Track Length Distributions Research Articles

The Fish Canyon Tuff: A new look at an old low-temperature thermochronology standard

The Fish Canyon Tuff (FCT) has served as an important source for geochronology standards, particularly for fission track, K–Ar and (40Ar/39Ar) dating, even though efforts to establish precise ages for its constituent minerals have proved to be unexpectedly complex. To evaluate the suitability of FCT apatite as a standard for apatite (U–Th–Sm)/He (AHe) thermochronometry, and to test underlying assumptions about its suitability for apatite fission track (AFT) thermochronometry, we analysed samples from a series of sites throughout the vertical and lateral extent of the host ignimbrite sheet. Samples were collected from the relatively lithic-rich, classic sampling location in the lower part of the thick proximal ignimbrite and a ∼330 m vertical section of FCT immediately above it. Average weighted mean AHe ages from multiple analyses at five sites in this profile range from 20.8±0.4 Ma from the classic site at the base, to an average of 28.4±0.2 Ma (all ±1σ) in the upper part of the section. The AHe age at the classic site is substantially reduced at 20.8±0.4 Ma relative to a reference age for ignimbrite emplacement of 28.2 Ma. Corresponding zircon (U–Th)/He (ZHe) ages for these samples are all concordant at 28.3±0.4 Ma. By contrast, apatite fission track (AFT) ages from the same vertical section are all concordant at 27.4±0.7 Ma with the central age of 28.8±0.8 Ma at the classic site, except for the uppermost sample (23.2±1.7 Ma) for which clear evidence for local, probably fire, disturbance is seen in the track length distribution. The AHe data at the classic site thus provide evidence for substantial post-eruptive Early Miocene cooling of the tuff consistent with its position at the bottom of a deeply incised valley with ∼800 m of local relief and probably >1000 m of removed section above it. The AFT age of the classic sampling site, however, is indistinguishable from ignimbrite emplacement and thus continues to be a useful standard for AFT (but not for AHe) geochronology. Apatites from this site have the highest measured Cl concentrations (0.82 wt%) of any of the FCT apatites analysed, contributing to this suitability. AFT and AHe ages at three distal localities, some 35–45 km to the East of the classic site, where the thickness of the FCT is reduced to <100 m, all yielded concordant ages with a weighted mean of 28.5±0.11 Ma with no evidence for post-emplacement thermal disturbance. One of the distal sites – a quarry in the upper part of the FCT – appears to be an ideal locality for a standard reference material that would be suitable for both AFT and AHe low-temperature thermochronometers. U–Pb ages for zircons and 40Ar/39Ar step heating age spectra for sanidine at this new distal site are essentially identical to those found at the classic site.

Thermal history of the Sabero Coalfield (Southern Cantabrian Zone, NW Spain) as revealed by apatite fission track analyses from tonstein horizons: implications for timing of coalification

Apatite fission track (AFT) central ages from Carboniferous (Stephanian) tonsteins of the Sabero Coalfield, NW Spain, range from 140.8 ± 7.5 to 65.8 ± 8.1 Ma (Cretaceous), with mean c-axis projected track length values ranging from 12.5 to 13.4 μm. Mean random vitrinite reflectance (R r) of these samples ranges from 0.91 to 1.20 %, which can be translated into maximum palaeotemperatures of ca. 130 to 180 °C. All analysed samples experienced substantial post-depositional annealing. The considerably younger AFT ages compared to the depositional ages of the samples and R r data indicate the certainty of the occurrence of at least one heating event after the deposition of strata. The unimodal track length distributions, the relatively short mean track length, and the rather low standard deviation (SD) (1.0–1.6 μm) indicate a relatively simple thermal history that could be related to the post-Late Variscan heating event followed by prolonged residence in the apatite partial annealing zone (APAZ). Geological data combined with thermal models of AFT data indicate that Stephanian strata reached the maximum palaeotemperatures in the Permian period, which was therefore the major time of the coalification processes. The Permian magmatic activity was responsible for a high heat flow, which, with the added effect of sedimentary burial, could account for the resetting of the AFT system. It appears that the fault-related hydrothermal activity could have redistributed heat in areas of significant subsidence. Cooling occurred in the Triassic–Cretaceous times after a high heat flow Permian regime. A post-Permian maturation of the Stephanian organic matter is not very likely, since there is no evidence of a high Mesozoic burial that was sufficient to cause a significant increase in the palaeotemperatures. Finally, exhumation and associated erosion rates may possibly have been faster in the Tertiary, causing the present exposure of the studied rocks.

Formation of passive-roof duplexes in the Colombian Subandes and Perú

Passive-roof duplexes are important features for accommodating shortening in active orogens, but their occurrences have been previously demonstrated only with significant subsurface data or after their exhumation. In this study, we describe a series of thin-skinned passive-roof duplexes along the Subandean front in Colombia and compare them with potential analogues in southern Peru. We suggest type localities for this structural style, which display conditions favorable for formation of these structures. It appears that passive-roof duplexes in the Subandean zones are mostly late Miocene features. Our main data for placing temporal and thermokinematic constraints on their development are 25 apatite fission-track (AFT) analyses, including track-length distributions. We show that these areas require high shortening, surface erosion, and downstream sedimentation rates at the time of their formation, as well as two distinct low-friction detachments. These features, which have been previously described by analog models, appear to have been conditioned by a phase of late Miocene topographic growth and denudation in the hinterland and subsequent increase in accommodation as well as sedimentation rates in the foreland.

A dosimetric study of Beta induced bremsstrahlung in bone

A dosimetric study of beta-induced bremsstrahlung in bone is important in the field of radiation protection. The beta-induced bremsstrahlung spectra produced by 113 pure beta nuclides in the bone are computed. The spectral shapes are primarily responsible for variations in the shapes by depth-dose distributions. They are intended to provide a quick and convenient reference for spectral shapes and to give an indication of the wide variation in these shapes. The computed bremsstrahlung spectrum is used in the evaluation of bremsstrahlung dose in bone. The evaluated beta bremsstrahlung dose as a function of distance for the studied nuclides is also presented. The beta bremsstrahlung dose decreases with the increase in distance. Present work also estimates the dosimetric parameters of bremsstrahlung such as yield, intensity and dose rate of bremsstrahlung by various pure beta nuclides in tissues of human skeleton such as cortical bone, red marrow, yellow marrow, spongiosa and cartilage. The yield, intensity and dose rate of bremsstrahlung in the cortical bone are higher than that of red marrow, yellow marrow, spongiosa and cartilage. Hence cortical bone is more beta/bremsstrahlung radiosensitive than that of other tissues of human skeleton. The estimated bremsstrahlung efficiency, intensity, photon spectra and photon track-length distributions determine the quality and quantity of the radiation. Precise estimation of this source term is very important in planning for radiotherapy and diagnosis.

Low‐temperature thermochronologic record of Eocene migmatite dome emplacement and late Cenozoic landscape development, Shuswap core complex, British Columbia

AbstractExhumed mid‐to‐lower crustal rocks offer an opportunity to determine the mechanisms, conditions, timing, and consequences of the ascent of hot rocks from deep to shallow crustal levels. We used results of low‐T thermochronology (zircon and apatite (U‐Th)/He, apatite fission track) to document the very shallow emplacement (&lt;2 km) of high‐grade metamorphic rocks and to determine the timing and rates of Cenozoic cooling, exhumation, and subsequent incision of the Thor‐Odin migmatite dome of the Shuswap metamorphic core complex, British Columbia (Canada). Samples collected at high elevation in the dome (&gt;1800 m) have preserved Eocene fission‐track ages and evidence of rapid cooling (≥60°C/Myr). This Eocene cooling event corresponds to rapid exhumation by upward flow of partially molten crust and final exhumation by detachment faulting. Samples collected below 1800 m in elevation display a wide range of apatite fission track ages (43–15 Ma) and track length distributions that reflect prolonged residence in the apatite partial annealing zone. These age‐elevation relations imply that the dome rocks reached the near surface (&lt;2 km) during initial upward flow and tectonic exhumation in the Eocene and that little erosion of the Eocene surface has occurred since that time. Thermal modeling of the lowest elevation samples (≤ ~600 m) and intrasample apatite (U‐Th)/He age variations reveal enhanced erosion and relief production at the onset of continental glaciations at ~3 Ma. Our work illustrates the dynamic links between deep and shallow crustal processes and the evolution of topography in a deeply incised hot orogen.

Photoelectron Track Length Distributions Measured in a Negative Ion Time Projection Chamber

We report photoelectron track length distributions between 3 and 8 keV in gas mixtures of Ne+CO2+CH3NO2 (260:80:10 Torr) and CO2+CH3NO2 (197.5: 15 Torr). The measurements were made using a negative ion time projection chamber (NITPC) at the National Synchrotron Light Source (NSLS) at the Brookhaven National Laboratory (BNL). We report the first quantitative analysis of photoelectron track length distributions in a gas. The distribution of track lengths at a given energy is best fit by a lognormal distribution. A powerlaw distribution of the form, f(E)=a(E/Eo)n, is found to fit the relationship between mean track length and energy. We find n=1.29 +/- 0.07 for Ne+CO2+CH3NO2 and n=1.20 +/- 0.09 for CO2+CH3NO2. Understanding the distribution of photoelectron track lengths in proportional counter gases is important for optimizing the pixel size and the dimensions of the active region in electron-drift time projection chambers (TPCs) and NITPC X-ray polarimeters.

Timing of coalification of the upper carboniferous sediments in the upper silesia coal basin on the basis of by apatite fission track and helium dating

Streszczenie Przeprowadzono datowania za pomocą metody trakowej i helowej dla apatytów z utworów karbońskich w Górnośląskim Zagłębiu Węglowym w celu określenia ram czasowych procesów uwęglcnia. Pomierzone centralne wieki trakowe apatytów mieszczą się w przedziale od 259±11 (późny perm) do 103±6 milionów lat (wczesna kreda), a średnia długość traków waha się od 11,7±0,2 do 13,7±0,1 (im. Wszystkie wieki trakowe są młodsze od wieku stratygraficznego analizowanych próbek, wskazując znaczne zaawansowanie procesów diagcnctycznych. Próbki z zachodniej i środ- kowej części GZW mają wieki trakowe od późnego permu do środkowego/późnego triasu (259±11 do 214±10 min lat). Jcdnomodalnc rozkłady długości traków i ich średnie wartości wskazują na poje- dyncze, względnie szybkie zdarzenie postwaryscyjskicgo wychładzania do temperatury poniżej 60°C, co jest zgodne zc znaczną erozją postinwersyjną utworów gómokarbońskich po fazie asturyjskicj. W pozostałej części mczozoiku następowało wolniejsze wychładzanie. Próbki zc wschodniej i NE części GZW mają wieki trakowe od późnego triasu do wczesnej kredy (210±10 do 103±6 milionów lat). Charakteryzuje je względnie krótsza średnia długość traków i wyższe odchylenia standardowe, a także w przypadku części próbek bimodalny i/lub mieszany charakter rozkładów długości. Jest to razem wskazówką bardziej złożonej historii termicznej, z długim okresem przebywania w PAZ i możliwym drugim zdarzeniem termicznym. Wieki helowe apatytów w całym basenie są wczcs- nokredowc(144,l±l 1 do 108,1±8 milionów lat), wskazując raczej na wolne postwaryscyjskic wychła- dzanie lub możliwe mczozoicznc podgrzanie karbonu do temperatury nic większej niż 60-70°C, które spowodowało częściową dyfuzję helu i odmłodzenie wieków helowych, ale równocześnie nic spo- wodowało znaczącego zabliźniania traków na większości obszaru GZW. Jedynie w NE części GZW podgrzanie mczozoicznc mogło być nieco wyższe, do temperatury 70-85°C, powodując odmłodzenie wieków trakowych, zwłaszcza przy długim okresie przebywania w PAZ. Mczozoiczny impuls ter- miczny był przypuszczalnie spowodowany cyrkulacją gorących roztworów związaną z procesami ekstensji. Powyższe zakresy temperatur i czas ich występowania świadczą, żc uwęglenie materii organicznej w GZW nastąpiło zasadniczo z końcem okresu karbońskiego.

Single-Molecule Motility: Statistical Analysis and the Effects of Track Length on Quantification of Processive Motion

In vitro, single-molecule motility assays allow for the direct characterization of molecular motor properties including stepping velocity and characteristic run length. Although application of these techniques in vivo is feasible, the challenges involved in sample preparation, as well as the added complexity of the cell and its systems, result in a reduced ability to collect large datasets, as well as difficulty in simultaneous observation of the components of the motility system, namely motor and track. To address these challenges, we have developed simulations to characterize motility datasets as a function of sample size, processive run length of the motor, and distribution of track lengths. We introduce the use of a simple bootstrapping technique that allows for the quantification of measurement uncertainty and a Monte Carlo permutation resampling scheme for the measurement of statistical significance and the estimation of required sample size. In addition, we have found that, despite conventional wisdom, the measured characteristic run length is directly coupled to the characteristic track length that describes the microtubule length distribution. To be able to make comparisons between motility experiments performed on different track populations as well as make measurements of motility when motors and tracks cannot be simultaneously resolved, we have developed a theoretical framework for the determination of the effect that track length has on observed characteristic run lengths. This shows good agreement with in vitro motility experiments on two kinesin constructs walking on microtubule populations of different characteristic track lengths.

The Exhumation Process of Mufushan Granite in Jiangnan Uplift Since Cenozoic: Evidence from Low‐Temperature Thermochronology

AbstractJiangnan uplift located in the collision zone between Yangtze and Cathaysia block is the key to interpret the tectonic evolution of South China block. The Cenozoic thermal history and the uplift processes of Mufushan pluton are constrained by the combination of analysis of apatite fission track and (U‐Th‐Sm)/He ages with modeling of fission track length distribution. Based on the analysis of thermochronological dating and regional tectonic setting, the authors probed into the relationship between the thermochronological evolution and the tectonic development. The exhumation of plutons arrived at about 4800 m since 80 Ma, and the uplift processes were not uniform in space, and different plutons had somewhat different uplift processes. Thermal history modeling of the AFT data shows that the Mufushan pluton experienced three episodes of cooling event, two rapid exhumation events happened in Upper Cretaceous‐Paleocene (about 80∼50 Ma) and Upper Miocene (since 10 Ma). There is a period of relative thermal stability between 50 and 10 Ma. The first stage of rapid uplifting during 80∼50 Ma in this region is coeval with extension episode of Mid‐Upper Yangtze during Himalayan which resulted in the formation of fault basin and exhumation of shoulder block; the Upper Miocene rapid cooling since 10 Ma is attributed to the westward movement and subduction of Pacific plate. The uplift and exhumation of Mufushan pluton since Late Yanshanian led to a good deposition response in basins and the cooling events of three phases just coincided with the regional tectonic settings of study area.

Apatite fission-track evidence for the Cretaceous development of kilometer-scale relief and steady-state Tertiary topography in New England

Apatite fission track (AFT) ages in samples collected along the 5000 foot relief (1500m) exposed at Mt. Washington in the Presidential Range of New Hampshire are used to constrain the Cretaceous cooling history of this area in the northern Appalachians. Nine AFT ages for samples of the Littleton and Rangeley formations collected along this profile range in age from ~150Ma at the highest elevations of ~1900m to ~100Ma at the base (~500m). Thermal histories based on these results and on model time–temperature histories based on the distribution of fission track lengths in the higher elevations samples show three stages of cooling, from 1.5–2.0°C/m.y. (170–130Ma) to 0.2°C (130–65Ma), to ~0.6°C/m.y. (65Ma to the present). In contrast, the thermal history calculated for the lower elevations sample shows cooling delayed until ~120Ma at ~2°C/m.y. (120–100Ma), followed by monotonic cooling of ~0.6°C/m.y. from 100Ma to the present. The convergence of these histories from high, intermediate, and low elevations suggests a common cooling history independent of elevation differences of >1km. Structural/tectonic explanations for this thermal convergence are implausible, and we conclude that the most likely explanation for the common cooling history across >1km of relief is that the relief was established by the end of the Cretaceous and has persisted with steady-state topography through the Tertiary to the present. The AFT results are consistent with an earlier relief method study employing 40Ar/39Ar muscovite cooling ages. Geothermal gradients calculated from the results of both studies yield ~40°C/km suggesting that this gradient persisted throughout Permian and Mesozoic times.

Links between orogenic wedge deformation and erosional exhumation: Evidence from illite age analysis of fault rock and detrital thermochronology of syn-tectonic conglomerates in the Spanish Pyrenees

We present new geochronologic data illuminating the tectonic and erosional history of the orogenic wedge exposed in the south-central Pyrenees, Spain. We interpret illite-age analyses from four fault gouges that record thrust-belt development and document the importance of out-of-sequence thrusting. Fault activity occurs in pulses, with slip occurring contemporaneously on multiple faults throughout the wedge. New apatite fission-track data from syn-orogenic sediments of the Sis Conglomerate body reveal ages of 48 to 42 Ma, with no consistent variation upsection in strata deposited from 41 to 30 Ma. We interpret these data, as well as thermal modeling of track-length distributions, to imply rapid cooling in the interior of the Pyrenean wedge during the Middle Eocene. The record of fault activity and erosion suggests that orogenic wedges may not evolve in a steady fashion, but generally exhibit significant changes in rates of deformation and exhumation. The observed correlation in the timing of tectonism and erosional exhumation provides evidence for links between tectonic and surface processes.

Determination of Onset of Uplifting for the Mid‐Upper Yangtze Area Since Indosinian Event

AbstractThe onset of uplifting of the Mid‐upper Yangtze area is constrained by the combination of analysis of apatite fission track (AFT) age and modeling of fission track length distribution. The results show that the general trend of onset of uplifting is characterized by decreasing westward in geological time for the different tectonic units in the Mid‐upper Yangtze area. Two episodes of uplifting in Jianghan basin occurred within 157~97 Ma and 10~0 Ma; Xiang'exi‐Wuling area experienced an uplifting in 137~0 Ma; E'xi Yudong area, Chuandong folds, Qianzhong Hights and Dian‐Qian‐Gui areas in 97~0 Ma; Northern Sichuan basin and Chuanzhong areas in 56~0 Ma; and Chuanxi depression and Dianxi region in 23~0 Ma. The major faults of Qiyueshan, Huanyinshan and Longquanshan controlled the boundaries of the onsets of uplifting. The onsets of uplifting for the areas east of Qiyueshan fault, in between Qiyueshan and Huayinshan faults, in between Huayinshan and Longquanshan faults, west of Longquanshan fault are late Jurassic‐early Cretaceous (J3‐K1), late Cretaceous (K2), Paleogene (E) and Neogene (N), respectively.

Denudation history of the Bocaina Plateau, Serra do Mar, southeastern Brazil: Relationships to Gondwana breakup and passive margin development

The Bocaina Plateau, which is situated on the eastern flank of the continental rift of southeastern Brazil, is the highest part of the Serra do Mar. Topographic relief in this area is suggested to be closely related to its complex tectono-magmatic evolution since the breakup of Western Gondwana and opening of the South Atlantic Ocean. Apatite fission track ages and track length distributions from 27 basement outcrops were determined to assess these hypotheses and reconstruct the denudation history of the Bocaina Plateau. The ages range between 303 ± 32 and 46 ± 5 Ma, and are significantly younger than the stratigraphic ages. Mean track lengths vary from 13.44 ± 1.51 to 11.1 ± 1.48 μm, with standard deviations between 1.16 and 1.83 μm. Contrasting ages within a single plateau and similar ages at different altitudes indicate a complex regional tectonothermal evolution. The thermal histories inferred from these data imply three periods of accelerated cooling related to the Early Cretaceous continental breakup, Early Cretaceous alkaline magmatism, and the Paleogene evolution of the continental rift of southeastern Brazil. The oldest fission track ages (> 200 Ma) were obtained in the Serra do Mar region, suggesting that these areas were a long-lived source of sediments for the Paraná, Bauru, and Santos basins.

Electrodynamic Disaggregation: Does it Affect Apatite Fission‐Track and (U‐Th)/He Analyses?

Apatite fission‐track and (U‐Th)/He analyses require the liberation of intact idiomorphic apatite grains from rock samples. While routinely being carried out by mechanical methods, electrodynamic disaggregation (ED) offers an alternative approach. The high‐voltage discharges produced during the ED process create localised temperature peaks (10000 K) along a narrow plasma channel. In apatite, such high temperatures could potentially reduce the length of fission tracks, which start to anneal at temperatures &gt; 60 °C, and could also enhance He diffusion, which becomes significant at 30–40 °C over geological time scales. A comparison of fission‐track analyses and (U‐Th)/He ages of apatites prepared both by mechanical (jaw crusher, disk mill) and ED processing provides a way of determining whether heating during the latter method has any significant effect. Apatites from three samples of different geological settings (an orthogneiss from Madagascar, the Fish Canyon Tuff, and a muscovite‐gneiss from Greece) yielded statistically identical track length distributions compared to samples prepared mechanically. Additionally, (U‐Th)/He ages of apatites from a leucogranite from Morocco prepared by both methods were indistinguishable. These first results indicated that during electrodynamic disaggregation apatite crystals were not heated enough to partially anneal the fission tracks or induce significant diffusive loss of He.

Evidence for surface uplift of the Atlas Mountains and the surrounding peripheral plateaux: Combining apatite fission-track results and geomorphic indicators in the Western Moroccan Meseta (coastal Variscan Paleozoic basement)

This work represents an initial attempt to link the evolution of the topography in relation to the general tectonic framework of western Morocco. For this purpose, in a section of the Western Moroccan Meseta different tools are combined in order to attain the general objective. Apatite fission-track (AFT) data of granitic rocks of the Rabat–Khenifra area give ages around 200 Ma with track length distributions which are compatible with the thermal models already established for the area. An inverse correlation between AFT ages and elevation is observed which is compatible with previous models indicating northward tilting of the whole Western Moroccan Meseta which is younger than 20–25 Ma. In order to test this possibility a detailed analysis of the topography at different scales in the Western Moroccan Meseta has been performed. Results indicate that two open folds with different amplitudes are recognized and that the one with wider wavelength could correspond to a lithospheric fold as previously stated by other authors on the basis of independent geological arguments. The northward tilting proposed based on the AFT data agrees with the results obtained in the analysis of the topography which reinforces the presence of a very open fold with a wavelength of 200–300 km in the north-western limb of the Western Moroccan Meseta.

Fission track data from the Mesohellenic Trough and the Pelagonian zone in NW Greece: Cenozoic tectonics and exhumation of source areas

The Mesohellenic Trough (MHT) trends parallel to the structural fabric of the Hellenides and covers the suture between the Apulian microplate and the Pelagonian block. It comprises an up to ∼4 km thick middle Eocene to upper Miocene sedimentary sequence. We have studied the thermal history of the Pelagonian basement and the provenance of the detrital material in the clastic sediments of the MHT by fission track dating. Apatite and zircon fission track (AFT, ZFT) analysis is applied to samples from the Pelagonian microcontinent along the eastern border of the MHT, and AFT analysis to the sedimentary rocks in the southern MHT. Eocene AFT age populations in the sedimentary strata indicate a proximal position of the Pelagonian microcontinent, which shows the same or even younger AFT ages, as the source area of the detrital material in the MHT. Late Cretaceous to Palaeocene age populations point to a more distant or structurally higher (now eroded) source area. The Eocene orogenic event caused only weak thermal overprinting in rocks of the Pelagonian microcontinent. In its eastern part, the AFT ages show only partial resetting, if any, whereas in its western part the ages were clearly reset during the Eocene event. AFT age‐elevation relations, correlation of zircon and apatite FT ages from the same samples, and thermal modelling based on AFT ages and track length distributions were all used to reconstruct the low‐temperature cooling history of the Pelagonian basement adjacent to the MHT. The results document fast cooling and exhumation in the Eocene that were possibly related to thrusting and associated erosion, followed by slow cooling and exhumation during Oligocene and Miocene time. This scenario is confirmed by the AFT data from the detrital material in the MHT sedimentary strata. The slow cooling period coincides with a stagnation period or crustal extension and possible reheating, which is probably also responsible for the partial rejuvenation of the ages of the detrital apatites in the oldest (Eocene) formation in the sediment sequence of the MHT.

^{207}Pb-^{206}Pb, ^{40}Ar-^{39}Ar and Apatite Fission-Track Geothermochronology Revealing the Emplacement, Cooling and Exhumation History of the Karaçayır Syenite (N Sivas), East-Central Anatolia, Turkey

The Karaçayır syenite, intruding the Palaeozoic crustal metamorphics and unconformably overlain by Upper Paleocene to Eocene Tokuş formation in the north of Sivas (east-central Anatolia), has been studied with respect to ^{207}Pb-^{206}Pb single zircon evaporation, biotite ^{40}Ar-^{39}Ar, and apatite fission-track geothermochronology. ^{207}Pb-^{206}Pb single zircon evaporation dating yields an age of 99.0±11.0 Ma (Cenomanian-Turonian) which is considered to be the intrusion age. Biotite ^{40}Ar-^{39}Ar age determination gives a cooling age of ca. 65 Ma. Apatite fission-track dating, combined with T-t modeling based on track-length distribution data, determines a fast tectonic exhumation with an uplift rate of > 1 mm/a which occurred 58-61 Ma ago. This Middle Paleocene fast tectonic uplift is considered to have resulted from a compressional regime induced by the collision between the Tauride-Anatolide platform (TAP) and the Eurasian plate (EP) along the İzmir-Ankara-Erzincan (İAE) suture zone following the closure of the north-dipping subducted İAE ocean which belongs to northern Neo-Tethyan realm. This compressional regime has also formed the peripheral foreland basins in central Anatolia.

Projected-length distributions of fission-fragment tracks from U and Th thin film sources in muscovite

Thin films of natural uranium and thorium deposited on muscovite were used as sources of neutron-induced fission fragments. Fragment energy loss in thin-source geometry is negligible. In this way, the observed fragment range results from the interaction of the fragment with the detector material. This characteristic enables the investigation of asymmetric fission and etching, through measurements of projected track length distributions in muscovite micas coupled with thin films. The means and standard deviations of the etchable length distributions of the heavy and light fission-fragment tracks were estimated by fitting a theoretical equation to the experimental data.The light fission-fragment accounts for ∼54% and the heavy fission-fragment for ∼46% of the etchable length of a full fission track. This average partition is the same for tracks from thermal-neutron-induced fission of 235U and fast-neutron-induced fission of 232Th. The mean etchable length of uranium fission tracks is ∼2.5% longer than that of thorium fission tracks. This difference is at the resolution limit of these measurements but correlates with the difference in the mean combined initial kinetic energies of the fission fragments. The mean etchable length of uranium fission tracks in muscovite is ∼5% shorter than their calculated latent track length, supporting earlier estimates of a length deficit of this magnitude. The length deficit and the standard deviation of the etchable length distribution of the light fission-fragment tracks are twice the equivalent values for the heavy fission-fragment tracks. This is interpreted in terms of a vt-profile (track etch rate) that depends on the mass of the track-forming particle.

Apatite fission-track thermochronology applied to the Chulyshman Plateau, Siberian Altai Region

The Chulyshman Plateau (CP) is located in the central Siberian Altai Mountains and is bordered by active strike-slip and thrust faults. The current mountainous morphology developed since the Late Miocene to Pliocene by transpressive tectonics as a distant effect of India–Eurasia convergence. The metamorphic basement of the plateau is widely intruded by Palaeozoic granitoids. To constrain the thermo-tectonic history, apatite fission-track (AFT) thermochronology on basement and granitoids was performed. Cretaceous AFT ages ( ∼ 130 – 90 Ma ) were obtained, while mean track lengths ( 13 – 14 μ m ) and length distributions indicate thermal track shortening and prolonged residence in the apatite partial annealing zone. The AFT data were modelled and a three-stage thermal history model was reconstructed. First, a Late Jurassic–Cretaceous cooling is associated with Mesozoic tectonic reactivation of the area. A second stage is a Late Cretaceous–Early Neogene period of stability linked to regional peneplanation. Final cooling is related to denudation of the CP during Late Cenozoic tectonic reactivation.

Measurements of fossil confined fission tracks in ion-irradiated apatite samples with low track densities

This investigation looks into the potential and consequences of combined ion-irradiation and sequential etching as means for increasing the number of measurable horizontal confined tracks in geological apatite samples with low fission-track densities. The gain is substantial but varies between samples depending on the size and aspect of the grains and the fraction of prismatic sections. The average maximum gain (ρion≈2×06cm−2; lion≈30μm; θion=15°; 35s etching in 4M HNO3 at 25°C) is ∼200 times. Greater gains are achieved in some cases but there are indications of saturation. For maximum effect, the beam parameters should be tailored to the properties of the sample. The effectiveness of sequential etching is much greater than expected from the ratio of the etching times or of the track cross-sections at successive etching times. The number of confined tracks increases ∼3.5 times between te=15s and 25s and a further ∼2 times between te=25s and 35s. The total increase due to etching (∼7 times) is still much lower than that from ion-irradiation alone, estimated at ∼30 times. Ion-irradiation has no significant effect on the etchable lengths of individual confined tracks and no pronounced effect on their angular distribution. Sequential etching has no apparent effect on their angular distribution; its effects on the lengths of individual tracks, mean confined-track length, length distribution and anisotropy are less clear. Overall, our results document a quasi-isotropic increase of the mean confined-track length between te=15s and 25s and an according quasi-uniform shift of the track-length distribution towards longer lengths but no significant further increase nor shift between te=25s and 35s. This trend varies between samples because the relationship between the mean track length and that of individual confined tracks is affected by complex sampling issues that require detailed investigation.