New 40Ar/39Ar ages, combined with selected ages from published studies, provide detailed insight into the late Quaternary (<500 ka) eruptive history and related hazards of the Rio Grande rift and Jemez lineament of New Mexico, USA. Most eruptions in the region during this time were within monogenetic volcanic fields, which largely produced cinder cones and mafic lava flows. 40Ar/39Ar ages of mafic groundmass determined using the high-sensitivity ARGUS VI multicollector mass spectrometer are significantly more precise, by as much as an order of magnitude, than prior 40Ar/39Ar dates. The high-precision data permit more rigorous interpretation of age spectra and isochrons, leading to a greater confidence in assigning eruption ages, and thus allowing more accurate and thorough calculations of eruptive rates and repose periods. For most fields, comprehensive dating identifies a greater number of late Quaternary eruptions than previously known and, for some fields, determines younger-than-previously established ages for the last eruptive events. Repose periods in the fields range from too short to measure with the 40Ar/39Ar method to a few hundred thousand years, which suggests that all 12 fields in the rift and lineament with late Quaternary activity should be considered dormant rather than extinct, with the possibility of future eruptions. Average recurrence intervals for these fields during the late Quaternary range from 16.5 k.y. to 170.8 k.y. Many fields display peak periods of activity where rates spike to a recurrence interval of 5 k.y. or less. At the scale of the entire rift and lineament, 75 late Quaternary eruptions were dated, yielding an average recurrence interval of 6.5 k.y., which is a minimum estimate considering the likelihood of undated eruptions (either not studied or buried). During the last 100 k.y., the volcanic record is better preserved, and the recurrence interval is 3.2 k.y., which indicates that the current hiatus of 3.9 ± 1.2 k.y. is typical for the region. Since ca. 36 ka, the average recurrence interval decreased to 2.3 k.y., which suggests a slight increase in recent activity. When ages are compared to vent locations, a previously unrecognized migrational pattern is observed in nearly all of the fields studied. Migration vectors vary from 1.0 cm/yr to 4.0 cm/yr, and always with an eastward component—similar to migration patterns at some other late Cenozoic fields throughout the American Southwest. Volcanic migration is attributed to a combination of mechanisms including asthenospheric convection along the margin of the Colorado Plateau, North American plate motion over partial mantle melt, and extensional tectonics. Developing similar high-precision chronologies for other Quaternary fields throughout southwestern North America will be necessary to better understand the volcanic hazards of the region.

Establishing the temporal and spatial eruptive patterns of late Quaternary volcanic fields is fundamental for quantifying associated hazards, exploring controls on eruptive behavior, and explaining the origin of magmas. Ages are used to determine recurrence rates and repose periods, two common parameters that characterize past activity and provide insight into future eruptions (Turrin et al., 1991; Heizler et al., 1999; Connor and Hill, 1995; Conway et al., 1998; Connor et al., 2000; Bebbington and Cronin, 2011; Linnell et al., 2016; Nieto-Torres and Del Pozzo, 2019). When ages are combined with spatial data, such as vent locations and volume estimates of volcanic deposits, this information can be used to identify vent migration patterns (Tanaka et al., 1986; Condit et al., 1989; Fleck et al., 2014) and changes in volcanic flux (Hora et al., 2007; Lipman and Calvert, 2013; Calvert et al., 2018, Stelten et al., 2020), two additional parameters that are useful for assessing volcanic hazards. Integration of geochemistry with geochronology often reveals the mechanisms of magmatic genesis, ascent timescales, and the processes that trigger eruptions (Spell and Harrison., 1993; McMillan et al., 2000; Andersen et al., 2019; Eichler and Spell, 2020). Lastly, many late Quaternary volcanic deposits are commonly intercalated with sediments in adjacent basins. Thus, the ages of these deposits constrain sedimentation rates, the timing of landscape evolution, and, if present, ages of fossil or archaeological assemblages (Deino and Potts, 1990; Walter, 1994; Nereson et al., 2013; Channer et al., 2015; Oppenheimer et al., 2019).

Several regions throughout the United Sates are identified as areas for current or future volcanic unrest (Ewert et al., 2018). These include the volcanoes along the Aleutian Arc in Alaska, the Cascade volcanoes of the Pacific Northwest, the Yellowstone caldera and nearby volcanic fields, and the numerous monogenetic volcanic fields throughout the western states. Recent and ongoing geologic, geophysical, and geochemical studies in each of these regions are establishing the eruptive histories and characterizing the potential for current and future volcanic activity (e.g., Smith et al., 2009; Chang et al., 2010; Calvert et al., 2018; Kiser et al., 2018; Burgess et al., 2019; O’Hara et al., 2020). However, the late Quaternary volcanic history (here defined as <500 ka) in the southwestern United States, particularly as it pertains to volcanic hazards, is arguably the least understood. This is, in part, because the region has not experienced any eruption since ca. 1100 CE (i.e., Sunset Crater and Little Springs volcano in Arizona, USA—Ort et al., 2002, 2008; Elson et al., 2011), nor is there any current indication of any renewed activity. Nevertheless, the Southwest region contains mid- and upper-crustal magma chambers (Sanford et al., 1977; Fialko and Simons, 2001; Aprea et al., 2002; Finnegan and Pritchard, 2009), small seismic events linked to magmatic movement (Schlue et al., 1987; Brumbaugh et al., 2014; Mesimeri et al., 2021), numerous Holocene eruptions (Giegengack, 1962; Ort et al., 2002; Dunbar and Phillips, 2004; Valentine et al., 2021), and multiple fields with prolonged (i.e., >1 m.y.) eruptive histories that show no signs of cessation (Tanaka et al., 1986; Condit et al., 1989; Spell and Harrison, 1993; Stroud, 1997; Appelt, 1998; Williams, 1999; Olmsted, 2000). Together, these factors suggest that the region is still susceptible to volcanic eruptions and related hazards.

This paper presents new, high-precision 40Ar/39Ar ages, along with relevant published ages determined by various dating techniques, to address three fundamental aspects of the late Quaternary eruptive history of the Rio Grande rift and along the Jemez lineament of New Mexico, USA, a region of significant Quaternary activity (Fig. 1). First, how many late Quaternary eruptions occurred within the rift and along the lineament? Second, are there changes in the eruptive rates or repose periods that indicate whether the tempo of volcanic activity is waxing or waning? Lastly, is the temporal-spatial distribution of vents random during the late Quaternary Period, or is there systematic migration? Answers to these questions provide important insight into future volcanic activity.

Thousands of late Quaternary volcanoes have been identified in the southwestern United States (Ulrich and Bailey, 1987; Laughlin et al., 1993; Dunbar, 2005; Ewert et al., 2018; Valentine et al., 2021). In October 2012, nearly 100 scientists and federal, state, and local officials began discussing hazard assessment priorities and possible volcanic eruption scenarios in the southwestern United States (Lowenstern, 2013). One outcome of this meeting was the recognition that a comprehensive, accurate, and precise record of the timing of volcanism in the Southwest is one of the most critical, yet lacking, data sets for understanding future eruptions in the region. The timing of southwestern late Quaternary volcanism is partially known from existing geochronologic studies, but that record is both incomplete and imprecise. Of the flows dated, many have large uncertainties (±>50 ka) that do not permit the calculation of repose periods between eruptions and do not accurately identify temporal-spatial patterns. For example, Valentine et al. (2021) estimated that only 15% of ~2229 Quaternary volcanoes of the American Southwest were dated with modern, high-precision techniques. Yet these temporal-spatial patterns of volcanism, coupled with detailed understanding of post-eruptive behavior, constitute some of the core data required for statistical assessment of volcanic hazards (Turrin et al., 1991; Connor and Hill, 1995; Wolff and Gardner, 1995; Conway et al., 1998; Heizler et al., 1999; Connor et al., 2000; Valentine and Perry, 2007). Furthermore, this information provides scenarios for volcanologists and first-responders tasked with disaster response (e.g., Fink and Ajibade, 2022).

The limited chronology of late Quaternary volcanism in the Southwest is, in part, related to two aspects of prior studies of young volcanism in the region. First, many published studies of late Quaternary volcanism are related to either landscape or geochemical evolution, neither of which requires a comprehensive suite of high-precision ages (e.g., Bachman and Mehnert, 1978; Baldridge et al., 1987; Crow et al., 2008, 2011; Peters et al., 2008; Nereson et al., 2013; Channer et al., 2015). Targeted dating of key units or using published ages with variable precision was typically sufficient. Second, dating of very young volcanic rocks is a unique challenge for geochronologists (Renne et al., 1997, 2009; Turrin et al., 2008; Wijbrans et al., 2011). Numerous methods have been used, each with their own benefits and limitations. The 14C method is extremely useful for dating very young eruptions because of the 5730 year half-life of 14C. However, the short half-life also limits dating events to eruptions that are ca. <50 ka (Bronk Ramsey, 2008). Furthermore, the technique requires finding carbon samples suitable for dating, which are sometimes uncommon in the arid American Southwest. Exposure dating methods (e.g., 36Cl and 3He) are also widely used to understand young volcanism. Flows with significant erosion or that were covered by younger deposits are not candidates for this technique (Kurz, 1986; Phillips et al., 1986; Dunbar and Phillips, 2004). U-series techniques have been successful for dating young volcanic rocks, but magmatic residence times and open-system magmatic processes can complicate age interpretations (Allegre and Condomines, 1976; Peate et al., 1996; Sims et al., 2007).

The 40Ar/39Ar dating method is perhaps the most common technique used to date Quaternary volcanic rocks because nearly all volcanic rocks contain K-bearing mineral phases, either as phenocrysts or as mineral constituents of the groundmass (McDougall and Harrison, 1999; Renne, 2000). In the early 2010s, the ability to determine the 40Ar/39Ar ages of young, potassium-poor, volcanic rocks—those typical of the late Quaternary distributed monogenetic fields of the Southwest—significantly improved due to the development and implementation of low-volume, high-sensitivity, multicollector noble gas mass spectrometers. For example, the ARGUS VI mass spectrometer used in this study provides approximately an order of magnitude improved precision compared to pre-ARGUS single-collector mass spectrometers (Heizler, 2011, 2012; Phillips and Matchan, 2013; Matchan and Phillips, 2014; Zimmerer et al., 2016), such as those used to date previous campaigns of late Quaternary southwestern U.S. volcanism. The enhanced precision also leads to more accurate age determinations by identifying samples with inheritance issues (e.g., xenocrysts or incompletely degassed antecrysts held within cold storage magma chambers), excess 40Ar, and 40Ar loss related to alteration.

Thus, this study began at the crossroads of two independent initiatives—a community interest in better characterizing the late Quaternary eruptive history, specifically as it pertains to hazards of the southwestern U.S. (Lowenstern, 2013), and the development and implementation of next-generation, high-sensitivity, multicollector noble gas mass spectrometers that are capable of yielding ages with the small uncertainties (±1–10 ka) necessary to precisely characterize late Quaternary eruptive histories.

Late Quaternary volcanism within the Rio Grande rift and along the Jemez lineament represents the most recent activity of a much longer duration volcanic and magmatic flare-up during the late Cenozoic. Although a complete summary of late Cenozoic volcanism of the southwestern U.S. is beyond the scope of this paper, a brief summary of the spatial, temporal, chemical, and geophysical characteristics is provided, particularly those that pertain to late Quaternary eruptive patterns and the related hazards. Baldridge (2004) and Dunbar (2005) provide additional summaries regarding the late Quaternary eruptive history of the rift and lineament.

Physical Characteristics of Late Quaternary Volcanism in the Rift and along the Lineament

Late Cenozoic volcanism in New Mexico is found within two zones (Fig. 1; Table 1): the north-trending Rio Grande rift of east–west extension and the Jemez lineament, a northeast-trending zone of crustal weakness that has focused magmatism since at least ca. 10 Ma (Baldridge and Olsen, 1989; Chapin et al., 2004). The origin of the Jemez lineament is still somewhat debated, though it is commonly thought to represent a Paleoproterozoic suture zone between two accreted terrains of the North American craton (Karlstrom and Bowring, 1988). Late Cenozoic volcanism is focused in approximately eight major volcanic fields; some have areas greater than several thousand square kilometers comprising tens of vents to more than 100 vents (e.g., Raton-Clayton and Potrillo fields), and several zones of isolated cones, flows, and fissure systems (e.g., Tusas Brazos field and Cat Hills volcanoes). The Valley of Fires (east-central New Mexico, USA) and the Animas Valley cones and flows (southwestern New Mexico) are not located along the lineament or within the central axis of the rift (Fig. 1), but instead are located within adjacent basins where the rift transitions into the Basin and Range physiographical province.

Most of the late Cenozoic fields of the rift and lineament display physical volcanologic characteristics (Fig. 2) that are common to monogenetic mafic volcanism (Valentine and Gregg, 2008). The majority of eruptions are sourced from distributed cinder and spatter cones (Figs. 2A2C). Cone heights range from as little as 20–50 m to as much as 300 m. Younger cones (i.e., <100 ka) typically have slopes near the angle of repose for cinders with limited development of erosional rills (Fig. 2A); older cones (i.e., >100 ka) typically have shallower slope angles with moderate to significant erosion (Fig. 2B). Most vents produced lava flows of limited aerial extent (i.e., distal flows reach a maximum of a few kilometers from the source; Fig. 2C), although some flows traveled many tens of kilometers. The abundance of vegetation on flow surfaces, in addition to cone morphology, is another common parameter used to determine relative ages of flows (Figs. 2C and 2D). Pyroclastic deposits, where exposed, are typically found within a radius of 1–2 km of the source vent(s) and are usually stratigraphically below the associated lava flow(s), which indicates that many eruptions commenced with explosive activity before transitioning to more effusive behavior. Many of the cones are linearly aligned (Kelley and Kudo, 1978; Maldonado et al., 2006), are parallel to or are co-located with regional structures, and likely represent fissure eruptions produced during a single event (Fig. 2E). Small shield volcanoes (Fig. 2F) and maar volcanoes (Figs. 2G and 2H) are also present (Hoffer, 1976; Hoffer and Corbitt, 1991), but they represent a minor eruptive style in the region. Eruptions from Mount Taylor stratovolcano and those related to Valles caldera in the Jemez Mountains are more silicic in composition (Fig. 2I). Eruptions from these latter two centers built large edifices that are typical of polygenetic centers (Goff et al., 2011, 2019).

Timing of Late Cenozoic Volcanism in the Rift and along the Jemez Lineament

Numerous studies have temporally characterized aspects of late Cenozoic volcanism within the rift and lineament (e.g., Spell and Harrison, 1993; McIntosh and Cather, 1994; Chapin et al., 2004; Dunbar and Phillips, 2004; Dunbar, 2005; Nereson et al., 2013). However, none attempted to produce a comprehensive suite of high-precision ages for the region, instead typically focusing on components of a single volcanic field or region. K/Ar dating of the 1970s and 1980s (e.g., Stormer, 1972; Bachman and Mehnert, 1978; Leavy and Shafiqullah, 1987) was replaced by the more-precise 40Ar/39Ar technique in the 1990s and 2000s (Hallet et al., 1997; McIntosh and Cather, 1994; Phillips et al., 2007; Singer et al., 2008). Several fields of northern New Mexico were targeted for extensive 40Ar/39Ar dating studies (Raton-Clayton—Stroud, 1997; Taos Plateau—Appelt, 1998; Ocate—Olmsted, 2000) that produced data sets identifying important temporal trends useful for assessing volcanic activity, characterizing changes in magmatic chemistry, and understanding landscape development (e.g., data of Stroud, 1997, within Nereson et al., 2013). Most of these studies focused on the late Miocene to early Pleistocene eruptive activity of the rift and lineament. Obtaining high-precision ages of late Quaternary eruptions with the single-detector mass spectrometers available required dating large volumes of material (>200–500 mg per sample) or calculating a weighted mean of replicate, often low-precision analyses to statistically reduce the uncertainty. Likewise, studies using techniques such as 14C and surface exposure dating typically focused on the youngest events (i.e., <50 ka) within selected fields (Anthony and Poths, 1992; Dunbar, 1999; Dunbar and Phillips, 2004; Sims et al., 2007). Thus, much of the late Quaternary eruptive history of the rift and lineament, particularly for those vents emplaced between ca. 50 ka and 500 ka, remain poorly constrained.

Nonetheless, early dating and mapping campaigns provided very important information upon which this study could be built. First, volcanic fields of the rift and lineament display a large diversity in eruptive durations. Initiation of volcanism within each of the volcanic fields was asynchronous (Table 1). For example, volcanism in the Ocate field began at ca. 8.3 Ma (Olmsted, 2000), at ca. 9.2 Ma in the Raton-Clayton field (Stroud, 1997; Nereson et al., 2013), and as early as ca. 25 Ma in the Jemez Mountains (Kelley et al., 2013). In contrast, volcanism in the Zuni-Bandera and Potrillo fields, both of which are comparable in area to the previously mentioned fields, appears to have initiated during the Quaternary Period (Laughlin and WoldeGabriel, 1997; Anthony and Poths, 1992; Williams, 1999). Second, several fields produced Holocene eruptions, including the 11.2 ka Bandera flow and 3.9 ka McCartys flow in the Zuni-Bandera volcanic field (Dunbar and Phillips, 2004), and the 5.2 ka Carrizozo flow in the Valley of Fires volcanic field (Dunbar, 1999). Finally, the available geochronology, regardless of precision, helped to identify targets for dating in this study, as well as potential gaps in the chronologic record that needed to be filled to develop a more comprehensive history. Some of the <50 ka eruptions are well-dated by cosmogenic surface (i.e., 36Cl and 3He) and/or 14C dating methods (Anthony and Poths, 1992; Dunbar, 1999; Dunbar and Phillips, 2004; Sims et al., 2007), which provides important comparative tests for assessing the accuracy and reproducibility of 40Ar/39Ar ages obtained by multicollector, high-sensitivity noble gas mass spectrometers.

Chemical Characteristics of Late Quaternary Volcanism in the Rift and along the Lineament

Volcanism in the region during the late Quaternary Period, and for most of the late Cenozoic Era, is dominantly mafic in composition (Stormer, 1972; Baldridge et al., 1987; Chapin et al., 2004). Volcanic rocks in small fields, such as the olivine tholeiite flows of the Albuquerque volcanoes (Figs. 1 and 2E; Kelley and Kudo, 1978; Baldridge, 1979), typically display a restricted range of compositions that is thought to reflect eruption from a single, possibly short-lived magmatic system. In contrast, most of the larger volcanic fields with longer life spans erupted both alkalic and tholeiitic mafic lavas, commonly in close spatial and temporal proximity (Baldridge et al., 1987; Baldridge, 2004; Peters et al., 2008). The Potrillo volcanic field is an exception, as it only produced alkali basalts (Thompson et al., 2005). Furthermore, monogenetic fields with longer lifespans display significant compositional diversity in their earlier eruptive histories before transitioning to solely mafic volcanism. For example, some >5 Ma eruptions in the Raton-Clayton field are dacitic, trachytic, and rhyolitic in composition, whereas volcanism during the last ca. 1 m.y. has been entirely mafic (Stormer, 1972).

No study has identified systematic compositional trends of alkalic and tholeiitic late Quaternary volcanism, either within individual fields or for the region. However, several studies have identified longer term temporal-chemical patterns. Lithospheric sources dominated the earlier magmatic history (>10 Ma) of the rift and lineament, whereas asthenospheric sources are more common for the most recent (i.e., <10 Ma) eruptions (Perry et al., 1988; McMillan et al., 2000; Baldridge, 2004; Chapin et al., 2004; Crow et al., 2011). This chemical pattern is commonly interpreted to represent protracted lithospheric extension and decompression melting from the rising asthenosphere (McMillan et al., 2000). Petrologic models for Pliocene to Quaternary activity typically indicate polybaric melt generation from a variety of lithospheric and asthenospheric sources (Baldridge et al., 1987; Peters et al., 2008). Crustal contamination appears to be an insignificant component in mafic magma generation, although some flows contain xenocrystic quartz and feldspar that indicate some minor crustal involvement (Baldridge, 2004), likely during melt transport through the upper crust. Similar melt-genesis models were proposed for other volcanically active regions of the Southwest (Arculus and Gust, 1995; Crow et al., 2011). The major exceptions to the mafic Quaternary volcanism of the rift and lineament are the silicic eruptions of Valles caldera (Goff et al., 2011) and Mount Taylor (Perry et al., 1990; Goff et al., 2019). However, only the 68.9 ka and 74.2 ka rhyolitic eruptions of the East Fork Member at Valles caldera occurred within the last 500 ka (Nasholds and Zimmerer, 2022), the time period considered here.

Geophysical Characteristics of Late Quaternary Magmatism

Geophysical studies of the rift and lineament provide great insight into current crustal and mantle structures that played a role in late Quaternary volcanism and will influence future volcanism. Seismic studies of the “LA RISTRA” (Rio Grande rift Seismic Transect) project indicate that the crust is 40–50 km thick on either side of the rift but thins to 30–40 km beneath the axis of the rift (Gao et al., 2004; Wilson et al., 2004). This cross-section is consistent with geochemical models that indicate late Cenozoic crustal thinning and upwelling of the asthenospheric mantle during the late Cenozoic. Geophysical studies (Schmandt and Humphreys, 2010; MacCarthy et al., 2014) indicate that a broad, ~150-km-wide, low-velocity zone underlies most of the fields of the rift and lineament (particularly the latter; see data of Schmandt and Humphreys, 2010, presented in fig. 1B of Channer et al., 2015). These low-velocity zones are imaged to as deep as 195 km, and many are interpreted to represent zones of partial melt (Schmandt and Humphreys, 2010; Sosa et al., 2014), which likely contributes to surface volcanism and possibly current uplift (Channer et al., 2015).

Geophysical investigations also identified two zones of crustal melt within the rift (Fig. 1). The largest of these is the 510 km Socorro magma body located ~19 km beneath the central rift (Sanford et al., 1977; Balch et al.,1997). The surface above the northern magma body is currently uplifting at maximum rates of 2.5–3.0 mm/yr (Fialko and Simons, 2001). No surface volcanism is associated with this magma body, although a seismic event in 1935 south of Albuquerque, New Mexico, may have been related to dike emplacement (Schlue et al., 1987). In the Jemez Mountains, an upper to mid-crustal magma body, mostly crystalline (>80% by volume) and likely silicic, is located beneath the western sector of Valles caldera at depths ranging from 5 km to 15 km. Numerous magma bodies interpreted to be mafic sills are also located between 20 km and 39 km depth beneath the caldera (Lutter et al., 1995; Steck et al., 1998; Aprea et al., 2002; Schmandt et al., 2019).

Sample Collection

The overarching sampling philosophy was to collect all <500 ka eruptions in the rift and along the lineament. A total of 187 late Quaternary samples were collected during this study. Mapped units with published ages of <500 ka, regardless of the associated uncertainty, were targeted for sampling and redating for better precision. Additionally, those units without any existing chronology, but displaying characteristics similar to those of other late Quaternary eruptions (e.g., edifices with slopes near the angle of repose, glassy flow surfaces, limited soil development, and the absence of dense vegetation; Fig. 2), were also sampled. Non-glassy, well-crystallized lava flow interiors were sampled where exposed. In the absence of flow interiors, dense, lava-cored bombs were sampled. For some units, neither flow interiors nor dense bombs were exposed or identified, and thus the least vesiculated, most crystalline material from near-flow surfaces was sampled. Commonly, multiple samples from the same unit were collected and dated to test for reproducibility. Exposed lavas with abundant vesiculation, significant amounts of carbonate or other secondary minerals (e.g., zeolites), or red (oxidized) glass were avoided.

Based on the available geochronology, fields without a significant number of <500 ka eruptions were not targeted for dating. For example, prior dating indicates that the youngest eruptions in the Ocate and Taos Plateau fields are 0.81 ± 0.14 Ma (O’Neill and Mehnert, 1988) and 1.04 ± 0.01 Ma (Appelt, 1998), respectively. No samples were collected or dated from these fields. Although prior K/Ar dating of volcanic units from the Lucero and Jornada del Muerto fields of the middle Rio Grande rift yielded ages >500 ka, many of the youngest vents and flows displayed characteristics of young eruptions (Fig. 2F) and thus were redated to evaluate the accuracy of the published ages. K/Ar dating of flows in the Springerville volcanic field at the southwestern limit of the Jemez lineament indicates that all activity was prior to ca. 300 ka (Aubele et al., 1986; Condit and Shafiqullah, 1985; Cooper et al., 1990). No attempt was made to redate the youngest flows of this field. The only silicic units that erupted since 500 ka are the East Fork Member rhyolites of Valles caldera (Fig. 2I). These units represent a very different style of volcanism compared to the more common monogenetic mafic volcanism of the region, and thus the geochronologic results of the East Fork Member were published in previous studies (Zimmerer et al., 2016; Nasholds and Zimmerer, 2022). These results are briefly mentioned in the discussion and are included in the late Quaternary recurrence rate calculations for the rift and lineament.

40Ar/39Ar Dating

Samples were processed using standard mineral separation techniques. Mafic groundmass was crushed, sieved, washed in a dilute 15% HCl solution to remove carbonate, tripled rinsed in deionized water, and passed through a Frantz magnetic separator to remove phenocrystic phases. Lastly, the concentrated groundmass was picked beneath a binocular microscope to remove grains with adhering phenocrysts, glassy vesicle walls, or alteration. Some samples contained sanidine or anorthoclase crystals, which were also separated for dating using the techniques described above. Feldspar crystals were also washed with dilute hydrofluoric acid (HF) and deionized water to remove adhering matrix.

Samples were dated at the New Mexico Geochronology Research Laboratory (NMGRL), Socorro, New Mexico. Before dating, groundmass concentrate and feldspar separates were co-irradiated with the 28.201 Ma FC-2 sanidine interlaboratory standard (Kuiper et al., 2008). Groundmass samples ranging from a few tens of milligrams to more than 100 mg of material were loaded into one or more 7 × 7 × 4 mm pits in a Cu tray (each pit holds up to ~30–40 mg of material). Samples were incrementally heated by rastering a focused 810 nm diode laser over the pit(s). Step-heating schedules varied throughout the course of the project, but were designed to evenly degas the sample, typically ending in a total fusion step. Single sanidine crystals were fused using a focused 55 W CO2 laser. Extracted gas from groundmass concentrate and feldspar was expanded into a fully automated extraction line shared between two ARGUS VI mass spectrometers. One ARGUS VI spectrometer was solely used to measure isotopic ratios of sanidine to limit blank and background values. Measurements of isotopic ratios of young groundmass concentrate and other mafic/hydrous minerals, which commonly have large atmospheric argon concentrations that increase blank and background values, were performed on the second ARGUS VI mass spectrometer. Near the end of this project, the NMGRL acquired a high-resolution Thermo Scientific Helix MC Plus multicollector noble gas mass spectrometer. Selected groundmass samples were redated using this instrument. Groundmass concentrate gas was passed through a cold finger (~–197 °C) and then exposed to SAES getter pumps to remove active species before expansion into the mass spectrometer. Blanks were measured between each step-heat experiment and every 5–15 single-crystal laser-fusion analyses. Atmospheric air (40Ar/36Ar, 295.5; Nier, 1950) and a synthetic gas with a 40Ar/39Ar ratio of 13.2343 (referred to as the “cocktail”) were routinely measured throughout data collection, typically before and after each step-heat experiment. The atmospheric and cocktail gases were used to intercalibrate detectors and the mass spectrometers, and to monitor and correct for any instrument drift. The footnote at the end of the Supplemental Material1 contains additional information regarding the sample preparation process, analytical procedures, blank values, J-values, and error calculations.

Vent Migration Assessment

Vent migration patterns were assessed by comparing both new and published ages to vent locations. Vent locations were determined from published maps and Google Earth. Linear regressions were fit to age versus northing and easting coordinates to quantify migration rates. The northing and easting migration rates were quadratically summed to calculate the net migration rate in each field, which is reported in centimeters per year. The direction of migration was determined by calculating the angle between the northing and easting vent-migration vectors.

40Ar/39Ar Dating

From the available sample suite, 138 samples representing as many as 104 different eruptive units were selected for 40Ar/39Ar dating. Including replicate analyses, 155 new 40Ar/39Ar ages were generated during this study. An additional 25 samples were dated but did not yield interpretable results (i.e., due to discordant spectra or poor fits on an inverse isochron). Table 2 contains new 40Ar/39Ar results as well as selected ages from published studies (see references within). Figures 35 show some of the various aspects and complexities of dating mafic groundmass concentrate with the 40Ar/39Ar method. The Supplemental Material contains all of the 40Ar/39Ar data tables and related figures.

Spectra from incremental heating of groundmass concentrate display significant variation, with ages determined by several different methods. Spectra characteristics include (1) those that yield large plateaus (a plateau is defined as three or more contiguous steps that overlap at 2σ and contain more than 50% of the gas released; Fleck et al., 1977) composed of 80–100% of the 39Ar released (Figs. 3A and 5A), (2) spectra with smaller plateaus composed of less 39Ar released (50–80%) bounded by low- and/or high-temperature discordant steps (Figs. 3B and 3C, Fig. 4A), and (3) spectra that did not yield a plateau (Fig. 3D). Plateau ages are weighted by the percent of 39Ar released in each step. All incremental heating data were plotted on inverse isochron to assess the trapped 40Ar/36Ar component (e.g., Figs. 4B, 5B, 5D, and 5F). Because of the low radiogenic yields of most samples, data tend to cluster near the 40Ar/36Ar intercept on an isochron. Thus, the trapped component is often determined to a high precision (~0.5%). The plateau ages are preferred if the inverse isochron yields an 40Ar/36Ar intercept within error (2σ) of the atmosphere (e.g., 295.5; Nier, 1950; Figs. 3A, 3B, 3C, and 5A), which indicates no excess 40Ar. In contrast, isochron ages are preferred if the 40Ar/36Ar intercept is significantly greater than the atmosphere (Fig. 4B), which indicates the presence of excess 40Ar (Kelley, 2002; Schaen et al., 2020). Some spectra did not yield plateaus because of minor discordance between steps yet display acceptable isochron regressions that have intercepts within uncertainty of 295.5, in which case the isochron age is used. Likewise, for a few spectra, a mean age weighted by 39Ar released was calculated for contiguous steps even though plateau criteria were not met (e.g., fewer than three steps or <50% of released gas or ages of steps are statistically distinguishable). These near-plateau weighted-mean ages are identified as “f-plateau” (i.e., force plateau) in Table 2. A notable characteristic of some groundmass concentrate spectra is that low-temperature steps yield negative ages (Figs. 5C and 5E). For these spectra, higher temperature steps increase in age to yield a plateau (Fig. 5C) or continue to increase in age (Fig. 5E). Inverse isochrons for these samples yield subatmospheric intercepts (Figs. 5D and 5F) with poorly fit regressions (Fig. 5F). The implications of using spectra with negative age steps and subatmospheric intercepts for assigning eruption ages is discussed later.

New groundmass ages range from 1692.2 ± 32.4 ka to 5.4 ± 2.7 ka (Table 2). For most samples, 2σ uncertainties are <± 10–15 ka, and many are <±5 ka. For samples dated multiple times, or units with multiple dated samples, weighted mean ages of all acceptable results are typically preferred. Inverse isochron ages are typically less precise than plateau ages because samples tend to have low radiogenic yields and steps cluster near the 40Ar/36Ar intercept rather than near the 40Ar/39Ar intercept. Despite the wide range of ages and spectra characteristics, radiogenic yields and K/Ca ratios display common systematic characteristics (Figs. 3 and 4). Radiogenic yields scale with age. Most young samples (i.e., <20–50 ka) have radiogenic yields of near 0–10%; older samples have higher radiogenic yields approaching 60–100%. K/Ca values of steps for most groundmass samples typically decrease from 1 to 3 to 0.1–0.5 during the course of step-heat experiments, which is interpreted to reflect the initial degassing of K-rich phases, such as interstitial glass and plagioclase, at low temperatures, followed by the degassing of Ca-rich phases, such as pyroxene and olivine, at higher temperatures.

Several anorthoclase megacrysts (NMLC-07, NMZB-07, and Bandera; Table 2) within basaltic lavas were also dated to compare to groundmass concentrate ages. The crystals, which are likely ante- or xenocrystic, were possibly heated to temperatures high enough to completely degas any pre-eruptive accumulation of radiogenic Ar and thus could yield accurate eruption ages (e.g., Heizler et al., 1999). At least one anorthoclase sample from the Bandera Crater and flow of the Zuni-Bandera field displays evidence of melting along the crystal face. After identifying and removing obvious outliers and plagioclase analyses, single-crystal laser-fusion of anorthoclase produced weighted mean ages with uncertainties of ±5 ka or less (e.g., Fig. 4C).

Vent Migration

Vent migration rates and directions were calculated for the Raton-Clayton, Zuni-Bandera, Red Hill–Quemado, Lucero, Valley of Fires, and Jornada del Muerto fields. Average migration rates range from 1.0 cm/yr to 4.0 cm/yr. Migration directions range from 018 (N18E) to 147 (S33E). However, the migration patterns for short durations (e.g., <100 k.y.) of eruptive activity commonly deviate from longer period migration patterns. Although quantifying eruption characteristics during the last 500 k.y. is the primary goal of this work, review of published data indicates that the initiation of vent migration at some fields commonly predates the <500 ka age range of units targeted in this study. Published ages of variable precision and quality that are >500 ka were necessary to constrain some migration patterns. Migration rates and direction do not incorporate uncertainty related to ages; many of the published ages are an order of magnitude less precise than the new data, and thus migration trends would be heavily weighted to the newly generated 40Ar/39Ar ages.

Interpretation and Reliability of Mafic Groundmass Concentrate 40Ar/39Ar Ages

Establishing the eruptive history and developing an age-based hazard assessment of late Quaternary activity within the rift and along the lineament hinges nearly completely on assigning accurate eruption ages from 40Ar/39Ar dating of groundmass concentrate. However, the accuracy of basaltic groundmass ages, particularly of samples with hypocrystalline or hypohyaline textures that are common to little-eroded late Quaternary flows, presents unique challenges related to alteration, 40Ar loss, 39Ar recoil, and isotope fractionation (McDougall and Harrison, 1999; Koppers et al., 2000; Brown et al., 2009; Fleck et al., 2014; Heath et al., 2018, 2020). Several tests were performed to assess the reproducibility and accuracy of the 40Ar/39Ar groundmass concentrate dates.

First, replicate dating and dating different phases of an eruptive sequence were used to evaluate reproducibility. In most cases, these experiments yield plateau or isochron ages that are indistinguishable at 2σ error (in particular, see results of the Raton-Clayton and Potrillo fields; Table 2). For example, replicate analyses of Malpie Mountain (NMRC-23), a cone in the Raton-Clayton field, yield indistinguishable plateau ages of 64.2 ± 2.3 ka and 61.5 ± 0.7 ka. Dating of another sample from this cone (NMRC-24) yields an isochron age of 60.6 ± 19.2 ka, which although imprecise is within error of the more precise plateau ages. Another example is the Santo Tomas Mountain cone and flow of the eastern Potrillo field. The flow (NMPV-21) contains excess 40Ar, and thus the isochron age of 162.9 ± 5.6 ka is used. The central intrusion does not contain excess 40Ar and yields an indistinguishable plateau age of 164.9 ± 5.6 ka. Although both samples yield indistinguishable ages, the different trapped 40Ar/36Ar components likely reflect slight mineralogical differences in the samples (e.g., the percent of phenocrysts with melt-inclusion–hosted 40Ar) or variable degassing of pre-eruptive 40Ar within parts of the cone and flow. This highlights how variable (and potentially sensitive) Ar systematics are for low-K mafic groundmass, as well as the value of multiple analyses for assessing reproducibility.

A test for assessing accuracy is 40Ar/39Ar dating of co-erupted alkali feldspar megacrysts within basaltic lava flows (e.g., Fig. 4C). Dating of alkali feldspar has numerous advantages compared to groundmass concentrate, such as higher K content, single-phase mineralogy, and less susceptibility to alteration and recoil compared to interstitial glass in groundmass. Although the alkali feldspar megacrysts in basalt are likely either antecrystic or xenocrystic in origin, incorporation of these phases into basaltic melt causes partial to complete degassing of the crystal (Gillespie et al., 1982), particularly if the crystal is small and retained in the melt for an extended duration. Dating of alkali feldspar in the Bandera and Oso Ridge Crater flows of the Zuni-Bandera field yields ages that are comparable to the corresponding groundmass ages. Single-crystal laser-fusion of anorthoclase in the Oso Ridge Crater yields a weighted mean age of 64.9 ± 3.3 ka (Fig. 4C), which is indistinguishable at 2σ uncertainty from the isochron and preferred eruption age of 65.5 ± 1.5 ka from the groundmass concentrate (Fig. 4B). Similarly, the anorthoclase in the Bandera flow yields an age of 11.0 ± 2.2 ka, which is also indistinguishable from the groundmass ages of 8.5 ± 2.4 ka, 9.3 ± 2.9 ka, and 9.9 ± 1.6 ka (Table 2). These results provide additional support that late Quaternary groundmass ages represent accurate eruption ages, because autocrystic anorthoclase and sanidine ages are typically regarded as robust eruption ages (e.g., McIntosh et al., 1992; Walter, 1994; McDougall and Harrison, 1999). However, 40Ar/39Ar ages of ante- or xenocrystic K-rich crystals in basalt should probably be considered maximum ages to reflect the possibility of incomplete degassing (Renne et al., 2012).

A final test of accuracy involves comparing new 40Ar/39Ar ages to available published 36Cl exposure ages for several <50 ka flows in the Zuni Bandera (Dunbar and Phillips, 2004) and Valley of Fires (Dunbar, 1999) fields. As discussed in the prior paragraph, groundmass concentrate and anorthoclase ages for the Bandera flow range from 8.5 ± 2.4 ka to 11.0 ± 2.2 ka, all of which are indistinguishable from the 36Cl age of 11.2 ± 0.6 ka. The 40Ar/39Ar age for the Twin Crater is 18.7 ± 1.8 ka, whereas the 36Cl age is 18.0 ± 1.8 ka. Replicate analyses of the Paxton Springs (north) flow yield 40Ar/39Ar ages of 24.3 ± 4.9 ka and 23.2 ± 2.2 ka, which are also indistinguishable from the published 36Cl age of 20.7 ± 2.2 ka. The weighted mean age of the Carrizozo flow, calculated from six 40Ar/39Ar step-heat analyses, is 5.6 ± 0.9 ka, which agrees with the 36Cl age of 5.2 ± 0.7 ka. In addition to demonstrating that new 40Ar/39Ar ages are capable of replicating ages determined by other methods, ages of the Bandera and Carrizozo flow, for example, support previous studies demonstrating that 40Ar/39Ar dating of groundmass concentrate can readily establish accurate Holocene eruptive ages similar in precision to exposure dating techniques (Hora et al., 2007; Turrin et al., 2008) without the complication of needing to assure continuous exposure.

Perhaps the least straightforward interpretation of the new groundmass ages is related to those spectra that have initial steps with negative ages, are commonly discordant, and have subatmospheric 40Ar/36Ar intercepts (Figs. 5C5F). Heath et al. (2018) suggested that ages calculated from isochron regressions with subatmospheric intercepts should only be considered maximum eruption ages. Results for this study generally agree with this conclusion. Figure 5 shows three samples dated from the Albuquerque volcanoes (Fig. 2E; Table 2). A proximal flow yields a plateau age of 194.9 ± 15.4 ka (mean square weighted deviation [MSWD] = 2.58) from 100% of the gas released (Fig. 5A), and a 40Ar/36Ar intercept within error of the atmospheric ratio (Fig. 5B). The spectrum of a distal flow (Fig. 5C), from sample NMAV-01, yields initial steps with negative ages or within error of 0 ka, an anomalously young plateau age of 127.2 ± 16.1 ka (~75% of the gas released), and a slight subatmospheric 40Ar/36Ar intercept of 292.8 ± 0.7 (Fig. 5D). The isochron age is 209.6 ± 29.2 ka, which is indistinguishable from the proximal flow eruption age, albeit with great uncertainty. Recasting the spectrum with this trapped 40Ar/36Ar value yields a plateau composed of ~90% of the gas released and an age indistinguishable from that of the proximal flow (Figs. 5A and 5C). For this sample, the lower precision isochron age is preferred, in part to account for geologic uncertainty related to subatmospheric intercepts. In contrast, a sample from the JA volcano (sample NMAV-05, from the largest of the Albuquerque volcanoes) yields a discordant spectrum with initial steps as negative as −1000 ka and higher temperature steps that increase in age to ca. 400 ka (Fig. 5E). The inverse isochron yields an erroneous and unrealistic age of 1030.4 ± 103.1 ka and an extremely low-40Ar/36Ar intercept of 284.2 ± 1.0 (Fig. 5F). Recasting of this spectrum with the trapped 40Ar/36Ar component does not yield a plateau and the ages of the steps are too old. These results suggest that maximum eruption ages can, in some cases, be obtained from samples that yield subatmospheric intercepts (e.g., Fig. 5D); however, those samples that yield ultra-subatmospheric intercepts (40Ar/36Ar intercepts <~290) may not provide reliable ages. The division between “useful” and “erroneous” samples with subatmospheric intercepts is unclear and likely varies from field to field, or even flow to flow. Furthermore, using maximum ages derived from subatmospheric samples is only possible where there are other well-dated flows with stratigraphic relations to guide interpretation.

Although not a primary focus of this study, new results provide some insight into the origin of excess 36Ar and related low-temperature negative age steps of some spectra. Numerous studies have identified and discussed the origin of subatmospheric 40Ar/36Ar intercepts in some groundmass concentrate and obsidian samples. Proposed mechanisms include: (1) the incorporation of primitive, mantle-derived argon (Dalrymple, 1969) coupled with incomplete degassing during eruption; (2) loss of 37Ar during recoil, which leads to underestimation of Ca-derived 36Ar, and thus overestimation of atmospheric 36Ar (Fleck et al., 2014); (3) mass-dependent fractionation either during step-heating (Trieloff et al., 2005; Heath et al., 2018) or during alteration and/or cooling of groundmass glass (Dalrymple, 1969; Brown et al., 2009); and (4) isobaric interferences at mass 36. Several aspects of the data collected in this study have implications for these proposed mechanisms. First, most spectra that yield subatmospheric 40Ar/36Ar intercept values typically have measured 40Ar and 36Ar values that are an order of magnitude larger than those that yield atmospheric or supra-atmospheric values. Second, excess 36Ar is more common in the youngest (i.e., <50 ka) and least dissected flows, where material sampled is from the near surface and typically has the most interstitial glass. These two common traits, although not definitive, suggest the preferential enrichment of atmospheric 36Ar relative to atmospheric 40Ar in glassy samples, likely during cooling and/or alteration of the groundmass. The correlation between the large absolute signal size of atmospheric isotopes and subatmospheric intercepts suggests that mass-dependent fractionation during step-heating is not a likely mechanism for the observed phenomenon, because subatmospheric intercepts are rarely observed in samples with “nominal” 40Ar and 36Ar intensities. Third, preliminary studies of nonirradiated samples, which previously yielded subatmospheric values after irradiation, also yield subatmospheric values, which indicates loss of 37Ar during recoil cannot explain spectra with negative ages. Lastly, some negative age samples originally analyzed with ARGUS VI were reanalyzed with the high-resolution Helix MC Plus multicollector noble gas mass spectrometer (sample NMAV-03; Table 2) that is capable of identifying HCl and C3 interferences at mass 36. Neither of the interferences were observed in the Helix analyses, and negative ages were still present in the low-temperature steps. This is consistent with several studies that show isobaric interferences are typically not the cause of subatmospheric intercepts (Phillips et al., 2017; Heath et al., 2018).

Eruptive Histories of Fields within the Rift and along the Lineament

Raton-Clayton Volcanic Field

Late Quaternary eruptions in the Raton-Clayton volcanic field represent the youngest activity associated with nearly 9.2 m.y. of volcanic unrest at the northeastern limits of the Jemez lineament (Fig. 1). New 40Ar/39Ar ages indicate at least nine eruptions, clustered into four pulses of activity, during the last 500 k.y. (Fig. 6). The two oldest late Quaternary eruptions/pulses are the southern Las Mesetas vent and Horseshoe Crater at 368.2 ± 7.3 ka and 238.2 ± 6.4 ka, respectively. The 130.0 ± 9.7 k.y. repose period between these two eruptions was followed by a similar repose period of 125.7 ± 8.6 k.y., before activity resumed along the New Mexico–Colorado, USA, border at the 112.5 ± 5.8 ka Trinchera Pass vent. Eruption of The Crater at 97.7 ± 5.0 ka near the center of the field constrains the duration of the third pulse of activity to 14.8 ± 7.7 k.y.

Following a ca. 36 k.y. period of quiescence, activity resumed at Malpie Mountain with emplacement of a 14.5-km-long lava flow at 61.7 ± 1.1 ka. No dates for this center existed prior to this study. Activity at Capulin Mountain (also called Capulin volcano) followed soon thereafter at 54.2 ± 1.8 ka, which is indistinguishable from, but more precise than, the previous age determinations of 57 ± 8 ka (40Ar/39Ar, Stroud, 1997) and 59 ± 6 ka (3He, Sayre et al., 1995). The new comprehensive dating confirms field investigations showing that three eruptions postdate Capulin Mountain and mark the final activity in the field (Sayre and Ort, 2011). These include the 44.8 ± 2.2 ka eruption of Baby Capulin, the 36.7 ± 2.2 ka eruption of Twin Mountain, and the 36.6 ± 6.0 ka eruption of the Purvine Hills fissure.

An important discovery is a newly identified eastward migration of volcanism during the last 1300 k.y., from the western margin of the field eastward to the Capulin Mountain area (Fig. 7). Between 1295 ± 60 ka and 738 ± 60 ka (ages from Stroud, 1997), volcanic activity produced as many as eight vents south and east of the present-day town of Raton, New Mexico. Volcanism then migrated eastward, emplacing four vents, oriented N–S, between 368.2 ± 7.3 ka (i.e., Horseshoe Crater) and 97.7 ± 5.0 ka (i.e., The Crater). The youngest pulse of activity that occurred between 61.7 ± 1.1 ka and 36.6 ± 6.0 ka is largely centered near Capulin Mountain. The average vent migration rate and direction during the entire ca. 1300 k.y. period is 2.3 cm/yr to the east (Fig. 7B). No systematic migration to the north or south is detected (Fig. 7C). Mud Hill, located only 1.5 km northeast of Capulin Mountain, erupted at 1692 ka (Fig. 6B), which indicates that the eastward vent migration is a relatively young feature superimposed on the long-lived field and began between ca. 1.3 Ma and 1.7 Ma. In general, the migration direction is subparallel to the greater Jemez lineament, suggesting a link between the vent location and deeper structures.

These newly determined ages and migration patterns for the Raton-Clayton volcanic field have significant implications for hazards of the region. First, the average recurrence interval2 during the late Quaternary Period is ca. 41.5 k.y., less than half the recurrence rate of one eruption per ~99 k.y. based on the previous dating. Second, the recurrence rate during the late Quaternary Period is nearly half that of the average frequency during the entire lifespan of the field. As many as 142 vents (Table 1) erupted between 9.2 Ma (Stroud, 1997) and 37 ka (this study) yielding a recurrence interval of 65 k.y. This suggests increased late Quaternary activity compared to long-term rates of the entire field. Third, the maximum recurrence rate during the youngest pulse of activity (i.e., the eruption of Malpie Mountain and younger) is one eruption per 6.3 k.y. further indicating increased activity even within the late Quaternary cycle of volcanism. Interestingly, although the field has been quiescent since the 36.6 ka Purvine Hills eruption, the current period of repose is of similar magnitude to the late Quaternary recurrence interval.

Tusas Brazos Field

The Tusas Brazos field is a cluster of six vents and related flows located along the crest and western flank of the Tusas Mountains of northern New Mexico (Fig. 8). Some of the vents in this area are informally named for their summit elevations (vents 10406’, 10186’, 10368’, and 10313’). Two of the three youngest flows in the area, based on vent morphology, were dated as part of this study. The Tierra Amarilla flow (Fig. 8A) and an unnamed flow sourced from vent 10186’ (Fig. 8B) yield ages of 179.0 ± 6.0 ka and 207.8 ± 10.5 ka, respectively. Previous geologic mapping (Doney, 1968; Muehlberger, 1968) identified vent 10406’ (Fig. 8B) as the likely source of the Tierra Amarilla flow, although additional dating and chemical analyses could strengthen this interpretation. This flow cascaded ~990 m into the Brazos Box Canyon and traveled 30 km down the Rio Brazos before stopping at the confluence with the Rio Chama. The length of this flow exemplifies the impact that even modest eruptions can have when lava is emplaced into highly dissected terrain. Furthermore, the age and pathway of the lava indicates that much of the landscape relief had developed by 179 ka. Vent 10313’ (Fig. 8B) has average slope angles of ~22–25%, similar to vents 10406’ and 10186’, suggesting it also produced a late Quaternary eruptive event. Undated vents to the south–southwest appear older based on morphology and erosion.

This study interprets the Tusas Brazos cones to be a different field than the Taos Plateau volcanic field, located to the east, for three reasons. First, eruption locations in the Taos field between ca. 3–4 Ma (i.e., eruptions at or near San Antonio Mountain; Fig. 8A) and 1 Ma appear to have migrated to the northeast (Appelt, 1998), or almost completely opposite from the direction of the Tusas Brazos field to the southwest. Second, the closest vents of the Taos Plateau are ~35–40 km to the east, such that the Tusas Brazos cones would represent an extreme spatial outlier of the Taos Plateau. This spatial relation is not observed at any other field in the region. Finally, the youngest eruption of the Taos Plateau field is the 1.04 ± 0.10 Ma Mesita vent eruption (Appelt, 1998), or ~800 ka older than the vents dated in the Tusas Brazos field, which would represent an unusually long eruptive hiatus in the field. Thus, previous interpretations of the Tusas Brazos cones as the youngest eruptions of the Taos Plateau volcanic field are not preferred (Lipman and Mehnert, 1979; Baldridge, 2004).

Jemez Mountains Volcanic Field

The only rhyolitic eruptions in the last 500 k.y. within the rift and along the lineament were sourced from postcaldera vents within Valles caldera in the Jemez Mountains volcanic field. The Battleship Rock ignimbrite and the co-erupted El Cajete pyroclastic beds erupted at 74.2 ± 1.1 ka. Following a short repose period of ~5 k.y., the Banco Bonito obsidian flow was emplaced at 68.9 ± 1.0 ka (Nasholds and Zimmerer, 2022). Two rhyolite domes, South Mountain and San Antonio, were emplaced at ca. 530 ka and 560 ka (Spell and Harrison, 1993; Nasholds and Zimmerer, 2022), respectively, older than the arbitrary 500 ka definition of “late Quaternary” used in this study. Regardless of the relatively low number of late Quaternary eruptions in the Jemez field, the presence of multiple magma bodies imaged below the caldera (Lutter et al., 1995; Steck et al., 1998; Aprea et al., 2002), an actively degassing geothermal system (Goff and Grigsby, 1982), and the high probability that future activity will be rhyolitic and potentially explosive, indicates that Valles caldera is one of the greatest volcanic threats in the southwestern United States (Goff et al., 2011; Ewert et al., 2018).

Zuni-Bandera Volcanic Field

The emplacement of abundant cinder cones with associated flows, short recurrence intervals, and multiple Holocene eruptions characterizes late Quaternary activity in the Zuni-Bandera field (Figs. 9A and 9B). New ages for the little-studied northern Chain of Craters (Fig. 9A) in the Zuni-Bandera field represent a significant addition to the understanding of late Quaternary volcanism along the Jemez lineament. Furthermore, new 40Ar/39Ar dating of the post-50 ka vents and flows confirms prior age determinations as well as sheds light on some of the youngest eruptive patterns in the southwestern U.S.

Ages of volcanic eruptions in the northern Chain of Craters region document ~700 k.y. of activity between ca. 800 ka and 100 ka (Fig. 9B). The oldest eruption dated in this part of the field is 792.9 ± 7.0 ka, at an unnamed cone. Between eruption of this cone and ca. 330 ka, new dating reveals only five eruptions, although evidence for additional activity during this time may be buried by younger flows. Many of the units dated that yield ages in this temporal range are characterized by discordant spectra, with subatmospheric inverse isochron intercepts and corresponding large uncertainties (±30–100 ka). These ages are considered maximum eruption ages. Between 332.1 ± 11.6 ka and 100.3 ± 3.4 ka, at least 16 vents erupted in the northern Chain of Craters, most of which are between ca. 200 ka and 100 ka. Repose periods between sequential eruptions range from as much as ~40 k.y. to too short to measure (i.e., the ages of successive eruptions are indistinguishable). The average recurrence interval during this episode of activity is 15.5 k.y. Eruptions from the southern Chain of Craters were not investigated during this study. Although many of these vents appear to be more eroded and older than those in the northern Chain of Craters, the southern Chain of Craters region likely contains some vents and flows that erupted within the last 500 k.y.

New 40Ar/39Ar ages, in combination with several published 36Cl ages (Table 2), indicate that following the eruptions within the Chain of Craters, the activity in the Zuni-Bandera field increased to make it the most active late Quaternary volcanic field along the lineament or within the rift. Most of these eruptions were located either to the east of the northern Chain of Craters (within El Malpais National Monument) or to the north within the Zuni Mountains. An exception is the 36.6 ± 3.3 ka Bluewater flow (Dunbar and Phillips, 2004; Sims et al., 2007) of the El Tintero cone ~20 km north of the Zuni Mountains. Following a ca. 35 k.y. hiatus after activity in the Chain of Craters, five eruptions were sourced from cones in the Zuni Mountains, with vents co-located on preexisting Laramide faults (Fig. 9A; Maxwell, 1986). After eruptions of the 65.4 ± 1.4 ka Oso Ridge Crater on the southwestern flank of the Zuni Mountains and a 51.0 ± 11.4 ka flow to the northeast, three eruptions were sourced from the Paxton Springs cones near the southeastern terminus of the range. Early eruptions from Paxton Springs at 22.2 ± 1.9 ka produced lavas that flowed ~28 km to the north, spilling into the valley that is now occupied by the town of Grants, New Mexico. A second lava erupted and flowed southward at 15.0 ± 1.0 ka (Dunbar and Phillips, 2004). The final eruption of lava flowed north of Paxton Spring cone and yields an age of 9.0 ± 1.9 ka (unit 2 of Fig. 9B), the first indication of Holocene eruptive activity in the Zuni Mountains. Although three cones are identified in the Paxton Springs area (Maxwell, 1986), the flows are not yet linked to specific source vents. At least six vents erupted in the El Malpais region between 50 ± 14 ka (Laughlin and WoldeGabriel, 1997) and 3.9 ± 1.9 ka (Dunbar and Phillips, 2004), the latter of which, the McCartys flow, is the youngest within the rift or lineament. Multiple vents, such as Lava Crater, an adjacent unnamed vent west of Lava Crater, Cerro Candelaria, and a cone north of Lost Woman, were not successfully dated, but were emplaced during this period based on mapping and stratigraphic constraints (Maxwell, 1986). Repose periods, calculated using only the 12 dated centers of the Zuni Mountains and El Malpais regions, range from as much as 14.4 ± 11.5 k.y. to as little as 1.9 ± 2.0 k.y. The corresponding average recurrence interval during this period is 5.6 k.y., or nearly a third of that in the older Chain of Craters. However, if the previously mentioned undated centers are included in the calculation, the recurrence interval decreases to 4.1 k.y.

Ages and vent locations indicate a northeastward migration of volcanism during activity in the northern Chain of Craters, Zuni Mountains, and El Malpais (Fig. 10A). Using all 34 of the vents and/or flows that were dated indicates a migration rate of 3.3 cm/yr at a direction of N40°E (040). However, several of the oldest eruptions are based on less-than-ideal age determinations. Excluding ages older than 200 ka yields a much faster migration rate of 15.9 cm/yr, in a similar direction. R2 values for the least-squares linear regressions for the vents <200 ka (dashed lines in Figs. 10B and 10C) show dramatic improvements in fits to the data, particularly in the eastward direction, compared to using all of the dated units, suggesting that (1) systematic vent migration is more predominant in the latter history of the field, and (2) the eastward component of migration is more prevalent than that of the northern component. Accordingly, the youngest eruption in the field is sourced from the easternmost vent (on trend), but from one of the southernmost vents (off trend). The migration direction to the northeast is closely parallel to multiple normal faults that strike N20–35°E (Fig. 9A), which suggests that vent location and migration are, in part, linked to structures of the region (further explored later in the discussion). Similar to the migration characteristics of the Raton-Clayton field, the migration pattern of volcanism for the Zuni-Bandera field was not identified until now. However, this migration pattern is easily recognizable from aerial imagery (Fig. 9A). The landscape of the northern Chain of Craters region is light brown to light green and characterized by somewhat mature vegetation, whereas the landscape coloration to the east is much darker, a characteristic of young flows with limited vegetation and soil development (Fig. 2D). The pattern of northeast vent migration may extend further back in time beyond the initiation of volcanism in the northern Chain of Craters to include eruptions in the southern Chain of Craters (Fig. 9A), which appear to constitute the earliest activity in the Zuni-Bandera field.

Red Hill–Quemado Volcanic Field

Late Quaternary volcanism in the Red Hill–Quemado field (Fig. 11) includes sparse eruptions of cinder cones and a maar that mark the youngest episode of ca. 8.0 m.y. of ongoing, yet sporadic, activity. The oldest late Quaternary eruption in the field is a poorly dated 208 ± 68 ka flow northeast of Blaines Lake (McIntosh and Cather, 1994). Although new dating of that flow yields an inverse isochron age of 69.4 ± 10.4 ka, this age is interpreted as inaccurate because the base of the flow is located ~30 m above the surrounding landscape, which would require implausibly rapid rates of erosion that are not documented in this region. Two vents erupted at ca. 120 ka—Cerro Pomo (Fig. 2C) at 120.4 ± 11.1 ka, and an unnamed vent north of Cerro Pomo at 119.0 ± 15.0 ka. The weighted mean age of two replicate analyses of a lava from the Red Hill cone yield an age of 66.7 ± 13.9 ka, which is indistinguishable from, but more precise than, the published age of 71.9 ± 24.2 ka (McIntosh and Cather, 1994).

The youngest eruption along the northern edge of the field is the world-famous Zuni Salt Lake maar (Fig. 2G). Replicate analyses of the partially exposed ring dike of the maar yield a weighted mean age of 24.8 ± 6.7 ka, which is indistinguishable from a prior 14C age of 22.9 ± 1.4 ka (Bradbury, 1966). McIntosh and Cather (1994) reported a 40Ar/39Ar plateau age of 87.1 ± 62.8 ka. Although the isochron for this sample in McIntosh and Cather (1994) yields an atmospheric intercept, the spectrum (figure b of appendix 1 [p. 216] of McIntosh and Cather, 1994) yields the classic “saddle-shaped” 39Ar release pattern indicative of excess 40Ar. Thus, this older age, albeit indistinguishable given the large uncertainty, may reflect the inability of low-precision data generated by older-generation mass spectrometers to identify excess 40Ar in Quaternary rocks. Recently determined 14C and optical spin luminescence ages of sediments that bracket Zuni Salt Lake volcanic deposits range from 13.4 ka to 9.9 ka (Onken and Forman, 2017). Explaining the discrepancy between the new 24.8 ± 6.7 ka 40Ar/39Ar age of Zuni Salt Lake and those ages reported in Onken and Forman (2017) is not straightforward. However, because of the relatively low radiogenic yields of the dated ring dike sample, the replicate analyses have high-precision 40Ar/36Ar intercepts of 294.9 ± 0.7 and 295.0 ± 1.9, indicating that the 40Ar/39Ar ages are not older than the 14C and optical spin luminescence ages because of excess 40Ar. Regardless, the Zuni Salt Lake maar is one of the youngest eruptions along the Jemez lineament.

Two additional eruptions took place during the late Quaternary Period in the Red Hill–Quemado volcanic field, but their sources and/or ages are poorly constrained. A xenolith block within the Zuni Salt Lake phreatomagmatic deposits yields an age of 83.1 ± 5.1 ka, which is ca. 40 ka younger than the neighboring eruption of Cerro Pomo to the south. Thus, the source vent and related flow is likely buried beneath the Zuni Salt Lake deposits. Several small, isolated outcrops of scoria deposits indicative of proximal vent facies are located outside the current exposure of the tuff ring (Cummings, 1968; Onken and Forman, 2017) and are partially covered by 3–5 m of basaltic tephra from the Zuni Salt Lake eruption. These undated deposits may represent remnants of the 83 ka vent complex. More dating is needed to evaluate this correlation. A cinder cone and flow are located southwest of Cerro Pomo (Fig. 11B), but dating of this unit was unsuccessful. However, the unit likely erupted during the late Quaternary Period. Slopes of the unnamed cinder cone are ~25°, similar to Cerro Pomo and Red Hill.

Recurrence calculations indicate that volcanic rates increased during the late Quaternary Period relative to the entire history of the field. The total number of eruptions in the 8.0 m.y. life of the field is not well constrained, but estimates range from 30 to 40 (Chamberlin et al., 1994; McIntosh and Cather, 1994; Scholle, 2003), indicating an average recurrence interval of ~200–275 k.y. At least seven eruptions are now identified during the last 500 k.y., between 208 ± 68 ka (i.e., flow at Blaines Lake; McIntosh and Cather, 1994) and 24.8 ± 6.7 ka (i.e., Zuni Bandera). Because of the large uncertainty of the age that constrains the older bounds of late Quaternary volcanism, the average recurrence interval is poorly constrained to between ca. 19 k.y. and 42 k.y., approximately an order of magnitude decrease compared to the long-term average eruption interval. Repose periods are variable—some successive eruptions are indistinguishable in age, and others indicate up to 40 k.y. of inactivity. The increased eruptive frequency in the last 200 k.y., combined with the presence of one of the youngest and most explosive eruptions along the Jemez lineament (i.e., Zuni Salt Lake maar), indicate that volcanic hazards in this field are significant (e.g., Ewert et al., 2018).

New ages, in conjunction with some previously determined K/Ar (Baldridge et al., 1989) and 40Ar/39Ar ages (McIntosh and Cather, 1994), help to quantify the magnitude and direction of volcanic migration. Beginning at ca. 2.5 Ma, vent locations migrated over 50 km to the north–northeast (N18°E) at a rate of 1.5 cm/yr (Fig. 12A). McIntosh and Cather (1994) originally identified a northward migration of volcanism. The new assessment of migration generally supports this with an R2 value of 0.82 for the northing trend (Fig. 12C). The new migration calculations suggest a weakly eastern component (R2 = 0.33) as well. However, the eastern component of the migration trend is sensitive because of the few data points. For example, exclusion of the 67 ka Red Hill cone, the second youngest in the field and located ~16 km southwest of the ca. 120 ka eruptions of Cerro Pomo (Fig. 11) and a neighboring vent, increases the R2 value of the eastern migration trend to 0.50. Characteristics of the migration pattern share affinities to other fields along the Jemez lineament. For example, the migration direction to the northeast broadly parallels fault orientations in the region that generally strike ~N20–50°E (Chamberlin et al., 1994; McIntosh and Cather, 1994), a notable similarity to vent and fault relations in the nearby Zuni-Bandera field. Furthermore, the migration pattern initiated late in the history of the field. Late Quaternary eruptions and the related migration overprint a belt of abundant 7.9–5.6 Ma vents (Fig. 12A). A similar spatiotemporal pattern is found in the Raton-Clayton area, where migration appears to have commenced at ca. 1.3 Ma, or nearly 8 m.y. after the inception of volcanism in the field.

Fissure Eruptions of the Middle Rio Grande Rift: Albuquerque and Cat Hills Volcanoes

Located within the middle Rio Grande rift are two isolated volcanic fields that formed from fissure eruptions (Figs. 13A13E). The Albuquerque volcanoes (Figs. 2E, 13B, and 13C), located west of Albuquerque, are a north-striking lineation of ~15 cinder and spatter cones co-located with several north-trending Rio Grande rift normal faults (Fig. 13B) along the western margin of the rift (Baldridge, 2004). Earlier mapping of this field suggested that six flows (Kelley and Kudo, 1978; Connell, 2008) erupted during the fissure cycle. However, more recent mapping (Thompson et al., 2020) shows that as many as 15 different flow units were emplaced during the eruption cycle. Seven plateau and inverse isochron ages from three different samples (NMAV-01, NMAV-03, and NMAV-13) yield a weighted mean eruption age of 199.5 ± 10.0 ka (Figs. 13A and 13D). The agreement between the new 40Ar/39Ar age and a previously published 40Ar/39Ar age of 219 ± 14 ka (Singer et al., 2008) supports the accuracy of the new age for the Albuquerque volcanoes and indicates that the published 238U-230Th isochron age of 156 ± 29 ka (Peate et al., 1996) is incorrect. Attempts to date proximal deposits of the five largest cones (the JA, Black, Vulcan, Bond, and Butte volcanoes) were not successful. These cones are directly located on the underlying flows without intervening sediments, which suggests little to no quiescence in eruptive activity at the Albuquerque volcanoes. This is further supported by paleomagnetic data that show the flows have indistinguishable pole directions (Geissman et al., 1990). However, the low-K tholeiitic basaltic lavas yield ages with large uncertainties of ~±15–40 ka (Table 2). Thus, the possibility of short-lived hiatuses cannot be ruled out for this field.

New 40Ar/39Ar dating of seven flows and vent facies of the Cat Hills volcanoes suggests at least three temporally distinct eruptive events (Figs. 13A and 13F). Similar to the Albuquerque volcanoes, the volcanoes are parallel to the rift (Figs. 13B and 13E) in a linear array of 23 cinder cones and seven associated flows, suggesting a fissure style-eruption (Kelley and Kudo, 1978). A 112.4 ± 3.7 ka dike (sample NMCH-06) and a 118.0 ± 3.3 lava (sample NMCH-05) from two central cones near the topographic high of the fissure system are statistically indistinguishable at 2σ, indicating that the youngest pulse of activity occurred at 115.5 ± 5.6 ka. Four samples, including a lava west of the central cones (sample NMCH-04), the southernmost cone (sample NMCH-11), the northernmost cone (sample NMCH-01; also known as the Blackbird Hill cone), and eastern distal lava (sample NMCH-14) yield a series of overlapping ages (Fig. 13F) between 136.4 ± 8.0 ka and 156.5 ± 7.7 ka, with a weighted mean age defining an earlier eruptive pulse at 146.6 ± 10.0 ka. The MSWD for the second pulse is 5.34, slightly higher than expected for a single population of four analyses, which suggests that this second pulse of activity may actually represent multiple events. The topographically highest cone, vent 5789, yields the oldest age of 268.1 ± 10.3 ka. Repose periods for activity at the Cat Hills volcanoes are 31.1 ± 11.5 k.y. (between the youngest and middle events) and 121.4 ± 14.4 k.y. (between the middle and oldest events). In general, the new 40Ar/39Ar ages of the Cat Hills volcanoes support the interpretation presented in Maldonado et al. (2006, 2007), suggesting multiple eruptions between 92 ± 27 ka and 493 ± 161 ka. However, the new 40Ar/39Ar ages better define the total number of eruptive events (i.e., three, possibly four) and indicate a shorter period of total activity (153 k.y. versus 401 k.y.). The presence of multiple events is further supported by data presented in Maldonado et al. (2006), which reports at least two geochemically distinct magmas (i.e., low- and high-MgO lavas).

Lucero Volcanic Field

The only confirmed late Quaternary eruption in the Lucero volcanic field is the 192.1 ± 5.1 ka Suwanee flow sourced from Cerro Verde (Fig. 14). Although this age is indistinguishable from the 184 ± 10 ka age determined by Channer et al. (2015), the newest date is the preferred eruption age given the higher precision. Two additional vents/flows may be related to late Quaternary activity in this field. A K/Ar age of 500 ± 100 ka for Badger Butte (Baldridge et al., 1987) suggests that this vent might have been emplaced during the late Quaternary Period, but attempts to date this unit to higher precision were unsuccessful. Located ~15 km northwest of the northern exposure of Suwanee flow is the partially buried 322 ± 11 ka Laguna Pueblo flow exposed along the Rio San Jose (Channer et al., 2015). The vent for this flow is unknown and thus could be a satellite eruption related to either the Lucero or Zuni-Bandera volcanic fields.

New dating does not indicate additional late Quaternary activity besides that previously mentioned. Six vents erupted between 659.0 ± 7.8 ka (Volcano Hill) and 549.3 ± 5.9 ka (Martin Tank cone; Fig. 14A), with four of these eruptions between 572 ka and 549 ka. A published age of 789 ± 9 ka for the lower flows at Volcano Hill (Channer et al., 2015), in comparison to the new age of 659.0 ± 7.8 ka, indicates that this volcano may be polygenetic (Fig. 14A). At least three vents remain undated. However, the exposed bases of these flows are elevated above the current landscape, similar to flows that erupted between 660 ka and 549 ka, and thus the three vents are unlikely to be <500 ka.

Repose periods in the Lucero volcanic field are extremely irregular, which highlights the challenges of using previous temporal patterns to assess future activity. For example, the repose period between eruptions at Gunn Mesa and Mush Mountain is 4.4 ± 4.0 k.y. In contrast, the repose period between the two youngest eruptions dated by the 40Ar/39Ar technique is ~80 times longer at 357.2 ± 7.8 k.y. Even more dramatically, the field appears to have sat in a period of repose for ~2.3 m.y. prior to resuming activity at 789 ka. However, more dating and detailed mapping are necessary to confirm this.

Age and vent locations indicate the migration of volcanic activity to the northeast at a rate of ~1.0 cm/yr (Fig. 15), although the timing of the migration initiation is somewhat poorly constrained. Eruptions at 3.1 Ma and older are found north, east, and south of the <1000 ka activity. Two of the oldest vents in the field, Mesa Lucero (8.2 Ma) and Lava Butte (6.2 Ma), are directly north of late Quaternary activity, which suggests that migration began after eruption of these two vents. Excluding sparse eruptions to the north and east, a cluster of vents that erupted between 5.78 Ma and 3.10 Ma appears to define an early migration that trends from the southwest to the northeast into the cluster of Quaternary vents (Fig. 15). The migration direction of N32°E broadly parallels the major fault zone between the Colorado Plateau and the Rio Grande rift (Fig. 15A) located to the east, the Jemez lineament to the north, as well as northeast-trending faults exposed within and adjacent to the Zuni-Bandera volcanic field (Fig. 9A) to the west. A few faults with similar orientations but with less displacement are located within the Lucero field.

Valley of Fires and Jornada del Muerto Fields

Two small volcanic fields within and adjacent to the middle Rio Grande rift produced volumetrically significant late Quaternary eruptions, including the Holocene Carrizozo flow, the youngest eruption within the rift (Fig. 16). Prior to the Carrizozo eruption, at least two, and possibly three, eruptions occurred near the Broken Back Crater region in the Valley of Fires volcanic field (also known as the Carrizozo lava flow field). The oldest dated flow is 347.2 ± 15.1 ka (Cather and Heizler, 2009), and was likely sourced from an unnamed vent (vent 6253’ of Table 2). Dating in this study of lava on the rim of Broken Back Crater and a flow ~0.8 km north of this cone yields distinguishable ages of 244.3 ± 12.6 ka and 309.6 ± 7.8 ka, respectively, despite displaying similar mineral assemblages (i.e., olivine >> plagioclase; no pyroxene) and field relations that indicate the northern flow sample was sourced from the cone. The northern lava age is the preferred eruption age for Broken Back Crater because the sample along the crater rim has a more oxidized groundmass and disturbed spectrum. An undated, older flow that crops out along and south of U.S. Route 380 was not sampled in this study and has yet to be dated successfully (Cather and Heizler, 2009).

Although prior 36Cl dating of the Carrizozo flows yielded a robust age of 5.2 ± 0.7 ka (Dunbar, 1999), the unit was redated to the test the limits of the ARGUS VI mass spectrometer to determine ages of basaltic Holocene eruptions. Dunbar (1999) found no age difference between the lower southern flow unit and the upper northern flow unit, which suggests monogenetic emplacement of the 4.3 km3 (Allen, 1951) flow field. This is further supported by the absence of sediment at the brecciated contact between the two flows. Six analyses (three replicate analyses from two samples) of the upper northern Carrizozo flow yield a weighted mean age of 5.6 ± 0.9 ka (MSWD = 1.34), indistinguishable from and nearly as precise as the 36Cl age, confirming the ability of the ARGUS VI mass spectrometer to measure precise and accurate ages of some Holocene lavas. The ca. 300 k.y. repose period prior to eruption of the Carrizozo flows also demonstrates the possibility of renewed activity in fields that have been quiescent for several hundred-thousand-year periods.

The Jornada del Muerto field is located ~80 km west of the Valley of Fire field. A subset of new 40Ar/39Ar ages for the Jornada del Muerto field was first published in Sion et al. (2020), because the ages and elevations of the flows above the present-day Rio Grande provide important constraints on the landscape evolution of the region. These ages are presented here for completeness, along with new ages that highlight additional complexities of the region.

The Jornada field consists of only two vents with associated flows. The largest, youngest, and only late Quaternary eruption is the 7.5 km3 Jornada del Muerto shield volcano eruption (Hoffer and Corbitt, 1991), one of the most volumetrically significant in the region. The new age of this volcano is 78.1 ± 3.2 ka (n = 2 replicate analyses of the distal sample; Sion et al., 2020), approximately one order of magnitude younger than the previously published ca. 760 ka K/Ar age (Bachman and Mehnert, 1978). Additional dating of the medial and proximal flow yields less precise but indistinguishable ages of 61.3 ± 15.9 ka and 85.4 ± 8.1 ka, which confirm that the eruption is late Quaternary in age as first presented in Sion et al. (2020). Dating of a hypocrystalline sample from the late-stage cinder cone in the central vent yields an imprecise and distinguishable age of 146.9 ± 21.6 ka (Fig. 2F). Likewise, a 3-m-thick rafted block of holocrystalline lava yields an older age of 179.2 ± 7.2 ka. The origin of these older ages from the cone and block is not completely understood. The rafted block contains up to 30% crystals, significantly more than the well-dated Jornada flows, which suggests that the block may represent a different, now-buried flow or more-crystallized part of the Jornada magmatic system that was carried to the surface during eruption. Alternatively, sandstone xenoliths observed in the block indicate the presence of crustal material, which may have added extraneous 40Ar to the lava, causing an anomalously old apparent age. The age of Mesa del Contadero is 817.3 ± 4.7 ka (Sion et al., 2020), or ca. 1.4 m.y. younger than the 2.2 Ma K/Ar age of Bachman and Mehnert (1978). Similar to the Jornada volcano, dating of near-vent facies (sample NMJV-06; bombs/blocks) yields older, non-reproducible ages of ca. 872 ka and 932 ka. The older ages of hypocrystalline to hypohyaline bombs compared to the holocrystalline lavas could be the result of extremely rapid cooling and incomplete degassing of pre-eruptive radiogenic 40Ar, in which case the ages of these near-vent samples should be considered maximum ages.

The ages and vent locations in both the Valley of Fires and Jornada del Muerto fields suggest a similar southeastern vent migration pattern, albeit poorly constrained because of the infrequent eruptions (Fig. 16). Vent migrations of 4.0 cm/yr at S67°E and 2.5 cm/yr at S33°E are calculated for the Valley of Fires and Jornada fields, respectively. These migration patterns, with an eastward component, share similarities with vent migration displayed in other fields, which all contain an eastward component, either directly east or northeast. The apparent southeastward migration in these fields, which is orthogonal to that of many others in this study, may reflect a poorly constrained pattern based on a limited number of eruptions.

Potrillo Volcanic Field

The Potrillo volcanic field is the largest mafic Quaternary volcanic field in the southern Rio Grande rift, with many established eruptions that are younger than 50 ka and a history of phreatomagmatic explosions (Hoffer, 1976, 2001; Anthony and Poths, 1992; Williams, 1999). Despite the size, youthfulness, and location proximal to several major population centers (i.e., Las Cruces, New Mexico; El Paso, Texas, USA; and Ciudad de Juarez, Mexico), the volcanic hazard potential of the region is poorly constrained. In particular, the remote and very inaccessible West Potrillo Mountains, with over 100 cones and vents (as many as ~160 appear on some generations of maps; Hoffer, 1976), remain poorly dated (Fig. 17). Work in this study largely focused on establishing higher-precision 40Ar/39Ar ages for the central and eastern cones, the youngest in the field, many of which have existing 40Ar/39Ar and 3He ages for comparison (Williams, 1999).

The oldest eruptions in the Potrillo field are located in the West Potrillo Mountains. Prior dating indicates eruptions as old as 927 ka, although with such limited dating, unrecognized older eruptions are certainly possible. Several magnetically reversed flows (Williams, 1999) indicate that Matuyama chron (i.e., >780,000 ka) eruptions may be common in the field. However, most of the available ages, paleomagnetic data, and cone morphology indicate cone construction after 780 ka (Williams, 1999; Hoffer, 2001; this study). Dating of the Dry Lake cone and an unnamed flow in the northwestern part of the field yield low-precision ages of 202.2 ± 34.9 ka and 262.2 ± 22.3 ka, respectively. Termination of activity in the West Potrillo Mountains was previously thought to be at 265 ± 12 ka (vent 4446’; lower left in Fig. 17B). Thus, the new age of ca. 202 ka extends the duration of West Potrillo volcanism and raises the possibility of additional younger volcanism. Dating of pre-, syn-, and post-Malpais maar eruptions yields ages of 603.4 ± 8.6 ka, 327.1 ± 12.3 ka, and 300.7 ± 7.1 ka, respectively. The available ages do not suggest any systematic migration patterns in the West Potrillo Mountains. However, radius-to-height ratios of the cones suggest the vents along the crest of the West Potrillo Mountains, which are co-located with the west Robledo Fault, are potentially younger than those located on the margins (fig. 1 of Hoffer, 2001). Furthermore, Hoffer (2001) showed that cone morphology correlates with mineral assemblages and xenolith contents, supporting at least two stages of West Potrillo volcanic activity.

Following eruptions in the West Potrillo Mountains, at least five vents and flows were emplaced along the eastern margin of the field between 190.1 ± 2.3 ka and 163.6 ± 3.1 ka, co-located with a normal fault along the western margin of the rift. Two eruptions occurred near the Black Mountain center. Dating of vent facies lava and related flow north of Black Mountain yields an age of 190.1 ± 2.3 ka, whereas a proximal flow at Black Mountain yields a distinguishably younger age of 171.1 ± 4.3 ka. The ages of the previously undated Santo Tomas Mountain and San Miguel Mountain eruptions are 165.8 ± 4.7 ka and 164.1 ± 3.5 ka. The new age for Little Black Mountain cone is 163.6 ± 3.1 ka, which agrees with the prior age of 169 ± 21 ka (Williams, 1999), but is nearly an order of magnitude more precise. 3He ages for this cone and flows are ~110–30 ka younger (Williams, 1999) than the new ages presented here, possibly due to burial by ephemeral eolian sand sheets. The ages of the Santo Tomas Mountain, San Miguel Mountain, and Little Black Mountain are indistinguishable at 2σ uncertainties (Fig. 17A), as is the age of the southern proximal flow at Black Mountain with the Santo Tomas and San Miguel flows. Excluding the northern flow at Black Mountain, this brief episode of activity of the eastern cones, now constrained between 171.1 ± 4.3 ka and 163.6 ± 3.1 ka, or a duration of 7.5 ± 5.3 k.y., has an extremely short recurrence interval of ca. 1.9 k.y.

The youngest stage of activity in the Potrillo field, which includes the emplacement of two lava flows and three maar eruptions, is located within the central portion of the field. Similar to prior activity, the vent location is spatially connected to faults of the region (Fig. 17B). The Afton flows, sourced from the Gardner cones, erupted at 87.0 ± 2.3 ka. Although mapping of the flows indicates at least three extrusive events (Hoffer, 1976), the absence of sediment between the flows and the indistinguishable ages presented here indicate a monogenetic eruption. The Potrillo maar, located along the U.S.–Mexico Border, erupted at 60 ± 10 ka (40Ar/39Ar age from Williams, 1999). No attempt was made to redate this cone because of access issues. However, the maar appears to have a complex eruptive history, with the emplacement of three cones and at least one flow in the southern half of the crater (in Mexico) and a single cone and associated flow in the northern half of the crater (in the U.S.). Future work involving mapping, geochronology, and geochemistry will be necessary to assess the nature of this internationally shared volcano.

40Ar/39Ar ages for Kilbourne Hole did not exist prior to this study. Eruptive deposits include undatable ash-rich, phenocryst-poor tephra and xenolithic blocks from the older Afton flow exposed in the maar crater wall (Fig. 2H). However, co-erupted peridotite mantle xenoliths have thin lava coatings, which was targeted for dating. One groundmass concentrate sample, dated twice, yields reproducible ages and a weighted mean age of 42.9 ± 3.6 ka. Two additional samples yielded discordant spectra. This first successful 40Ar/39Ar age for Kilbourne Hole agrees with field relations and the new age of the underlying Afton flow and indicate that this eruption must be younger than 87.0 ± 2.3 ka. The new 40Ar/39Ar age of Kilbourne Hole is slightly older than prior estimates of ca. 29 ka, based on comparing 3He ages of the Afton basalt flow surface to those in the maar crater wall (Williams, 1999), and 24 ka, based on pedogenic carbonate formation (Gile, 1987).

The final two eruptions in the Potrillo field include Aden lava flow and Hunt’s Hole maar, but both new and published ages have yet to determine the age relation. Two of three samples of Aden flow yield indistinguishable ages of 23.5 ± 5.1 ka and 19.5 ± 2.6 ka, with a weighted mean age of 20.3 ± 3.2 ka. This agrees well with 3He ages that range from 20 ka to 24 ka but is significantly younger than the previous 40Ar/39Ar age of 42 ± 6 ka (Williams, 1999). A third sample of the Aden flow yields an indistinguishable, yet very imprecise age of 45.6 ± 33.9 ka. Dating of a small vent just north of Hunt’s Hole, interpreted as late-stage activity of the maar volcano, yields an age of 16.4 ± 6.3 ka, which is indistinguishable from the age of the Aden flow eruption. The 40Ar/39Ar age of Hunt’s Hole is a considerable improvement over the 3He ages of 14 ± 14 ka and 17 ± 18 ka. The uncertainty associated with the new 40Ar/39Ar ages for Hunt’s Hole suggests the possibility of previously unknown Holocene activity in the southern Rio Grande rift.

The available ages suggest a possible decrease in eruptive activity during the lifespan of the Potrillo field. The eruptive history in the central and eastern cone is well understood. Here, 10 cones and maars erupted between 190.1 ka and 16.4 ka, with an average recurrence interval of 19.3 k.y. As previously discussed, most eruptions in the West Potrillo Mountains are undated, with the exception of the few ages presented here and in Williams (1999). Even the total number of cones/vents in this part of the field is poorly known, with estimates ranging from ~115 to 150 using various generations of maps. If the age range currently established for the western volcanoes is close to accurate, this suggests an average recurrence interval of 6.1–8.0 k.y. However, considering the limited data, significant uncertainty is associated with this calculation. Repose periods, much like those of all the fields discussed in this paper, are highly variable and range from too short to measure to at least 76.6 ± 4.2 k.y. (between the eruptions at San Miguel Mountain and the Gardner cones/Afton flows).

In general, ages of eruptions are younger on the eastern side of the field compared to those in the west, but the migration pattern appears more complicated than at other fields in the rift and lineament (Fig. 17C). Even without full dating coverage of the West Potrillo Mountains, this sector of the field is demonstrably older than the central and east based on the available ages, cone morphology, Landsat imagery, and soil and vegetation development (Hoffer et al., 1998; this study). Activity then migrated to the very eastern edge of the field, only to then migrate to the center. This final migration westward, to the center of the field, is unique among all other fields studied (i.e., the youngest eruptions are typically located in the most eastern sectors). The vents in the Potrillo field are co-located with major faults of the region (Fig. 17B), and thus a simple explanation is that the youngest magmas intersected the East Robledo and Fitzgerald faults at depth, which then focused volcanism in that area.

Animas Valley Flows

New 40Ar/39Ar ages were determined for the largest basalt flow in Animas Valley (Fig. 18) to improve upon contrasting published K/Ar dates that possibly indicate late Quaternary volcanism in southwestern New Mexico, where the rift transitions into the Basin and Range province. K/Ar dating for the Animas flow includes indistinguishable ages of 511 ± 30 ka (Deal et al., 1978) and 544 ± 50 ka (Lynch, 1978), as well as a significantly younger age of 140 ± 20 ka (Marvin et al., 1978). Two samples dated in this study yield indistinguishable 40Ar/39Ar ages of 348.7 ± 3.3 ka and 348.5 ± 4.5 ka, for a preferred eruption age of 348.6 ± 2.7 ka (weighted mean of the two analyses). Four undated cones are located ~13–20 km southwest of the Animas Valley cone along the western flank of the Peloncillo Mountain. The two southernmost cones display limited erosion similar to that of the Animas Valley cone, whereas the two northernmost cones appear to be more eroded and thus are likely older.

The relation of the Animas Valley cones and associated flows to regional Quaternary volcanism is not well established. At least one previous study suggested the Animas Valley flows are distal eruptive centers of the exceptionally large San Bernardino volcanic field located ~40 km to the southwest in southeastern Arizona (Lynch, 1978). The eruptive history of the San Bernardino volcanic field is poorly understood, constrained by only seven K/Ar ages, including four with reported locations—3.30 ± 0.12 Ma from a vent along the western margin of the field, 1000 ± 100 ka and 975 ± 100 ka at Paramore maar crater near the middle of the field, and 274 ± 50 ka in the southwestern part of the field (Lynch, 1978; Marvin and Cole, 1978; Reynolds et al., 1986). Using the ages and locations of the oldest eruption and the dated Animas flow located ~53 km to the northeast yields an apparent net migration rate of 1.8 cm/yr to the northeast, which mimics many of the migration trends already presented. However, as previously discussed, K/Ar ages for Quaternary basalts are commonly inaccurate. Thus, this migration trend is preliminary at best and is yet another area for future work.

An Assessment of Volcanic Hazards in the Rift and along the Lineament

Temporal Trends and Implications for Hazards

A total of 15 late Cenozoic volcanic fields are located within the rift and along the Jemez lineament of New Mexico (Fig. 19). Twelve of these fields contain late Quaternary eruptions. New dating indicates younger-than-previously-known eruptions in the Raton-Clayton, Tusas Brazos, Cat Hills, Jornada del Muerto, Lucero, Red Hill, Potrillo, and Animas Valley fields. Average recurrence intervals during the late Quaternary Period for individual fields, using the respective oldest and youngest eruptions, range from 16.5 k.y. to 170.8 k.y., although many fields have periods of increased activity when recurrence intervals decrease to 5 k.y. or less. Similar recurrence rates were estimated for volcanic fields distributed throughout the American Southwest (Condit and Connor, 1996; Conway et al., 1998; Valentine et al., 2021). For fields with long-term histories that extend beyond the last 500 k.y., three fields show an increase in activity during the late Quaternary Period compared to their entire lifespan (Raton-Clayton, Zuni-Bandera, and Red Hill–Quemado), and three fields appear to decrease in activity (Jemez Mountains, Lucero, and Potrillo). However, given the highly variable repose periods found within all of the fields, some of which display several hundred-thousand-year hiatuses between eruptions, all 12 fields with late Quaternary eruptions are considered dormant with the possibility of future activity. The Taos Plateau, Ocate, and Mt. Taylor fields have been in periods of inactivity that are many times greater than their long-term average recurrence intervals. A return to activity in these fields seems unlikely compared to those fields with late Quaternary activity.

New 40Ar/39Ar dating, combined with a compilation of selected published ages (Table 2), provides new insight into the regional tempo of late Quaternary eruptions within the Rio Grande rift and along the Jemez lineament. During the last 500 k.y. (Fig. 20A), 75 eruptions are identified in the region by dating alone, which yields an average recurrence interval of 6.5 k.y. for the rift and lineament. This rate is a minimum estimate, considering that undated centers remain in most fields. For example, at least eight undated volcanoes almost certainly erupted during the late Quaternary Period within the northern Chain of Craters and El Malpais region of the Zuni-Bandera field as constrained by stratigraphy and morphology. Although there is great uncertainty as to whether all undated centers of the West Potrillo Mountains erupted in the last 500 k.y., an estimate (Table 2) suggests that as many as 66 undated late Quaternary eruptive events may exist. This brings the total number of eruptions to ~141, with an average recurrence interval of 3.4 k.y. Even if the number of undated late Quaternary volcanoes is overestimated by a factor of two, this yields a rate of one eruption per 4.5 ka for the region.

As previously discussed, repose periods within individual fields are highly variable, ranging from too short to measure with the 40Ar/39Ar method to several hundred thousand years (e.g., Valley of Fires and Lucero). However, at the regional scale of the entire rift and lineament, repose periods between successive eruptions are much shorter than at individual fields. Although age uncertainties for a few samples are large enough to permit eruptive hiatuses on the order of ~10–20 k.y., in general, there appears to be little evidence to suggest that volcanism in the rift and lineament paused for any extended period of time (i.e., several tens to hundreds of thousands of years). For example, within the last 100 k.y., eruptive hiatuses range from as short as 0.0 ± 6.8 k.y. (between the El Tintero [Zuni-Bandera] and Purvine Hills [Raton-Clayton] eruptions at 36.6 ± 3.3 ka and 36.6 ± 6.0 ka, respectively) to as long as 10.7 ± 5.7 k.y. (between The Crater [Raton-Clayton] and the Gardner Cones/Afton flow [Potrillo] eruptions at 97.7 ± 5.0 ka and 87.0 ± 2.8 ka, respectively).

An alternative to assessing the eruptive tempo of the region for hazard characterization is to only use eruptions during the last 100 k.y. Because eruptions <100 ka are well preserved in the current landscape (as opposed to older eruptions that might be buried) and careful attention was made to date these units in this and other studies, it is unlikely that the <100 ka summary excludes any sub-100 ka eruptive units in New Mexico (Fig. 20B). The average recurrence interval, based solely on dated units, is 3.2 k.y. However, this again is almost certainly an underestimate. For example, Lava Crater and Cerro Candelaria, two morphologically young cones in the El Malpais region of the Zuni-Bandera field, where all known activity occurred at <50 ka, do not yet have established ages. Including these units yields a slightly shorter recurrence interval of 3.0 k.y. The <100 ka summary is interpreted as a more accurate assessment of recent volcanic temporal trends in the rift and along the lineament compared to the <500 ka average frequency.

Five important aspects of the eruptive history during the last 100 k.y. have particular relevance to volcanic hazards of the region. These are:

  • (1)

    Between 105.8 ka (the poorly dated Ramah flow) and 42.9 ka (Kilbourne Hole), the recurrence interval is 3.7 k.y. Between 36.6 ka and 3.9 ka, a total of 15 vents erupted, yielding a recurrence interval of 2.3 k.y. Once again, if the two undated flows in the El Malpais region are included, the recurrence interval decreases to 2.0 k.y. Thus, at the sub-100 ka scale (Fig. 20B), there is high confidence for an increase in eruptive activity with time on the regional scale.

  • (2)

    The current eruptive hiatus, starting at 3.9 ± 1.2 ka (Dunbar and Phillips, 2004), is not indicative of the end of volcanism in the region. The average rate and repose periods described above indicate that the current hiatus is typical for eruption frequencies and repose times throughout the last 100 k.y.

  • (3)

    The number of established Holocene eruptions (<11.7 ka; Walker et al., 2009) is greater than previously known. Prior to this study, there were only three confirmed Holocene eruptions in the region—Bandera, Carrizozo, and the McCartys flows (Dunbar, 1999; Dunbar and Phillips, 2004). In light of the new 40Ar/39Ar ages, there is at least a fourth Holocene eruption—a 9.0 ± 1.9 ka flow at Paxton Springs in the Zuni Mountains—and possibly a fifth Holocene eruption—the 16.4 ± 6.3 ka cone related to Hunt’s Hole in the Potrillo field. More precise dating of the latter eruption is needed.

  • (4)

    The Zuni-Bandera field has been the most active volcanic field of the rift and lineament during the last 100 k.y. Fifteen to 17 eruptions (15 dated and two not yet dated) occurred in this field during this period. This represents ~50% of all known sub-100 ka eruptions in the rift and lineament. Eleven to 13 (the latter estimate includes two flows not yet dated) of those eruptions are younger than or within error of 50 ka.

  • (5)

    Not all of the eruptions were characterized by mild effusion of mafic lava. Of the 33 eruptions in the last 100 k.y., five (~15%) were explosive. This includes four maar eruptions (Zuni Salt Lake, Potrillo, Kilbourne Hole, and Hunt’s Hole), three of which are located in the Potrillo field, and a silicic pyroclastic eruption (i.e., the Battleship Rock Ignimbrite and co-erupted the El Cajete pyroclastic deposits) at Valles caldera. Thus, although effusive activity is the most likely style of future activity, explosive eruptions (Figs. 2G2I) cannot be ruled out.

Spatial Trends and Hazard Implications

Combining new 40Ar/39Ar ages, published ages, and the locations of eruptive centers indicates a prominent eastward component of migration in many of the fields of the Rio Grande rift and along the Jemez lineament (Fig. 19). Of the 11 mafic fields studied here (i.e., excluding the silicic eruptions in the Jemez Mountains), seven have newly identified vent migration patterns (Raton-Clayton, Zuni-Bandera, Red Hill–Quemado, Lucero, Valley of Fires, Jornado del Muerto, and Potrillo). Migration magnitudes vary from 1.0 cm/yr to 4.0 cm/yr. Migration directions range from N18°E (018) to S33°E (147) but always have an eastern component. The migration patterns in the Jornada del Muerto and Valley of Fires fields are poorly constrained because of the few eruptions, but in both cases the most recent activity is east of older vents. At Potrillo, the five youngest eruptions are in the center of the field rather than to the east. However, the oldest activity that produced the most eruptions and largest combined volume lies within the Western Potrillo Mountains. Migration trends were not identified in the Cat Hills and Albuquerque volcanoes; these fissure eruptions are somewhat unique in terms of eruptive style relative to the distributed monogenetic fields of the region. Additional dating of the Tusas Brazos and the Animas Valley vents and flows will be necessary to document any migration trends. In agreement with interpretations from prior studies, new dating did not reveal any systematic migration of volcanism along the length of the Jemez lineament or along the axis of the rift (Chapin et al., 2004; Goff and Kelley, 2021).

The onset of vent migration within individual fields is also variable. For some fields with protracted histories (i.e., greater than several million years) that extend before the late Quaternary Period, only the latest eruptions exhibit the eastward migration pattern. For example, volcanism in the Raton-Clayton field began at ca. 9.2 Ma, but the eastward migration trend only begins at ca. 1.3 Ma. Similarly, eruptions began at ca. 8.0 Ma within the Red Hill–Quemado area, but migration to the north–northeast initiated much later at ca. 2.5 Ma. In contrast, fields with shorter eruptive histories mostly confined to the Quaternary Period (e.g., Zuni-Bandera) appear to show vent migration during their entire lifespans.

The newly discovered pattern of volcanism has significant implications for hazards in the region. The data suggest that, if the migration pattern continues, future activity is most likely to occur near or slightly east of the most recent activity in each field. However, it is important to note that the observed vent migration describes long-term patterns of activity (hundreds of thousands of years to a few million years). Thus, although the established migration patterns are potentially useful for assessing the locations of future eruptions, deviation to the long-term trends is possible, and perhaps expected, at much shorter timescales and for fields with few eruptions to constrain migration.

Two late Cenozoic fields in New Mexico potentially have migration patterns with an eastward component but are less understood and were not studied here in detail because of the lack of late Quaternary activity. The ages of volcanic eruptions in the Ocate field are between 8.3 Ma and 810 ka (O’Neill and Mehnert, 1988; Olmsted and McIntosh, 2004). Most early eruptions between ca. 7 Ma and 3 Ma are located in the northwestern sector of the field. In contrast, volcanism between 3 Ma and 0.8 Ma is located in the central and southeastern parts of the field (Fig. 21A). Dating by Appelt (1998) suggests that the majority of volcanism in the Taos Plateau field migrated in a clockwise manner between ca. 6 Ma and 1 Ma. The final segment of migration started with 3–4 Ma activity near San Antonio Mountain on the western margin of the field, then migrated northeast to the 2–3 Ma Ute Mountain center, followed by northeast migration to the ca. 1 Ma Mesita vent of south-central Colorado (Fig. 21B). Therefore, volcanism in the Taos Plateau somewhat mimics activity at the Raton-Clayton, Lucero, and Red Hill–Quemado fields, where only the most recent episodes of activity display migration with an eastward component.

Implications for Processes Driving Volcanic Vent Migration in the Southwestern U.S.

The behavior of and mechanisms driving volcanic vent migration in the Southwest, at various temporal and spatial scales, have been topics of interest for many decades, with intertwined implications for volcanic hazards, mantle processes, and landscape evolution (e.g., Tanaka et al., 1986; Condit et al., 1989; Roy et al., 2009; Crow et al., 2011; Nereson et al., 2013; Karlstrom et al., 2017; Walk et al., 2019; Golos and Fischer, 2022). The established migration patterns presented here for late Cenozoic, and particularly late Quaternary, volcanic activity of the rift and lineament were previously unknown or poorly described. However, many southwestern U.S. distributed volcanic fields active during the late Cenozoic Era show migration trends similar to those of this study, though many of those trends have not been fully vetted with comprehensive dating using the most modern techniques and instrumentation. For example, Tanaka et al. (1986) proposed eastward vent migration at rates of 1.2–2.9 cm/yr during the ca. 6.0 m.y. lifespan of the San Francisco volcanic field of north-central Arizona (Fig. 21C). Likewise, eruptive activity in the 2.1–0.3 Ma Springerville field, east-central Arizona (Fig. 21D), is characterized by eastward migration at similar rates (Condit et al., 1989; Condit and Connor, 1996). Migration patterns in these Arizona fields were first attributed to plate motion over fixed mantle sources because the vector of vent migration was broadly opposite, but of similar magnitude, to that of North American plate motion (Tanaka et al., 1986; Condit et al., 1989). Published ages for the less-studied Pinacate field located along the Arizona–Mexico border also suggest an eastward migration of volcanism (Fig. 21E). During and following the construction of the Santa Clara shield volcano in the central region of the Pinacate field between 1.5 Ma and 0.8 Ma, distributed volcanic eruptions began at ca. 1.0 Ma on the western margin of the field and then migrated to near the center of the field. Six well-dated eruptions between ca. 38 ka and 4 ka are located along the eastern edge of the field (Lynch, 1981; Damon et al., 1997; Guttmann et al., 2000; Guttmann and Turrin, 2006; Turrin et al., 2008; Alva-Valdivia et al., 2019; Rodríguez-Trejo et al., 2019).

For longer timescales, several studies have suggested that much of the volcanism in the American Southwest has migrated toward the center of the Colorado Plateau due to asthenospheric convection and lithospheric erosion of the plateau. This seems particularly true for regions along the western and southwestern margins of the plateau. Estimates of vent migration along the western Grand Canyon (Fig. 21E) range from ~1.0 cm/yr to 5.0 cm/yr to the northeast during the last 10–25 m.y. (Crow et al., 2011; Karlstrom et al., 2017; Walk et al., 2019). Likewise, Nelson and Tingey (1997) determined that northeast vent migration in the Pahranagat–San Rafael belt of Nevada and Utah, USA, occurred between ca. 15 Ma and 3.5 Ma at an average rate of 3.5 cm/yr. Roy et al. (2009) and Crow et al. (2011) suggest that volcanism in southern, central, and northern New Mexico has migrated to the northwest or west during the last 25 m.y. This latter migration trend is key—linking migration patterns to asthenospheric convection along the edges of the Colorado Plateau rather than to the plate motion over near-stationary mantle sources requires western to northwestern migration in the volcanic fields of New Mexico (figs. 4 and 5 of Crow et al., 2011). Crow et al. (2011) also showed that migration toward the center of the plateau is well correlated with an increase in εNd for erupted magmas, which further supports a model of lithospheric erosion at the edge of the plateau by convection of modern depleted-mantle asthenosphere.

In light of new and published volcanic spatial trends, multiple processes appear to have controlled eruption migration in the southwestern U.S. at different temporal and spatial scales. First, a migrating front of volcanism toward the interior of the Colorado Plateau caused by asthenospheric convection and erosion along the plateau margins played a major role in the locations of eruptions between the mid-Cenozoic Era and Quaternary Period (Roy et al., 2009; van Wijk et al., 2010; Crow et al., 2011). However, the near ubiquitous post-5 Ma eastern migration observed in volcanic fields distributed throughout the region (Figs. 19 and 21A21E), both proximal and distal to the Colorado Plateau, suggests that additional processes are now possibly involved in volcanic migration. Perhaps most importantly, none of the New Mexico fields investigated in this study display a northwestern or western migration of late Cenozoic to late Quaternary volcanism toward the Colorado Plateau that match longer term trends. As many workers have observed, and is well documented in this study, the vent migration vectors are of similar magnitude (i.e., 2–3 cm/yr) but approximately opposite to the western to southwestern absolute plate motion of North America (Minster and Jordan, 1978; Gripp and Gordon, 2002; Calais et al., 2003). Recent high-resolution tomographic imaging of the mantle shows slow velocity zones due to partial melts beneath many of the southwestern Quaternary fields, particularly at ~80 km depth near the lithospheric–asthenospheric boundary (Schmandt and Humphreys, 2010; Sosa et al., 2014; Golos and Fischer, 2022). One possibility is that observed late Quaternary vent migration trends are linked to plate motion over partial melt zones in the asthenospheric mantle. Perhaps, once magma genesis transitioned from dominantly in situ lithospheric melting to asthenospheric decompression melting during the late Cenozoic Era, as constrained by isotopic evidence (e.g., McMillan et al., 2000; Crow et al., 2011; Reid et al., 2012), only then did the eastward migration within individual volcanic fields begin to develop. Or, more simply, the dominant source of melting transitioned from the moving lithosphere into relatively stable sources of the asthenosphere, which is overridden by a North American plate moving to the southwest. Although dating alone cannot fully resolve the various mechanisms responsible for driving vent migration, future modeling efforts, largely built upon geodynamic and geochemical observations, may find the newly discovered migration patterns useful for testing hypotheses.

Lastly, the role of structures cannot be ignored as a significant controlling factor for eruption location. With few exceptions, most vents are broadly co-located with mapped structures, including faults of various ages and displacement as well as larger regional structures such as Precambrian suture zones (Condit and Connor, 1996; Nelson and Tingey, 1997; Conway et al., 1997; Chapin et al., 2004; this study). Although the migration directions of volcanic activity in the fields described in this study show an eastern component, the migration vector is inconsistent between fields (Fig. 19). This may be because crustal flaws function as pathways for magmatic ascent, such that plate motion and asthenospheric convection may drive long-term and large-scale migration, but the final location where magma intersects the Earth’s surface is more strongly influenced by structures in the uppermost crust (i.e., a few kilometers to a few tens of kilometers). This interpretation potentially explains why the locations of some eruptions appear off trend compared to long-term migration within their fields, such as the final east-to-west migration in the Potrillo field. Further support for the role of extension and related structures in promoting magmatic ascent and influencing eruption locations is found along the northeastern margin of the Colorado Plateau where Quaternary volcanism is nearly absent (fig. 1D of van Wijk et al., 2010; fig. 2B of Roy et al., 2009), with the exception of the Holocene Dotsero volcano. In contrast to the western, southern, and eastern margins of the plateau, where Quaternary extension is well documented and volcanism abundant, southwestern and western Colorado show no current extension, which is possibly preventing magmatic transport and eruption.

An extensive 40Ar/39Ar dating campaign, coupled with a compilation of published ages from various dating techniques, allows for time-space analyses of vent locations that provides a leap in understanding late Quaternary volcanism and related hazards for the Rio Grande rift and Jemez lineament. Twelve fields (11 mafic and one silicic) in these provinces produced late Quaternary eruptions. All are considered dormant with the possibility of resuming activity. The tempo of volcanism within each field is highly variable, with some fields showing an increase in activity with time and others a decrease. However, at the scale of the entire rift and lineament, there is demonstrable evidence of an increase in volcanic activity in the last 100 ka, when the average recurrence interval decreased to 3.2 k.y. The newly identified eastward migration of volcanism in many of these fields provides insight into the location of future eruptions should patterns of past activity continue into the future. Establishing the relative roles of mantle convection, plate motion, and crustal extension is needed to better assess migration patterns for distributed volcanic fields in the southwestern U.S. This work will hopefully serve as a foundation to do so. Fundamental to this study was the development of the high-sensitivity, multicollector ARGUS VI mass spectrometer and related software for generating abundant high-precision 40Ar/39Ar ages for young, K-poor rocks, which further highlights the vital role of technological development in improving our understanding of Earth processes.

  • (1)

    More dating is needed. Although the data presented here represent a significant advance in understanding volcanic patterns of the rift and lineament, many geochronologic gaps remain, such as centers and flows that are not yet dated or have not yet yielded accurate, reproducible ages. Paleomagnetic data could be combined with geochronologic data to assess whether indistinguishable ages for neighboring volcanoes represent co-eruptive events. Future dating campaigns could be extended to include early- to mid-Quaternary centers that would provide even greater timelines for assessing eruptive trends. Likewise, additional Quaternary volcanic fields are found throughout the southwestern U.S. in Arizona, Utah, Colorado, Nevada, and California. The geochronology of many of these fields has yet to be fully explored.

  • (2)

    More mapping is needed. Most of the late Quaternary eruptive deposits throughout the American Southwest are poorly mapped and without modern digital geodatabases. Comprehensive field mapping would provide better stratigraphic constraints for interpreting ages as well as establishing scenarios of past monogenetic eruptions, which could prove invaluable if an eruptive episode were to begin. Furthermore, combining ages with volume estimates would determine relations among eruptive fluxes, tempos, and migration patterns. Mapping would also enhance our understanding of how faults and other structures impact Quaternary volcanism.

  • (3)

    More petrology and geochemistry are needed. Similar to existing mapping efforts, very few of the eruptive centers, let alone entire fields, have comprehensive petrologic and geochemical data sets. The new age data set presents an excellent opportunity to assess magmatic genesis and evolution, which would help assess causes of vent migration.

  • (4)

    More monitoring is needed. The only volcanic field is this study with a dedicated seismic network is the Jemez Mountains (Roberts et al., 2019). None of the remaining volcanic fields have dedicated seismic networks, which creates a precarious situation where small-magnitude precursor activity could be missed or poorly located. Furthermore, the absence of a dedicated seismic network and geophysical assessment and imaging of melt (or lack thereof) below these volcanic fields provides no baseline for assessing future melt movement or seismic activity. The four volcanic fields with activity that is less than 50 ka (Raton-Clayton, Red Hill–Quemado, Valley of Fire, and Potrillo) should be prioritized, as should the Cat Hills and Albuquerque volcanoes, where prior seismic events were attributed to magmatic migration (Schlue et al., 1987; Roberts et al., 2019).

  • (5)

    More outreach and communication are needed. Volcanic hazards in the southwestern U.S. are a real threat. However, without any historical eruptions as “reminders,” this threat is somewhat forgotten. Outreach opportunities exist in the numerous parks, monuments, and public lands that showcase these youthful volcanic landscapes as well as in public and tribal schools near zones of past volcanism. Continuously developing relationships that involve federal and state governments, universities, tribal leaders, and various public and private landowners will prove invaluable when volcanism returns to the southwestern U.S.

1Supplemental Material. 40Ar/39Ar data tables and related figures. Please visit https://doi.org/10.1130/GEOS.S.24978252 to access the supplemental material, and contact editing@geosociety.org with any questions.
2Recurrence intervals calculated using the oldest and youngest ages during the last 500 k.y. (not the entire 500 k.y. period) divided by N-1, where N is the total number of vents emplaced.
Science Editor: Christopher J. Spencer
Associate Editor: Nancy Riggs

This project was funded by National Science Foundation Division of Earth Sciences grant no. 1322089. Reviewers Mark Stelten and Michael Ort, together with Associate Editor Nancy Riggs, provided useful comments and suggestions that greatly improved the readability and accuracy of the manuscript. Matt Heizler, William McIntosh, and numerous New Mexico Bureau of Geology researchers provided invaluable feedback during this project and preparation of this manuscript. Capulin Volcano, Petroglyph, and El Malpais National Monuments, Valles Caldera National Preserve, and Valley of Fires recreation area provided access to protected lands for sampling. Likewise, Isleta Pueblo and numerous private landowners allowed access to their lands for sampling.

Gold Open Access: This paper is published under the terms of the CC-BY-NC license.