Abstract Fission-track and (U–Th–Sm)/He thermochronology on apatites are radiometric dating methods that refer to thermal histories of rocks within the temperature range of 40°–125 °C. Their introduction into geological research contributed to the development of new concepts to interpreting time-temperature constraints and substantially improved the understanding of cooling processes within the uppermost crust. Present geological applications of apatite thermochronological methods include absolute dating of rocks and tectonic processes, investigation of denudation histories and long-term landscape evolution of various geological settings, and basin analysis.

Abstract We report on new image-analysis techniques that, for the first time, provide a practical solution to the problem of fully automated counting of fission tracks in natural minerals, a long-desired goal in fission-track dating. Specific challenges to be overcome have been the discrimination of fission tracks from non-track defects, polishing scratches, etc.; resolving multiple track overlaps; and reliable identification of small tracks amongst a similarly sized background of surface defects, fluid inclusions, etc. Most previous attempts at automated image analysis have failed in one or more of these tasks. The central component of our system is called ‘coincidence mapping’ and utilizes two images of the same tracks obtained in transmitted and reflected light. The complementary nature of the information in these two images allows a powerful discrimination of true fission tracks from most non-track features. The much smaller average track size in the reflected light image allows the resolution of most track overlaps apparent in transmitted light. The discrimination is achieved by segmenting the two images using a custom-developed thresholding routine and extracting the coincidence of features in the two binary images. The analysis is computationally efficient and takes only a few seconds to complete the processing of images that may contain up to many hundreds of tracks. Preliminary indications are that error rates are about the same as, or better than, those achieved by a human operator using normal counting conditions in transmitted light. The performance is even better at high track densities (>10 7 cm −2 ) giving the potential for measuring track densities up to an order of magnitude greater than a human operator can count. Automated counting should significantly increase the speed and consistency of analysis and improve data quality in fission-track dating through better counting statistics, increased objectivity and measurement of additional track description parameters that are not currently determined.

Abstract LA–ICP-MS (laser ablation–inductively coupled plasma-mass spectrometry) has the potential to measure uranium concentration for fission-track (FT) chronometry as an alternative to thermal neutron-induced fission of 235 U. This study examines the effect that chemical etching, required to reveal spontaneous fission tracks of 238 U, has upon LA–ICP-MS analyses. Uranium concentrations were measured before and after etching for six large gem-quality apatite crystals and six zircon samples – three large crystals and three FT age standards. Comparison of the results shows no significant difference in 238 U concentrations measured on the etched and unetched mineral surfaces. The 238 U concentrations determined by the LA–ICP-MS provide reasonable FT ages for the zircon age standards, which, with the previously reported LA–ICP-MS apatite FT results, promotes the use of the LA–ICP-MS for FT chronometry.

Abstract Despite the potential of zircon He thermochronometry for constraining rock thermal histories, it remains less commonly exploited than the apatite He chronometer. In part, this is due to the more challenging analytical techniques required to extract He, U and Th. Here we present a new method for the routine determination of zircon (U–Th)/He ages, and demonstrate how it can be used to constrain thermal histories and to quantify cooling in different tectonic settings. We present zircon (U–Th)/He ages that place a firm upper limit on the extent of denudation-induced cooling ( c . 3 km) on the SE Australian passive margin; a region where synrift apatite fission-track and apatite (U–Th)/He ages have previously prevented quantitative constraint. We have also used the zircon (U–Th)/He thermochronometer to quantify the cooling of early Tertiary mafic plutons from Skye, Scotland, where the rate and timing of cooling cannot be determined using other thermochronometers.

Abstract Detrital thermochronology is commonly used to locate source and improve the understanding of a region's long-term development. Provenance detrital age data typically comprise a number of distinct age modes representing contributions from a number of different sources. For methods such as apatite fission track (FT) this can present a challenge in that direct assignment of age modes to particular lithological or tectono-stratigraphic units is rarely possible, particularly when ancient orogenic sediments are examined. A new approach described here is based on measuring Nd isotopic data on single apatites by laser ablation ICPMS where the 143 Nd/ 144 Nd ratios are diagnostic of grain source. By combining Nd isotopic measurement with apatite FT analysis it is possible to link detrital apatite FT ages to specific rock unit sources. The methodology and wider benefits of this new approach are discussed using examples from the Himalaya and Andaman Islands.

Abstract Understanding time–temperature histories using apatite fission-track thermochronology involves sample preparation, analysis and then thermal modelling using an appropriate annealing algorithm. A subtle point in this sequence is ascertaining that the sample preparation utilized is compatible with the methodology used in obtaining the data for constructing the annealing data set. This issue is important if one wishes to utilize the relatively new multikinetic annealing algorithm of Ketcham et al. that is implemented in their AFTSolve and HeFTy models which is based on a different etching recipe than those previously used. A preliminary calibration step involves comparing published etch pit diameters for a suite of samples with those analysed by an operator. Results show that the operator can reliably reproduce the calibration data set. We then report a laboratory experiment using samples from Finland and Spain that compares the results obtained using two different etching methodologies (7% nitric acid with qualitative etching conditions and 5.5 M nitric acid at constant conditions). The two raw data sets yield variable results. Comparing the two etching methodologies reveals the influence of this procedure on the kinetic parameter D par .

Abstract The effects of convective heat transfer by hydrothermal fluid flow on fission-track (FT) thermochronology are studied using numerical modelling techniques. Parameter studies are carried out on two-dimensional crustal segments with a steeply dipping fault zone exposed to constant denudation to evaluate the relative importance of different variables, including denudation rate as well as hydraulic and material properties. Time–temperature histories of particle points are calculated in the vicinity and also a few kilometres away of the fault zone. These time–temperature paths are then used in a forward-modelling approach to determine the expected FT cooling ages and track-length distributions. Modelling results indicate that hydrothermal fluid flow can significantly disturb the background conductive thermal state of the upper crust, and the interpretation of FT data using a steady-state geothermal gradient can result in erroneous denudation rates that overestimate the true erosion rates by more than 80%. A pattern of highly varied FT cooling ages from samples at the same elevation does not necessarily ask for differential tectonic movements, instead it can be generated by deep circulation of groundwater within a few million years (Ma). Denudation rates inferred from FT cooling age–elevation plots are likewise inaccurate in a hydrothermally active area because the important assumption about closure temperature isotherms being horizontal or at a constant depth below the surface is not met.

Abstract Thermochronology can provide information on palaeotopography and its evolution. We present a new method to reconstruct the shape of the palaeotopography from the palaeoisotherm derived from low-temperature thermochronological data. While deriving palaeoisotherm and reconstructing palaeotopography, the exhumation rate may also be constrained. The proposed method is independent of the relationship between the ancient and modern topography; however, it requires that the palaeotopography maintains its shape and the subsurface thermal field is invariant with time during the period when a set of samples passed through the closure temperature. It is shown that the inherent uncertainties in sample age and the limited sampling density will inevitably induce errors in the reconstructed topography, and these errors can be reduced by eliminating the noise in the derived isotherm. Increasing the number of samples helps to reduce the noise, and if, before sampling, we can make rough estimates about the geothermal gradient, the exhumation rate, the uncertainties in sample age, and the wave components and the relief of the palaeotopography to be reconstructed, the number of samples can be tentatively decided. It is also shown that the reconstructed topography is sensitive to exhumation rate and geothermal gradient, and a concrete sensitivity analysis is needed for a given dataset. By reinterpreting the (U–Th)/He data from the Sierra Nevada, California, we show that our method can reveal palaeorelief as well as other valuable information on the palaeotopography of the studied area.

Abstract Interpretation of low-temperature thermochronological data usually relies on assumptions on the shape of isotherms. Recently, a number of thermal modelling approaches investigate and predict the theoretical influence of topography on isotherms. The application and proof of these predictions is not well confirmed by measured data. Here we present apatite fission-track (AFT) data from samples collected along the Gotthard road tunnel and its corresponding surface line to test these predictions. AFT ages broadly cluster around 6 Ma along the tunnel. No correlation of tunnel ages with superimposed topography is seen, which means that topography-induced perturbation of isotherms under given boundary conditions (topographic wavelength 12 km; relief 1.5 km; exhumation rate 0.45 km Ma −1 ) can be neglected for the interpretation of AFT ages. Thus, in areas characterized by similar topographies and exhumation rates, apparent exhumation rates deduced from the age–elevation relationship (AER) of AFT data need no correction for topography-induced perturbation of isotherms. Three-dimensional (3D) numerical thermal modelling was carried out incorporating thermally relevant parameters and mechanisms, such as topography, geology, thermal conductivities and heat production. Modelling reveals a strong influence on the shape of isotherms caused by spatially variable thermal parameters, especially heat production rates. Therefore, not only topography has to be considered for interpreting low-temperature thermochronological data, but also other parameters like heat production rates. Supplementary material: 1. Electron microprobe analyses, 2. Topography and model extend, 3. Model parameters are all available online at http://www.geolsoc.org.uk/SUP18380 .

Abstract Detrital fission-track (FT) dating can be successfully used in provenance studies of siliciclastic sediments to define the characteristic cooling ages of the source regions during erosion and sedimentation. In order to obtain more specific information about potential source regions we have developed the pebble population dating (PPD) method in which pebbles of specific lithotype are merged and dated. Dating of both zircon and apatite crystals from such pebble populations yields age distributions, which reflect the cooling ages of the given lithotype in the source area at the time of sedimentation. By this technique it is possible to define ‘FT litho-terrains’ in the source regions and thus outline palaeogeological maps. Two examples are presented from the Eastern Alps. (i) Comparison of FT ages from a sandstone sample and a gneiss PPD sample from an Oligocene conglomerate of the Molasse Basin shows that the youngest age cluster is present only in the sand fraction and derived from the Oligocene volcanic activity along the Periadriatic zone. The lack of the youngest ages in the gneiss pebble assemblage excludes the Oligocene exhumation of the crystalline basement from mid-crustal level. (ii) Pebble assemblages of red Bunter sandstone, gneiss and quartzite were collected from an Upper Miocene conglomerate of the Molasse Foreland Basin and merged as PPD samples. Apatite and zircon FT grain age distributions of these PPD samples, representing the largest ancient East Alpine catchment, allow generating a new combination of palaeogeological and palaeo-FT-age maps of the Eastern Alps for the Late Miocene.

Abstract Fission-track dating on detrital apatites from modern sands of the Po Delta is used for a provenance study of sediments in the Po River basin. Analysed samples show a fission-track grain-age distribution characterized by two prominent peaks at 7.7 Ma and 17 Ma. The youngest peak accounts for 46% of the total population of dated grains. This young component in the grain-age distribution is consistent with bedrock cooling ages observed in the Western Alps between the External Massifs and the Houiller unit, as well as in the Lepontine dome of the Central Alps and in the Miocene foredeep units of the Apennines, that overall represent only 12% of the orogenic source area. Results suggest that most of the sediment load in the last 10 2 –10 5 years was supplied by focused erosion of relatively small areas that experienced short-term erosion rates one order of magnitude higher than in the rest of the belt.

Abstract We present new fission-track and (U–Th)/He data from apatite and zircon in order to reconstruct the exhumation of the Sierra de Cameros, in the northwestern part of Iberian Range, Spain. Zircon fission-track ages from samples from the depocentre of the basin were reset during the metamorphic peak at approximately 100 Ma. Detrital apatites from the uppermost sediments retain fission-track age information that is older than the sediment deposition age, indicating that these rocks have not exceeded 110 °C. Apatites from deeper in the stratigraphic sequence of the central part of the basin have fission-track ages of around 40 Ma, significantly younger than the stratigraphic age, recording the time of cooling after peak metamorphic conditions. Apatite (U–Th)/He ages in samples from these sediments are 31–40 Ma and record the last period of cooling during Alpine compression. The modelled thermal history derived from the uppermost sediments indicates that the thermal pulse associated with peak metamorphism was rapid, and that the region has cooled continuously to the present. The estimated palaeogeothermal gradient is around 86 °C km −1 and supports a tectonic model with a thick sedimentary fill ( c . 8 km) and explains the origin of the low-grade metamorphism observed in the oldest sediments.

Abstract Apatite fission-track (AFT) analyses were performed on 13 Late Palaeozoic samples in order to unravel the late- to post-Variscan evolution of the Ardennes. The dated AFT ages cover a range from 290±33 Ma to 168±12 Ma, and the mean confined track lengths correspond to a unimodal distribution, with means varying between 13.1±0.1 µm and 11.7±0.3 µm. These ages for the sedimentary rocks are clearly younger than the respective stratigraphic ages, indicative of a cooling through the apatite partial annealing zone after post-depositional complete annealing. All available AFT data (290–146 Ma) from this region might be classified as three groups, that is 290–229 Ma, 218–198 Ma and 190–146 Ma, at least in correlation with three exhumation events. Using an inverse model, four major cooling episodes are identified from the modelled temperature–time ( T – t ) paths. The first rapid cooling (4.2–5.4 °C Ma −1 , 320–300 Ma) corresponds to the late-Variscan rapid thrusting that ceased at about 300 Ma. The second cooling episode (0.2–4.0 °C Ma −1 , up to 230 Ma) activated differentially, and was probably controlled by the post-Variscan transtension. The third cooling regime (0.1–0.3 °C Ma −1 , 230–45 Ma) in the Ardennes Allochthon is slow, and represents a long-term and slow exhumation. In the Brabant Parautochthon, however, it is subdivided into 0.7 °C Ma −1 (225–110 Ma) and 0.2 °C Ma −1 (110–45 Ma). The last accelerated cooling (0.7–1.1 °C Ma −1 , since 45 Ma) that affected the whole Ardennes is associated with a south–north compression during the Pyrenean phase.

Abstract The Lusatian Block in eastern Germany is part of the Variscan Bohemian Massif. It is located at the intersection of two regional fault–thrust systems that have been controlling the regional geological and landscape evolution since at least late Mesozoic times, the Elbe Fault System and the Eger Graben. Although the Lusatian Block has traditionally been described as a morphological high throughout the Mesozoic, timing, style and amount of denudation have not yet been quantified. Apatite fission-track (AFT) analysis of basement rocks yields ages varying between approximately 70 and 95 Ma, with mean track-lengths ranging from 13.6 to 14.1 µm. Thermal history modelling of the AFT data points to a minimum denudation of 3 km of the Lusatian Block in the Late Cretaceous. Onset of denudation can be related to transpression along the Lusatian Thrust as a main element of the Elbe Fault System. Moreover, the thermal history models provide tentative evidence for a second cooling–denudation event, which affected Lusatia in the late Palaeogene. This final denudation of at least 1 km probably occurred as a far-field response to the uplift of the northern shoulder of the Cenozoic Eger Graben.

Abstract We assess the proposal of Hendriks & Redfield ( Earth and Planetary Science Letters , 236 , 443–458, 2005) that cross-over of the predicted apatite fission track (AFT)>(U–Th–Sm)/He (AHe) age relationship in the southeastern Fennoscandian shield in southern Finland reflects α-radiation-enhanced annealing (REA) of fission tracks at low temperatures and that more robust estimates of the denudation history are recorded through reproducible AHe data. New AHe results from southern Finland showing variable dispersion of single-grain ages may be biased by different factors operating within grains, which tend to give a greater weighting towards older age outliers. AHe ages from mafic rocks show the least dispersion and tend to be consistently lower than their coexisting AFT ages. In general, it is at the younger end of the single-grain variation range from such lithologies where most meaningful AHe ages can be found. AHe data from multigrain aliquots are, therefore, of limited value for evaluating thermal histories in southern Finland, especially when compared against coexisting AFT data as supporting evidence for REA. New, large datasets from the southern Canadian and Western Australian shields show the relationship between AFT age, single-grain age or mean track length as a function of U content (determined by the external detector method). These do not display the moderately strong inverse correlations previously reported from southern Finland in support of REA. Rather, the trends are inconsistent and generally exhibit weak positive or negative correlations. This is also the case for plots from both shields, as well as those from southern Finland, where AFT parameters are plotted against effective U concentration [eU] [based on U and Th content determined by inductively coupled plasma-mass spectroscopy (ICP-MS)], which weights decay of the parents more accurately in terms of their α-productivity. Further, samples from southern Finland yield values of chi-square χ 2 >5%, indicating that there is no significant effect of the range of uranium content between grains within samples on the AFT ages, and that they are all consistent with a single population. The oldest AFT ages in southern Finland apatites (amongst the oldest recorded from anywhere) are found in gabbros, which also have the highest Cl content of all samples studied. We suggest, that it is Cl content rather than REA that has influenced the annealing history of the apatites, which have experienced a history including reburial into the partial annealing zone by Caledonian Foreland basin sedimentation. The study of apatite from low U and Th rocks, with relatively low levels of α-radiation damage may provide the most practical approach for producing reliable results for AFT and AHe thermochronometry studies in cratonic environments.

Abstract In situ 10 Be concentrations in granites on Corsica (Mediterranean), exposed to subalpine climate, yield weathering rates of between 9 and 20 mm ka −1 , when averaged over the last 30–100 ka. Weathering rates rise with increasing precipitation and brittle deformation. Thermal history modelling of apatite fission-track (AFT) data confirms that average denudation rates of 5–20 mm ka −1 were typical during the Neogene. Short- and long-term denudation rates are in the same range and indicate that a late Neogene global increase in denudation rates has probably not affected geomorphically stable uplifted palaeosurfaces of hilly to low mountainous local relief. A Southwards decrease in short- and long-term denudation rates indicate a linear relationship with decreasing precipitation. Post-glacial downwearing of moraine matrix, as constrained by soil-derived humic etching of dated glacial boulders, yields 40–140 mm ka −1 , which indicates a poor preservation potential of moraines older than the last glaciation. This weathering rate is of the order of 3 times faster than that found for long-term regolith formation from granites based on of geomorphic evidence.

Abstract The Altai Mountains form an intracontinental, transpressive deformation belt in the NW Central Asian orogenic system. Using a multi-method chronometric approach, the thermo-tectonic history of the basement underlying the Teletskoye graben area is constrained in more detail. The results provide new insights into the Siberian Altai basement evolution from the Early Palaeozoic to the present. Zircon SHRIMP (sensitive high-resolution ion microprobe) U–Pb ages (Late Ordovician–Early Silurian, 460–420 Ma) indicate an earlier crystallization age for the basement granitoids than previously thought (Late Devonian–Early Carboniferous, 370–350 Ma), while new multi-mineral 40 Ar/ 39 Ar age spectra suggest continuous basement cooling throughout the Devonian–Carboniferous. Reactivation of long-lived Palaeozoic structures controls the Teletskoye graben formation since the Plio-Pleistocene as a distant effect of India–Eurasian convergence. Deformation is propagated through Central Asia and Siberia along an inherited structural network closely associated with its basement fabric. A similar reactivation affected the Altai during the Mesozoic. Modelled apatite fission-track data suggest Late Jurassic–Cretaceous (150–80 Ma) cooling, interpreted to be related to denudation and the tectonic reactivation that we link to the coeval Mongol–Okhotsk orogeny. From the Late Cretaceous until the Pliocene, the thermal history models indicate a period of stability. Roughly around 5 Ma ago renewed cooling is observed that possibly represents the denudation and growth of the present-day Altai, and provides the context for the Teletskoye graben formation. A modelled Late Cenozoic cooling can be a result of, or overemphasized by, a modelling artefact. Some caution should be taken not to overinterpret this cooling phase.

Abstract This research paper investigates the thermo-tectonic history of the north Mozambican basement subsequent to the Pan-African metamorphism. Six 40 Ar/ 39 Ar hornblende, three 40 Ar/ 39 Ar biotite and 25 titanite fission-track data place new constraints on the earliest timing of rifting in the central sector of Gondwana, and demonstrate a close linkage between the geometric rift configuration and the ductile metamorphic basement fabrics during the initial dispersal of the supercontinent. The 40 Ar/ 39 Ar hornblende and biotite ages range from c . 542 to 456 Ma and from c . 448 to 428 Ma, respectively. These data record slow basement cooling after the latest Pan-African metamorphism at rates of c . 7–11 °C Ma −1 between Early and Late Ordovician times. Locally, syn- to post-tectonic granitoid emplacements around 500–450 Ma delayed basement cooling to Late Ordovician–Early Silurian times. The titanite fission-track (TFT) ages fall into two age groups of c . 378–327 Ma and c . 284–219 Ma. The older TFT ages record very slow cooling from the Late Ordovician–Early Silurian to below 275±25 °C in the Late Devonian–Early Carboniferous at slow rates of less than 1 °C Ma −1 . This slow cooling is related to decreasing denudation in association with the establishment of pre-Karoo peneplains in central Gondwana. The younger TFT ages record denudation due to rift flank uplift in the context of initial Gondwana disintegration in the Mozambican sector. Corresponding Early–Late Permian crustal extension proceeded obliquely to a NW–SE tensional palaeo-stress field and was associated with a brittle reactivation of easterly trending ductile basement fabrics. In total, up to ≤9–12 km of denudation is deduced from the TFT results since Permo-Carboniferous times.

Abstract Apatite fission-track (AFT) data from two traverses across the Great Escarpment of the western coast of South Africa are used to reconstruct the tectonic evolution and denudation history of this sector of the Atlantic passive margin. Fission-track ages range between 180 and 86 Ma. Modelling of this data identifies two distinct cooling events. The first event, between 160 and 138 Ma, is recorded only by the rocks above the escarpment in the Karoo area, and is tentatively linked to post-Karoo magmatism ( c. 180 Ma) thermal relaxation. The second, between 115 and 90 Ma, results instead from a tectonically induced denudation episode responsible for the removal of up to 2.5 km of crust across the coastal zone in front of the escarpment and less than 1 km on the elevated interior plateau. Based on these results, it is suggested that the Cretaceous is the time when most of the elevated topography of Southern Africa was generated, with only a minor Cenozoic contribution.