silver Creek fault), resulting in topographic closure and the development of playa deposits within the lower Santa Fe Group (Popotosa Formation). As stratal-tilt rates further increased in the late Miocene, fluvial-lacustrine sedimentation offlapped toward narrow zones of maximum subsidence along active faults, causing the carving of widespread unconformities on older sediments on hanging-wall dip slopes. As tectonism waned in the latest Miocene-early Pliocene, sediment supply exceeded the development of accommodation space, resulting in onlap and aggradation of the upper Santa Fe Group (Sierra Ladrones Formation) that enabled the eventual integration of an axial-fluvial system (ancestral Rio Grande) between the longitudinally arrayed half grabens to the north and south of the Socorro Basin. The late Oligocene episode of rapid extension appears to be related to strain focusing by local thermal weakening due to voluminous concurrent magmatism in the area. In contrast, the late Miocene extensional event may be of regional scale, as shown by evidence for contemporaneous tectonism in other areas and the development of widespread unconformities between Miocene and Pliocene deposits of the Santa Fe Group in many basins of the Rio Grande rift.

We investigated a Plio-Pleistocene alluvial succession in the Albuquerque Basin of the Rio Grande rift in New Mexico using geomorphic, stratigraphic, sedimentologic, geochronologic, and magnetostratigraphic data. New 40 Ar/ 39 Ar age determinations and magnetic-polarity stratigraphy refine the ages of the synrift Santa Fe Group. The Pliocene Ceja Formation lies on the distal hanging-wall ramp across much of the Albuquerque Basin. The Ceja onlapped and buried a widespread, Upper Miocene erosional paleosurface by 3.0 Ma. Sediment accumulation rates in the Ceja Formation decreased after 3.0 Ma and the Ceja formed broad sheets of amalgamated channel deposits that prograded into the basin after ca. 2.6 Ma. Ceja deposition ceased shortly after 1.8 Ma, forming the Llano de Albuquerque surface. Deposition of the Sierra Ladrones Formation by the ancestral Rio Grande was focused near the eastern master fault system before piedmont deposits (Sierra Ladrones Formation) began prograding away from the border faults between 1.8 and 1.6 Ma. Widespread basin filling ceased when the Rio Grande began cutting its valley, shortly after 0.78 Ma. Although the Albuquerque Basin is tectonically active, the development of through-going drainage of the ancestral Rio Grande, burial of Miocene unconformities, and coarsening of upper Santa Fe Group synrift basin fill were likely driven by climatic changes. Valley incision was approximately coeval with increased northern- hemisphere climatic cyclicity and magnitude and was also likely related to climatic changes. Asynchronous progradation of coarse-grained, margin-sourced detritus may be a consequence of basin shape, where the basinward tilting of the hanging wall promoted extensive sediment bypass of coarse-grained, margin-sourced sediment across the basin.

The Albuquerque Basin is one of the largest and deepest basins of the Rio Grande rift. The latest Oligocene to middle Pleistocene Santa Fe Group is the major basin fill unit. Paleogene deposits underlie the Santa Fe and indicate that a depositional center predated the Albuquerque Basin. Pre-Santa Fe Tertiary deposits are subdivided into a lower unit that is at least partly correlative with the Eocene Galisteo and Baca Formations and an upper unit, the unit of Isleta #2, that is equivalent to the Datil Group and the overlying sequence of Oligocene volcanic rocks. Thickness of the Santa Fe Group ranges from 1,000 to 2,000 m along the basin margins to as much as 4,407 m in the basin center. Galisteo and Baca thicknesses are as much as 454 m; the unit of Isleta #2 is up to 2,185 m in the Shell West Mesa well. Galisteo-Baca sediments were deposited in basins that predated the Albuquerque Basin. These depocenters continued to receive sediments of the unit of Isleta #2 during early to middle Oligocene time. Lower Santa Fe sediments (30 to 5 Ma) were deposited into two internally drained basins. After 10 Ma, the Santa Fe had filled the basins to the point where a single, internally drained Albuquerque Basin was formed. At about 5 Ma, the basin drainage became through-flowing with the development of the ancestral Rio Grande. The first major episode of Rio Grande Valley entrenchment at about 1.0 Ma ended Santa Fe deposition. Sedimentation rates ranged from 20 to 30 m/m.y. during early and late phases of Santa Fe deposition to as much as 600 m/m.y. during middle Miocene deposition.

The Winston graben is located on the western margin of the Rio Grande rift in south-central New Mexico in an area of intense Neogene faulting. It is a symmetrical graben, 5 to 10 km wide, approximately 56 km long, and trends north-northwest to north-northeast. The structure is bounded by high-angle normal faults with about 2 km of stratigraphic separation across both margins. Most of its eastern boundary fault is believed to be a reactivated Eocene strike-slip fault. Northeast-trending accommodation zones (structural highs) terminate the graben at both north and south ends. Rocks exposed within the graben include Pennsylvanian and Permian sedimentary formations, several Eocene-Oligocene volcanic and volcaniclastic units, the upper Oligocene-Quaternary Santa Fe Group, a Miocene andesite flow that is intercalated with the Santa Fe Group, and a Pliocene basalt flow. Some of the volcanic units occur only within the Winston graben. Initial development of the graben began in the late Oligocene, but most of its growth was in the Neogene. Boundary faults of the Winston graben have been inactive for at least the past 4.8 m.y. When placed in a tectonic setting with surrounding structures of the Rio Grande rift, the Winston graben is interpreted as having formed over a deep detachment surface.

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.

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).

Discrepancies among previous models of the geometry of the Albuquerque Basin motivated us to develop a new model using a comprehensive approach. Capitalizing on a natural separation between the densities of mainly Neogene basin fill (Santa Fe Group) and those of older rocks, we developed a three-dimensional (3D) geophysical model of syn-rift basin-fill thickness that incorporates well data, seismic-reflection data, geologic cross sections, and other geophysical data in a constrained gravity inversion. Although the resulting model does not show structures directly, it elucidates important aspects of basin geometry. The main features are three, 3–5-km-deep, interconnected structural depressions, which increase in size, complexity, and segmentation from north to south: the Santo Domingo, Calabacillas, and Belen subbasins. The increase in segmentation and complexity may reflect a transition of the Rio Grande rift from well-defined structural depressions in the north to multiple, segmented basins within a broader region of crustal extension to the south. The modeled geometry of the subbasins and their connections differs from a widely accepted structural model based primarily on seismic-reflection interpretations. Key elements of the previous model are an east-tilted half-graben block on the north separated from a west-tilted half-graben block on the south by a southwest-trending, scissor-like transfer zone. Instead, we find multiple subbasins with predominantly easterly tilts for much of the Albuquerque Basin, a restricted region of westward tilting in the southwestern part of the basin, and a northwesterly trending antiform dividing subbasins in the center of the basin instead of a major scissor-like transfer zone. The overall eastward tilt indicated by the 3D geophysical model generally conforms to stratal tilts observed for the syn-rift succession, implying a prolonged eastward tilting of the basin during Miocene time. An extensive north-south synform in the central part of the Belen subbasin suggests a possible path for the ancestral Rio Grande during late Miocene or early Pliocene time. Variations in rift-fill thickness correspond to pre-rift structures in several places, suggesting that a better understanding of pre-rift history may shed light on debates about structural inheritance within the rift.

The Sunshine Valley–Costilla Plain, a structural subbasin of the greater San Luis Basin of the northern Rio Grande rift, is bounded to the north and south by the San Luis Hills and the Red River fault zone, respectively. Surficial mapping, neotectonic investigations, geochronology, and geophysics demonstrate that the structural, volcanic, and geomorphic evolution of the basin involves the intermingling of climatic cycles and spatially and temporally varying tectonic activity of the Rio Grande rift system. Tectonic activity has transferred between range-bounding and intrabasin faults creating relict landforms of higher tectonic-activity rates along the mountain-piedmont junction. Pliocene–Pleistocene average long-term slip rates along the southern Sangre de Cristo fault zone range between 0.1 and 0.2 mm/year with late Pleistocene slip rates approximately half (0.06 mm/year) of the longer Quaternary slip rate. During the late Pleistocene, climatic influences have been dominant over tectonic influences on mountain-front geomorphic processes. Geomorphic evidence suggests that this once-closed subbasin was integrated into the Rio Grande prior to the integration of the once-closed northern San Luis Basin, north of the San Luis Hills, Colorado; however, deep canyon incision, north of the Red River and south of the San Luis Hills, initiated relatively coeval to the integration of the northern San Luis Basin. Long-term projections of slip rates applied to a 1.6 km basin depth defined from geophysical modeling suggests that rifting initiated within this subbasin between 20 and 10 Ma. Geologic mapping and geophysical interpretations reveal a complex network of northwest-, northeast-, and north-south–trending faults. Northwest- and northeast-trending faults show dual polarity and are crosscut by north-south– trending faults. This structural model possibly provides an analog for how some intracontinental rift structures evolve through time.

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.