Fission Track Length Research Articles

Landform evolution of the Qilian Shan since 120 Ma revealed by apatite fission track data

The Cenozoic uplift of the Qilian Shan mountain range is intimately connected with the collision of the Indian and Eurasian plates, although the mechanism of deformation is still unclear due to the large distance between the Qilian Shan and the plate collision boundary. The first requirement if we are to determine this mechanism is to obtain the uplift process of the Qilian Shan range, which remains a matter of debate. We compiled apatite fission track data from previous studies of the Qilian Shan range to investigate the spatial and temporal disparities or similarities of the exhumation process. Most of the age-evolution profiles and thermo-modelling results show a low exhumation rate from 80 to 20 Ma, corresponding to shorter apatite fission track lengths, indicating a lower rate of erosion and lower relief across the whole Qilian Shan region. The results also reveal two stages of rapid exhumation: during the Cretaceous (120–80 Ma) and since the Miocene (20–0 Ma). The exhumation history of the Qilian Shan range shows no significant spatial difference and outward growth was limited at the southern and northern edges after 5 Ma. This temporal and spatial pattern for the exhumation of the Qilian Shan range suggests that there was probably no obvious uplift at the time of the initial collision of the Indian–Tibetan plates and support the proposal that the whole Qilian range has uplifted synchronously since 20 Ma.

Apatite Fission Track Thermochronology of Granite from the Xiazhuang Uranium Ore Field, South China: Implications for Exhumation History and Ore Preservation

AbstractXiazhuang uranium ore field, located in the southern part of the Nanling Metallogenic Belt, is considered one of the largest granite‐related U regions in South China. In this paper, we contribute new apatite fission track data and thermal history modeling to constrain the exhumation history and evaluate preservation potential of the Xiazhuang Uranium ore field. Nine Triassic outcrop granite samples collected from different locations of Xiazhuang Uranium ore field yield AFT ages ranging from 43 to 24 Ma with similar mean confined fission track lengths ranging from 11.8 ± 2.0 to 12.9 ± 1.9 μm and Dpar values between 1.01 and 1.51 μm. The robustness time‐temperature reconstructions of samples from the hanging wall of Huangpi fault show that the Xiazhuang Uranium ore field experienced a time of monotonous and slow cooling starting from middle Paleocene to middle Miocene (∼60–10 Ma), followed by relatively rapid exhumation in the late Miocene (∼10–5 Ma) and nearly thermal stability in the Pliocene–Quaternary (∼5–0 Ma). The amount of exhumation after U mineralization since the Middle Paleogene was estimated as ∼4.3 ± 1.8 km according to the integrated thermal history model. Previous studies indicate that the ore‐forming ages of U deposits in the Xiazhuang ore field are mainly before Middle Paleocene and the mineralization depths are more than 4.4 ± 1.2 km. Therefore, the exhumation history since middle Paleocene plays important roles in the preservation of the Xiazhuang Uranium ore field.

Fission-track age calibration: Phi and Zeta, never the twain shall meet?

The fission-track method was intended to be an autonomous dating method requiring two track counts and a neutron fluence measurement. It became clear that it was beset with methodological problems, and neutron fluence measurements were abandoned in favour of age standards. The coming of mass-spectrometric uranium measurements eliminated the need for irradiations altogether, and stimulated renewed efforts to establish fission-track dating as an autonomous method. This ran into similar problems as before, and recent publications signal a return to age standards. The attempts at independent dating nevertheless revealed that experts agree on the recommended fission constant and the need to consider experimental factors related to the track counts. In spite of that, the renewed reliance on standards makes that fission-track ages are again not independent, but relative, ages with an uncertain meaning, and from their nature untestable.We report an attempt at standardless and standard-based dating of the Durango and Duluth apatites using neutron irradiation and the external detector method. We start from the assumption that neutron fluence measurements are the most precise and accurate achievable in fission-track dating, a consensus on the recommended fission constant, and on the need to consider experimental factors. One factor (ζ0) accounts for the combined experimental influences; ζ0 can be calculated from identifiable contributions related to track registration, etching and counting; ζ0 can also be determined by experiment and calculated from the ages of standards. This gives three separate, consistent, dimensionless calibration factors that, in contrast to Z and ζ, are independent of the neutron fluence or the uranium content of a reference glass; ζ0 remains a personal factor, however.The standardless fission-track ages of the Durango age standard are in agreement with its reference age and the standardless and standard-based ages of the Duluth apatites are consistent with each other. Furthermore, if ζ0 is used to correct conventional Z- (and by implication ζ-) values for experimental effects, the results are consistent with their definitions in terms of physical parameters. Although ζ0 is a personal factor, the small number of measurements to date is consistent with each other. In contrast, annealing corrections based on the fossil and induced confined fission track lengths overestimate the reference age of Durango and produce incompatible standardless and standard-based ages of the dated apatites. We argue that this points to unresolved issues with the relationship between the age and the fossil track length, and suggest a connection with etching.Plots of the single-grain ages against uranium content and radial plots of the data exhibit conspicuous structure. However statistical factors alone account for the nature and magnitude of the observed correlations. This implies that in the case of the Duluth apatites there is no identifiable effect of radiation damage on the single-grain ages despite their high accumulated α-doses and protracted thermal histories. Spurious correlations must be addressed before attempting to interpret the observed data structure in terms of radiation damage, apatite composition or age components.

How many vs which: On confined track selection criteria for apatite fission track analysis

Inter-laboratory and inter-analyst experiments on confined fission-track length measurements in apatite have reported considerable variation, which can be attributed primarily to differences in analyst decision-making. Here we explore how different etching sequences, track selection approaches, and analytical methods influence track length and density measurements. The first experiment focuses on analyst judgement across different track selection criteria and light sources, and variation in effective etching time, using moderately annealed induced tracks in Durango apatite. Tracks are selected either with the usual approach of only choosing well-etched tracks, which we term the sufficiently-etched criterion, or a more relaxed criterion of simply being measurable without judging how etched they are. For the sufficiently-etched criterion, we separately analyze tracks found using exclusively reflected or transmitted light. We then follow the evolution of all selected tracks through further etching steps, and compare results based on length, angle, and dip, and placed in the context of a variable along-track etching rate model. All step etches show evidence of along-track etching rate falling from core to tip, and tracks measured using the sufficiently-etched criterion are slightly under-etched compared to the full latent track zone with enhanced etching rate. Minimizing effective etch time variation, by only using tracks selected after 10 s of etching, produced tracks averaging ~0.3 μm longer after 20 s than tracks selected using the sufficiently-etched criterion. Ensuring a minimum track etching time by first selecting measurable tracks after 20 s and etching for 10 s more produced tracks ~0.4 μm longer, but correctable to the latent length. Utilizing exclusively transmitted light diminishes mean track dip, and likely reflects lower efficiency in finding dipping tracks. In the second experiment, we repeatedly measure dipping confined tracks at various stages of annealing with and without using a cross-sectional view of the track, finding that the latter avoids an increase in scatter with increasing dip. We propose a two-step etching procedure that may provide more plentiful and reproducible data by reducing the impact of human decision-making, while still being relatable to published annealing data sets.

Denudation Process of Crystalline Nappes in a Continental Collision Zone Constrained by Inversion of Fission‐Track Data and Thermokinematic Forward Modeling: An Example From Eastern Nepalese Himalaya

AbstractThermochronological methods were applied to the Higher Himalayan Crystalline (HHC) nappe and the underlying Lesser Himalayan Sequences (LHS) to elucidate the denudation process for the middle‐ and upper‐crust of eastern Nepal over millions of years. Thermochronological inverse modeling was undertaken for new results of fission‐track (FT) age and FT length data of zircon and apatite in order to reconstruct the time‐temperature (t‐T) paths in the temperature range of 60–350°C. Eight t‐T paths calculated along the across‐strike section show that the cooling process of the HHC nappe in this study area is characterized by the following three aspects: (a) gradual cooling followed by rapid cooling and subsequent gradual cooling, (b) northward‐younging of the timing of the rapid cooling, and (c) gradual cooling followed by <2 Myr rapid cooling in the frontmost part of the HHC nappe. The observed FT ages and t‐T paths were then compared with those predicted by forwarding thermokinematic modeling. The results of the thermokinematic modeling for the “Flat‐Ramp‐Flat MHT model”, in which the HHC and the underlying LHS are denudated in direct proportion to the uplift of rocks transported along the Main Himalayan Thrust (MHT), reproduced the observed t‐T paths and FT ages in eastern Nepal. This indicates that the observed FT ages and t‐T paths reflect a denudation process driven by the movement of the MHT with a flat‐ramp‐flat geometry and that the denudation rate and its spatial distribution have roughly been constant in eastern Nepal since ca. 9 Ma.

Uplift-exhumation and preservation of the Yumugou Mo-W deposit, East Qinling, China: Insights from multiple apatite low-temperature thermochronology

As a result of the continued depletion of shallow ore resources, there has been renewed focus on the tectonic preservation of mineralized systems. Specifically, it is vital to constrain the post-mineralization exhumation and preservation history of existing economic deposits, in order to identify new regions suitable for mineral exploration. In this study, we present results from apatite U-Pb, fission-track and (U-Th-Sm)/He thermochronological and associated thermal history modeling studies on the Yumugou Mo-W deposit within the world-class East Qinling Mo polymetallic metallogenic belt, central China. Our major aim is to constrain the exhumation history and to evaluate preservation potential of the Yumugou Mo-W deposit, and provide related guidance for mineral exploration in the East Qinling Orogen. Apatite U-Pb dating yield Early Cretaceous lower intercept Terra-Wasserburg ages in the range of 153–141 Ma, interpreted as crystallization age of apatite, coeval with the formation of Yumugou deposit (ca. 145 Ma). Integrated apatite fission-track and apatite (U-Th-Sm)/He dating give ages varying from Late Cretaceous to Early Paleogene, representing the post-mineralization cooling ages. We also integrate these age data with confined fission-track length measurements and geological constraints from previous studies into the thermal history models, the resulting models suggest three cooling pulses: 1) a phase of Early Cretaceous (125–100 Ma) rapid cooling, followed by 2) a phase of Late Cretaceous (100–73 Ma) slow cooling, before finally, 3) a phase of Late Cretaceous to Paleogene (73–50 Ma) rapid cooling. The Early Cretaceous rapid cooling pulse correlates temporally with extension tectonic setting in the East Qinling Orogen, the magma emplacement and post-magmatic rapid cooling at ca. 160–130 Ma in the Luanchuan region might cause related exhumation at ca. 125–100 Ma to upper crustal temperatures. The Late Cretaceous slow cooling possibly corresponds to regionally slow extensive erosion in response to crustal shortening triggered by the westward subduction of the Paleo-Pacific Plate at this time. The Late Cretaceous-Paleogene rapid cooling likely result from the major far-field effects of the Pacific Plate subduction and the sinistral strike-slip motion of the Tan-Lu Fault Zone, as well as possible minor expansion effects derived by the Paleogene India-Eurasia collision. Moreover, we adopt thermal history modeling method to determine cooling rate, exhumation rate and depth of different cooling stages. In comparison with the calculated exhumation depth (∼5.6 km) and previously reported metallogenic depth (3–7 km) in the Luanchuan ore cluster, we infer that more than one kilometer metallogenic depth is theoretically preserved in the Yumugou Mo-W deposit. Combined the similar metallogenic setting and features with other giant Mo deposits in the East Qinling Orogen, we further suggest that the Mo deposits in this orogen possibly have underwent a similar degree of exhumation and show good prospecting potential.

Deconvolution of fission-track length distributions and its application to dating and separating pre- and post-depositional components

Abstract. To enable the separation of pre- and post-depositional components of the length distribution of (partially annealed) horizontal confined fission tracks, the length distribution is corrected by deconvolution. Probabilistic least-squares inversion corrects natural track length histograms for observational biases, considering the variance in data, modelization, and prior information. The corrected histogram is validated by its variance–covariance matrix. It is considered that horizontal track data can exist with or without measurements of angles to the c axis. In the latter case, 3D histograms are introduced as an alternative to histograms of c-axis-projected track lengths. Thermal history modelling of samples is not necessary for the calculation of track age distributions of corrected tracks. In an example, the age equations are applied to apatites with pre-depositional (inherited) tracks in order to extract the post-depositional track length histogram. Fission tracks generated before deposition in detrital apatite crystals are mixed with post-depositional tracks. This complicates the calculation of the post-sedimentary thermal history, as the grains have experienced different thermal histories prior to deposition. Thereafter, the grains share a common thermal history. Thus, the extracted post-depositional histogram without inherited tracks may be used for thermal history calculation.

Etching of fission tracks in monazite: Further evidence from optical and focused ion beam scanning electron microscopy

Abstract A series of experiments on monazites from Victoria, Australia, is presented to further understand their fission track etching properties. Using a 6 M HCl etchant at 90 °C, SEM images on crystal (100) pinacoid faces reveal well-etched rhombic spontaneous fission track openings. Average rhombic etch pit diameters Dpc and Dpb, parallel to the crystallographic c- and b-axes are 0.81 ± 0.20 µm and 0.73 ± 0.26 µm, respectively. An angular distribution experiment on (100) faces found that spontaneous fission tracks initially etch anisotropically, being preferentially revealed at an azimuth of 90° to the crystallographic c-axis up to ~60 min of etching. As etching continues, however, the distribution becomes progressively more uniform and is essentially isotropic by 90 min. Two experimental methods determined the rate at which the etchant penetrated along the lengths of implanted 252Cf fission tracks. This involved the application of a focused ion beam scanning electron microscope (FIB-SEM) to mill progressively into slightly etched monazite crystals followed by an etch-anneal-etch approach. Results indicate that at least the greater part of the etchable ranges of the latent fission tracks were penetrated by the 6 M HCl etchant within the first few minutes. Continued etching to 5 min indicates that track etching slows down toward the ends of the tracks, but the maximum ranges are estimated to be reached after 5–15 min, which represents the longest time the latent segments of the tracks are exposed to potential annealing at the etchant temperature. Taking into account that implanted 252Cf fission tracks in monazite anneal on average ~4% of their length at 90 °C after 1 h (Jones et al. 2019), suggests that a much shorter duration for exposure to this temperature causes less than ~1% of fission track length reduction during etching.

Post-ore change and preservation of the late Paleozoic Tuwu porphyry Cu deposit in Eastern Tianshan, NW China: Constraints from geology and apatite fission track thermochronology

The late Paleozoic Tuwu porphyry Cu deposit is located in the Dananhu–Tousuquan arc in Eastern Tianshan, northwest China. In this study, we present new apatite fission track data of the intermediate-felsic intrusions (i.e., diorite porphyry, tonalite porphyry, quartz porphyry, and quartz albite porphyry) from Tuwu, together with inverse thermal modeling and previous study, to reveal exhumation and cooling history. Apatite fission track length analyses indicate that the track annealing type belongs to a transition from the undisturbed basement distribution to the mixed distribution. As a whole, two phases of cooling were recognized from inverse thermal modeling including a protracted slow cooling period with an average cooling rate of 0.32 ℃/Ma during late Cretaceous to Miocene and moderate slow cooling with a cooling rate of 1.89 ℃/Ma from Miocene to the present day. The first phase of cooling can be mainly attributed to a tectonically quiet and arid to semiarid climate whereas the later phases of cooling related to structural activation occurred at ~25–15 Ma in response to a distant effect of the India-Eurasia convergence. In addition, the exhumation rates of Tuwu intrusions from the two stages are 10.7 m/Ma and 63 m/Ma, respectively. The calculated total exhumation thickness at Tuwu is 1.2–2.3 km since late Cretaceous. Based on the integration of stratigraphic sequence, tectonic setting, paleogeographic, geochronology, and low temperature thermochronology study, we suggest that the thick deposition of coal-bearing stratum during the early and middle Jurassic, the extended duration slow cooling and exhumation controlled by arid to semiarid climate, and the differential lifting movement of fault blocks are key factors for preservation of the early Carboniferous Tuwu Cu deposit.

Burial and exhumation of late paleozoic arc-related rocks in the Tulasu basin, Western Chinese Tianshan: Implication for the preservation of epithermal deposits in old orogenic belts

Epithermal deposits are susceptible to erosion owing to their shallow emplacement depth of 1 to 2 km, resulting in the observation that the majority of epithermal deposits worldwide are generally Cenozoic in age. Several large and super-large epithermal deposits likely associated with Late-Devonian to Carboniferous arc magmatism were recognized in the Tulasu volcanic-clastic sedimentary basin, western Chinese Tianshan. In order to further clarify the preservation processes of these old epithermal orebodies, apatite fission track (AFT) analysis has been undertaken in this study. Our AFT results gave a range of ages from 54.7 ± 6.3 Ma to 272.3 ± 8.7 Ma, and the mean confined fission track lengths are between 12.1 ± 0.2 μm and 13.5 ± 0.1 μm. Our new data, in combination with the 1:50,000 geological mapping, suggest that the ore-hosting Tulasu basin may have undergone multiple burial and exhumation episodes postdating the mineralization. The main burial scenario, probably occurring in the late Early Carboniferous (350–330 Ma) and Permian (300–250 Ma), may be related to subsidence on syn-volcanic faults concurrent with retreat of the Junggar Ocean and strong uplift of the neighboring mountain range induced by final closure of the Paleo-Asian Ocean, respectively. The calculated thickness of ca. 4.83 km of sedimentary rocks deposited during the burial events form a critical shield for the epithermal ore bodies that have been protected from substantial erosion by subsequent geological processes. Exhumation occurred in two episodes: Late Triassic-Cretaceous (ca. 240–100 Ma) and Oligocene (ca. 35 Ma). The Late Triassic-Cretaceous erosion is characterized by a slow exhumation rate (ca. 0.012 mm/y), corresponding to a total unroofing thickness of ca. 1.67 km. In contrast, the Oligocene event exhibits as a higher exhumation rate (up to ca. 0.057 mm/y), with removal of ca. 3.83 km of sedimentary cover. Therefore, the epithermal deposits have undergone a complex geological path from formation to final exposure. Initial burial and the following sluggish exhumation are two fundamental factors for the preservation of epithermal deposits in the old orogenic belts.

Mesozoic-Cenozoic cooling history of the Eastern Qinghai Nan Shan (NW China): Apatite low-temperature thermochronology constraints

Understanding the formation of the Qilian Shan in the NE Tibetan Plateau provides insights into the growth mechanisms of the northern portion of the plateau through time. The onset time of exhumation in the Qilian Shan is still debated. The Qinghai Nan Shan subrange, located in the southeastern Qilian Shan, is cut by the Yellow River that forms Longyang Gorge, providing a good vertical profile for thermochronological investigation of exhumation. In this paper, we reconstruct the cooling and exhumation history of the Eastern Qinghai Nan Shan using apatite fission-track(AFT) and apatite (UTh)/He(AHe) thermochronological dating on basement rocks. The fission-track data display a marked age-elevation trend from Early Cretaceous (114–122 Ma) ages at the bottom of the canyon to Early-Middle Jurassic ages (165–180 Ma) at the top. Mean (UTh)/He ages are mainly Early Cretaceous (97–128 Ma) with one Eocene outlier (42 Ma). Combined with fission track lengths, Dpar values and thermal modeling results, these data indicate that the study area resided above the closure temperature of AFT and AHe until Early Cretaceous. Rapid cooling occurred during the Early Cretaceous and early Cenozoic likely driven by the Lhasa and Indian blocks collision with the southern margin of the Asian continent. Nonetheless, since then exhumation was limited to less than a few kilometers in the study area. Combined with regional geological studies, the thermochronological analysis demonstrates that, since the Mesozoic, intense exhumation occurred simultaneously along the northern and southern margins of the Qilian Shan. Hence, we suggest that the geomorphic framework of the northeast margin of the Tibetan Plateau was established during the Early Cretaceous-early Cenozoic.

Apatite Fission Track Signatures of the Ross Sea Ice Flows During the Last Glacial Maximum

AbstractThe catchment for the Ross Sea ice includes both the East and the West Antarctic ice sheets, but the mass balance is a direct response to climate change. Our work is aimed to reconstruct the ice flows after the Last Glacial Maximum and is based on apatite fission track data from samples collected from 18 piston cores across the Ross Sea embayment. Fission track ages have been divided into meaningful populations and then compared with bedrock ages from West and East Antarctica. Furthermore, fission track lengths have been measured on each population and then compared through forward modeling with thermal histories derived from literature. The widespread presence of apatites with cooling ages of about 30–40 Ma reveals a main exhumation phase of the Transantarctic Mountains during the Oligocene associated to the last phases of the West Antarctic Rift System. Furthermore, the presence of key marker apatites (e.g., younger than 21 Ma or older than 230 Ma) allows to identify the Central High as a major ice flow divide.

The along-track etching structure of fission tracks in apatite: Observations and implications

Interpretation of apatite fission-track length data rests on the assumptions that etching fully reveals latent tracks, and that laboratory annealing of induced tracks constitutes a good proxy for geological annealing of spontaneous tracks. Recent work using improved step-etching procedures questions both of these assumptions. Step etching experiments interrogating the etching structure of tracks near and beyond their tips have shown that tracks are often not etched to their full extent of latent track damage in a standard single-step, 20-s etch, and that the etching structure in this outer region differs between fossil and induced tracks. In this study, we develop a more complete picture of track revelation for fossil and unannealed and annealed induced tracks in Durango apatite. Using first steps as early as 10 s after the beginning of etching, we discern differences in etching structure between various track types, and document a gradual fall-off in etching velocity from the track interior toward its tips. When effective etching time is constrained by selecting earlier-etching confined tracks for later analysis, mean lengths are longer, but length dispersion is not diminished. Although some proportion of unannealed tracks are under-etched after 20 s of etching, with increased levels of annealing the probability of an induced track being fully etched increases. We use an etch-anneal-etch procedure, consisting of partially etching tracks, annealing the apatite grains, and then continuing etching, to establish bulk etching rates at track tips. Bulk etching is effectively isotropic, as is etching velocity in the central portions of tracks, away from their tips. We use our data to compare two end-member possibilities for along-track etching structure, which provides a first step toward a deeper understanding of annealing and development of a more robust standard etching protocol.

Exhumation and landscape evolution in eastern South China since the Cretaceous: New insights from fission-track thermochronology

Widespread extensional half-/grabens with red bed sedimentation, dome structures, and magma intrusions/eruptions with polymetallic mineralization within South China during the Cretaceous-Cenozoic provide a natural laboratory to understand intracontinental deformation and plate tectonics. In order to decipher the tectonic and landscape evolution of eastern South China, this study presents new low-temperature thermochronology from a broad region covering major tectonic units including the Xuefeng, Jiuling, Nanling, and Wuyi mountains. Zircon and apatite fission track dates, length distribution, and thermal history modeling reveal a prominent and rapid cooling phase during ~125–80 Ma across the paleo-highland in eastern South China. A synthesis of existing thermochronologic data and geological observations suggests that NE/NNE-striking syn-sedimentary faults system with half-/graben basins, rapid exhumation of Cretaceous magma and pre-Cretaceous basement, and final extension of dome structures were all initiated in a back-arc setting during this period (~80-125 Ma). Eastern South China during the Paleogene was characterized by a tectonic quiescence during which insignificant exhumation, peneplanation, and local sedimentation occurred, forming regional low-amplitude, long-wavelength topography. Accelerated regional cooling commenced in the late Oligocene or early Miocene in response to the far-field effect of the India-Asia continental convergence. Additionally, the termination of red-colored gypsum-salt-bearing sediment in the Neogene suggests a paleo-environmental change within South China, from an arid to a monsoonal climate; such change was likely the result of the topographic uplift of the Tibetan Plateau and its margins.

Analyst and etching protocol effects on the reproducibility of apatite confined fission-track length measurement, and ambient-temperature annealing at decadal timescales

Abstract Previous inter-laboratory experiments on confined fission-track length measurements in apatite have consistently reported variation substantially in excess of statistical expectation. There are two primary causes for this variation: (1) differences in laboratory procedures and instrumentation, and (2) personal differences in perception and assessment between analysts. In this study, we narrow these elements down to two categories, etching procedure and analyst bias. We assembled a set of eight samples with induced tracks from four apatite varieties, initially irradiated between 2 and 43 years prior to etching. Two mounts were made containing aliquots of each sample to ensure identical etching conditions for all apatites on a mount. We employed two widely used etching protocols, 5.0 M HNO3 at 20 °C for 20 s and 5.5 M HNO3 at 21 °C for 20 s. Sets of track images were then captured by an automated system and exchanged between two analysts, so that measurements could be carried out on the same tracks and etch figures, in the same image data, allowing us to isolate and examine the effects of analyst bias. An additional 5 s of etching was then used to evaluate etching behavior at track tips. In total, 8391 confined fission-track length measurements were performed; along with 1480 etch figure length measurements. When the analysts evaluated each other's track selections within the same images for suitability for measurement, the average rejection rate was ~14%. For tracks judged as suitable by both analysts, measurements of 2D and 3D length, dip, and c-axis angle were in excellent agreement, with slightly less dispersion when using the 5.5 M etch. Lengths were shorter in the 5.0 M etched mount than the 5.5 M etched one, which we interpret to be caused by more prevalent under-etching in the former, at least for some apatite compositions. After an additional 5 s of etching, 5.0 M tracks saw greater lengthening and more reduction in dispersion than 5.5 M tracks, additional evidence that they were more likely to be under-etched after the initial etching step. Systematic differences between analysts were minimal, with the main exception being likelihood of observing tracks near perpendicular to the crystallographic c axis, which may reflect different use of transmitted vs. reflected light when scanning for tracks. Etch figure measurements were more consistent between analysts for the 5.5 M etch, though one apatite variety showed high dispersion for both. Within a given etching protocol, each sample reflected a decrease of mean track length with time since irradiation, giving evidence of 0.2–0.3 μm of annealing over year to decade timescales.

Post-Variscan tectonic and landscape evolution of the Elbe Fault Zone and the Lusatian Block based on apatite fission-track data and geomorphologic constraints

The Elbe Fault Zone is a continental scale NW striking structure that contains faults of different kinematics. Some faults were active at different times since at least the Variscan orogenesis in the Late Carboniferous. The Lusatian Fault (LF) was one of the major structures of the Elbe Fault Zone in the Late Cretaceous, when NW striking faults of Central Europe experienced basin inversion and thrusting during a short lived pulse of contraction. The LF displays a curved appearance on earth surface, what is not compatible with its activity at only one well defined event. Possibly, different faults where active along strike of the LF at different times.Apatite fission-track thermochronology can distinguish differential exhumation or burial of neighbouring fault blocks, if the vertical difference comprised at least 1–2 km. This study presents apatite fission-track data from the footwall and the hangingwall block of the LF and some other faults in its vicinity. Our new data show a clear difference between the hangingwall block of the LF (the Lusatian Block) with uniform 70–85 Ma apparent ages and relatively long track lengths and the footwall-block of the LF with 66–173 Ma apparent ages and shorter and more variable track lengths. For one section of the LF, we can show, that the Cretaceous fault is not visible geomorphologically, but was substituted by a neighbouring Cenozoic reverse fault with a distinct fault scarp. We used thermal history modelling to determine the timing and magnitude of exhumation and burial events from apparent apatite fission track ages and track length distributions. We infer, that the present morphological expression of the LF consist of smaller faults of different kinematics and age and present an evolution model for this section of the LF.This study establishes the existence of similar striking reverse faults in direct vicinity and partly intersecting each other in the Elbe Fault Zone. This observation emphasizes the significance of the Elbe Fault Zone as a continental scale zone of crustal weakness. It has not only been reactivated several times since the Variscan orogenesis, but represents a cluster of similar striking faults of different age and kinematics.

Constraints of new apatite fission-track ages on the tectonic pattern and geomorphic development of the northern margin of the Tibetan Plateau

The northern margin of the Qilian Shan is the northernmost edge of the Tibetan Plateau. The deformation timing and geomorphic development along the northern margin of the Qilian Shan are critical to understanding the dynamics of plateau growth. Although previous studies have suggested that thrusting along the middle and westernmost sections of the northern margin of the Qilian Shan began in the middle Miocene, the >600 km mountain range front may have experienced differential thrusting and exhumation history. We collected 11 apatite fission-track samples along an elevation transect to the south of Jiayuguan to unravel the cooling history and erosion variation of the northern margin of western section of the Qilian Shan. The apatite fission-track ages, fission-track lengths, and thermal modeling suggest a period of rapid exhumation initiated at ∼15–10 Ma along the northern margin of the Qilian Shan. The new fission-track ages can be divided into three domains with different mean ages. As the distribution of the fission-track ages is tightly confined by tectonic structures among them, we infer that tectonics plays the first-order control on erosion variations. Meanwhile, the strong linkage between erosion rates and glacier distribution may indicate that glacial buzzsaw exerts an important role in mountain building in the northern Qilian Shan. Additionally, combining this result with published data, we suggest that the northern margin of the Qilian Shan experienced a phase of synchronous deformation in the middle Miocene.

Thermochronological insights into the morphotectonic evolution of the Eastern Highlands, Zimbabwe: Implications for thermal history modelling of multi-thermochronometer data

The Eastern Highlands form a north-south trending high relief region along the eastern border of Zimbabwe, which is also spatially coincident with the eastern Zimbabwe cratonic margin. Uncertainty surrounds when this mountain range formed and how the region has evolved, particularly as there are no Phanerozoic cover rocks preserved. Here, we use low-temperature thermochronology (apatite fission track, apatite (U–Th-Sm)/He and zircon (U–Th)/He dating) and inverse thermal history modelling to reveal the morphotectonic evolution of this region since the mid-Paleozoic. The data suggest that structural reactivation occurred in the Late Devonian and Jurassic-Cretaceous that caused tectonic denudation and crustal cooling in the northern Eastern Highlands. The first period is thought to be associated with post-orogenic extension following Pan-African activity. The ensuing denudation continued into the Triassic and substantiates previous suggestions that these mountains were a source region of Lower Karoo sediments. The Jurassic-Cretaceous episodes are most likely associated with stress transmission from the progressive breakup of Gondwana. These events include the mid-to Late Cretaceous reactivation of the Mutare Shear Zone, which caused upward displacement (∼800 m) of the southern Eastern Highlands. The Eastern Highlands then entered a relatively quiescent phase with all samples residing close to the surface (<60 °C) since at least the beginning of the Cenozoic, which limits subsequent denudation of these mountains to less than ∼3 km. An investigation of the different multi-thermochronometer modelling combinations reinforces concerns with the current zircon radiation damage accumulation and annealing model (ZRDAAM). However, the findings also suggest that presenting the observed versus predicted apatite fission track length distribution should be an essential reporting requirement for thermal history modelling.

Thermal History since the Paleozoic in the Eastern Qaidam Basin, Northwest China

The Qaidam Basin is the one of the three major petroliferous basins in northeastern Tibetan Plateau, which has experienced multiphase superimposition and transformation. The study of thermal history not only plays an important role on revealing the tectonic origin of the Qaidam Basin and the forming mechanism and uplift history of the Tibetan Plateau, but also can provide scientific evidence for the assessment of oil and gas resources. This work used balanced cross‐section technique and apatite fission track ages with modeling of fission track length distribution to infer that the eastern Qaidam Basin has experienced significant tectonic movement in the Early Jurassic movement (∼200 Ma), which caused the carboniferous uplift and denudation, the geological movement in the Late Cretaceous, characterized by early stretching and late northeast‐southwest extrusion; the Himalayan movement in multi‐stage development in eastern Qaidam Basin, which can be divided into the early Himalayan movement (41.1–33.6 Ma) and the late Himalayan movement (9.6–7.1 Ma, 2.9–1.8 Ma), and large‐scale orogeny caused pre‐existing faults reactivated in late Himalayan movement. On the basis of burial history reconstruction, the thermal history of eastern Qaidam Basin was restored. The result shows that the thermal history in eastern Qaidam Basin shows slow cooling characteristics; the paleo‐geothermal gradient of eastern Qaidam Basin was 38–41.5°C/km, with an average value of 39.0°C/km in the Late Paleozoic, 29–35.2°C/km, with an average value of 33.0°C/km in the Early Paleogene; the geothermal gradient of the Qaidam Basin increased in the Late Paleogene, which was similar to the present geothermal gradient in the Late Neogene. The characteristics of the tectono‐thermal evolution since Paleozoic in the eastern Qaidam Basin are mainly controlled by magmatic thermal events in the study area.

Observations on three-dimensional measurement of confined fission track lengths in apatite using digital imagery

We report the results of a comparative study to explore the usefulness of 3D measurements of confined fission track lengths (TINTs) relative to confined track length measurements (dips ≤10°), and evaluate their suitability for thermal history modeling. Confined fission track lengths were measured in 10 annealed Fish Canyon Tuff apatites containing synthetic mixtures of different length components, and two Durango apatites containing spontaneous fission tracks. Measurements were primarily carried out using a digital image-based microscope system, and they were compared to those from a regular optical drawing tube-digitizing tablet setup, and a confocal laser scanning microscope. The results indicate that 3D measurements of confined track lengths are closely comparable to conventional track measurements, and the mean track lengths of inclined (dips > 10°) and (dips ≤10°) confined tracks from the one sample are equivalent within the measurement uncertainty. A strong dip-bias was observed, so that almost all the confined tracks measured were dipping at < 30°, and the great majority (∼70%) were dipping at ≤10°, thereby qualifying as horizontal confined tracks. Our results suggest that a useful increase of more than 40% in sample size can be achieved from including dip-and refraction-corrected 3D track length measurements. Some evidence was seen for a small bias in favor of shorter tracks at higher dip angles but this has very little influence on the mean lengths or length distributions up to the practical limit of dips (∼30°) observed in these measurements. Results obtained using the same measurement system by a single analyst over time and between six different observers in the one laboratory show good reproducibility. These results also agree well with conventional confined track length measurements in the same samples in the second laboratory involved. We conclude that 3D measurements of confined track lengths, including both and inclined tracks, are suitable for use in current fission track annealing models derived from experiments using confined tracks.