Raymond Jonckheere

On the densities of etchable fission tracks in a mineral and co-irradiated external detector with reference to fission-track dating of minerals

Abstract The ratio R of the densities of etchable fission tracks in a mineral and co-irradiated external detector is shown to depend on the length ratios of the latent ( L f / L f ′) and etched fission tracks ( L e / L e ′) in the two media. Calculations of L f for apatite, titanite zircon and L f ′ for muscovite external detectors allow a precise determination of R , and confirm that neglecting R could constitute the most significant source of systematic error in fission-track dating with the external detector method. The ratios of the measured fission-track densities in an external and internal surface of apatite and a muscovite external detector are calculated from R and the track counting efficiencies ηq . The fact that the results are consistent with the experiment supports the accuracy of R and of the ηq -factors. The external detector ages of a basal and prismatic section of Durango apatite are consistent with its reference age, without the need for a length correction. A critical examination of the uncertainties related to the thermal neutron fluence measurements, fission-decay constant, calculated R -value, independently determined ηq -factors and the reference age of the standard, demonstrates that the assumption that a reduction of the confined track length of the spontaneous tracks in Durango apatite necessitates a proportional correction of its apparent fission-track age is the element most in question. Published U/Th–He ages lend support to the conclusion that such a correction is not required. The significance of R and of the ratio Q of the ηq -factors for the induced and spontaneous track counts leads us to propose a formalisation of the age equation, in which these are included, as well as a formal length correction L , which is, however, not set equal to the ratio of the confined track lengths of the induced and spontaneous tracks. The accuracy of the thermal neutron fluence and the decay constant leads us to propose a possible deconvolution of the ζ -calibration factor. If, for suitable age standards L =1, the proposed deconvoluted calibration factor ζ 0 is a function of R and Q , which can be determined in ways that are independent of the reference ages of the standards. This can either be interpreted as a multiple calibration or as establishing the fission-track method as an independent dating method.

On methodical problems in estimating geological temperature and time from measurements of fission tracks in apatite

Abstract The results of apatite fission-track modelling are only as accurate as the method, and depend on the assumption that the processes involved in the annealing of fossil tracks over geological times are the same as those responsible for the annealing of induced fission tracks in laboratory experiments. This has hitherto been assumed rather than demonstrated. The present critical discussion identifies a number of methodical problems from an examination of the available data on age standards, borehole samples and samples studied in the framework of geological investigations. These problems are related to low- ( 60°C) annealing on a geological timescale and to the procedures used for calculating temperature–time paths from the fission-track data. It is concluded that it is not established that the relationship between track length and track density and the appearance of unetchable gaps, observed in laboratory annealing experiments on induced tracks, can be extrapolated to the annealing of fossil tracks on a geological timescale. This in turn casts doubt on the central principle of equivalent time. That such uncertainties still exist is in no small part due to an insufficient understanding of the formation, structure and properties of fission tracks at the atomic scale and to a lack of attention to the details of track revelation. The methodical implications of discrepancies between fission track results and the independent geological evidence are rarely considered. This presents a strong case for the re-involvement of track physicists in fundamental fission track research.

Building the Pamir‐Tibetan Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Central Pamir: 2. Timing and rates

Geothermochronologic data outline the temperature-deformation-time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before ~35 Ma and peaked at ~23–20 Ma, reflecting top-to- ~N thrust-sheet and fold-nappe emplacement that tripled the thickness of the upper ~7–10 km of the Asian crust. Multimethod thermochronology traces cooling through ~700–100°C between ~22 and 12 Ma due to exhumation along dome-bounding normal-sense shear zones. Synkinematic minerals date normal sense shear-zone deformation at ~22–17 Ma. Age-versus-elevation relationships and paleoisotherm spacing imply exhumation at ≥3 km/Myr. South of the domes, Mesozoic granitoids record slow cooling and/or constant temperature throughout the Paleogene and enhanced cooling (7–31°C/Myr) starting between ~23 and 12 Ma and continuing today. Integrating the Central Pamir data with those of the East (Chinese) Pamir Kongur Shan and Muztaghata domes, and with the South Pamir Shakhdara dome, implies (i) regionally distributed, Paleogene crustal thickening; (ii) Pamir-wide gravitational collapse of thickened crust starting at ~23–21 Ma during ongoing India-Asia convergence; and (iii) termination of doming and resumption of shortening following northward propagating underthrusting of the Indian cratonic lithosphere at ≥12 Ma. Westward lateral extrusion of Pamir Plateau crust into the Hindu Kush and the Tajik depression accompanied all stages. Deep-seated processes, e.g., slab breakoff, crustal foundering, and underthrusting of buoyant lithosphere, governed transitional phases in the Pamir, and likely the Tibet crust.

Cenozoic intra-continental deformation and exhumation at the northwestern tip of the India-Asia collision—southwestern Tian Shan, Tajikistan and Kyrgyzstan

Along the Ghissar-Alai Range of the southwestern Tian Shan (southwestern Kyrgyzstan, northern Tajikistan), the deformation front of the India-Asia collision—the Pamir-Tibet orogen—is interacting with the intra-continental Tian Shan orogen without the intervening Tarim Craton. Apatite fission-track (n = 33, ~3.3–145.6 Ma, 27% <10 Ma) and (U-Th)/He (n = 32, ~1.9–26.1 Ma, 56% <10 Ma) thermochronologic ages suggest approximate isothermal holding (very slow cooling to weak reheating) during relative tectonic quiescence between ~150–15 Ma. Accelerated exhumation (~0.2–1.0 km/Myr, median ~0.5 km/Myr) and cooling (11–16 °C/Myr) occurred over the last ~10 Myr. Geomorphologic parameters—incision, and river steepness and concavity—confirm the youth of the southwestern Tian Shans mountain building. High exhumation/cooling rates correlated with pronounced local relief, produced by Cenozoic faults reactivating inherited (Late Paleozoic) structures. Regions with similarly young exhumation are centered along rims of rigid crustal blocks in the central and eastern Tian Shan. Structurally, the Ghissar-Alai Range is a broad, ~east-trending zone of dextral transpression that includes the northern Tajik Basin (Illiak Fault Zone) and the Pamir Thrust System of the frontal northern Pamir. It is the particular deformation field at the northwestern tip of the India–Asia collision—the interaction of the westward gravitational collapse of the Pamir Plateau into the Tajik Basin with the bulk northward motion of the Pamir—that transformed the southwestern Tian Shan into a dextral transpression belt. The dextral transpression in the southwestern Tian Shan contrasts with sinistral strike-slip shear localized along inherited fault zones, accommodating dominant ~ north–south shortening, in the central and eastern Tian Shan. The deformation field influenced by the Pamir and the associated young exhumation make the Ghissar-Alai Range a unique feature in the Tian Shan orogen.

Correction factors for systematic errors related to the track counts in fission-track dating with the external detector method

Abstract Tracks from uranium fission are the basis of fission-track dating of minerals. The fission-track method has not attained the status of an independent dating method because of problems related to neutron fluence measurements and the decay constant. Fission-track ages are instead determined relative to the reference ages of standards using an empirical calibration factor ( ζ -method). Methodical advances during the last decade imply that these problems need not remain obstacles to establishing fission-track dating as an independent method. However, sources of systematic error related to the track density measurements need to be corrected before this can be the case. A correction factor (GQR), determined by experiment, is in good agreement with its calculated value. The ages of apatites of known age determined with the φ -method are also in good agreement with their reference ages. When applied to a set of samples of unknown age, the φ -method produces results that are consistent with those obtained with the Z - and ζ -methods. These results lend support to the method of neutron fluence measurement, to the recommended value of the decay constant and to the calculated and measured GQR-value. On the other hand, these results call into question the validity of age corrections based on confined track length measurements.

Late-stage foreland growth of China’s largest orogens (Qinling, Tibet): Evidence from the Hannan-Micang crystalline massifs and the northern Sichuan Basin, central China

This paper addresses the timing of final foreland growth of China’s largest orogens: the Mesozoic Qin Mountains (Qinling) and the Cenozoic Tibetan Plateau. In particular, we ask when the front of the Qinling orogen fold-thrust belt was emplaced, and when the northern Sichuan Basin was affected by the eastward growth of the Tibetan Plateau. We employ zircon and apatite fission-track and (U-Th)/He dating in the Proterozoic crystalline rocks of the Hannan-Micang massifs and the sedimentary rocks of the northern Sichuan Basin. The Hannan-Micang rocks remained in the zircon fission-track partial annealing zone (240 ± 30 °C) throughout the Paleozoic–Middle Triassic (481–246 Ma). From the late Middle Jurassic (ca. 165 Ma) to the early Late Cretaceous (ca. 95 Ma), enhanced cooling and exhumation, with rates of 1.2–2.5 °C/m.y. and 0.04–0.10 mm/yr, respectively, record propagation of the Qinling orogen into its leading foreland; the timing of foreland growth is supported by sedimentologic evidence, i.e., regional variation in sediment thickness and depocenter migration. Negligible cooling and exhumation since the Late Cretaceous (ca. 95 Ma) likely mark the end of the foreland fold-thrust belt formation and the subsequent persistence of a low-relief landscape that occupied extensive parts of central China; cooling and exhumation rates of 0.38–0.70 °C/m.y. and <0.02 mm/yr characterize this tectonic stagnation period. Accelerated cooling (4–5 °C/m.y.) since the Late Miocene (13–8 Ma), derived from apatite fission-track temperature-time path models, signifies involvement of the Hannan-Micang massifs and the northern Sichuan Basin into the eastward-growing Tibetan Plateau. Their inclusion into the plateau growth initiated faulting and stripped off 1.4–2.0 km of rock from the Hannan-Micang massifs and northern Sichuan Basin.

Sichuan Basin and beyond: Eastward foreland growth of the Tibetan Plateau from an integration of Late Cretaceous‐Cenozoic fission track and (U‐Th)/He ages of the eastern Tibetan Plateau, Qinling, and Daba Shan

Combining 121 new fission-track and (U-Th)/He ages with published thermochronologic data, we investigate the Late Cretaceous–Cenozoic exhumation/cooling history of the eastern Tibetan Plateau, Qinling, Daba Shan, and Sichuan Basin of east-central China. The Qinling orogen shows terminal southwestward foreland growth in the northern Daba Shan thrust belt at 100–90 Ma and in the southern Daba Shan fold belt at 85–70 Ma. The eastern margin of Tibet Plateau experienced major exhumation phases at 70–40 Ma (exhumation rate 0.05–0.08 mm/yr), 25–15 Ma (≤1 mm/yr in the Pengguan Massif; ~0.2 mm/yr in the imbricated western Sichuan Basin), and since ~11–10 Ma along the Longmen Shan (~0.80 mm/yr) and the interior of the eastern Tibet Plateau (Dadu-River gorge, Min Shan; ~0.50 mm/yr). The Sichuan Basin records two basin-wide denudation phases, likely a result of the reorganization of the upper Yangtze-River drainage system. The first phase commenced at ~45 Ma and probably ended before the Miocene; >1 km of rocks were eroded from the central and eastern Sichuan Basin. The second phase commenced at ~12 Ma and denudated the central Sichuan Basin, Longmen Shan, and southern Daba Shan; more than 2 km of rocks were eroded after the lower Yangtze River had cut through the Three Gorges and captured the Sichuan-Basin drainage. In contrast to the East Qinling, which was weakly effected by Late Cenozoic exhumation, the West Qinling and Daba Shan have experienced rapid exhumation/cooling since ~15–13 Ma, a result of growth of the Tibetan Plateau beyond the Sichuan Basin.

Single-track length measurements of step-etched fission tracks in Durango apatite: “Vorsprung durch Technik”

Abstract Fossil and induced confined fission-tracks in the Durango apatite do not etch to their full etchable lengths with the current protocols. Their mean lengths continue to increase at a diminished rate past the break in slope in a length vs. etch-time plot. The mean length of the fossil tracks increases from 14.5(1) to 16.2(1) μm and that of the induced tracks from 15.7(1) to 17.9(1) μm between 20 and 60 s etching (5.5 M HNO3; 21 °C); both are projected to converge toward ~18 μm after ~180 s. This increase is due to track etching, not bulk etching. The irregular length increments of individual tracks reveal a discontinuous track structure in the investigated length intervals. The mean lengths of the fossil and induced tracks for the standard etch time (20 s) for the (5.5 M HNO3; 21 °C) etch are thus not the result of a shortening of the latent fission tracks but instead of a lowering of the effective track-etch rate νT. The rate of length increase of individual fossil confined tracks correlates with their length: older tracks are shorter because they etch slower. Step etching thus makes it possible to some extent to distinguish between older and younger fossil fission tracks. Along-track νT measurements could reveal further useful paleo-temperature information. Because the etched length of a track at standard etch conditions is not its full etchable length, geometrical statistics based on continuous line segments of fixed length are less secure than hitherto held.