AHe Dates Research Articles

Segmentation of the Tashkurgan normal fault in the eastern Pamir: Insights from geomorphology and thermochronology and implications for fault-slip transfer

At the northwestern end of the India–Asia collision zone, in the eastern Pamir interior, the Kongur Shan extensional system extends for ∼250 km as a composite system of normal faults. As the southernmost segment of the extensional system, the Tashkurgan fault can be divided into a northern and southern segment by the intersection of the Tahman and Tashkurgan faults. To evaluate the tectonic activity along the Tashkurgan fault and explore the extensional characteristics of the southern Kongur Shan extensional system, we integrate geomorphologic analysis with thermochronologic data and modeling. Geomorphic indices, including local relief, normalized channel steepness index (ksn), and river χ-plots, show that the southern Tashkurgan fault is more active than the northern Tashkurgan fault. On the footwall of the northern Tashkurgan fault, most thermochronologic data, which include biotite 40Ar/39Ar (Arbt), zircon (U-Th)/He (ZHe), apatite fission track (AFT), and apatite (U-Th-Sm)/He (AHe) dates, record emplacement-related cooling, and only a few AHe dates possibly record exhumation of <2 km since ∼7 Ma ago. On the footwall of the southern Tashkurgan fault, almost all of the ZHe, AFT, and AHe dates record significant late Miocene to present exhumation; thus, we perform three-dimensional thermokinematic modeling for the normal slip along this fault segment. Modeling results indicate a probably constant slip rate on the southern Tashkurgan fault at a value of 1.4–1.5 mm/a, which corresponds to 6–7 km of both horizontal extension and footwall uplift since 6.5 Ma ago. All together, the geomorphic and thermochronologic evidences imply that the southern Tashkurgan fault is responsible for the ∼E–W extension along the southernmost portion of the Kongur Shan extensional system, while the northern fault segment plays a minor role in the extension. This result, combined with the previously published magnitude of extension along the southern Kongur Shan fault, indicates that the extension at any site along the southern Kongur Shan extensional system can be primarily attributed to normal faulting along a single border fault, which is a typical characteristic of an interbasin transfer zone with a transfer fault.

Cenozoic Exhumation Across the High Plains of Southeastern Colorado from (U-Th)/He Thermochronology

Abstract Colorado’s High Plains stand at anomalously high elevations (~1300–2100 m) for their continental interior setting, but when and why this region became elevated is poorly understood. The Cenozoic history of the High Plains is also likely linked with that of the Rocky Mountains, where the timing and cause(s) of uplift are similarly debated. We present apatite (U-Th)/He (AHe) data for 10 samples from Tertiary intrusives along a ~200 km west-to-east transect across the High Plains of southeastern Colorado to constrain the timing of exhumation and to gain insight into when and why regional elevation gain occurred. Mean sample AHe dates for the ~24–22 Ma East Spanish Peak pluton and associated radial dikes from the westernmost High Plains are 18.8 ± 1.4 to 14.1 ± 1.7 Ma, recording substantial postemplacement erosion. AHe results for the mafic to ultramafic Apishapa Dikes (oldest ~37 Ma, youngest ~14 Ma) located ~20–40 km farther north and east on the High Plains range from 12.0 ± 1.4 to 6.2 ± 1.9 Ma, documenting continued exhumation on the western High Plains during the ~12–5 Ma deposition of the Ogallala Formation farther east and suggesting that the western limit of Ogallala deposition was east of the Apishapa Dikes. In far southeastern Colorado, the Two Buttes lamprophyre was emplaced at 36.8 ± 0.4 Ma and yields a Late Oligocene AHe date of 27.1 ± 4 Ma. Here, the Ogallala Formation unconformably overlies Two Buttes, indicating that the regional ~12 Ma age for the base of the Ogallala is a minimum age for the exposure of the pluton at the surface. The AHe data presented here document that kilometer-scale erosion affected all of the southeastern Colorado High Plains in Oligo-Miocene time. While exhumation can have multiple possible causes, we favor contemporaneous surface uplift capable of elevating the region to modern heights.

Phanerozoic Burial and Erosion History of the Southern Canadian Shield from Apatite (U-Th)/He Thermochronology

Patterns of Phanerozoic burial and erosion across the cratonic interior of North America can help constrain the continental hypsometric history and the potential influence of dynamic topography on continental evolution. Large areas of the Canadian Shield currently lack Phanerozoic sedimentary cover, but thermochronology data can help reconstruct the previous extent, thickness, and erosion of Phanerozoic strata that once covered the craton. Here, we report apatite (U-Th)/He (AHe) data for 15 samples of Precambrian basement rocks and 1 sample of Triassic kimberlite from a 1400 km–long east–west transect across the southern Canadian Shield. Single-grain basement AHe dates range from &gt;500 Ma in the west to &lt;250 Ma in the east. AHe dates for the kimberlite in the middle of the transect overlap with the pipe’s Triassic eruption age. These data, combined with previous apatite fission-track data, geologic constraints, and thermal history modeling, are used to constrain the first-order regional thermal history that we interpret in the context of continental burial and erosion. Our burial and erosion model is characterized by Paleozoic burial that was greater to the east, unroofing that migrated eastward through Jurassic time, and little to no post-Triassic burial. This pattern suggests dynamic and tectonic forces related to Appalachian convergence, subduction cessation, and later rifting as drivers. The AHe data contribute to efforts to collect thermochronology data across the Canadian Shield to map out continental-scale burial and erosion patterns. The outcomes can be used to refine mantle dynamic models and test how dynamic topography, far-field tectonics, and other effects influence the surface histories of continental interiors.

Ediacaran-Ordovician tectonic and geodynamic drivers of Great Unconformity exhumation on the southern Canadian Shield

The Great Unconformity erosion surface between Paleozoic sedimentary rocks and Archean-Proterozoic basement has been related to erosion occurring during Snowball Earth glaciations, eustatic sea level fluctuations, and a range of tectonic and/or geodynamic mechanisms. Each class of mechanism predicts distinct timing and spatial patterns of exhumation. Snowball Earth glacial erosion is limited to 717-635 Ma and concentrated in narrow ice streams on continental margins. Sea-level related erosion is unconstrained in time but also spatially limited to continental margins. Tectonic and geodynamic mechanisms, in contrast, can result in exhumation distributed more broadly in time and space. We combine new zircon and apatite (U-Th)/He thermochronology data (ZHe, AHe) with independent paleodepth information from a continental interior location in southeastern Ontario, Canada to constrain the timing, magnitude, and regional pattern of exhumation associated with the Great Unconformity along a ∼650 km-long transect across the southern Canadian Shield. Here, the unconformity is defined by Middle Ordovician carbonates atop Archean-Proterozoic basement. ZHe analyses for seven basement samples display a range of dates from 960 ± 20 Ma to 37.5 ± 0.9 Ma that correlate negatively with radiation damage. AHe dates are ∼300-200 Ma and generally consistent across samples regardless of radiation damage. Independent evidence supports emplacement of both the 590 +2/-1 Ma Grenville dikes and the 577 ± 1 Ma Callander Complex in the study region at depths ≥6 km. The combined data require ≥6 km of exhumation between ca. 590-577 Ma and 470 Ma in the middle of our transect, with multi-km erosion up to ≥5 km elsewhere in the study area, well after the ca. 717-635 Ma Snowball Earth glaciations. These outcomes expand the spatial extent of an Ediacaran to early-Paleozoic exhumation signal previously inferred elsewhere across the Shield to ∼1.1 million km2. Thick Ediacaran successions on the Laurentian margins are complementary depositional signals of this erosion. The enormous spatial extent of Great Unconformity exhumation across the continental interior of the Canadian Shield is incompatible with glacial erosion and eustatic sea-level change as the primary causes. Instead, we attribute exhumation to tectonic and geodynamic mechanisms, which may include isostatic rebound, dynamic topography, plume activity associated with the Central Iapetus Magmatic Province, rifting during opening of the Iapetus Ocean, and development of the Transcontinental Arch.

Reconstruction of the topographic evolution of the coastal mountain system in southeastern China

The coastal mountain system (CMS) of southeastern China has been a natural place to study paleotopographic changes primarily due to its marked topographical contrast since the Late Cretaceous. For a quantitative reconstruction of the paleotopographic evolution of the CMS, this paper provides key information from along the Ningde–Jianyang and Huilai–Longchuan cross-sections using apatite (U-Th)/He (AHe) and zircon (U-Th)/He (ZHe) data combined with thermal history and paleotopographic modeling. For the Ningde–Jianyang section, the AHe dates are in the range of 44.3–70.8 Ma, and the ZHe dates are between 58.5 and 102.9 Ma. For the Huilai–Longchuan section, the resulting AHe dates are in the range of 37.9–73.8 Ma, and ZHe dates are between 65.5 and 119.7 Ma. The modeling results suggest that: 1) the CMS had an asymmetric topography during the Late Cretaceous with the SE segment being significantly higher (peak elevation = 3.8 km) than the NW segment (peak elevation = ~2 km). The marked decreases in elevation and relief of the CMS occurred during 80–40 Ma, followed by relatively minor topographic changes since ~40–30 Ma; 2) The SE segment of the southern CMS experienced significantly faster exhumation (~0.44 km/Myr) than northern CMS (0.16 km/Myr) during 70–60 Ma, which was probably related to thinning of the crust and orogenic collapse linked to the initial-rifting of the South China Sea; 3) The SE segment of southern CMS experienced slower exhumation than northern CMS during 60–40 Ma, likely resulting from the lower elevations in the southern CMS than in the northern CMS during 60–40 Ma.

A practical method for assigning uncertainty and improving the accuracy of alpha-ejection corrections and eU concentrations in apatite (U–Th) ∕ He chronology

Abstract. Apatite (U–Th) / He (AHe) dating generally assumes that grains can be accurately and precisely modeled as geometrically perfect hexagonal prisms or ellipsoids in order to compute the apatite volume (V), alpha-ejection corrections (FT), equivalent spherical radius (RFT), effective uranium concentration (eU), and corrected (U–Th) / He date. It is well-known that this assumption is not true. In this work, we present a set of corrections and uncertainties for V, FT, and RFT aimed (1) at “undoing” the systematic deviation from the idealized geometry and (2) at quantifying the contribution of geometric uncertainty to the total uncertainty budget for eU and AHe dates. These corrections and uncertainties can be easily integrated into existing laboratory workflows at no added cost, can be routinely applied to all dated apatite, and can even be retroactively applied to published data. To quantify the degree to which real apatite deviates from geometric models, we selected 264 grains that span the full spectrum of commonly analyzed morphologies, measured their dimensions using standard 2D microscopy methods, and then acquired 3D scans of the same grains using high-resolution computed tomography (CT). We then compared our apatite 2D length, maximum width, and minimum width measurements with those determined by CT, as well as the V, FT, and RFT values calculated from 2D microscopy measurements with those from the “real” 3D measurements. While our 2D length and maximum width measurements match the 3D values well, the 2D minimum width values systematically underestimate the 3D values and have high scatter. We therefore use only the 2D length and maximum width measurements to compute V, FT, and RFT. With this approach, apatite V, FT, and RFT values are all consistently overestimated by the 2D microscopy method, requiring correction factors of 0.74–0.83 (or 17 %–26 %), 0.91–0.99 (or 1 %–9 %), and 0.85–0.93 (or 7 %–15 %), respectively. The 1σ uncertainties in V, FT, and RFT are 20 %–23 %, 1 %–6 %, and 6 %–10 %, respectively. The primary control on the magnitude of the corrections and uncertainties is grain geometry, with grain size exerting additional control on FT uncertainty. Application of these corrections and uncertainties to a real dataset (N=24 AHe analyses) yields 1σ analytical and geometric uncertainties of 15 %–16 % in eU and 3 %–7 % in the corrected date. These geometric corrections and uncertainties are substantial and should not be ignored when reporting, plotting, and interpreting AHe datasets. The Geometric Correction Method (GCM) presented here provides a simple and practical tool for deriving more accurate FT and eU values and for incorporating this oft neglected geometric uncertainty into AHe dates.

Assessing the long-term low-temperature thermal evolution of the central Indian Bundelkhand craton with a complex apatite and zircon (U-Th)/He dataset

Modern approaches in low-temperature thermochronometry are capable of extracting long-term thermal histories from cratonic settings that may elucidate potential drivers of deep-time phases of intracontinental burial and erosion. Here, we assess the utilization of the Radiation Damage Accumulation and Annealing Model for apatite (RDAAM) and zircon (ZRDAAM) to track the long-term low-temperature thermal evolution of the Archean Bundelkhand craton and the surrounding undeformed strata of the ∼1.7–0.9 Ga Vindhyan successions in central India. We correspondingly interpret a complex basement and detrital zircon and apatite (U-Th)/He (ZHe and AHe, respectively) dataset in light of observed model limitations and known geologic context. ZHe and AHe dates from across the craton reveal a significant (&gt;300 Myr) date inversion between the two systems within grains with moderate to high effective uranium (eU) concentrations. Inverse thermal models utilizing current ZRDAAM and RDAAM parameters are not capable of reproducing observed coupled basement ZHe and AHe data for the same thermal history. However, meaningful thermal information can be extracted from AHe inverse models coupled with a forward modeling approach applied to detrital ZHe data from Vindhyan deposits, which have notably lower eU concentrations and yield significantly older ZHe dates (between ∼1,475 and 575 Ma) than basement zircon. Resulting thermal models indicate that the Bundelkhand craton experienced peak burial temperatures of ∼150°C between 850 and 475 Ma, followed by a major crustal cooling event at ∼350–310 Ma, possibly driven by late Paleozoic glaciations and/or epeirorogenic uplift. Inverse models including AHe data require a Deccan Traps related thermal perturbation between ∼66 and 65 Ma, and we suspect that this event overprinted basement zircon with moderate to high eU concentrations. Although the effects of zonation, grain morphology, and/or uncertainties in damage-annealing parameters contribute to disparities between predicted and observed AHe and ZHe dates, these factors alone cannot account for the major ZHe and AHe date inversion observed from the Bundelkhand craton. Instead, it is likely the case that current damage-dependent models for ⁴He diffusion are not adequately calibrated at the resolution necessary to predict short-lived thermal perturbations that occurred in a late phase relative to a prolonged period of extensive damage accumulation.

Investigating apatite (U-Th)/He thermochronologic ages to understand exhumation history of the Ethiopian Plateau

Although the Ethiopian Plateau is a prominent feature in East Africa, its exhumation history is not well constrained. For the last few decades, this history has been a topic of scientific interest due to the plateau's complicated geologic setting and thermal response to surface topographic changes. Studies of the plateau's thermal history have the potential to cast light on the numerous tectonic and geodynamic events that have affected the region since the beginning of the Mesozoic, such as Gondwana rifting, Afar mantle plume upwelling, continental flood basalt emplacement, Cenozoic continental rifting, and shield and cinder cone volcanism that has continued until the present day. We present new single-grain apatite (U-Th)/He (AHe) thermochronologic cooling ages from the Blue Nile Canyon to address the Ethiopian Plateau's exhumation history. AHe dates of 55 grains from nine Neoproterozoic basement and Mesozoic sedimentary rock samples, collected from a vertical transect of the Blue Nile Canyon, range from 58.9 ± 2.7 to 460.2 ± 34.7 Ma. Interpretation of these cooling ages is complex due to intra-sample age dispersion. We address this dispersion by systematically evaluating the ages based on standard deviation percentage, effective Uranium concentration [eU], and equivalent spherical grain radius r, which also helps select samples to generate a geologically meaningful thermal history model. Our inverse thermal history model using the Radiation Damage Accumulation and Annealing Model (RDAAM) reveals a protracted Paleozoic cooling, a gradual reheating during Permo-Mesozoic time (between ∼300 and ∼100 Ma), and a rapid cooling in the Cretaceous (∼100 Ma). This Paleozoic and Mesozoic thermal history reflects post-Pan-African orogenic denudation followed by Gondwana rift-related burial and then exhumation. A period of tectonic quiescence is reflected between ∼100 and ∼30 Ma, during which there was little to no cooling. Although our thermal model shows burial-related reheating after the flood basalt emplacement around 30 Ma and eventual cooling to present-day surface temperature, our thermochronologic data likely lack sufficient resolution to determine the details of the Neogene cooling related to incision of the Blue Nile Canyon. Our study suggests that the thermal history of the Ethiopian Plateau is dynamic and warrants further high-resolution, low-temperature thermochronologic investigation to constrain post-30 Ma river incision of the plateau.

Long-lagged (∼19 Myr) response of accelerated river incision to rock uplift on the northern margin of the Tibetan Plateau

Quantifying tectonic-climatic interaction in shaping the margins of Tibetan Plateau is essential for understanding outward plateau growth. Here we investigate the histories of rock uplift and erosion along the northwestern margin of the Tibetan Plateau in the Altyn Shan, using low-relief high-elevation surfaces (LRHES) and combined bedrock and detrital apatite (U-Th)/He (AHe) analyses. Bedrock AHe dates and associated thermal modeling of samples collected from the mountain flank reveal a phase of very fast cooling starting at ∼36 Ma, followed by subsequent moderate cooling since ∼31 Ma. We argue that this pattern results from rock uplift caused by tectonic activity of the North Altyn fault (NAF), which separates the uplifted Altyn Shan from the low-lying Tarim Basin to the north. However, detrital AHe analysis and associated numerical modeling indicate that accelerated river incision in the Altyn Shan was delayed to as late as ∼17 Ma merely ∼10 km away from the NAF. Taken together, the data imply a long-lagged (∼19 Myr) response of accelerated river incision to tectonic-driven rock uplift. This delayed incision likely results from very low erosional efficiency associated with arid climate during the interval of ∼36 – 17 Ma, with increased erosional efficiency during the relatively wet climate of the middle Miocene climate optimum (∼17 – 14 Ma). These findings illustrate the interplay between tectonics and climate in developing landforms on the northern margin of the Tibetan Plateau, and bear implications for both the Cenozoic evolution of this margin, and, more broadly, thermochronometer-based study of rock uplift and erosion.

Linking exhumation, paleo-relief, and rift formation to magmatic processes in the western Snake River Plain, Idaho, using apatite (U-Th)/He thermochronology

Abstract The western Snake River Plain (WSRP) in southwest Idaho has been characterized as an intracontinental rift basin but differs markedly in topography and style from other Cordilleran extensional structures and structurally from the down-warped lava plain of the eastern Snake River Plain. To investigate mechanisms driving extension and topographic evolution, we sampled granitoid bedrock from Cretaceous and Eocene-aged plutons from the mountainous flanks of the WSRP to detail their exhumation history with apatite (U-Th)/He (AHe) thermochronometry. AHe cooling dates from seventeen samples range from 7.9 ± 1.4 Ma to 55 ± 10 Ma. Most cooling dates from Cretaceous plutons adjacent to the WSRP are Eocene, while Eocene intrusions from within the Middle Fork Boise River canyon ~35 km NE of the WSRP yield Miocene cooling dates. The AHe dates provide evidence of exhumation of the Idaho batholith during the Eocene, supporting a high relief landscape at that time, followed by decreasing relief. The Miocene AHe dates show rapid cooling along the Middle Fork Boise River that we take to indicate focused river incision due to base level fall in the WSRP. Eocene AHe dates limit magnitudes of exhumation and extension on the flanks of the WSRP during Miocene rift formation. This suggests extension was accommodated by magmatic intrusions and intrabasin faults rather than basin-bounding faults. We favor a model where WSRP extension was related to Columbia River Flood Basalt eruption and enhanced by later eruption of the Bruneau-Jarbidge and Twin Falls volcanic fields, explaining the apparent difference with other Cordilleran extensional structures.

Thermochronological transect across the Basin and Range/Rio Grande rift transition: Contrasting cooling histories in contiguous extensional provinces

AbstractThe Basin and Range and Rio Grande rift (RGR) are regions of crustal extension in southwestern North America that developed after Laramide-age shortening, but it has not been clear whether onset and duration of extension in these contiguous extensional provinces were the same. We conducted a study of exhumation of fault blocks along a transect from the southeastern Basin and Range to across the RGR in southern New Mexico. A suite of 128 apatite and 63 zircon (U-Th)/He dates (AHe and ZHe), as well as 27 apatite fission-track (AFT) dates, was collected to investigate the cooling and exhumation histories of this region. Collectively, AHe dates range from 3 to 46 Ma, ZHe dates range from 2 to 288 Ma, and AFT dates range from 10 to 34 Ma with average track lengths of 10.8–14.1 µm. First-order spatiotemporal trends in the combined data set suggest that Basin and Range extension was either contemporaneous with Eocene–Oligocene Mogollon-Datil volcanism or occurred before volcanism ended ca. 28 Ma, as shown by trends in ZHe data that suggest reheating to above 240 °C at that time. AHe and ZHe dates from the southern RGR represent a wider range in dates that suggest the main phase of cooling occurred after 25 Ma, and these blocks were not reheated after exhumation. Time-temperature models created by combining AHe, AFT, and ZHe data in the modeling software HeFTy were used to interpret patterns in cooling rate across the study area and further constrain magmatic and/or volcanic versus faulting related cooling. The Chiricahua Mountains and Burro Mountains have an onset of rapid extension, defined as cooling rates in excess of &amp;gt;15 °C/m.y., at ca. 29–17 Ma. In the Cookes Range, a period of rapid extension occurred at ca. 19–7 Ma. In the San Andres Mountains, Franklin Mountains, Caballo Mountains, and Fra Cristobal range, rapid extension occurred from ca. 23 to 9 Ma. Measured average track lengths are longer in Rio Grande rift samples, and ZHe dates of &amp;gt;40 Ma are mostly present east of the Cookes Range, suggesting different levels of exhumation for the zircon partial retention zone and the AFT partial annealing zone. The main phase of fault-block uplift in the southern RGR occurred ca. 25–7 Ma, similar to what has been documented in the northern and central sections of the rift. Although rapid cooling occurred throughout southern New Mexico, thermochronological data from this study with magmatic and volcanic ages suggest rapid cooling was coeval with magmatism in the Basin and Range, whereas in the Rio Grande rift cooling occurred during an amagmatic gap. These observations support a model where an early phase of extension was facilitated by widespread ignimbrite magmatism in the southeastern Basin and Range, whereas in the southern Rio Grande rift, extension started later and continues today and may have occurred between local episodes of basaltic magmatism. These differences in cooling history make the Rio Grande rift tectonically distinct from the Basin and Range. We infer based on geologic and thermochronological evidence that the onset of extension in the southern Rio Grande rift occurred at ca. 27–25 Ma, significantly later than earlier estimates of ca. 35 Ma.

Apatite (U-Th)/He thermochronometric constraints on the northern extent of the Deccan large igneous province

The volcanic emplacement and subsequent weathering of the Deccan Traps of India is believed to have had a significant influence in driving global climatic shifts from the Late Cretaceous and through the Cenozoic. The magnitude of the Deccan Traps' impact on Earth's surface environment is largely dependent on the speculated original footprint of the large igneous province. To test established estimates for the pre-erosive northern extent of the Deccan Traps, we applied low-temperature apatite (U-Th)/He thermochronology (AHe) on rocks from the Bundelkhand craton and overlying Proterozoic Vindhyan successions of central India ∼150–200 km northeast of the northernmost preservation of Deccan basalts. New AHe data reveal young ∼5–85 Ma AHe dates with low effective uranium concentrations (eU) between 5–22 ppm, with a steep positive date-eU correlation that plateaus at ∼350 Ma in grains with eU values >50 ppm. Inverse thermal history modeling—utilizing AHe diffusion parameters of the Radiation Damage Accumulation and Annealing Model (RDAAM)—indicate that observed AHe date-eU correlations are most consistent with thermal histories that require the craton and Vindhyan strata to be at or near surface temperatures by ∼66 Ma, followed by a discrete reheating event associated with Deccan volcanism. These results establish new minimal areal constraints for the northern extent of Deccan volcanism which thermally perturbed much of the Vindhyan succession. Thermal alteration of organic rich Vindhyan sediment may have provided an additional source of volatile emissions that facilitated late Maastrichtian warming at the onset of Deccan volcanism. New minimal northern constraints on Deccan volcanism additionally confirm that large volumes of Deccan basalts have been stripped away since the time of their emplacement, which poses considerable implications for unraveling their role in Cenozoic cooling.

Cenozoic Exhumation History of the Eastern Margin of the Northern Canadian Cordillera

AbstractNew low‐temperature thermochronology data from clastic sedimentary rocks in the northern Richardson Mountains, Canada, indicate significant exhumational cooling during late Eocene–early Oligocene time. Apatite (U‐Th‐Sm)/He (AHe) data were collected from 19 Proterozoic–Paleocene rocks across a 115 km transect. Eighty‐eight single‐grain AHe dates range from 16–300 Ma and are generally younger than stratigraphic ages, indicative of thermal resetting by burial. Additionally, zircon (U‐Th)/He (ZHe) dates from two Proterozoic–Cambrian rocks range from 49–123 Ma and suggest burial to &gt;160°C. In contrast, ZHe dates from Jurassic sandstones are older than the stratigraphic age, which limits maximum burial to &lt;160°C. Thermal history modeling reveals three phases of cooling, during the Paleocene–early Eocene (&gt;65–50 Ma), late Eocene–early Oligocene (40–30 Ma), and late Oligocene–early Miocene (30–15 Ma). Most samples cooled during the first and second phases, whereas the third phase is less well constrained. In general, most rocks were below the sensitivity of AHe analysis since the early–middle Miocene. The results suggest a previously unrecognized phase of inferred deformation in the northern Richardson Mountains between 40–30 Ma. Our findings contribute to previous work that recognizes Late Cenozoic deformation along the eastern margin of the Northern Cordillera. We further investigated the potential mechanisms of this widespread deformation and suggest exhumation may relate to kinematic changes of the North American plate relative to structural trends along the margin of the Northern Cordillera.

Apatite (U-Th-Sm)/He date dispersion: First insights from machine learning algorithms

Numerous parameters impact apatite (U-Th-Sm)/He (AHe) thermochronological dates, such as radiation damage, chemical content, crystal size and geometry, and their knowledge is essential for better geological interpretations. The present study investigates a new method based on advanced data mining techniques, to unravel the parameters that could play a role in He retention and thus on AHe date. The purpose is to decipher which factors influence the AHe date dispersion, and to exclude the impact of other parameters on helium retention. As an example, we use a dataset previously collected on apatite from basements rocks, sampled in French Brittany, where all samples underwent the same thermal history, and for which were reported a set of physical and chemical parameters. The dataset includes dimension and geometry, He, U, Th, Sm and major and trace element content for ∼35 crystals. The algorithm ranks the parameters according to their influence on helium retention, using predictive trees, which are commonly used in computing sciences. After looking at 100 simultaneous predictions, we compared the predicted and measured He content for each analyzed apatite crystal. For this particular case, the predictions confirmed the prominent role of the parent nuclides in the He production, as AHe dates can be predicted accurately with these parameters (especially U and Th). Additionally, the predictions without knowledge of the apatite chemical composition and dimension provided better results than using all available parameters (median error of 14% instead of 18%). Therefore, for this specific study, the apatite chemistry and crystal dimensions do not influence significantly He retention nor AHe date dispersion. Nevertheless, detailed inspection of analysis results suggests which parameters have the most discrimination ability, which in this study include crystal length, height, and Mn content. The latter may reveal an eventual influence on alpha damage annealing kinetics. Finally, this approach shows that some grains could never achieve good predictions, indicating that for these crystals the input parameters are not enough to predict the He content. We propose that such crystals are statistically different from the remaining dataset, and this suggests that machine learning has a strong potential to correct errors, or to detect anomalies.

Exhumation history of the north-central Shanxi Rift, North China, revealed by low-temperature thermochronology

We present new apatite U-Th-Sm/He (AHe; n = 51), apatite fission track data (AFT; n = 12), and zircon U-Th/He (ZHe; n = 8) data for two elevation transects in the north-central Shanxi Rift, North China. Low-temperature thermochronologic data combined with forward and inverse time-temperature history models reveal a Precambrian to Quaternary thermal history characterized by: (1) cooling to <∼50°C during the Proterozoic, consistent with the development of a regional unconformity above Neoarchean–Paleoproterozoic cratonic basement rocks; (2) reheating to <∼180°C due to sediment burial during the Paleozoic to Mesozoic; (3) cooling at a rate >3.5°C/Ma during the Late Jurassic to earliest Cretaceous Yanshanian orogeny; (4) a possible ca. 120-90 Ma reheating event due to elevated geothermal gradients and/or local sediment burial; (5) Late Cretaceous (ca. 110-65 Ma) cooling contemporaneous with regional extension in eastern Asia and denudation of the paleo-Taihangshan highlands; and finally, (6) post ca. 10 Ma cooling associated with extension in the Shanxi Rift. AFT dates from the deepest exhumed structural positions of the sampled footwall blocks are mostly >65 Ma and AHe dates tend to be highly dispersed within samples. AFT inverse and AHe forward model results indicate that samples were at temperatures of <∼75°C by ca. 70 Ma. Despite the early Cenozoic and older AFT and AHe dates, metamict zircon grains with high effective uranium (eU >∼750 ppm) yield young ZHe dates of ca. 13-9 Ma, consistent with Late Miocene exhumation. We argue for the onset of latest cooling by ca. 10 Ma based on these ZHe dates; however, the precise timing for the onset of rifting remains uncertain. The results further suggest that Late Miocene–Quaternary extension in the north-central Shanxi Rift is responsible for ≤∼2.5 km of exhumation, such that published Quaternary extension and fault throw rates are significantly (>100%) higher than long-term rates inferred from the thermochronologic data.

Mesozoic denudation history of the lower Orange River and eastward migration of erosion across the southern African Plateau

Abstract Topographic uplift of the southern African Plateau is commonly attributed to mantle causes, but the links between mantle processes, uplift, and erosion patterns are not necessarily straightforward. We acquired apatite (U-Th)/He (AHe) dates from eight kimberlite and basement samples from the lower reaches of the large westward-draining Orange River system with the goal of evaluating the roles of lithospheric modification and river incision on the erosion history here. Average AHe dates range from 79 to 118 Ma and thermal history models suggest that most samples are consistent with a main erosion phase at ca. 120–100 Ma, with some variability across the region indicating a complex erosion history. Major erosion overlaps with the timing of strong lithospheric thermochemical modification as recorded in xenoliths from the studied kimberlites, but the denudation pattern does not mimic the northward progression of lithospheric alteration across the study region. We attribute this area’s denudation history to a combination of mantle effects, rifting, establishment of the Orange River outlet at its current location, and later faulting. When considering these results with other kimberlite-derived surface histories from an ∼1000-km-long E-W transect across the plateau, an eastward-younging trend in denudation is evident. The interplay of mantle processes and the shape of the large, west-draining Orange River basin likely control this first order-pattern.

Thermal history of the Mackenzie Plain, Northwest Territories, Canada: Insights from low-temperature thermochronology of the Devonian Imperial Formation

Abstract Devonian strata from the Mackenzie Plain, Northern Canadian Cordillera, have undergone two major burial and unroofing events since deposition, providing an excellent natural laboratory to assess the effects of protracted cooling history on low-temperature thermochronometers in sedimentary basins. Apatite and zircon (U-Th)/He (AHe, ZHe) and apatite fission track (AFT) thermochronology data were collected from seven samples across the Mackenzie Plain. AFT single grain ages from six samples span the Cambrian to Miocene with few Neoproterozoic dates. Although there are no correlations between Dpar and AFT date or track length distribution, a relationship exists between grain chemistry and age. We calculate the parameter rmr0 from apatite chemistry and distinguish up to three discrete kinetic populations per sample, with consistent Cambrian–Carboniferous, Triassic–Jurassic, Cretaceous, and Cenozoic pooled ages. Fifteen ZHe dates range from 415 ± 33 Ma to 40 ± 3 Ma, and AHe dates from 53 analyses vary from 225 ± 14 Ma to 3 ± 0.2 Ma. Whereas several samples exhibit correlations between date and radiation damage (eU), all samples demonstrate varying degrees of intra-sample date dispersion. We use chemistry-dependent fission track annealing kinetics to explain dispersion in both our AFT and AHe data sets and detail the thermal history of strata that have experienced a protracted cooling history through the uppermost crust. Thermal history modeling of AFT and AHe samples reveals that the Devonian strata across the Mackenzie Plain reached maximum burial temperatures (∼90 °C–190 °C) prior to Paleozoic to Mesozoic unroofing. Strata were reheated to lower temperatures in the Cretaceous to Paleogene (∼65 °C–110 °C), and have a protracted Cenozoic cooling history, with Paleogene and Neogene cooling pulses. This thermal information is compared with borehole organic thermal maturity profiles to assess the regional burial history. Ultimately, these data reflect the complications, and possibilities, of low-temperature thermochronology in sedimentary rocks where detrital variance results in a broad range of diffusion and annealing kinetics.

A Tortonian onset for the Algerian margin inversion: Evidence from low‐temperature thermochronology

AbstractIn North Africa, the Algerian margin is made of basement blocks that drifted away from the European margin, namely the Kabylia, and docked to the African continental crust in the Early Miocene. This young margin is now inverted, as dated Miocene (17 Ma) granites outcrop alongshore, evidencing kilometre‐scale exhumation since their emplacement. Age of inversion is actually unknown, although Pliocene is often considered in the offshore domain. To decipher the exhumation history of the margin between 17 and 5 Ma, we performed a coupled apatite fission track (AFT) and (U–Th–Sm)/He (AHe) study in the Cap Bougaroun Miocene granite. AFT dates range between 7 ± 1 and 10 ± 1 Ma, and mean AHe dates between 8 ± 2 and 10 ± 1 Ma. These data evidence rapid and multi‐kilometre exhumation during Tortonian times. This event cannot be related to slab break‐off but instead to the onset of margin inversion that has since developed as an in‐sequence north‐verging deforming prism.

Low-temperature thermochronology of Anticosti Island: A case study on the application of conodont (U-Th)/He thermochronology to carbonate basin analysis

Abstract The Ordovician to Silurian carbonate succession preserved on Anticosti Island is a near continuous record of the changing depositional and tectonic history of the Early Paleozoic Laurentian margin. These strata are a prime natural laboratory to test the applicability of conodont apatite (U-Th)/He thermochronology (CAHe) for carbonate basin analysis in a setting with a well-understood burial history and abundant paleotemperature constraints. We report 63 CAHe dates from seven samples collected from two boreholes. X-ray micro-computed tomography (XR-μCT) was used to assess individual conodont element size, volume and alpha ejection correction. CAHe dates are negatively correlated with effective uranium concentration (eU), with single conodont dates enigmatically older than the age of deposition to conodont dates as young as 2.7 ± 0.2 Ma. We rationalize this dispersion as a product of several factors, including diagenetic processes and pervasive conodont crystalline porosity and permeability. Additionally, one basement and two sandstone samples were collected for apatite and zircon (U-Th)/He (AHe, ZHe) and apatite fission track (AFT) thermochronology. In the Precambrian basement, ZHe dates range from 581.7 ± 46.5 Ma to 761.2 ± 60.9 Ma (n: 4) and AHe dates range from 26.7 ± 1.6 to 48.4 ± 2.9 Ma (n: 4). AHe dates from two Lower Ordovician sandstones (n: 8) are positively correlated with eU, increasing from 13.7 ± 0.8 Ma to 105.2 ± 6.3 Ma over a range of 186 ppm eU. Thermal history modeling of these data suggests two plausible thermal histories, both of which are novel as they imply that far-field effects related to the opening of the Atlantic Ocean have influenced the thermal history of Anticosti Island. Ultimately, these data highlight the complexities associated with, and necessity of, low-temperature thermochronology in carbonate basins.

Distinguishing slow cooling versus multiphase cooling and heating in zircon and apatite (U-Th)/He datasets: The case of the McClure Mountain syenite standard

New zircon, titanite, and apatite (U-Th)/He (ZHe, THe, AHe) data for four samples of the 524 Ma McClure Mountain syenite (MMS) in the Wet Mountains of Colorado constrain the <200 °C thermal history of this 40Ar/39Ar geochronology standard. ZHe dates vary from 420 to 567 Ma, do not correlate with their limited eU range (66–388 ppm), have dispersion attributable to eU zonation, and show no systematic variation between samples. THe dates are 488 to 515 Ma. AHe dates range from 70 to 211 Ma, with some inter-sample variability that could be due to modest post-70 Ma displacements along faults. Our dataset is consistent with recently published (U-Th)/He data for the MMS, although the published ZHe results extend to higher eU and define a negative date-eU correlation not captured by our lower eU zircons. Existing U-Pb and 40Ar/39Ar thermochronologic data, together with the THe and ZHe results, document rapid post-emplacement cooling of the MMS to ≤200 °C by ~500 Ma. A recent (U-Th)/He study inferred that the MMS subsequently underwent simple slow monotonic cooling until present-day. However, geologic relationships are incompatible with such a history, instead requiring that rocks in the vicinity of the MMS were at the surface at ~480 Ma, ~180 Ma, and ~60 Ma, with possible phases of burial and erosion of varying magnitude between these times. Inverse thermal history modeling can simultaneously satisfy the new and previously published ZHe and AHe data while honoring these geologic constraints. The outcomes of this work highlight that date-eU patterns can be generated by protracted cooling as well as by multiphase heating and cooling histories, and underscore the importance of integrating geologic data to help identify the most probable explanation for a (U-Th)/He dataset.