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Abstract Predictions for the southwestern US with warming often suggest increased aridity. We investigate the sedimentary record of the Miocene Climate Optimum and Transition (MCO and MCT; ∼17–14 Ma) in northern New Mexico to understand the impact of warmer global temperatures and higherpCO₂on southwestern US hydroclimate. The MCO and MCT comprised a globally warmer period with elevatedpCO₂similar to end‐of‐the‐century (∼400–800 ppm) projections. We present new stable isotope (δ¹⁸O and δ¹³C) records of vadose‐zone and groundwater terrestrial carbonates and of modern precipitation, stream, and groundwater from the Española basin in northern New Mexico and establish a high‐resolution age model using new⁴⁰Ar/³⁹Ar ages. We interpret δ¹⁸O as reflecting the balance between summertime monsoonal and wintertime precipitation and δ¹³C as a reflection of plant productivity. Terrestrial carbonate δ¹⁸O is lowest during the MCO and MCT and is correlated with terrestrial carbonate δ¹³C and anti‐correlated with the benthic δ¹⁸O record. We interpret these data as recording an overall winter‐wet climate during the MCO and MCT, but that precipitation seasonality varied in response to changes in global climate during this period. The further correlation with carbonate δ¹³C suggests that plant productivity was driven by the amount of wintertime precipitation. Comparison with middle Miocene climate model simulations reveals that higher CO₂drives a shift toward wintertime precipitation. Though paleogeographic changes may obscure a direct comparison to modern warming, overall, our findings suggest that prolonged global warmth may be associated with increased wintertime precipitation and greater primary productivity in northern New Mexico. 

40Ar/39Ar detrital sanidine (DS) dating of river terraces provides new insights into the evolution and bedrock incision history of the San Juan River, a major tributary of the Colorado River, USA, at the million-year time scale. We dated terrace flights from the San Juan−Colorado River confluence to the San Juan Rocky Mountains. We report >5700 40Ar/ 39Ar dates on single DS grains from axial river facies within several meters above the straths of 30 individual terraces; these yielded ∼2.5% young (<2 Ma) grains that constrain maximum depositional ages (MDAs) and minimum incision rates. The most common young grains were from known caldera eruptions: 0.63 Ma grains derived from the Yellowstone Lava Creek B eruption, and 1.23 Ma and 1.62 Ma grains derived from two Jemez Mountains eruptions in New Mexico. Agreement of a DS-derived MDA age with a refined cosmogenic burial age from Bluff, Utah, indicates that the DS MDA closely approximates the true depositional age in some cases. In a given reach, terraces with ca. 0.6 Ma grains are commonly about half as high above the river as those with ca. 1.2 Ma grains, suggesting that the formation of the terrace flights likely tracks near-steady bedrock incision over the past 1.2 Ma. Longitudinal profile analysis of the San Juan River system shows variation in area-normalized along-stream gradients: a steeper (ksn = 150) reach near the confluence with the Colorado River, a shallower gradient (ksn = 70) in the central Colorado Plateau, and steeper (ksn = 150) channels in the upper Animas River basin. These reaches all show steady bedrock incision, but rates vary by >100 m/Ma, with 247 m/Ma at the San Juan−Colorado River confluence, 120−164 m/Ma across the core of the Colorado Plateau, and 263 m/Ma in the upper Animas River area of the San Juan Mountains. The combined dataset suggests that the San Juan River system is actively adjusting to base-level fall at the Colorado River confluence and to the uplift of the San Juan Mountains headwaters relative to the core of the Colorado Plateau. These fluvial adjustments are attributed to ongoing mantle-driven differential epeirogenic uplift that is shaping the San Juan River system as well as rivers and landscapes elsewhere in the western United States. 

Continental alkaline magmatism produces a wide variety of igneous rock types because of varying degrees of partial melting of heterogenous mantle sources, fractional crystallization, and magma contamination during transit through the continental crust. The Mount Overlord Volcanic Field (MOVF) is a continental alkaline volcanic province in northern Victoria Land, Antarctica. Mount Overlord and the associated vents that make up the volcanic field are some of the least-explored volcanic rocks in the western Ross Sea. The MOVF sits within the Transantarctic Mountains, which form the rift shoulder of the extensive West Antarctic Rift System. The compositions of volcanic rocks in the MOVF range widely from basanite to evolved trachyte and comendite with a suite of intermediate rock types. Here we present 40Ar/39Ar ages, petrography, and whole-rock and mineral geochemistry to establish the temporal and magmatic evolution of the magmatic system. Volcanic activity occurred from 21.2 to 6.9 Ma, making it one of the longest records of volcanism in the western Ross Sea area. Mount Rittmann, an active volcano that is part of the MOVF, is not discussed here but extends the timing of volcanism of the MOVF into the Holocene. At Mount Overlord and surrounding areas, there were eruptions of lava flows, domes, and pyroclastic rocks. Localized deposits of hyaloclastites formed by magma-ice interactions provide an insight into former ice levels. Geochemically the MOVF shows a single magma differentiation trend except for Navigator Nunatak lavas which have a potassic affinity rarely seen in northern Victoria Land. Partial melting of an amphibole-bearing mantle lithology at or near the base of the continental lithospheric mantle (CLM) was the main source of the parental basaltic magmas. Polybaric crystal fractionation of the primary basaltic magmas mainly occurred at lower crustal depths and involved fractionation of clinopyroxene, olivine, kaersutite, feldspars, biotite, Fe–Ti oxides, apatite, and sodalite. Crustal assimilation of c. 10% granite harbor igneous complex granitoids was important in the evolution of intermediate composition magmas. Trachyte, phonolite, and comendite magmas stagnated and evolved at shallow crustal depths (c. <8 km). Over 95% crystal fractionation was required to generate the comendites. Extraction of the comendite melt from a felsic crystal mush was an important process. The potassic Navigator Nunatak magma required partial melting of phlogopite-bearing metasomatized CLM. The metasomes had ‘HIMU-like’ or FOZO isotopic compositions that ultimately originated from recycling of materials in the mantle. The MOVF displays a stronger affinity toward FOZO than other northern Victoria Land basaltic rocks. This suggests that the interaction between parental melt and juvenile CLM was limited, which is similar to volcanic rocks from the oceanic Adare Basin seamounts. Our result emphasizes the critical importance of a thick CLM for the genesis of diverse alkaline magma compositions in a continental rift system. 

Crustal thickening along the Indus-Yarlung suture zone during the India-Asia collision was primarily accommodated by slip along the north-dipping Gangdese thrust (ca. 27–18 Ma) and south-dipping Great Counter thrust (ca. 25–10 Ma). However, the along-strike continuities, geometries, and timings of these thrusts remain unclear, resulting in an inadequate understanding of Himalayan-Tibetan orogenesis. In this study, we performed geologic mapping, strain analyses, geo/thermochronology, and thermobarometry across the easternmost Himalayan orogen (i.e., the northern Indo-Burma thrust belt), specifically: (1) the Tidding thrust and easternmost Indus-Yarlung suture zone (i.e., Tidding mélange complex) in its hanging wall; and (2) the Lohit thrust and Jurassic–Cretaceous Gangdese batholith and Mesoproterozoic basement (i.e., Lohit Plutonic Complex) in its hanging wall. The Tidding thrust is a north-dipping, top-south mylonitic shear zone that was active by ca. 36–30 Ma, during which hanging-wall mélange rocks were exhumed from ~33–38 km depth. The geometry, kinematics, and initiation age of the Tidding thrust contrast those of the top-north Great Counter thrust at the same structural position to the west. North of the Tidding thrust, the Lohit thrust is a ~5-km-wide, subvertical, north-side-up mylonitic shear zone that contains a basal, discrete “Lohit thrust fault”. Results of electron backscatter diffraction analyses across the Lohit thrust shear zone show that deformation fabric intensity and finite strain magnitudes decrease southwards toward the discrete thrust fault. This spatial relationship may be the result of transient peak strain during the lifespan of the shear zone. The Lohit thrust was active by ca. 25–23 Ma, during which hanging-wall basement and batholithic root rocks were exhumed to mid-crustal depths. The Lohit thrust and Gangdese thrust to the west are located at the same structural position and have comparable geometries, kinematics, and timings. Based on these similarities and previous findings, we interpret that the Lohit and Gangdese thrusts are correlative segments of a single, orogen-wide thrust system that accommodated crustal thickening along the Indus-Yarlung suture zone during the Oligocene–Miocene. 

Crustal thickening has been a key process of collision-induced Cenozoic deformation along the Indus-Yarlung suture zone, yet the timing, geometric relationships, and along-strike continuities of major thrusts, such as the Great Counter thrust and Gangdese thrust, remain inadequately understood. In this study, we present findings of geologic mapping and thermo- and geochronologic, geochemical, microstructural, and geothermobarometric analyses from the easternmost Indus-Yarlung suture zone exposed in the northern Indo-Burma Ranges. Specifically, we investigate the Lohit and Tidding thrust shear zones and their respective hanging wall rocks of the Lohit Plutonic Complex and Tidding and Mayodia mélange complexes. Field observations are consistent with ductile deformation concentrated along the top-to-the-south Tidding thrust shear zone, which is in contrast to the top-to-the-north Great Counter thrust at the same structural position to the west. Upper amphibolite-facies metamorphism of mélange rocks at ∼9−10 kbar (∼34−39 km) occurred prior to ca. 36−30 Ma exhumation during slip along the Tidding thrust shear zone. To the north, the ∼5-km-wide Lohit thrust shear zone has a subvertical geometry and north-side-up kinematics. Cretaceous arc granitoids of the Lohit Plutonic Complex were emplaced at ∼32−40 km depth in crust estimated to be ∼38−52 km thick at that time. These rocks cooled from ca. 25 Ma to 10 Ma due to slip along the Lohit thrust shear zone. We demonstrate that the Lohit thrust shear zone, Gangdese thrust, and Yarlung-Tsangpo Canyon thrust have comparable hanging wall and footwall rocks, structural geometries, kinematics, and timing. Based on these similarities, we interpret that these thrusts formed segments of a laterally continuous thrust system, which served as the preeminent crustal thickening structure along the Neotethys-southern Lhasa terrane margin and exhumed Gangdese lower arc crust in Oligocene−Miocene time. 

Abstract The Ruby Mountains–East Humboldt Range–Wood Hills–Pequop Mountains (REWP) metamorphic core complex, northeast Nevada, exposes a record of Mesozoic contraction and Cenozoic extension in the hinterland of the North American Cordillera. The timing, magnitude, and style of crustal thickening and succeeding crustal thinning have long been debated. The Pequop Mountains, comprising Neoproterozoic through Triassic strata, are the least deformed part of this composite metamorphic core complex, compared to the migmatitic and mylonitized ranges to the west, and provide the clearest field relationships for the Mesozoic–Cenozoic tectonic evolution. New field, structural, geochronologic, and thermochronological observations based on 1:24,000-scale geologic mapping of the northern Pequop Mountains provide insights into the multi-stage tectonic history of the REWP. Polyphase cooling and reheating of the middle-upper crust was tracked over the range of <100 °C to 450 °C via novel 40Ar/39Ar multi-diffusion domain modeling of muscovite and K-feldspar and apatite fission-track dating. Important new observations and interpretations include: (1) crosscutting field relationships show that most of the contractional deformation in this region occurred just prior to, or during, the Middle-Late Jurassic Elko orogeny (ca. 170–157 Ma), with negligible Cretaceous shortening; (2) temperature-depth data rule out deep burial of Paleozoic stratigraphy, thus refuting models that incorporate large cryptic overthrust sheets; (3) Jurassic, Cretaceous, and Eocene intrusions and associated thermal pulses metamorphosed the lower Paleozoic–Proterozoic rocks, and various thermochronometers record conductive cooling near original stratigraphic depths; (4) east-draining paleovalleys with ∼1–1.5 km relief incised the region before ca. 41 Ma and were filled by 41–39.5 Ma volcanic rocks; and (5) low-angle normal faulting initiated after the Eocene, possibly as early as the late Oligocene, although basin-generating extension from high-angle normal faulting began in the middle Miocene. Observed Jurassic shortening is coeval with structures in the Luning-Fencemaker thrust belt to the west, and other strain documented across central-east Nevada and Utah, suggesting ∼100 km Middle-Late Jurassic shortening across the Sierra Nevada retroarc. This phase of deformation correlates with terrane accretion in the Sierran forearc, increased North American–Farallon convergence rates, and enhanced Jurassic Sierran arc magmatism. Although spatially variable, the Cordilleran hinterland and the high plateau that developed across it (i.e., the hypothesized Nevadaplano) involved a dynamic pulsed evolution with significant phases of both Middle-Late Jurassic and Late Cretaceous contractional deformation. Collapse long postdated all of this contraction. This complex geologic history set the stage for the Carlin-type gold deposit at Long Canyon, located along the eastern flank of the Pequop Mountains, and may provide important clues for future exploration. 

Abstract Our study used zircon (U-Th)/He (ZHe) thermochronology to resolve cooling events of Precambrian basement below the Great Unconformity surface in the eastern Grand Canyon, United States. We combined new ZHe data with previous thermochronometric results to model the <250 °C thermal history of Precambrian basement over the past >1 Ga. Inverse models of ZHe date-effective uranium (eU) concentration, a relative measure of radiation damage that influences closure temperature, utilize He diffusion and damage annealing and suggest that the main phase of Precambrian cooling to <200 °C was between 1300 and 1250 Ma. This result agrees with mica and potassium feldspar 40Ar/39Ar thermochronology showing rapid post–1400 Ma cooling, and both are consistent with the 1255 Ma depositional age for the Unkar Group. At the young end of the timescale, our data and models are also highly sensitive to late-stage reheating due to burial beneath ∼3–4 km of Phanerozoic strata prior to ca. 60 Ma; models that best match observed date-eU trends show maximum temperatures of 140–160 °C, in agreement with apatite (U-Th)/He and fission-track data. Inverse models also support multi-stage Cenozoic cooling, with post–20 Ma cooling from ∼80 to 20 °C reflecting partial carving of the eastern Grand Canyon, and late rapid cooling indicated by 3–7 Ma ZHe dates over a wide range of high eU. Our ZHe data resolve major basement exhumation below the Great Unconformity during the Mesoproterozoic (1300–1250 Ma), and “young” (20–0 Ma) carving of Grand Canyon, but show little sensitivity to Neoproterozoic and Cambrian basement unroofing components of the composite Great Unconformity. 

Abstract The Great Unconformity of the Rocky Mountain region (western North America), where Precambrian crystalline basement is nonconformably overlain by Phanerozoic strata, represents the removal of as much as 1.5 b.y. of rock record during 10-km-scale basement exhumation. We evaluate the timing of exhumation of basement rocks at five locations by combining geologic data with multiple thermochronometers. 40Ar/39Ar K-feldspar multi-diffusion domain (MDD) modeling indicates regional multi-stage basement cooling from 275 to 150 °C occurred at 1250–1100 Ma and/or 1000–700 Ma. Zircon (U-Th)/He (ZHe) dates from the Rocky Mountains range from 20 to 864 Ma, and independent forward modeling of ZHe data is also most consistent with multi-stage cooling. ZHe inverse models at five locations, combined with K-feldspar MDD and sample-specific geochronologic and/or thermochronologic constraints, document multiple pulses of basement cooling from 250 °C to surface temperatures with a major regional basement exhumation event 1300–900 Ma, limited cooling in some samples during the 770–570 Ma breakup of Rodinia and/or the 717–635 Ma snowball Earth, and ca. 300 Ma Ancestral Rocky Mountains cooling. These data argue for a tectonic control on basement exhumation leading up to formation of the Precambrian-Cambrian Great Unconformity and document the formation of composite erosional surfaces developed by faulting and differential uplift. 

Abstract Crooked Ridge and White Mesa in northeastern Arizona (southwestern United States) preserve, as inverted topography, a 57-km-long abandoned alluvial system near the present drainage divide between the Colorado, San Juan, and Little Colorado Rivers. The pathway of this paleoriver, flowing southwest toward eastern Grand Canyon, has led to provocative alternative models for its potential importance in carving Grand Canyon. The ∼50-m-thick White Mesa alluvium is the only datable record of this paleoriver system. We present new 40Ar/39Ar sanidine dating that confirms a ca. 2 Ma maximum depositional age for White Mesa alluvium, supported by a large mode (n = 42) of dates from 2.06 to 1.76 Ma. Older grain modes show abundant 37–23 Ma grains mostly derived ultimately from the San Juan Mountains, as is also documented by rare volcanic and basement pebbles in the White Mesa alluvium. A tuff with an age of 1.07 ± 0.05 Ma is inset below, and hence provides a younger age bracket for the White Mesa alluvium. Newly dated remnant deposits on Black Mesa contain similar 37–23 Ma grains and exotic pebbles, plus a large mode (n = 71) of 9.052 ± 0.003 Ma sanidine. These deposits could be part of the White Mesa alluvium without any Pleistocene grains, but new detrital sanidine data from the upper Bidahochi Formation near Ganado, Arizona, have similar maximum depositional ages of 11.0–6.1 Ma and show similar 40–20 Ma San Juan Mountains–derived sanidine. Thus, we tentatively interpret the <9 Ma Black Mesa deposit to be a remnant of an 11–6 Ma Bidahochi alluvial system derived from the now-eroded southwestern fringe of the San Juan Mountains. This alluvial fringe is the probable source for reworking of 40–20 Ma detrital sanidine and exotic clasts into Oligocene Chuska Sandstone, Miocene Bidahochi Formation, and ultimately into the <2 Ma White Mesa alluvium. The <2 Ma age of the White Mesa alluvium does not support models that the Crooked Ridge paleoriver originated as a late Oligocene to Miocene San Juan River that ultimately carved across the Kaibab uplift. Instead, we interpret the Crooked Ridge paleoriver as a 1.9–1.1 Ma tributary to the Little Colorado River, analogous to modern-day Moenkopi Wash. We reject the “young sediment in old paleovalley” hypothesis based on mapping, stratigraphic, and geomorphic constraints. Deep exhumation and beheading by tributaries of the San Juan and Colorado Rivers caused the Crooked Ridge paleotributary to be abandoned between 1.9 and 1.1 Ma. Thermochronologic data also provide no evidence for, and pose substantial difficulties with, the hypothesis for an earlier (Oligocene–Miocene) Colorado–San Juan paleoriver system that flowed along the Crooked Ridge pathway and carved across the Kaibab uplift.