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Abstract Low‐temperature thermochronometric data can reveal the long‐term evolution of erosion, uplift, and thrusting in fold‐thrust belts. We present results from central Idaho and southwestern Montana, where the close spatial overlap of the Sevier fold‐thrust belt and Laramide style, basement‐involved foreland uplifts signify a complex region with an unresolved, long‐term tectono‐thermal history. Inverse QTQt thermal history modeling of new zircon (U‐Th)/He (ZHe,n = 106), and apatite (U‐Th)/He dates (AHe,n = 43) collected from hanging walls of major thrusts systems along a central Idaho to southwestern Montana transect, and apatite fission track results from 6 basement samples, reveal regional thermal and spatial trends related to Sevier and Laramide orogenesis. Inverse modeling of foreland basement uplift samples suggest Phanerozoic exhumation initiated as early as ∼80 Ma and continued through the early Paleogene. Inverse modeling of interior Idaho fold‐thrust belt ZHe samples documents Early Cretaceous cooling at ∼125 Ma in the Lost River Range (western transect), and a younger cooling episode in the Lemhi Arch region (mid‐transect) at ∼90–80 Ma through the late Paleogene. This cooling in the Lemhi Arch temporally overlaps with cooling in southwestern Montana's basement‐cored uplifts, which we interpret as roughly synchronous exhumation related to contractional tectonics and post‐orogenic collapse. These data and models, integrated with independent timing constraints from foreland basin strata and previously published thermochronometric results, suggests that middle Cretaceous deformation of southwestern Montana's basement‐cored uplifts was low magnitude and preceded tectonism along the classic Arizona‐Wyoming Laramide “corridor.” In contrast, Late Cretaceous and Paleogene thrust‐related exhumation was more significant and largely complete by the Eocene. The basement‐involved deformation was contemporaneous with and younger than along‐strike Sevier belt thrusting in central Idaho. 

Abstract Archean rocks exposed in the Beartooth Mountains, Montana and Wyoming, have experienced a complex >2.5 Gyr thermal history related to the long‐term geodynamic evolution of Laurentia. We constrain this history using “deep‐time” thermochronology, reporting zircon U‐Pb, biotite⁴⁰Ar/³⁹Ar, and zircon and apatite [U‐Th(‐Sm)]/He results from three transects across the basement‐core of the range. Our central transect yielded a zircon U‐Pb concordia age of 2,805.6 ± 6.4 Ma. Biotite⁴⁰Ar/³⁹Ar plateau ages from western samples are ≤1,775 ± 27 Ma, while those from samples further east are ≥2,263 ± 76 Ma. Zircon (U‐Th)/He dates span 686.4 ± 11.9 to 13.5 ± 0.3 Ma and show a negative relationship with effective uranium—a proxy for radiation damage. Apatite (U‐Th)/He dates are 109.2 ± 23.9 to 43.6 ± 1.9 Ma and correlate with sample elevation. Multi‐chronometer Bayesian time‐temperature inversions suggest: (a) Cooling between ∼1.90 and ∼1.80 Ga, likely related to Big Sky orogeny thermal effects; (b) Reheating between ∼1.80 Ga and ∼1.35 Ga consistent with Mesoproterozoic burial; (c) Cooling to ≤100°C between Mesoproterozoic and early Paleozoic time, likely reflecting continental erosion; (d) Variable Paleozoic–Jurassic cooling, possibly related to Paleozoic tectonism and/or low eustatic sea level; (e) Rapid Cretaceous–Paleocene cooling, preceding accepted proxies for flat‐slab subduction; (f) Eocene–Miocene reheating consistent with reburial by Cenozoic volcanics and/or sediments; (g) Post‐20 Ma cooling consistent with Neogene development of topographic relief. Our results emphasize the utility of multi‐chronometer thermochronology in recovering complex, non‐monotonic multi‐billion‐year thermal histories. 

Abstract Crystalline basement rocks of southwestern Montana have been subjected to multiple tectonothermal events since ∼3.3 Ga: the Paleoproterozoic Big Sky/Great Falls orogeny, Mesoproterozoic extension associated with Belt‐Purcell basin formation, Neoproterozoic extension related to Rodinia rifting, and the late Phanerozoic Sevier‐Laramide orogeny. We investigated the long‐term (>1 Ga), low‐temperature (erosion/burial within 10 km of the surface) thermal histories of these tectonic events with zircon and apatite (U‐Th)/He thermochronology. Data were collected across nine sample localities (n = 55 zircon andn = 26 apatite aliquots) in the northern and southern Madison ranges, the Blacktail‐Snowcrest arch, and the Tobacco Root uplift. Our zircon (U‐Th)/He data show negative trends between single aliquot date and effective uranium (a radiation damage proxy), which we interpreted with a thermal history model that considers the damage‐He diffusivity relationship in zircon. Our model results for these basement ranges show substantial cooling from temperatures above 400°C to near surface conditions between 800 and 510 Ma. Subsequent Phanerozoic exhumation culminated by ∼75 Ma. Late Phanerozoic cooling is coincident with along‐strike Sevier belt thin‐skinned thrusting in southeastern Idaho, and older than exhumation in basement‐involved uplifts of the Wyoming Laramide province. Our long‐term, low‐temperature thermal record for these southwestern Montana basement ranges shows that: (a) these basement blocks have experienced multiple episodes of upper crustal exhumation and burial since Archean time, possibly influencing Phanerozoic thrust architecture and (b) the late Phanerozoic thick‐skinned thrusting recorded by these rocks is among the earliest thermochronologic records of Laramide basement‐involved shortening and was concomitant with Sevier belt thin‐skinned thrusting. 

Deep-time thermochronology by the zircon (U-Th)/He (ZHe) method is an emerging field of study with promise for constraining Precambrian rock thermal and exhumation histories. The Grand Canyon provides an opportunity to further explore this method because excellent geologic constraints can be integrated with multiple thermochronometers to address important questions about the spatial variability of basement erosion below the sub-Cambrian Great Unconformity composite erosional surface. In this study, we synthesize new ZHe results (n = 26) and published (n = 77) ZHe data with new K-feldspar 40Ar/39Ar data and models (n = 4) from Precambrian basement rocks of the Grand Canyon, USA. We use HeFTy and QTQt thermal history modeling to evaluate the ability of the individual ZHe and K-feldspar 40Ar/39Ar thermochronometric data sets to resolve Precambrian thermal histories and compare those results with jointly modeled data using the QTQt software. We also compare Precambrian basement thermal histories of the eastern and western Grand Canyon, where the eastern Grand Canyon has ∼4 km of Grand Canyon Supergroup strata deposited and preserved, and the western Grand Canyon, where the Supergroup was either never deposited or not preserved. In all locations, models constrained only by ZHe data have limited resolving power for the past ∼600 m.y., compared to models that combine K-feldspar 40Ar/39Ar and ZHe data, which extends the recorded history into the Mesoproterozoic. Our model results suggest that two regional basement unroofing events occurred. A ca. 1350−1250 Ma cooling event is interpreted to record basement exhumation from depths of ∼10 km, and a second cooling episode (∼200−100 °C total) records exhumation from a depth of ∼3 km to 7 km to near-surface conditions between ca. 600 Ma and 500 Ma. Easternmost Grand Canyon models suggest that the preserved maximum ∼4 km thickness of the Grand Canyon Supergroup (with burial heating at ∼100 °C) approximates the total original Mesoproterozoic and Neoproterozoic stratal thickness. Whether these Supergroup rocks were present and then eroded in the western Grand Canyon, as suggested by regional geologic studies, or were never deposited is not constrained by thermochronological data. 

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 Classically held mechanisms for removing mountain topography (e.g., erosion and gravitational collapse) require 10-100 Myr or more to completely remove tectonically generated relief. Here, we propose that mountain ranges can be completely and rapidly (<2 Myr) removed by a migrating hotspot. In western North America, multiple mountain ranges, including the Teton Range, terminate at the boundary with the relatively low relief track of the Yellowstone hotspot. This abrupt transition leads to a previously untested hypothesis that preexisting mountainous topography along the track has been erased. We integrate thermochronologic data collected from the footwall of the Teton fault with flexural-kinematic modeling and length-displacement scaling to show that the paleo-Teton fault and associated Teton Range was much longer (min. original length 190-210 km) than the present topographic expression of the range front (~65 km) and extended across the modern-day Yellowstone hotspot track. These analyses also indicate that the majority of fault displacement (min. 11.4-12.6 km) and the associated footwall mountain range growth had accumulated prior to Yellowstone encroachment at ~2 Ma, leading us to interpret that eastward migration of the Yellowstone hotspot relative to stable North America led to removal of the paleo-Teton mountain topography via posteruptive collapse of the range following multiple supercaldera (VEI 8) eruptions from 2.0 Ma to 600 ka and/or an isostatic collapse response, similar to ranges north of the Snake River plain. While this extremely rapid removal of mountain ranges and adjoining basins is probably relatively infrequent in the geologic record, it has important implications for continental physiography and topography over very short time spans.