Plant water usage strategies and source tracing provide valuable insights into ecosystem hydrology, plant lipid biosynthesis, and their applications towards paleoclimate reconstructions. While coupled hydrogen isotope analyses of leaf water (δD lw ) and leaf wax (δD wax ) are integral to such studies, multi-species calibrations in seasonal, semi-arid environments remain sparse. This study reports 77 pairs of leaf water δD lw (δ 18 O lw ) and leaf wax δD wax values across five distinct taxonomic groups (broadleaf trees, grasses, pines, forbs, and shrubs) across two transects in Montana, USA, during peak late-summer. Using a modified Craig-Gordon model that integrates environmental uncertainties, we back-calculate source water δD w values. Most taxa systematically utilize source water matching the δD w signature of winter and spring precipitation, or catchment-integrated deep soil and groundwater. Notably, half of the sampled pines tap old groundwater with δD w values lower than winter precipitation, whereas a subset of grasses and shrubs exploit evaporatively enriched shallow ground moisture or summer rains. Biosynthetic fractionation (ε bio ) values across broadleaves, shrubs, pines, and forbs are remarkably consistent, averaging -148±25‰ (1s) and aligning with the global average. Consequently, these taxa share a common apparent fractionation factor of -79±29‰ (1s). Grasses exhibit significantly lower ε bio (by ~30 ‰). These results demonstrate that under moisture stress, distinct growth forms maintain predictable hydrogen isotope fractionation, but, partition subsurface water resources differently, likely driven by root depth. Our findings demonstrate that δD wax tracks seasonal recharge and root-zone hydrology rather than mean annual or growing-season precipitation in semi-arid landscapes and have implications on interpreting sedimentary leaf wax records.

Western North America is the archetypical Cordilleran orogenic system that preserves a Mesozoic to Cenozoic record of oceanic Farallon plate subduction-related processes. After prolonged Late Jurassic through mid-Cretaceous normal-angle Farallon plate subduction that produced the western North American batholith belt and retroarc fold-thrust belt, a period of low-angle, flat-slab subduction during Late Cretaceous-Paleogene time caused upper plate deformation to migrate eastward in the form of the Laramide basement-involved uplifts, which partitioned the original regional foreland basin. Major questions persist about the mechanism and timing of flat-slab subduction, the trajectory of the flat-slab, inter-plate coupling mechanism(s), and the upper-plate deformational response to such processes. Critical for testing various flat-slab hypotheses are the timing, rate, and distribution of exhumation experienced by the Laramide uplifts as recorded by lowtemperature thermochronology. In this contribution, we address the timing of regional exhumation of the Laramide uplifts by combining apatite fission-track (AFT) and (UTh-Sm)/He (AHe) data from 29 new samples with 564 previously published AFT, AHe, and zircon (U-Th)/He ages from Laramide structures in Arizona, Utah, Wyoming, Colorado, Montana, and South Dakota, USA. We integrate our results with existing geological constraints and with new regional cross sections to reconstruct the spatial and temporal history of exhumation driven by Laramide deformation from the mid-Cretaceous to Paleogene. Our analysis suggests a two-stage exhumation of the Laramide province, with an early phase of localized exhumation occurring at ca. 100-80 Ma in Wyoming and Montana, followed by a more regional period of exhumation at ca. 70-50 Ma. Generally, the onset of enhanced exhumation occurs earlier in the northern Laramide province (ca. 90 Ma) and later in the southern Laramide province (ca. 80 Ma). Thermal history models of selected samples along regional cross sections through Utah-Arizona-New Mexico and Wyoming-South Dakota show that exhumation occurred contemporaneously with deformation, implying that Laramide basement block exhumation is coupled with regional deformation. These results have implications for testing proposed migration pathway models of Farallon flat-slab and for how upper-plate deformation is expressed in flat-slab subduction zones in general

Dinoceratans or uintatheres are a phylogenetically enigmatic clade of extinct mammalian herbivores that were among the first eutherians to obtain truly massive body size. Uintatheres were reasonably common elements of North American mammal faunas spanning from the late Paleocene (late Tiffanian, Ti-5) through the middle Eocene (Uintan). However, prior records of North American uintatheres have been geographically restricted to the central Rockies, Texas, and California. Here, we report the first occurrence of a uintathere from the Kishenehn Formation in northern Montana, similar to 750 km northwest of their previously known range. The Kishenehn uintathere is currently documented by a single upper molar bearing several characters that may indicate it pertains to a new taxon. Because various lineages of uintatheres are known to have inhabited North America and Asia, additional fossils from higher latitudes of North America could be essential in untangling their evolution, dispersal, and biogeographic history.

The timing of deformation within and adjacent to the Helena salient of west-central Montana is poorly constrained relative to other segments of the Sevier fold-and-thrust belt. This study presents low-temperature thermochronology data from the Little Belt Mountains, a basement-cored Laramide uplift that is juxtaposed with the leading edge of the salient. We analyzed eight samples of Paleoproterozoic basement for apatite fission-track (AFT) and zircon (U-Th)/He (ZHe) thermochronology. Four samples yielded AFT ages ranging from ca. 80 Ma to 73 Ma and associated long, unimodal confined track lengths, indicating rapid cooling and exhumation of Little Belt Mountains basement during the Late Cretaceous. The other four samples are characterized by younger AFT ages (ca. 55 Ma), which suggest a combination of prolonged residence in the apatite partial annealing zone and postexhumation magmatic reheating. In total, 20 new ZHe dates range from ca. 236 Ma to 28 Ma and show a correlation between date and effective uranium. Forward model results for ZHe data are consistent with upper-crustal residence during the Proterozoic followed by Phanerozoic burial and rapid Late Cretaceous cooling. Cross sections across the Little Belt Mountains display the geometry of the Volcano Valley fault zone, an array of down-to-the-south Proterozoic normal faults that profoundly influenced the development of the Cordilleran thrust belt. Our new constraints from the Little Belt Mountains when integrated with published kinematic constraints from the Helena salient reveal significant out-of-sequence deformation in this portion of the thrust belt between ca. 80 Ma and 55 Ma. A kinematic model is proposed that involves Late Cretaceous (ca. 80 Ma) exploitation of rheologically incompetent units at the base of the Belt Supergroup within the Helena Embayment, facilitating early exhumation in the Little Belt Mountains. Our new data and synthesis are consistent with previous interpretations in which an inherited stratigraphic and structural architecture of Proterozoic ancestry was the predominant control on the development of the Helena salient during Cretaceous−early Eocene time.

The Mesoproterozoic Belt Basin of the northwestern United States and southwestern Canada contains a 5-20-km-thick metasedimentary succession deposited during an important transition in the Precam-brian development of North America. Key unresolved issues for the Belt Basin include the chronology of deposition, sources of siliciclastic sediment, and regional paleogeography during Laurentian orogenesis. To address these topics, we acquired detrital zircon U-Pb geochronologic data for eastern exposures of the Belt-Purcell Supergroup in the Lewis thrust salient along the USA-Canada border. To define an integrated chronostratigraphic and provenance framework for the Belt Basin, we calculated maximum depositional ages and qualitatively and quantitatively compared our geochronologic data set to a com-pilation of Laurentian igneous and metamorphic zircon U-Pb ages using multidimensional scaling and an inverse Monte Carlo model. The results suggest a stratigraphic age range of ca. 1495-1380 Ma, constituting a depositional duration of similar to 115 m.y. with an average sediment accumulation rate of similar to 40 m/m.y. for the studied locality (extrapolated to similar to 155 m/m.y. for the basin depocenter). Variations in sediment provenance are expressed by three distinct intervals within the Belt- Purcell Supergroup. The lower Belt Supergroup succession (Waterton to lower Helena Formations; ca. 1495- 1440 Ma) is dominated by Paleoproterozoic and Archean grains derived from the northeastern Canadian Shield. The middle Belt Supergroup succession (upper Helena to Sheppard Formations; ca. 1440-1420 Ma) displays mixed early Mesoproterozoic, late Paleoproterozoic, and Archean zircon age groups. The upper Belt Supergroup succession (Gateway to Roosville Formations; ca. 1420-1380 Ma) contains almost entirely late Paleo-protero zoic zircons sourced from the south (Yavapai- Mazatzal and Mojave crustal provinces). We interpret sediment provenance to reflect a continental- scale, fluvial drainage reorganization during middle Belt Supergroup deposition that can be linked to the recently recognized Picuris orogeny.

Detrital zircon data sets that include sample locations, U-Pb zircon analyses of 27 samples, Laurentian Precambrian metamorphic and igneous ages, proportions of Precambrian age populations throughout Laurentia and Belt Basin, MDA calculations, modeling results, and associated material with MDS plots.

The Kishnehn Formation crops out in Glacier National Park of northwest Montana where a rich fossil record of plant macrofossils, pollen and spores, insects, terrestrial and aquatic mollusks, and fish has been unearthed. Past research has also described an extensive mammal fauna from the Eocene (Uintan-Chadronian). Oligocene-aged fossil mammals have been reported before, but none has ever been published in the peer-reviewed literature. Here, we present the first Arikareean-aged fossil mammals from the Kishenehn Formation, the youngest fossil mammals ever discovered in the park. The fossils consist of a set of lower jaws of the leptomerycid Pronodens transmontanus (Douglas, 1903) and a partial lower jaw of the rodent Paciculus montanus Black, 1961, both endemics of the northern Rocky Mountains. These new fossils enable us to explore the morphological variation in Pronodens Koerner, 1940 and Paciculus Cope, 1879. Our analyses suggest the existence of a single widely distributed and sometimes locally abundant species of Pronodens, which may co-occur with a rare and very large second species. Our revised diagnoses for the genus and species show the need for additional work on this little-studied artiodactyl genus. Similar efforts on the systematics of cricetid rodents will benefit from building upon our analysis of tooth morphology in Paciculus to shed light on the rise of leidymines. The last fossil we describe, partial paired dentaries of Miohippus Marsh, 1874, is the northern-most occurrence of the genus in the Rocky Mountains and shows the potential for future work in the Kishenehn Formation to enable the study of faunal change across the EoceneOligocene boundary in the northern Rocky Mountains.

Broken foreland basins are caused by crustal-scale contractional basement structures that compartmentalize (or break) a contiguous retroarc or collisional foreland basin into smaller disconnected basins. Broken foreland basins differ from their unbroken counterparts in their deformational, depositional, and geodynamic framework. Whereas contiguous (unbroken) foreland basins are generated mainly by regional flexural loading due to shortening of supracrustal cover strata and uppermost basement in organized ramp-flat thrust systems, broken foreland basins are governed principally by isolated topographic loads and structural tilting associated with widely spaced crustal-scale reverse faults that accommodate intraplate basement shortening. These structural contrasts foster either de acute accent collement-style fold-thrust belts (orogenic wedges) with large integrated erosional drainage systems (watersheds) spanning diverse sediment source regions (including thin-skinned fold-thrust belts, elevated hinterland zones, accreted terranes, and magmatic arcs) or independent foreland block uplifts with local drainage systems dominated by basement sources. Although the genesis of broken foreland basins has been uniquely attributed to flat slab subduction, these basins are also sensitive to inherited structural, strati -graphic, thermal, and rheological configurations, as well as synorogenic mass redistribution in relationship to climate, erosion, sediment transport efficiency, and sediment accumulation.Despite the many modern and ancient examples, questions persist over the underlying geodynamic processes that promote development of a broken or compartmentalized foreland basin instead of a single regionally unified flexural foreland basin. Additional uncertainties and misconceptions surround the criteria used to define broken foreland basins and their linkages to subduction dynamics (chiefly slab geometry), strain magnitude, and structural reactivation. Here we review the tectonic framework of broken foreland basins-with emphasis on South and North America (Pampean and Laramide provinces)-and propose that their genesis can be ascribed to a combination of: (i) underlying conditions in the form of tectonic inheritance, including precursor structural, stratigraphic, thermal, and rheological heterogeneities and anisotropies; and (ii) mechanical triggers, such as increased stress, enhanced horizontal stress transmission, and/or selective crustal strengthening or weakening.

The Andes of western Argentina record spatiotemporal variations in morphology, basin geometry, and structural style that correspond with changes in crustal inheritance and convergent margin dynamics. Above the modern Pampean flat‐slab subduction segment (27–33°S), retroarc shortening generated a fold‐thrust belt and intraforeland basement uplifts that converge north of ∼29°S, providing opportunities to explore the effects of varied deformation and subduction regimes on synorogenic sedimentation. We integrate new detrital zircon U‐Pb and apatite (U‐Th)/He analyses with sequentially restored, flexurally balanced cross sections and thermokinematic models at ∼28.5–30°S to link deformation with resulting uplift, erosion, and basin accumulation histories. Tectonic subsidence, topographic evolution, and thermochronometric cooling records point to (a) shortening and distal foreland basin accumulation at ∼18–16 Ma, (b) thrust belt migration, changes in sediment provenance, and enhanced flexural subsidence from ∼16 to 9 Ma, (c) intraforeland basement deformation, local flexure, and drainage reorganization at ∼12–7 Ma, and (d) out‐of‐sequence shortening and exhumation of foreland basin fill by ∼8–2 Ma. Thrust belt kinematics and the reactivation of basement heterogeneities strongly controlled tectonic load configurations and subsidence patterns. Geo/thermochronological data and model results resolve increased shortening and combined thrust belt and intraforeland basement loading in response to ridge collision and Neogene shallowing of the subducted oceanic slab. Finally, this study demonstrates the utility of integrated flexural thermokinematic and erosion modeling for evaluating the geometries, rates, and potential drivers of retroarc deformation and foreland basin evolution during changes in subduction.

Oligocene and early Miocene displacement on the Catalina–San Pedro detachment fault and its northern correlatives uncovered mylonitic fabrics that form the greater Catalina metamorphic core complex in southeastern Arizona, USA. Gently to moderately dipping mylonitic foliations in the complex are strongly lineated, with a lineation-azimuth average of 064–244° and dominantly top-southwest shear sense over the entire 115-km-long mylonite belt. Reconstruction of detachment fault displacement based on a variety of features indicates 40–60 km of displacement, with greater displacement in more southern areas. Widespread 26–28 Ma volcanism during early extensional basin genesis was followed by 24–26 Ma granitoid magmatism. Cooling of footwall mylonites continued until 22–24 Ma, as indicated by 40Ar/39Ar mica dates. Lower temperature thermochronometers suggest that footwall exhumation was still underway at ca. 20 Ma. Tectonic reconstruction places a variety of unmetamorphosed supracrustal units in the Tucson and Silver Bell Mountains above equivalent units that were metamorphosed and penetratively deformed in the Tortolita and Santa Catalina Mountains. This restored juxtaposition is interpreted as a consequence of older Laramide thrust burial of the deformed units, with northeast-directed thrusting occurring along the Wildhorse Mountain thrust in the Rincon Mountains and related but largely concealed thrusts to the northwest. Effective extensional exhumation of lower plate rocks resulted from a general lack of internal extension of the upper plate wedge. This is attributed to a stable sliding regime during the entire period of extension, with metamorphic core complex inflation by deep crustal flow leading to maintenance of wedge surface slope and detachment fault dip that favored stable sliding rather than internal wedge extension.

Datasets of molluscan taxa, zircon U-Pb geochronology, and stable isotopes.

Double-dating using the apatite U-Pb and fission-track systems is becoming an increasingly popular method for resolving mid- to upper- crustal cooling. However, these thermochronometers constrain dates that are often difficult to link through geological time due to the large difference in temperature window between the two systems (typically >250 degrees C). In this study, we apply apatite U-Pb, fission-track, and apatite and whole rock geochemistry to fourteen samples from four tectonic domains common in Cordilleran orogenic systems: (1) basement-cored uplifts, (2) plutons intruded through a thick crustal column, (3) metamorphic core complexes and associated detachment faults, and (4) rapid, extrusive volcanic cooling, in order to provide a link between in situ geochemical signatures and cooling mechanisms. Comparisons of trace element partitioning between apatite and whole rock provide insights into initial apatite-forming processes and/or subsequent modification. Apatite trace element geochemistry and the Th/U and La/LuN ratios provide tools to determine if an apatite is primary and representative of its parent melt or if it has undergone geochemical perturbation(s) after crystallization. Further, we demonstrate that by using a combined apatite U-Pb, FT, trace element, and whole rock geochemistry approach it is possible to determine if a rock has undergone monotonic cooling since crystallization, protracted residence in the middle crust, and provide unique structural information such as the history of detachment faulting. Insights provided herein offer new applications for apatite thermochronology.

The cause of Cenozoic uplift of the Colorado Plateau is one of the largest remaining problems of Cordilleran tectonics. Difficulty in discriminating between two major classes of uplift mechanisms, one related to lithosphere modification by low-angle subduction and the other related to active mantle processes following termination of subduction, is hampered by lack of evidence for the timing of uplift. The carbonate member of the Pliocene Bouse Formation in the lower Colorado River Valley southwest of the Colorado Plateau has been interpreted as estuarine, in which case its modern elevation of up to 330 m above sea level would be important evidence for late Cenozoic uplift. The carbonate member includes laminated marl and claystone interpreted previously in at least one locality as tidal, which is therefore of marine origin. We analyzed lamination mineralogy, oxygen and carbon isotopes, and thickness variations to discriminate between a tidal versus seasonal origin. Oxygen and carbon isotopic analysis of two laminated carbonate samples shows an alternating pattern of lower δ18O and δ13C associated with micrite and slightly higher δ18O and δ13C associated with siltstone, which is consistent with seasonal variation. Covariation of alternating δ18O and δ13C also indicates that post-depositional chemical alteration did not affect these samples. Furthermore, we did not identify any periodic thickness variations suggestive of tidal influence. We conclude that lamination characteristics indicate seasonal genesis in a lake rather than tidal genesis in an estuary and that the laminated Bouse Formation strata provide no constraints on the timing of Colorado Plateau uplift.

The Santa Catalina Mountains in southeastern Arizona include extensive mylonitic fabrics developed within granitic and gneissic rocks that make up most of the range. These fabrics are strongest at the southern foot of the range where they dip south and project beneath the rangebounding Catalina – San Pedro detachment fault. Shear sense in the mylonitic rocks is primarily top-southwest, consistent with shearing down-dip from the detachment fault during early normal faulting and exhumation to form the metamorphic core complex. Two to three kilometers north of the foot of the Santa Catalina Mountains the mylonitic foliation is horizontal and farther north it dips to the north. The mylonitic fabrics thus dip outward from the axis of the Forerange arch with primarily top-northeast shear-sense indicators on the north side of the arch. This dominantly topnortheast fabric forms the Molino Basin shear zone, which continues eastward toward the Bellota Ranch area that is the subject of this study. We found that mylonitic fabrics in the Bellota Ranch area are generally subhorizontal, weak, and without clear shear-sense indicators in by far ost outcrops. Four days of field study yielded clear shear-sense indicators at only eight outcrops, with six of them indicating top-southwest shear sense. We were not able to divide mylonitic rocks into top-southwest and top-northeast zones as in a previous study by Bykerk-Kauffman (2008). Lineation trend gradually changes laterally from more northeasterly in the Molino Basin area to more northerly in the Bellota Ranch area. As with the Molino Basin area, lineations associated with top-northeast shearing generally trend more northerly than lineations associated with top-southwest shearing. We conclude that mylonitic lineation in the Bellota anch area is not a simple continuation of the Molino Basin shear zone as it is dominated by top-southwest shear-sense indicators rather than top-northeast as with the Molino Basin shear zone.