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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. [ABSTRACT FROM AUTHOR]

In this study, we present new evidence for changes in magma storage conditions that preceded the 1232 ka caldera-forming eruption of the Bandelier magmatic system in the Jemez Mountains Volcanic Field. Using high precision 40Ar/39Ar sanidine dating we determine that at least eight rhyolites erupted within 8.6 ± 3.4 kyr of the ~400 km3 eruption that formed Valles caldera. Some of those rhyolites contain fayalite with or without biotite, others contain only biotite. An eruption of fayalite-bearing rhyolite at 1240.5 ± 2.1 ka ended an eruption hiatus of at least 100 kyr. Following that first post-hiatus episode of volcanism, at least four more eruptions of fayalite-bearing rhyolite and three eruptions of biotite-bearing rhyolite occurred prior to the caldera-forming eruption. We use phase equilibrium experiments and geothermobarometry to infer the storage conditions and processes that led to these differing crystal cargos and ultimately generated ~400 km3 of predominantly fayalite rhyolite ignimbrite (Tshirege Member of the Bandelier Tuff). We find that biotite-bearing rhyolites were stored at 695–750°C, 75–160 MPa, and at an oxygen fugacity more oxidizing than the quartz-fayalite-magnetite (QFM) buffer reaction. Fayalite-bearing rhyolites were similarly stored at 695–745°C and 70–190 MPa, but at more reducing conditions (⁠|${f}_{O_2}$| ≤ QFM). We suggest that the reduced, fayalite-bearing rhyolite was most likely produced via interaction of crystal-poor rhyolitic magma with a reducing, potentially Cl-bearing, and H2O-rich supercritical fluid phase. This fluid flux event was a key component of the substantial magmatic rejuvenation that enabled the mobilization of ~400 km3 of mostly fayalite-bearing rhyolite prior to not only the Tshirege event, but the older Otowi event as well. [ABSTRACT FROM AUTHOR]

The combination of new 40Ar/39Ar and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U/Pb zircon ages with published geochemistry of the volcanic and plutonic rocks of the Organ caldera complex (New Mexico) provides a framework for understanding the origin of these silicic magmas and the time scales of caldera magmatism. The Organ caldera complex erupted three ignimbrites: the 36.45 ± 0.08 Ma Cueva Tuff, the 36.23 ± 0.14 Ma Achenback Park Tuff, and the 36.03 ± 0.16 Ma Squaw Mountain Tuff. The ignimbrite sequence is zoned from a crystal-poor, high-SiO2 rhyolite at the base to a crystal-rich, low-SiO2 rhyolite at the top. The ignimbrite sequence is intruded by the zoned Organ Needle pluton, which has previously been interpreted to be the nonerupted silicic cap and less-differentiated residual crystal mush of the caldera-forming magma chamber. The geochronology of the Organ Needle pluton indicates that these silicic magmas were generated via shallowcrustal in situ differentiation. U/Pb zircon and many 40Ar/39Ar biotite ages of the different phases of the Organ Needle pluton are temporally indistinguishable from the Squaw Mountain Tuff eruption age, indicating that this pluton was emplaced and rapidly cooled during or shortly after the youngest caldera eruption. New ages also suggest that Organ caldera magmatism was characterized by protracted emplacement of magmas following caldera collapse. Volcanism continued after the caldera eruptions until at least 35.7 Ma. Three silicic postcaldera plutons were emplaced between 36.0 and 34.3 Ma. Multiple diffusion domain thermal modeling of plutonic K-feldspar suggests reheating events, possibly related to postcaldera magmatism, at 34 Ma, 32-30 Ma, and as young as 26 Ma. Geochronology, geochemistry, and field-based observations of the Organ Needle pluton and caldera-forming ignimbrites support the hypothesis that some plutonic rocks are the nonerupted, geochemically complementary residues of large-volume silicic eruptions. [ABSTRACT FROM AUTHOR]

Volcanic and plutonic rocks exposed in the Latir volcanic field, northern New Mexico, provide an opportunity to examine the timing of caldera-related magmatic processes. Fifty-five mineral separates were dated using the 40Ar/39Ar method. Volcanism began at 28.5 Ma and ended at 22.7 Ma, climaxing with the eruption of the high-SiO2 peralkaline Amalia Tuff from the Questa caldera at 25.39 ± 0.04 Ma. Following caldera collapse, four resurgent plutons were emplaced between 25.4 and 25.0 Ma. Previous geochemical studies suggested that the peralkaline phases of two resurgent plutons and a peralkaline ring dike represent an intrusive equivalent to the Amalia Tuff. A new analysis of K-feldspar from the ring dike yielded a plateau age of 25.38 ± 0.04 Ma, supporting this previous conclusion. Only these small-volume peralkaline intrusions are temporally indistinguishable from syncaldera magmatism. The structurally lower resurgent plutons were emplaced ∼100--400 k.y. after the Amalia Tuff eruption and are therefore too young to be the non-erupted, more-mafic parts of Amalia Tuff magma chamber, implying that the Amalia Tuff magma was not generated via in situ differentiation in the upper crust. Following resurgent plutonism, five additional plutons were emplaced between 24.9 and 19.3 Ma along the southern caldera margin and south of the caldera. Thermochronology indicates that most plutons rapidly cooled following emplacement, but that some were subsequently reheated during protracted post-caldera pluton emplacement. The timing of pluton emplacement at the Questa caldera indicates that only small volumes of the intrusive rocks are the same age as the caldera eruption and that most of the intrusive rocks record postcaldera magmatic processes. [ABSTRACT FROM AUTHOR]

We combine 15 new 40Ar/39Ar ages with existing age constraints of basalts to investigate the incision and denudation history of the ~150-km-long Rio San Jose (RSJ) of west-central New Mexico (USA) over the past 4 Ma. Temporal and spatial scales of differential incision may help evaluate the relative importance of neotectonic, geomorphic and climatic forcings. The RSJ is a southeast-flowing river that orthogonally crosses the northeast-trending Jemez volcanic lineament, which is underlain by a zone of low-velocity mantle. Preserved basalt flows along the length of the river at different elevations that directly overlie river gravels are used to construct paleoprofiles of the RSJ and give insight into the differential incision history, which can test the hypothesis that epeirogenic uplift associated with the Jemez lineament influenced differential incision of the RSJ. Observations include (1) a northeast- trending graben along the central reach of the RSJ (El Malpais valley graben) which is parallel to the Jemez lineament, (2) the present-day east tilt of the originally west-flowing 3.7 Ma Mesa Lucero flow along the eastern edge of the Jemez lineament, and (3) modern profile convexities that are colocated with ca. 3 Ma paleoprofile convexities and are centered above the Jemez lineament. The arched ca. 3 Ma paleoprofile defined by the pre-Mount Taylor strath has greater convexity than younger profiles, suggesting neotectonic bowing of ~135 m (~50 m/Ma) in this reach over the past ~3 Ma relative to areas off axis of the Jemez lineament, in spite of graben subsidence and aggradational fill in this reach exceeding 100 m. Differential incision of the 184 ka Suwanee flow at the edge of the Colorado Plateau may be attributable to base-level fall in downstream reaches of the RSJ and/or headwater uplift, and more erosive climate in the past several hundred thousand years. However, these observations, when considered together, cannot be explained entirely by geomorphic or climatic forcings. Rather, they are best interpreted as resulting from surface uplift centered over the northeast-trending Jemez lineament, and our model suggests that both the faulting and broad bending may relate to mantle driven epeirogeny that caused differential river incision. Several interacting neotectonic and magmatic mechanisms may have contributed to postulated uplift. Magmatically driven geodynamic uplift forcings may include construction of the Mount Taylor stratovolcano just north of the RSJ that changed surface elevation by several kilometers at the volcanic peak itself. However, semisteady denudation and similar incision rates in other rivers in the region indicate that a regional erosional landscape was the primary driver of differential river incision over the past 5-8 Ma. Our focus on the pre-Mount Tayler RSJ paleoprofile reinforces this conclusion. Other mantle-related uplift mechanisms that may have generated mantle buoyancy include thermal buoyancy or magmatic inflation due to dike and sill networks related to the building of the Mount Taylor stratovolcano and eruption of Zuni-Bandera volcanic fields. Both could have contributed to uplift, but their relative importance is unknown. Broad epeirogenic uplift is also possible due to small-scale upper mantle convection beneath a thin elastic plate and resulting dynamic topography. [ABSTRACT FROM AUTHOR]

The Rio Grande rift hosts a remarkable record of Quaternary river incision preserved in an alluvial terrace sequence that has been studied for more than a century. However, our understanding of Rio Grande incision history in central New Mexico since the end of basin filling ca. 0.78 Ma remains hampered by poor age control. Robust correlations among Rio Grande terrace sequences in central and southern New Mexico are lacking, making it difficult to address important process-related questions about terrace formation in continental-scale river systems. We present new age controls using a combination of 40Ar/39Ar, 36Cl surface-exposure, and 14C dating techniques from alluvial deposits in the central New Mexico Socorro area to document the late Quaternary incision history of the Rio Grande. These new age controls (1) provide constraints to establish a firm foundation for Socorro basin terrace stratigraphy, (2) allow terrace correlations within the rift basin, and (3) enable testing of alternative models of terrace formation. We identified and mapped a high geomorphic surface interpreted to represent the end of basin filling in the Socorro area and five distinct, post-Santa Fe Group (ca. 0.78 Ma) alloformations and associated geomorphic surfaces using photogrammetric methods, soil characterization, and stratigraphic descriptions. Terrace deposits exhibit tread heights up to 70 m above the valley floor and are 5 to >30 m thick. Their fills generally have pebble-to-cobble bases overlain by fine-to-pebbly sand and local thin silt and clay tops. Alluvial-fan terraces and associated geomorphic surfaces grade to former valley levels defined by axial terrace treads. Carbon-14 ages from detrital charcoal above and below a buried tributary terrace tread show that the most recent aggradation event persisted until ca. 3 ka during the transition from glacial to modern climate conditions. Drill-log data show widespread valley fill ~30 m thick that began aggrading after glacial retreat in northern New Mexico and southern Colorado (ca. 14 ka). Aggradation during this transition was likely due to hillslope destabilization, increased sediment yield, decreased runoff, and reduced stream competence. Chlorine-36 ages imply similar controls on earlier terraces that have surface ages of ca. 27-29, 64-70, and 135 ka, and suggest net incision during glacial expansions when increased runoff favored down-cutting and bedload mobilization. Our terrace chronology supports existing climate-response models of arid environments and links tributary responses to the axial Rio Grande system throughout the central Rio Grande rift. The terrace chronology also reflects a transition from modest (60 m/m.y.) to rapid (300 m/m.y.) incision between 610 and 135 ka, similar to patterns observed throughout the Rio Grande rift and the western United States in general. [ABSTRACT FROM AUTHOR]