Crystallographic orientation dependence of mean etchable fission track length in apatite: An empirical model and experimental observations

On Confined-Track Samples for Constraining Thermal Histories

Preprint

Full-text available

Sep 2024

Fission-track modelling rests on etching, counting and measuring the lattice damage trails from uranium fission. The tools for interpreting fission-track data are advanced but the results are never better than the data. Confined-track samples must be an adequate size for statistical analysis, representative of the track population, consistent with the model assumptions and with the calibration data. Geometrical and measurement biases are understood and can be dealt with up to a point. The interrelated issues of etching protocol and track selection are however more difficult to untangle. Our investigation favours a two-step protocol. The duration of the first step is inversely proportional to the apatite etch rate, so that different apatites etch to the same Dpar. A long immersion reveals many more confined tracks, terminated by basal and prism faces. This allows consistent length measurements and permits to orient each track relative to the c-axis. Long immersion times combined with deep ion irradiation reveal confined tracks deep inside the grains. Provided it is long enough, the precise immersion time is not important if the effective etch times of the selected tracks are calculated from their measured widths. Then, whether the sample is mono- or multi-compositional, we can, post hoc, select tracks with the desired properties. The second part of the protocol has to do with the fact that fossil tracks in geological samples appear to be under-etched compared to induced tracks etched under the same conditions. This should be assumed if the semi-axes (lC, lA) of a fitted ellipse plot above the induced-track line. In that case an additional etch can increase the track lengths to a point where they are consistent with the model based on lab-annealing of induced tracks, a condition for valid thermal histories. Here too, it is possible to select a subset with effective etch times consistent with the model if the widths of confined tracks are measured along with their lengths and orientations.

Sampling Confined Fission Tracks for Constraining Geological Thermal Histories

Article

Full-text available

Oct 2024

Fission-track modeling rests on etching, counting and measuring the lattice damage trails from uranium fission. The tools for interpreting fission-track data are advanced but the results are never better than the data. Confined-track samples must be an adequate size for statistical analysis, representative of the track population and consistent with the model assumptions and with the calibration data. Geometrical and measurement biases are understood and can be dealt with up to a point. However, the interrelated issues of etching protocol and track selection are more difficult to untangle. Our investigation favors a two-step protocol. The duration of the first step is inversely proportional to the apatite etch rate so that different apatites etch to the same Dpar. A long immersion reveals many more confined tracks, terminated by basal and prism faces. This allows consistent length measurements and permits orienting each track relative to the c-axis. Long immersion times combined with deep ion irradiation reveal confined tracks deep inside the grains. Provided it is long enough, the precise immersion time is not important if the effective etch times of the selected tracks are calculated from their measured widths. Then, whether the sample is mono- or multi-compositional, we can, post hoc, select tracks with the desired properties. The second part of the protocol has to do with the fact that fossil tracks in geological samples appear to be under-etched compared to induced tracks etched under the same conditions. This should be assumed if the semi-axes of a fitted ellipse plot above the induced-track line. In that case, an additional etch can increase the track lengths to a point where they are consistent with the model based on lab-annealing of induced tracks, a condition for valid thermal histories. Here too, it is possible to select a subset of tracks with effective etch times consistent with the model if the widths of confined tracks are measured along with their lengths and orientations.

Traçage des sédiments dans les séries syn-orogéniques du bassin d'Aquitaine

Thesis

Jun 2020

Stéphane M. Al Reda

La dynamique de formation d’une chaîne de montagne peut être reconstruite à partir de l’étude des bassins réceptacles des produits d’érosion. Les travaux présentés ont été réalisés dans la partie orientale du Bassin d’Aquitaine (Corbières), à partir de l’étude de la série de Palassou. Ces sédiments, en majorité conglomératiques, traduisent une phase d’érosion majeure des reliefs, accompagnant la continentalisation des bassins au cours de l’Yprésien et la période de raccourcissement principal de l’orogenèse pyrénéenne. Trois unités tectono-stratigraphiques ont été défini dans cette série : l’unité 1 - Yprésien supérieur-Lutétien – caractérisée par la présence de galets Méso-Cénozoïques, l’unité 2 – Bartonien – caractérisée par la présence des galets de socle et l’unité 3 – Priabonien- caractérisée par la présence à nouveau de galets Méso-Cénozoïques. L’objectif de cette thèse est de comprendre le mode de remplissage du bassin d’avant-pays nord pyrénéen et son évolution au cours de l’Éocène. Ceci permettra d’appréhender la formation des reliefs à l’intérieur de la chaîne et de caractériser les différentes sources d’apport. Trois approches ont été utilisées pour reconstruire le routage des sédiments dont la première est l’étude sédimentaire et la caractérisation des environnements de dépôts des deux premières unités. Les résultats obtenus montrent l’identification de quatre séquences de remplissage sédimentaire dans l’unité 1, dont l’essentiel s’est déposé dans le synclinal de Talairan jusqu’à son débordement au cours du dépôt de la troisième séquence. Ceci amène à interpréter le synclinal de Talairan comme un bassin en piggy back. La deuxième approche est la thermochronologie basse température appliquée sur des clastes de granites issus de l’unité 2. Les résultats de traces de fission sur apatite montrent des âges plus jeunes d’est en ouest alors que les résultats de (U-Th-Sm)/He sur apatite montrent une dispersion des âges. La modélisation thermique de ces résultats indique un réchauffement post-Bartonien, traduisant une série sédimentaire plus épaisse au moment du dépôt puis partiellement érodée. La phase d’érosion est estimée comme pré-Langhienne suite aux résultats de modélisation thermique et des contraintes stratigraphiques. La troisième approche utilisée au cours de cette thèse est la datation U/Pb sur zircon. Des échantillons de matrice de conglomérat ainsi que des galets de granite issus des trois unités ont fait l’objet d’étude par cette approche. Les résultats obtenus montrent un signal Varisque majeur. Ils ont été couplés avec des analyses Raman et des directions de paléo-courant mesurés afin de caractériser les sources des sédiments pour chaque unité. Les dépôts de l’unité 1 sont issues de la Zone Nord Pyrénéenne. Les dépôts de l’unité 2 ont comme source la Zone Axiale des Pyrénées alors que les dépôts de l’unité 3 présentent un spectre d’âges assez large dont la source est la partie orientale et centrale des Pyrénées. Ces résultats ont permis de reconstruire le cheminement des sédiments dans les Corbières au cours de l’Éocène.

Structural evolution of the Kohat and Potwar fold and thrust belts of Pakistan

Thesis

Jan 2019

Humaad Ghani

Fold and thrust belts are characteristic features of collisional orogen that grow laterally through time by deforming the upper crust in response to stresses caused by convergence. The deformation propagation in the upper crust is accommodated by shortening along major folds and thrusts. The formation of these structures is influenced by the mechanical strength of décollements, basement architecture, presence of preexisting structures and taper of the wedge. These factors control not only the sequence of deformation but also cause differences in the structural style. The Himalayan fold and thrust belt exhibits significant differences in the structural style from east to west. The external zone of the Himalayan fold and thrust belt, also called the Subhimalaya, has been extensively studied to understand the temporal development and differences in the structural style in Bhutan, Nepal and India; however, the Subhimalaya in Pakistan remains poorly studied. The Kohat and Potwar fold and thrust belts (herein called Kohat and Potwar) represent the Subhimalaya in Pakistan. The Main Boundary Thrust (MBT) marks the northern boundary of both Kohat and Potwar, showing that these belts are genetically linked to foreland-vergent deformation within the Himalayan orogen, despite the pronounced contrast in structural style. This contrast becomes more pronounced toward south, where the active strike-slip Kalabagh Fault Zone links with the Kohat and Potwar range fronts, known as the Surghar Range and the Salt Range, respectively. The Surghar and Salt Ranges developed above the Surghar Thrust (SGT) and Main Frontal Thrust (MFT). In order to understand the structural style and spatiotemporal development of the major structures in Kohat and Potwar, I have used structural modeling and low temperature thermochronolgy methods in this study. The structural modeling is based on construction of balanced cross-sections by integrating surface geology, seismic reflection profiles and well data. In order to constrain the timing and magnitude of exhumation, I used apatite (U-Th-Sm)/He (AHe) and apatite fission track (AFT) dating. The results obtained from both methods are combined to document the Paleozoic to Recent history of Kohat and Potwar. The results of this research suggest two major events in the deformation history. The first major deformation event is related to Late Paleozoic rifting associated with the development of the Neo-Tethys Ocean. The second major deformation event is related to the Late Miocene to Pliocene development of the Himalayan fold and thrust belt in the Kohat and Potwar. The Late Paleozoic rifting is deciphered by inverse thermal modelling of detrital AFT and AHe ages from the Salt Range. The process of rifting in this area created normal faulting that resulted in the exhumation/erosion of Early to Middle Paleozoic strata, forming a major unconformity between Cambrian and Permian strata that is exposed today in the Salt Range. The normal faults formed in Late Paleozoic time played an important role in localizing the Miocene-Pliocene deformation in this area. The combination of structural reconstructions and thermochronologic data suggest that deformation initiated at 15±2 Ma on the SGT ramp in the southern part of Kohat. The early movement on the SGT accreted the foreland into the Kohat deforming wedge, forming the range front. The development of the MBT at 12±2 Ma formed the northern boundary of Kohat and Potwar. Deformation propagated south of the MBT in the Kohat on double décollements and in the Potwar on a single basal décollement. The double décollement in the Kohat adopted an active roof-thrust deformation style that resulted in the disharmonic structural style in the upper and lower parts of the stratigraphic section. Incremental shortening resulted in the development of duplexes in the subsurface between two décollements and imbrication above the roof thrust. Tectonic thickening caused by duplexes resulted in cooling and exhumation above the roof thrust by removal of a thick sequence of molasse strata. The structural modelling shows that the ramps on which duplexes formed in Kohat continue as tip lines of fault propagation folds in the Potwar. The absence of a double décollement in the Potwar resulted in the preservation of a thick sequence of molasse strata there. The temporal data suggest that deformation propagated in-sequence from ~ 8 to 3 Ma in the northern part of Kohat and Potwar; however, internal deformation in the Kohat was more intense, probably required for maintaining a critical taper after a significant load was removed above the upper décollement. In the southern part of Potwar, a steeper basement slope (β≥3°) and the presence of salt at the base of the stratigraphic section allowed for the complete preservation of the stratigraphic wedge, showcased by very little internal deformation. Activation of the MFT at ~4 Ma allowed the Salt Range to become the range front of the Potwar. The removal of a large amount of molasse strata above the MFT ramp enhanced the role of salt in shaping the structural style of the Salt Range and Kalabagh Fault Zone. Salt accumulation and migration resulted in the formation of normal faults in both areas. Salt migration in the Kalabagh fault zone has triggered out-of-sequence movement on ramps in the Kohat. The amount of shortening calculated between the MBT and the SGT in Kohat is 75±5 km and between the MBT and the MFT in Potwar is 65±5 km. A comparable amount of shortening is accommodated in the Kohat and Potwar despite their different widths: 70 km Kohat and 150 km Potwar. In summary, this research suggests that deformation switched between different structures during the last ~15 Ma through different modes of fault propagation, resulting in different structural styles and the out-of-sequence development of Kohat and Potwar.

11. Forward and Inverse Modeling of Low-Temperature Thermochronometry Data: Techniques, Interpretations, and Applications

Chapter

Dec 2005

Richard Ketcham

Structural variation within the Himalayan fold and thrust belt: A cCase study from the Kohat-Potwar Fold Thrust Belt of Pakistan

Article

Aug 2018
J STRUCT GEOL

Abstract The Kohat and Potwar fold thrust belts (KP-FTB) in Pakistan exhibit structural variations over 250 km along strike within the Himalayan fold and thrust system. Our 3D deformation model shows that Kohat surface structures evolved above an active roof thrust in Eocene evaporites. The ramp-forming duplexes in the Kohat were stacked and passively transported toward the foreland above new ramps, resulting in up to 5 km of thickening between the two decollements. Ramps from the Kohat extend into the Potwar as thrust tips of fault propagation folds. The basement slope changes from flat (β < 1°) below the northern part to north-dipping (β > 1°) below the southern part, corresponding to the change in structural style and complexity of the KP-FTB. The Kalabagh Fault Zone, linking the two belts, is interpreted as a zone of complex dextral strike-slip rotational faulting. Salt expulsed from the hanging walls of normal faults and under synclines in the Kalabagh Fault Zone moved toward the footwall of normal faults, accumulated in the cores of anticlines, and formed lobe structures at the deformation front. The fundamental reasons for the variable structural styles are changes in decollement strength, basement slope, preexisting normal faulting, presence of a secondary decollement and spatially-variable salt mobility and accumulation.

Fission-Track Annealing: From Geologic Observations to Thermal History Modeling

Chapter

Jul 2019

Richard Ketcham

This chapter reviews the evolving state of knowledge concerning fission-track (FT) annealing, primarily in apatite and zircon, based on theory, experiments, and geological observations. Multiple insights into track structure, formation, and evolution arise from transmission electron microscopy, small-angle X-ray scattering, atomic force microscopy, and molecular dynamics computer modeling. Our principal knowledge, however, comes from experiments in which spontaneous or induced tracks are annealed, etched, and measured, the results statistically fitted, and their predictions compared against geological benchmarks. This empirical approach has proven effective and resilient, though physical understanding remains an ultimate goal. The precise mechanism by which lattice damage anneals, and how it varies among minerals and damage types, remains unknown. Multiple similarities between apatite and zircon suggest equivalent underlying processes. Both minerals demonstrate annealing anisotropy, and its characterization is crucial for understanding both track shortening and density reduction. The fanning curvilinear equation, featuring curved iso-annealing lines on an Arrhenius-type diagram, has been the most successful for matching data spanning timescales from seconds to hundreds of millions of years. A super-model featuring a single set of iso-annealing lines describes all apatite experimental data to date. Annealing rates vary with both anion and cation substitutions, and more work is required to ascertain how these substitutions interact. Other areas for further research include differences between spontaneous and induced tracks, and possible additional processes affecting length and density evolution, such as seasoning. Thermal history inversion simultaneously leverages and tests our models, and accounting for kinetic variation is key for doing it soundly.

Single-track length measurements of step-etched fission tracks in Durango apatite: "vorsprung durch Technik"

Article

May 2017

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.

THE LONG-TERM THERMAL EVOLUTION OF CENTRAL FENNOSCANDIA, revealed by integrated low temperature thermochronometry.

Thesis

Full-text available

Oct 2003

Glen Murrell

Modelling the thermal history of onshore Ireland, Britain and its offshore basins using low-temperature thermochronology

Thesis

Full-text available

Jan 2017

Daniel Doepke

The main goal of this PhD thesis is to investigate the early Cenozoic exhumation history of Ireland and Britain. The causal mechanism for early Cenozoic exhumation on this sector of the NW European margin remains contentious, but broadly can be divided into two main competing hypotheses: i) exhumation caused by mantle-driven processes associated with the development of the proto-Iceland mantle plume and ii) exhumation related to far-field compressional stress associated with the effects of the opening of the North Atlantic Ocean and the Alpine collision. Here, a combination of AFT and AHe dating was employed to discriminate between these two hypotheses. Sample locations were focused around the Irish Sea, and extended into Wales, central and western Ireland and Scotland. Overall, 47 samples were analysed for AFT and AHe dating, including four pseudo-vertical profiles. The AFT results range from 43.3 ± 5.8 Ma (1σ) on the Isle of Arran (IoA-3) to 353.6 ± 12.1 Ma (1σ) in central Scotland (Sct-6). The AHe ages (1σ, FT-corrected) range from 5.2 ± 0.4 Ma (Ire-10, a borehole at 1702 m) to 290.9 ± 23.3 Ma in central Scotland (Sct-9). The key findings of the thermal history modelling reveal that late Permian to mid-Jurassic onshore exhumation in Ireland and Wales shows a clear temporal relationship between exhumation and rifting in the immediately adjacent offshore basins, which is related to the complex break-up of Pangea and the subsequent initiation of rifting in the North Atlantic region. The Scottish massif appears rather unaffected by Pangea break-up and subsequent Mesozoic rifting, with most samples from Scotland generally showing slow cooling during Paleozoic and Mesozoic times. A key finding of this study is that a combination of both AFT and AHe dating with pseudo-vertical profile analysis is necessary in order to resolve small amounts of exhumation. Thereby, it was possible to sub-divide the early Cenozoic exhumation history of Ireland and Britain into four different zones, depending on intensity of the exhumation signal. The first zone is centred on the northern part of the Irish Sea and experienced 2 to 3 km of early Cenozoic exhumation, assuming a paleogeothermal gradient of 25°C/km. The second zone in Wales and western Scotland experienced between 1.5 and 2 km exhumation and the third zone in southeast Ireland and in part of the Northern Highlands of Scotland experienced up to 1 km of exhumation. The final zone in south Wales, central Scotland and west Ireland shows no sign of early Cenozoic exhumation. The results indicate a clear “dome-like” exhumation pattern centred on the northern part of the Irish Sea, and more limited exhumation in south Wales and western Scotland. This focused exhumation event is in good agreement with previously published exhumation patterns but is better resolved, and is synchronous with the emplacement of the BIPIP. This study identifies the emplacement of an elongate body of hot mantle material beneath the Irish Sea associated with the development of the proto-Iceland mantle plume as the main cause of early Cenozoic exhumation. It is also possible that a component of the observed early Cenozoic exhumation was caused by dynamically supported uplift and mechanical erosion of the lithosphere. The AFT and AHe dating systems are not temperature-sensitive enough to resolve the Neogene thermal history, but the presented thermal histories suggest small amounts of Neogene exhumation for Ireland, whereas the Scottish samples tend to show monotonic cooling during the Neogene.

Composition and orientation dependent annealing of ion tracks in apatite - Implications for fission track thermochronology

Article

Dec 2016
CHEM GEOL

The annealing behaviour of swift heavy-ion tracks in apatite from different origins is studied as a function of their crystallographic orientation and the mineral composition. The tracks were generated by irradiating the apatite samples with 2.3 GeV Bi ions, which have a comparable rate of energy loss to fission tracks in this mineral. The track radius was investigated using synchrotron-based small-angle x-ray scattering (SAXS) combined with ex situ annealing. Results indicate that tracks parallel to the c-axis are initially larger and anneal slower than those perpendicular to the c-axis. Natural variation in the mineral composition shows stronger annealing resistance of ion tracks with higher chlorine content. The SAXS results are consistent with previous studies on etched tracks and provide evidence that the orientation and composition effects are directly linked to the property of the un-etched track and not to preferential etchability. The study helps to connect the empirical studies on etched fission tracks to more fundamental solid-state processes.

Mineral detection of neutrinos and dark matter. A whitepaper

Article

Full-text available

May 2023

Mineral Detection of Neutrinos and Dark Matter. A Whitepaper

Preprint

Jan 2023

Minerals are solid state nuclear track detectors - nuclear recoils in a mineral leave latent damage to the crystal structure. Depending on the mineral and its temperature, the damage features are retained in the material from minutes (in low-melting point materials such as salts at a few hundred degrees C) to timescales much larger than the 4.5 Gyr-age of the Solar System (in refractory materials at room temperature). The damage features from the O(50) MeV fission fragments left by spontaneous fission of

^{238}

U and other heavy unstable isotopes have long been used for fission track dating of geological samples. Laboratory studies have demonstrated the readout of defects caused by nuclear recoils with energies as small as O(1) keV. This whitepaper discusses a wide range of possible applications of minerals as detectors for

E_R \gtrsim O(1)

keV nuclear recoils: Using natural minerals, one could use the damage features accumulated over O(10) Myr

-O(1)

Gyr to measure astrophysical neutrino fluxes (from the Sun, supernovae, or cosmic rays interacting with the atmosphere) as well as search for Dark Matter. Using signals accumulated over months to few-years timescales in laboratory-manufactured minerals, one could measure reactor neutrinos or use them as Dark Matter detectors, potentially with directional sensitivity. Research groups in Europe, Asia, and America have started developing microscopy techniques to read out the

O(1) - O(100)

nm damage features in crystals left by

O(0.1) - O(100)

keV nuclear recoils. We report on the status and plans of these programs. The research program towards the realization of such detectors is highly interdisciplinary, combining geoscience, material science, applied and fundamental physics with techniques from quantum information and Artificial Intelligence.

Differential exhumation of cratonic and non-cratonic lithosphere revealed by apatite fission-track thermochronology along the edge of the São Francisco craton, eastern Brazil

Article

Full-text available

Feb 2022

Lithosphere of cratons and orogens generally reacts differently to tectonic events. Although these differences are mostly clear during the orogenic phases, understanding how they respond to tectonic reactivation is still challenging. Here, we report the first detailed apatite fission‑track (AFT) study pinpointing the gradual transition between cratonic and orogenic lithosphere, using the case study of the São Francisco craton (SFC) and the adjacent Araçuaí‑West Congo Orogen (AWCO), eastern Brazil. The collision that built the AWCO partially affected the inherited rift structures of the Paramirim Aulacogen, embedded in the São Francisco‑Congo paleocontinent. Our data reveal a differential Phanerozoic exhumation between closely interspaced areas affected and not affected by the AWCO deformation. Samples from the SFC present slow and protracted basement cooling during the Phanerozoic, while samples from the orogen display rapid exhumation since the Eocene. An intermediate ~ N–S zone of c.40 km shows lower magnitude basement cooling during the Cenozoic, possibly because the propagation of AWCO deformation decreases towards the craton interior. Within the orogen, the Rio Pardo salient is the main reactive structure and probably results from the deformation of a master fault, inherited from its precursor rift. Here, we show how the magnitude of Phanerozoic denudation may be deeply associated with previous events of lithosphere weakening.

A new phenomenological model for annealing of fission tracks in apatite: laboratory data fitting and geological benchmarking

Article

Full-text available

May 2021
PHYS CHEM MINER

Rana track annealing model 2007, based on an Arrhenius approach, is significantly modified, including new assumptions, fitting parameters, and analysis, to yield a new phenomenological model for annealing of fission tracks in apatite. Construction and optimization of the new six parameter model have been described in detail. The model shows a hybrid behavior. In Arrhenius plot, for higher temperatures (laboratory data included), iso-annealing lines are curved while, for low temperatures (geological time scale), iso-annealing lines become parallel straight lines. This is a new feature for any such model and has produced promising results. C-axis projected lengths from laboratory experiments on Durango apatite are employed to find model parameters. KTB borehole fission-track data are added to build the temperature function that modifies the original equation. Partial annealing zone (PAZ), closure temperature (TC) and total annealing temperature (TA) were calculated and compared to geological benchmarks. The predictions of the present model agreed well with low and high temperature benchmarks. PAZ, TC and TA predictions were also compared to the Fanning Curvilinear Arrhenius model predictions, resulting in good agreement. The present model is flexible enough to be applied to other fission-track systems, like zircon, muscovite and titanite.

Thermo-tectonics of the Calabrian Arc, southern Italy: Constraints from fission track analysis

Thesis

Jan 1993

Stuart Nigel Thomson

This study uses fission track analysis to provide temperature and time constraints on the cooling and exhumation history of the basement rocks of the Calabrian Arc of southern Italy. Fission track analysis also provides information on the provenance and burial history of the terrigenous Oligo-Miocene Stilo-Capo d'Orlando Formation. 65 samples from the basement rocks have yielded 57 apatite fission track ages, 54 zircon fission track ages and 25 apatite track length distributions. 9 samples from the Stilo- Capo d'Orlando Formation have yielded 8 apatite fission track ages, 8 zircon fission track ages and 6 apatite track length distributions. Qualitative and quantitative analysis of the fission track data reveals that the majority of the basement rocks underwent a phase of increased cooling related to exhumation between about 35 Ma (Early Oligocene) and 15 Ma (Middle Miocene). Evidence from the local sedimentary record indicates that erosion played an important role in the exhumation process. Extensional tectonism also contributes to some of the increased exhumation. Analysis of the fission track results obtained from the Stilo-Capo d'Orlando Formation confirm a Calabrian basement provenance for the sediments. The previously debated origin of volcanic conglomerate clasts from the formation is also resolved. Finally apatite fission track analysis indicates post-depositional burial at the base of the formation to temperatures greater than 80° C. The final part of this thesis uses the fission track age and temperature constraints to produce an improved tectonic model for the Oligo-Miocene tectonic evolution of the Calabrian Arc. This model proposes that increased exhumation is a consequence of the dynamics of an overthickened orogenic wedge. The model is related to the overall plate dynamics of the western Mediterranean orogeny.

Reconstructing deep‐time histories from integrated thermochronology: An example from southern Baffin Island, Canada

Article

Full-text available

Jun 2019

We present a multi-chronometric approach for reconstructing deep-time thermal histories using southern Baffin Island as a case study. This continuous thermal history begins with the Palaeoproterozoic Trans-Hudson Orogeny and is derived from in- verse and forward models that integrate thermochronometers spanning some 500°C: new apatite U–Pb ages and K-feldspar 40Ar/39Ar multi-diffusion domain data, pub- lished (U–Th)/He zircon ages and new multi-kinetic fission-track results. Integration of data from a wider temperature range reduces ambiguities in thermal-history mod- elling and permits us to constrain the timing of geological processes including, extended post-orogenic cooling, enhanced later Proterozoic cooling, and then episodic burial and exhumation in the Palaeozoic–Mesozoic.

MOUVEMENTS VERTICAUX DE LA BORDURE NORD DU CRATON OUEST AFRICAIN (ANTI-ATLAS CENTRAL, MAROC). APPORT DE LA THERMOCHRONOLOGIE BASSE TEMPERATURE

Thesis

Full-text available

May 2013

Mostafa Oukassou

L’Anti-Atlas se situe à la bordure nord du Craton Ouest-Africain (WAC). Cet orogène intracontinental plissé au Carbonifère est caractérisé par des reliefs très importants (sommets à ~ 2500 m) énigmatiques pour une chaîne très ancienne. La connaissance de l’évolution géodynamique de ce domaine et des mouvements verticaux qui sont responsables de son relief ont été éclairés par l’application de la thermochronologie de basse température (traces de fission sur apatite et zircon (TF) et (UTh)/He sur apatite. En Anti-Atlas central (Siroua, Zenaga et Bou Azzer-El Graara), les âges traces de fission sur zircon (ZFT) déterminés sur des granitoïdes et des gneiss précambriens (13 échantillons) sont compris entre 340±27 et 306±21 Ma. Les âges traces de fission sur apatite (AFT) (18 échantillons) varient entre 171±7 Ma et 133±5 Ma, avec des longueurs de traces confinées comprises entre 10.8±2.4 μm et 12.8±1.7 μm. Les âges (U Th)/He sur apatite (AHe) (6 échantillons) sont groupés entre 26±4 Ma et 61±29 Ma. En s’appuyant sur des calages chrono-stratigraphiques contraints par la géologie régionale, des modélisations de l’histoire thermique à partir des donnés TF et (U-Th)/He ont été réalisées avec les algorithmes AFTSolve® et HeFty®. L’exhumation post-varisque de l’Anti-Atlas central est complexe, les chronomètres (UTh)/He et TF nous ont permis de déterminer un début de phase de surrection vers 320 Ma. Vers 140-120 Ma le taux d’exhumation s’est accéléré jusqu’à 0.1 km/Ma. Entre 120 et 40 Ma il y a eu un réchauffement jusqu’à des températures de l’ordre de 65-70 °C à 55-40 Ma. Enfin, une exhumation finale jusqu’à la surface s’est produite dans les derniers 40 millions d’années. L’âge moyen ZFT obtenu dans ce travail est 321±21 Ma, similaire aux résultats trouvés dans l’Anti-Atlas occidental. Ce résultat est comparable aux datations 40K/40Ar sur micas blanc et biotite. Le fait que ces différents systèmes donnent des résultats équivalents suggère un événement thermique court au Viséen supérieur-Namurien, suivi par un refroidissement rapide en-dessous de 240±20°C. Le réchauffement est attribué au plissement varisque et à la circulation des fluides hydrothermaux. Les phases d’exhumation méso-cénozoïques sont liées à la réactivation des structures varisques (rifting de l’Atlantique central et de la Téthys alpine, orogenèse alpine). La plus récente est en relation avec une anomalie mantélique régionale à la bordure nord du craton Ouest-Africain, connue pour être responsable d’une partie du soulèvement des Atlas marocains. L’histoire thermique post-varisque confirme qu’une couverture sédimentaire épaisse (~1.2 km) d’âge Crétacé-Paléogène a existé dans les régions etudiées, dont l’essentiel a été érodé au cours du Néogène. Au regard des résultats thermochronologiques basse température de ce travail, ainsi que la synthèse des données thermochronologiques disponibles pour l’Anti-Atlas occidental, nous concluons que tout l’Anti-Atlas centro-occidental a subi une évolution tectonique et thermique commune depuis la fin du plissement varisque, tandis que la comparaison avec l’Anti-Atlas oriental reste discutable, vu l’absence apparente du réchauffement Crétacé-Paléogène dans ce domaine. Mots clés : Géodynamique, Anti-Atlas, exhumation, thermochronologie, traces de fission, (U-Th)/He.

Relating differential crustal architecture to passive margin evolution: A case study from the Colatina Fracture Zone (SE Brazil) using apatite fission-track thermochronology

Article

Jul 2024

The Colatina Fracture Zone (CFZ) defines a distinct NNW-SSE-oriented linear zone of fractures and brittle faults that represents an inherited weak zone in the current crustal structure of the (Pre)Cambrian Araçuaí Orogen. In the Early Cretaceous, the CFZ was reactivated during rifting of West Gondwana and subsequent opening of the South Atlantic Ocean, as evidenced by the emplacement of dykes along its structural network and the development of major depocentres of the Campos Basin in the offshore segments of the CFZ. Previous thermochronological studies have demonstrated that the CFZ was also rejuvenated during the drift phase of the South Atlantic. However, a number of questions regarding differential surface uplift and basement exhumation between the CFZ and its surrounding areas, such as the Doce River Valley (DRV), are still unresolved. In this study, we aim to investigate the CFZ as a distinctive structure in the tectonic rejuvenation of the passive margin of southeast Brazil. Samples from the CFZ and the DRV were collected for apatite fission-track (AFT) analyses. In the DRV, samples yield AFT central ages from 87 to 97 Ma with mean track lengths (MTL) from 12.6 to 13.3 μm. In contrast, in the CFZ, AFT central ages from 70 to 83 Ma with MTL values from 13.2 and 13.4 μm are obtained. The correlation between AFT age and elevation suggests that the tectonic development of these regions was markedly different and uncoupled. The thermal history models from the AFT data further constrain this differential evolution. On the one hand, thermal history modelling for the DRV indicates a slower and protracted cooling since the incipient Atlantic rifting in the Early Cretaceous. On the other hand, the models for CFZ reveal a rapid cooling phase between the Late Cretaceous to the Palaeocene. In the DRV, the observed basement cooling was most probably triggered by erosion of the uplifted rift shoulder generated by Gondwana break-up. The more recent, Late Cretaceous-Palaeocene rock cooling, localized in the CFZ, was synchronous with a major phase of the Andean orogeny. This suggests that reactivations and erosional exhumation of the CFZ basement could be a consequence of far-field propagation of intraplate compressional stress. The higher susceptibility of the CFZ to reactivating over its surroundings shows that structural inheritance is a key factor in the differential tectonic evolution of passive margins. Further research on the Late

Extensional exhumation of cratons: insights from the Early Cretaceous Rio Negro–Juruena belt (Amazonian Craton, Colombia)

Article

Full-text available

Mar 2024
SE

This study presents results from apatite fission track (AFT) thermochronology to investigate the thermal history and exhumation dynamics of the Rio Negro–Juruena basement, situated within the western Guiana Shield of the Amazonian Craton. AFT dating and associated thermal history modeling in South America has largely been restricted to the plate's margins (e.g., Andean active margin, Brazilian passive margin, and others). Our paper reports on low-temperature thermochronological data from the internal part of the western Guiana Shield for the first time. This area is part of a vast cratonic lithosphere that is generally thought to be stable and little influenced by Mesozoic and Cenozoic tectonics. Our data, however, show AFT central ages ranging from 79.1 ± 3.2 to 177.1 ± 14.8 Ma, with mean confined track lengths of ca. 12 µm. Contrary to what might be expected of stable cratonic shields, inverse thermal history modeling indicates a rapid basement cooling event in the early Cretaceous. This cooling is interpreted as a significant exhumation event of the basement that was likely driven by the coeval extensional tectonics associated with back-arc rifts in the Llanos and Putumayo–Oriente–Maranon basins. The extensional tectonics facilitated both basement uplift and subsidence of the adjoining basins, increasing erosional dynamics and consequent exhumation of the basement rocks. The tectonic setting shifted in the late Cretaceous from extensional to contractional, resulting in reduced subsidence of the basins and consequential diminishing cooling rates of the Guiana Shield basement. Throughout the Cenozoic, only gradual, slow subsidence occurred in the study area due to regional flexure linked to the Andean orogeny. Comparative analysis with low-temperature thermochronology data from other west Gondwana cratonic segments highlights that exhumation episodes are highly controlled by tectonic inheritance, lithospheric strength, and proximity to rift zones. This study underscores the complex interplay between tectonic events and the response of cratonic lithosphere over geological timescales and highlights extensional settings as an important geological context for craton exhumation.

Interpreting and reporting fission-track chronological data

Article

Full-text available

Feb 2024

Fission-track dating is based on the analysis of tracks—linear damage trails—produced by the spontaneous fission of 238U in a range of natural accessory minerals and glasses. The retention of tracks is sensitive to elevated temperatures, and the data serve principally as a tool for recording thermal histories of rocks, potentially over the range of ∼20−350 °C, depending on the specific minerals studied. As such, in most cases, fission-track data generally bear little or no direct relationship to the original formation age of the material studied. The age range of fission-track dating is related to the product of age and uranium content, and ages from several tens of years to older than 1 Ga are reported. Fission-track analysis led to the development of powerful modeling techniques. When used with appropriate geological constraints, these modeling techniques allow important geological processes to be addressed in a broad range of upper crustal settings. Since early attempts to standardize the treatment of fission-track data and system calibration over more than 30 years ago, major advancements were made in the methodology, necessitating the development of new, updated data reporting requirements. Inconsistencies in reporting impede public data transparency, accessibility and reuse, Big Data regional syntheses, and interlaboratory analytical comparisons. This paper briefly reviews the fundamentals of fission-track dating and applications to provide context for recommended guidelines for reporting and supporting essential meta fission-track data for publication and methodological archiving in structured formats that conform with FAIR (Findable, Accessible, Interoperable, and Reusable) data principles. Adopting such practices will ensure that data can be readily accessed, interrogated, and reused, allowing for further integration with other numerical geoscience techniques.

New detrital petrographic and thermochronologic constraints on the Late Cretaceous–Neogene erosional history of the equatorial margin of Brazil: Implications for the surface evolution of a complex rift margin

Article

Full-text available

Aug 2023

The equatorial margin of Brazil is an example of a rift margin with a complex landscape, dominated by an escarpment perpendicular to the continental margin, which testifies to an equally complex rift and post‐rift surface and tectonic evolution. This has been the focus of a long debate on the driving mechanism for post‐rift tectonics and on the amount of exhumation. This study contributes to this debate with new petrographic and thermochronologic data on 152 samples from three basins, Pará‐Maranhão, Barreirinhas and Ceará, on the offshore continental platform. Our detrital record goes back to the rift time at ca. 100 Ma ago and outlines three major evolutionary phases of a changing landscape: a rift phase, with the erosion of a moderate rift escarpment, a Late Cretaceous‐Palaeogene post‐rift phase of major drainage reorganization and significant vertical erosion and a Late Oligocene‐to‐Recent post‐rift phase of moderate vertical erosion and river headwater migration. We estimate that along the equatorial margin of Brazil, over a large onshore area, exhumation since the Late Cretaceous has totalled locally up to 2–2.5 km and since the late Oligocene did not exceed 1 km.

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

Article

Jun 2023
Chem Geol Isot Geosci

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 20s 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 datasets.

Control of inherited structural fabric on the development and exhumation of passive margins – Insights from the Araçuaí Orogen (Brazil)

Article

Full-text available

May 2023

A reappraisal of the principle of equivalent time based on physicochemical methods

Article

Apr 2023
CHEM GEOL

The main feature of the Fission-Track Thermochronology is its ability to infer the thermal histories of mineral samples in regions of interest for geological studies. The ingredients that make the thermal history inference possible are the annealing models, which capture the annealing kinetics of fission tracks for isothermal heating experiments, and the Principle of Equivalent Time (PET), which allows the application of the annealing models to variable temperatures. It turns out that the PET only applies to specific types of annealing models describing single activation energy annealing mechanisms (parallel models). However, the PET has been extensively applied to models related to multiple activation energy mechanisms (fanning models). This procedure is an approximation that has been overlooked due to the lack of a suitable alternative. To deal with this difficult, a formalism, based on physicochemical techniques, that allows to quantify the effects of annealing on the fission tracks for variable temperatures, is developed. It is independent of the annealing mechanism and, therefore, is applicable to any annealing model. In the cases in which the PET is valid, parallel models, the proposed method and the PET predict the same degrees of annealing. However, deviations appear when the methods are applied to the fanning models, with the PET underestimating annealing effects. The consequences for the inference of thermal histories are discussed.

The Tectonic Evolution of the Paleozoic Tannuola Terrane of Tuva in the Mesozoic and Cenozoic: Data of Fission-Track Thermochronology of Apatite

Article

Full-text available

Nov 2022

Intra-continental evolution of the Chinese Tianshan and Junggar orogenic collage

Thesis

Full-text available

May 2022

Zhiyuan He

The Central Asian Orogenic Belt (CAOB) lies between the Baltica, Siberia and Tarim-North China cratons, and is one of the largest Phanerozoic orogenic belts on Earth. The development of the CAOB initiated in the Neoproterozoic and it further grew during the Paleozoic via the accretion of various island arcs, seamounts, accretionary wedges and micro-continents. This vast orogenic system was eventually amalgamated by the final closure of the Paleo-Asian Ocean during the late Paleozoic, resulting in the docking of the Tarim-North China craton from the south. Since the late Paleozoic, parts of the CAOB (e.g., the Tianshan and Altai) have served as typical examples of intra-continental orogens where the relationship between plate margin processes and the occurrence of compressive intra-plate deformation can be studied. Throughout the Mesozoic, the southwestern (SW) part of the CAOB experienced several major periods of intra-continental deformation, which have been interpreted to be related with a series of Cimmerian collisions (e.g., the collisions of Qiangtang, Lhasa and Karakorum blocks with Eurasia) occurring along the southern Eurasia margin. The evolution of the SW CAOB continued with active deformation in response to far-field effects of the convergence between the Indian plate and the Eurasia continent throughout the Cenozoic. Stress-fields as a result of these distal tectonic events propagated through the inherited Paleozoic structures of the CAOB resulting in progressive and punctuated exhumation and mountain building events that shaped the prominent Tianshan and Altai-Sayan mountainous landscapes that are seen today. This study focuses on the intricate intra-continental evolution of the Chinese Tianshan and Junggar orogenic collage, a key component of the SW CAOB. After the initial establishment in the late Paleozoic, this orogenic belt was immediately reworked by the movement of several deep-rooted strike-slip faults probably until the earliest Triassic, then subjected to large-scale reactivation events during the Meso-Cenozoic. As the architecture of the Tianshan and Junggar orogenic belt is complicated and its intra-plate evolution long-lasting, several issues regarding its thermo-tectonic history since the late Paleozoic remain unclear. Main objectives of this research are to better unravel late Paleozoic tectonic wedging due to strike-slip movements and to further elucidate the Meso-Cenozoic reactivation history of the Tianshan and Junggar systems, focusing on some of their uninvestigated or poorly constrained key regions. Regarding the late Paleozoic strike-slip faults system developed along the Chinese Tianshan belt, we carried out structural and geochronological studies on the poorly investigated Xiaergou and Wulasitai shear zones around and in the Central Tianshan block (Chapter 4). The Xiaergou shear zone is the connecting segment between the North Tianshan fault and Main Tianshan shear zone along the northern margin of the Yili - Central Tianshan blocks, it strikes NW-SE with a width of ~3-5 km and shows predominant dextral kinematics. Zircon U-Pb ages of pre- and syn-kinematic granitic dykes within the Xiaergou shear zone indicate that the dextral shearing was active at ~312-295 Ma. The Wulasitai shear zone is a high-strain belt occurring in the interior of the Central Tianshan block, it extends NW-SE for more than 40 km with variable widths of ~1-5 km, steep mylonitic foliations and sub-horizontal stretching lineation are well developed and various kinematic indicators suggest prevailing sinistral shearing. New biotite 40Ar/39Ar ages of two meta-sedimentary rock units, together with the published metamorphic zircon U-Pb ages constrain the timing of the sinistral shearing at ~312-301 Ma. Our new results combined with the previous studies reveal that the dextral strike-slip shear zones framing the Central Tianshan formed almost simultaneously in the latest Carboniferous (~310 Ma) and lasted until the middle to late Permian. They resulted from the eastward tectonic wedging and relative rotations between continental blocks in the SW CAOB. The sinistral shearing of the Wulasitai shear zone within the Central Tianshan was likely generated due to differential eastward motions of the northern and southern parts of the Central Tianshan. New apatite fission track (AFT) data on the Paleozoic rocks in and adjacent to the Chinese Central Tianshan were obtained, including two age-elevation profiles in the Alagou and Gangou areas. Inverse thermal history modeling reveals that the basement of the Central Tianshan experienced regional slow to moderate cooling during most of the Mesozoic, and that the present-day topography was mainly built by Cenozoic surface uplift and erosion. Geomorphological observation reveals several remnant fragments of flat, low-relief surfaces within the Central Tianshan, which were likely to have formed in the Mesozoic as evidenced by thermal history modeling of the Alagou age-elevation profile. Furthermore, the new data suggests that the Chinese Central Tianshan and its adjacent terranes did not undergo intensive relief building during its long-term Mesozoic evolution, as several pre-Mesozoic deep-rooted regional faults did not record evidence for a significant Mesozoic reactivation. Finally, differential exhumation of the basement in the western Chinese Tianshan and Junggar has been studied, and shows that the development of regional brittle faults significantly influences the processes of intra-continental deformation (Chapter 5). The Chinese Eastern Tianshan and East Junggar orogenic belts are major constituents of the SW CAOB, and low-temperature thermochronology was applied to constrain the thermo-tectonic history of these two domains (Chapter 6). AFT dating of Paleozoic basement samples from the region dominantly yields Cretaceous (~126-70 Ma) AFT ages, except for two granitic samples from the East Junggar with older ages of ~239 and ~157 Ma, respectively. Thermal history modeling reveals that the Eastern Tianshan and southern part of the East Junggar experienced moderate to rapid basement cooling throughout the Cretaceous. We interpret this as a far-field effect of accretion and collision along the southern Eurasia margin since the Early Cretaceous. Major faults were reactivated and thus may have played an important role in controlling localized rapid basement uplift and cooling. We also dated seven Mesozoic sandstone samples collected from the eastern margin of the Junggar Basin. The detrital AFT age peaks, together with inverse thermal history modeling of the basement, reveal that the East Junggar underwent late Permian to Early Jurassic basement cooling episodes. These cooling events are thought to be related to post-orogenic transpression along major faults as a distal effect of the coeval Qiangtang-Eurasia collision. Combined with already published evidence, our new data suggests that the Eastern Tianshan and East Junggar did not undergo significant exhumation (> ~2-3 km) during the Cenozoic. The Yili block in the western Chinese Tianshan forms the easternmost part of the Kazakhstan paleocontinent, and exploring its thermo-tectonic history is important to reconstruct the intra-continental evolution of the Tianshan belt. We report new AFT data from the basement rocks from the northern (i.e. the Wenquan complex) and southern (i.e. the Dahalajunshan - Nalati range) margins of the Yili block (Chapter 7). Thermal history modeling reveals that the Wenquan complex underwent moderate basement cooling in the Cretaceous, possibly due to far-field effects of the Tethys closure and convergent deformation and the ensuing Lhasa-Qiangtang collision. These events at the southern Eurasian margin propagated tectonic stress to the northern Yili and triggered localized deformation. Early Triassic-middle Jurassic moderate cooling is also identified in the Dahalajunshan - Nalati range, and is interpreted to be related to the post-orogenic strike-slip motion along the major shear zones and the effects of the Qiangtang and Kunlun-Qaidam collision. Combined with the published thermochronological data, it is suggested that the northern and southern parts of the Yili block experienced a distinctly different Mesozoic thermo-tectonic evolution. Basement cooling of the northern Yili block generally took place before the Cretaceous, exhuming shallower crustal levels as compared with the southern one. The intermontane Yili basin may have accommodated substantial propagated contraction induced by the Cretaceous collisional events, resulting in less strain reaching the northern Yili. Based on our new results and the previously published thermochronological data, it is suggested that the intra-continental reactivation of the North Tianshan and Nalati faults probably did not invoke significant regional exhumation during the Meso-Cenozoic. Instead, small-scale brittle faults controlled localized enhanced denudation. In general, the research conducted in this dissertation provides new constraints and valuable improvements on our knowledge of the timing and nature of intra-continental deformation and reactivation of the Chinese Tianshan and Junggar orogenic collage since the late Paleozoic. Meanwhile, it lays the framework for a systematic review of low-temperature thermochronological data that could now be undertaken as many of the regional gaps in the Tianshan-Junggar have been filled (Chapters 8 and 9).

Thermochronology and landscape evolution of the Serra do Caparaó, an onshore segment of the Brazilian South Atlantic continental margin

Thesis

Full-text available

Apr 2022

Tobias Fonte-Boa

The origin and evolution of elevated passive continental margins have intrigued geoscientists since their prominent topography is more reasonably explained in the context of an active rift than a mature `passive’ setting. A classic example is the Brazilian continental margin which evolved from a rift in Cretaceous, ca. 130 Ma ago, yet presents elevations that reach more than 2000 m above sea level. The Brazilian South Atlantic marginal upwarp extends for more than 3000 km and shows substantial morphological variation. Nevertheless, geoscientists have been mainly focused on studying the escarpment-form segments (e.g., the Serra do Mar in southeast Brazil) and frequently assuming the Brazilian elevated continental margin as a well-defined and continuous marginal upwarp instead of a diversified and more complex landscape. Attempting to understand the landscape variation along the Brazilian South Atlantic passive margin, this work uses the low-temperature thermochronology to explore how time-space denudation patterns differ along the southeast Brazilian coast with a particular focus on the Serra do Caparaó segment; a relatively less-studied area found adjacent to the Serra do Mar and where the typical escarpment form is absent. Overall, the thermochronology analysis shows an asymmetric denudation profile in which AFT ages tend to increase with elevation and toward the continental interior. The continental drainage divide delimits a dissected coastal region (i.e., where AFT ages are generally younger or similar to the rifting event) from a less denudated continental interior area (i.e., where AFT ages are older than continental breakup). This asymmetric denudation profile is associated with the generalized differential exhumation of the coastal region due to the continued erosion and unloading isostatic rebound triggered by the continental breakup processes, which are expected in generic passive continental margins evolution models. However, the Brazilian continental margin shows AFT ages substantially younger than rifting event, suggesting that the margin has experienced increased post-rift exhumation. Thermal models frequently show a pulse of cooling occurring long after rifting event, indicating that an additional transient factor than continued rifting-related denudation may have influenced the exhumation of the margin. The post-rift exhumation seemed to have been controlled by the reactivation of lithospheric inherited structures that triggered the relative uplift/subsidence between basement blocks resulting in contrasting denudation patterns along the coastal region, including the onshore segment related to the Espirito Santo basin (i.e., the Serra do Capraó region). In this situation, the Serra do Capraó is currently a high-elevation coastal interfluve segment that includes relicts of ancient long-lived syn-rift coastal catchment areas (i.e., syn-fit less-denudated coastal interfluve region) that has been changed and reestablished during the post-rift reactivation. Finally, this study highlights that the thermochronology pattern corroborates the morphological differences along the margin, supporting that the Brazilian continental margin is not a single and continuous tectonic setting. Furthermore, besides the variety of proposed mechanisms operating over the post-rifting evolution of the margin, tectonic inheritance (i.e., structural framework, lithology, the effective flexural strength of the lithosphere) has strongly impacted the denudation pattern contributing to the geomorphic diversification along the Brazilian South Atlantic coast.

Tectonic Evolution of the SE West Siberian Basin (Russia): Evidence from Apatite Fission Track Thermochronology of Its Exposed Crystalline Basement

Article

Full-text available

Jun 2021

The West Siberian Basin (WSB) is one of the largest intracratonic Meso-Cenozoic basins in the world. Its evolution has been studied over the recent decades; however, some fundamental questions regarding the tectonic evolution of the WSB remain unresolved or unconfirmed by analytical data. A complete understanding of the evolution of the WSB during the Mesozoic and Cenozoic eras requires insights into the cooling history of the basement rocks as determined by low-temperature thermochronometry. We presented an apatite fission track (AFT) thermochronology study on the exposed parts of the WSB basement in order to distinguish tectonic activation episodes in an absolute timeframe. AFT dating of thirteen basement samples mainly yielded Cretaceous cooling ages and mean track lengths varied between 12.8 and 14.5 µm. Thermal history modeling based on the AFT data demonstrates several Mesozoic and Cenozoic intracontinental tectonic reactivation episodes affected the WSB basement. We interpreted the episodes of tectonic activity accompanied by the WSB basement exhumation as a far-field effect from tectonic processes acting on the southern and eastern boundaries of Eurasia during the Mesozoic-Cenozoic eras.

Alpha-decay induced shortening of fission tracks simulated by in situ ion irradiation

Article

Full-text available

Feb 2021
GEOCHIM COSMOCHIM AC

The diffusion of elements used for dating (e.g., He and Pb) and defects (e.g., vacancies and interstitials) in a mineral structure is a thermal process: This is the primary assumption used to determine the age and thermal history of minerals. For instance, thermal history reconstruction, through the number and length distribution of tracks produced by spontaneous fission of ²³⁸U, is obtained by assuming a thermal event to be the only energy source for shortening of fission tracks. Here, we report a new, non-thermal energy source that induces additional shortening of fission tracks by the irradiation of alpha-recoils from the alpha-decay of ²³⁸U and ²³²Th. We simulate alpha-decay induced track-shortening by combining ion accelerator irradiations with transmission electron microscopy. This allows for the first observation of track-shrinkage during in situ ion irradiation. We show that rather than alpha-particles, alpha-recoils induce a significant shortening of fission tracks by nuclear-collisions. The shortening of track-length can be quantified as a function of alpha-decay event dose. However, apatite is less sensitive than zircon to this non-thermal process. The findings exemplify the interactions among different types of self-irradiation from alpha-particles, alpha-recoils and fission-fragment nuclei in single mineral grains and have important implications for the use of zircon and apatite for radiometric dating and thermochronology.

Differential Phanerozoic evolution of cratonic and non-cratonic lithosphere from a thermochronological perspective: São Francisco Craton and marginal orogens (Brazil)

Article

Jan 2021
GONDWANA RES

The São Francisco Craton (SFC) and its marginal Araçuaí and Brasília orogens exhibit a significant diversity in their lithospheric architecture. These orogens were shaped during the Neoproterozoic–Cambrian amalgamation of West Gondwana. The rigid cratonic lithosphere of the SFC and the relatively weak lithosphere of the Araçuaí Orogen were disrupted during the Cretaceous opening of the South Atlantic Ocean, whereas the Brasília Orogen remained in the continental hinterland. In earlier research, the thermal effects of the Phanerozoic reactivations in the shallow crust of the Araçuaí Orogen have been revealed by low-temperature thermochronology, mainly by apatite fission track (AFT) analysis. However, analyses from the continental interior are scarce. Here we present new AFT data from forty-three samples from the Brasília Orogen, the SFC and the Araçuaí Orogen, far from the passive margin of the Atlantic coast (~150 to 800 km). Three main periods of basement exhumation were identified: (i) Paleozoic, recorded both by samples from the SFC and Brasília Orogen; (ii) Early Cretaceous to Cenomanian, recorded by samples from the Araçuaí Orogen; and (iii) Late Cretaceous to Paleocene, inferred in samples from all domains. We compare the differential exhumation pattern of the different geotectonic provinces with their lithospheric strengths. We suggest that the SFC likely concentrated the Meso-Cenozoic reactivations in narrow weak zones while the Araçuaí Orogen displayed a far-reaching Meso-Cenozoic deformation. The Brasília Orogen seems to be an example of a stronger orogenic lithosphere, inhibiting reworking, confirmed by our new AFT data. Understanding the role of the lithosphere rigidity may be decisive to comprehend the processes of differential denudation and the tectonic–morphological evolution over Phanerozoic events.

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

Article

Aug 2020
CHEM GEOL

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.

Devonian to Permian post-orogenic denudation of the Brasília Belt of West Gondwana: insights from apatite fission track thermochronology

Article

May 2020
J GEODYN

The Brasília Belt in southern Brazil is a Neoproterozoic orogenic belt that represents the northern border of the Paraná Basin, a long-lived early Paleozoic intracratonic basin. The belt is surrounded by cratonic domains and were modestly affected and reactivated by the opening of the South Atlantic Ocean during the Cretaceous. Here we present new apatite fission track data from crystalline rocks of the Brasília Belt. The apatite fission track central ages range from 386 to 243 Ma and the mean track lengths range between 13.4 and 14.5 μm. Thermal history modeling reveals Devonian to Permian cooling, which we interpret as the main final exhumation of the Brasília Belt, implying that indeed the influence of posterior Mesozoic and Cenozoic geodynamic history on the belt is limited. The Paraná Basin and surrounding elevated terranes therefore provide a time window to study the tectonic history of Gondwana and the effect of far-field intraplate stresses on the interior of West Gondwana. Rapid basement exhumation of the Brasília Belt is coeval with extension and tectonic subsidence in the Paraná Basin as a result of continent-scale tectonic forces occurring over entire West Gondwana.

Is Low‐Temperature Fission‐Track Annealing in Apatite a Thermally Controlled Process?

Article

Full-text available

Mar 2020
GEOCHEM GEOPHY GEOSY

We report a new series of experiments to explore the phenomenon of low‐temperature annealing of fission tracks in apatite that feature a number of improvements over previous work. Grain mounts were preirradiated using ²⁵²Cf to increase confined track detection and allow briefer thermal neutron irradiation. We coirradiated and etched four apatite varieties (Durango, Fish Canyon, Renfrew, and Tioga) over five time steps equally spaced from 3.66 to 15 ln(s). A length standard was coetched with all experiments to ensure that subtle differences are within detection limits. Finally, we used a standard etching protocol, allowing the data to be comodeled with extensive high‐temperature data sets and recent analyses of induced tracks that underwent ambient‐temperature annealing over year‐to‐decade time scales. Ambient‐temperature annealing occurs at two different rates, with faster annealing at early stages that decreases to a slower rate that converges with empirical fanning linear or curvilinear models. The nature of this decrease varies among the apatite species examined, but no patterns could be determined. The fitted models make geological time‐scale predictions consistent with those based on high‐temperature data only and also make predictions consistent with reasonable inferred low‐temperature histories for all four apatite varieties. The empirical fanning curvilinear equation encompasses low‐temperature annealing at month‐to‐decade time scales, but low‐temperature annealing at shorter time scales may occur by a distinct mechanism. We consider but rule out annealing by radiation from short‐lived activated isotopes. We also reconsider the notion of the initial track length, and the appropriate length for normalizing confined track length measurements.

Is low-temperature fission-track annealing in apatite a thermally controlled process?

Preprint

Full-text available

Dec 2019

Raman microspectroscopy: a new tool for calibration of apatite fission-track analysis

Article

Jan 2007
CHEM GEOL

Compositional control on the annealing kinetics of fission-tracks (FT) in apatite requires routine measurement of sample grain composition. However, for practical reasons the bulk composition of analysed grains is not routinely measured and instead grain chlorine content or etch-pit dimensions are used to characterise a samples annealing behaviour. A more desirable approach is to measure crystallographic parameters (i.e. unit cell dimension) of a grain as these represent the summed effect of all substitutions and crystal defects. We show how Raman microspectrometry can be used as a routine non-destructive tool to obtain rapid measurement of the crystallographic structure of apatite grains etched for FT analysis. Variations of unit cell parameter a are found to correspond to a systematic variation of Raman shift in the range of 452–440 cm− 1 for measurements made on c-parallel sections of apatite where the direction of the polarized incident beam is parallel to the c axis.

New insights into the Gondwana breakup at the Southern South America by apatite fission-track analyses

Article

Full-text available

Jan 2019
ADGEO

Apatite fission-track (AFT) analyses, applied to Southern Brazil and Uruguay samples, was employed aiming to understand the low temperature history of the Dom Feliciano Belt Segment. The Dom Feliciano Belt formed during the Neoproterozoic to Early Paleozoic, linked to the Brasiliano/Pan-African Orogeny. Twenty-four samples were dated, and confined track lengths of twenty samples were measured. The spatial distribution of ages shows three domains with different evolution cut by shear zones and, or suture zones in the Dom Feliciano Belt. The Western Domain exhibits AFT ages > 250 Ma (Permian to Devonian) while the Eastern Domain shows AFT ages < 230 Ma (Paleogene to Triassic). In the Central Domain, the AFT ages range from ∼196 to 130 Ma (Jurassic to Early Cretaceous). The thermal modeling in the domains revealed a complex evolution, with cooling and reheating phases, and a denudation of ∼2600 m. The AFT ages clearly postdate the Gondwanide, Paraná-Etendeka and Rio Grande Cone exhumation history of the Dom Feliciano Belt.

A GENETIC ALGORITHM APPLICATION IN THERMOCHRONOLOGY

Conference Paper

Full-text available

May 1999

This paper propose an optimization algorithm for modeling thermal histories with apatite fission tracks. In fact, thermal evolution of sedimentary basins and their margins is of major importance for petroleum exploration, as it controls the timing of hydrocarbon generation. This kind of problems is characterized by their nonlinear and multi-parameter aspects. For this purpose we have used genetic algorithms (GA). The adopted GA, which is in other respects perfectly adapted to the problem’s requirements (encoding scheme, genetic operations, etc.) provide satisfactory results for both synthetic and real data. The re-examination of some samples studied earlier, from the French western Alps, confirm the obtained results. Keywords : Thermal histories, Optimization, Genetic algorithms.

Analysis of exhumation history in Misho Mountains, NW Iran: Insights from structural and apatite fission track data

Article

Full-text available

Mar 2017
Neues Jahrbuch Geol Palaontol Abhand

The Misho complex in Northwest Iran is a prominent topographic massif bounded by well known active faults. Our new structural analysis of this area indicates that faulting has important role in the exhumation of this complex. The conjugate orientation of the North and South Misho Faults caused uplift in the Misho and exhumation of the Precambrian crystalline basement. Our structural and stratigraphic data shows that rapid uplift could have been initiation since the 21-22 Ma and exhumation rate was about 0.16 to 0.24 km/Ma. To refine this age, we performed U/Pb analysis of detrital zircon from the Upper Red Formation using LA-ICP-MS. We conducted AFT analysis on 6 basement samples from the hanging wall and 1 sample from the Upper Red Formation in the footwall NMF. Uplift in the hanging wall of NMF led to resting of sample 916 marl. This geochronologic and thermochronologic data shows that exhumation in the MC is diachronously along strike and affected by faults. The phase of exhumation is documented in the study area and entire Iranian plateau is related to the final closure of the Neo-Tethys and northward motion of the Arabian Plate. © 2017 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany.

Neogene extension and volcanism in the Kunlun fault zone, northern tibet: new constraints on the age of the Kunlun fault

Article

Full-text available

Jan 2003

Cenozoic intracontinental deformation and exhumation at the northwestern tip of the India-Asia collision-southwestern Tian Shan, Tajikistan, and Kyrgyzstan: India-Asia Collision in the Southwestern Tian Shan

Article

Full-text available

Sep 2016

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 Shan's 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.

Investigation of ion-track morphology and annealing behavior using small-angle X-ray scattering

Thesis

Jan 2013

Boshra Afra

When heavy ions with energies in the range of hundreds of MeV to GeV penetrate a solid, they lose their energy through inelastic interactions with the target electrons and can leave narrow cylindrical trails of permanent damage along their path, known as ion tracks. Ion tracks are typically a few nanometers in diameter and can be up to tens of micrometers long. When these tracks are annealed at elevated temperatures, they shrink in size and eventually the damage inside the material can recover. In this project ion tracks are studied in Durango apatite, San Carlos olive and synthetic quartz. In minerals such as apatite track formation can result from spontaneous fission of naturally occurring uranium inclusions that produces high energetic fragments. These so called “fission tracks” are used for dating and constraining the thermal history of geological samples. The current dating methods, however, utilize chemical etching, which destroys the primary damage such that essential information on the actual scale of the underlying radiation damage is irrevocably lost. A detailed understanding of the un-etched track damage in minerals and its dependence on geologically relevant conditions is of fundamental importance for the application of etched tracks in geo- and thermochronology. Tracks in olivine are used for identification of cosmic rays in meteorites. Many meteorites contain different amounts of olivine which is a crystalline mineral susceptible to ion track formation from high energetic cosmic particles. Studying the annealing behavior of ion tracks in meteorites can lead to estimation of the temperature of the mineral during track formation. In quartz, swift heavy ion irradiation leads to a change in the refractive index of the material inside the narrow tracks and provides new possibilities for fabrication and micromachining of optical devices. Annealing of ion tracks in quartz is of interest as for example device fabrication processes often involve elevated temperatures. In this work synchrotron based small angle x-ray scattering in combination with in situ and ex situ annealing experiments is used to study the morphology and damage recovery of un-etched ion tracks. It is demonstrated that SAXS is a powerful tool for studying ion track damage in a variety of materials as it is sensitive to small density changes at the nanometer scale that often occur in the damaged regions. It is a non-destructive technique and can be used to determine changes in the track radii with sub-nanometer precision. Short acquisition times make it well suited for studying track annealing kinetics in situ. The work presented in this thesis has aided in developing the possibilities that small angle x-ray scattering can provide for studying the morphology and annealing behavior of nano sized damage structures. The high accuracy with which the track radii can be determined using SAXS, the nondestructive, artifact-free measurement methodology, as well as the data analysis models introduced in this work provide an effective means for in-depth studies of ion-track morphology and annealing behavior in a variety of materials.

Cooling and Denudation of the Norwegian and Barents Sea Margins, Northern Scandinavia - Constrained by apatite fission track and (U-Th)/He thermochronology

Thesis

Full-text available

Jun 2003

Bart WH Hendriks

In this study, the history of cooling and denudation of the Northern Scandes range and the adjacent Norwegian and Barents Sea margins is investigated. This is done with Fission Track and (U-Th)/He thermochronology. Several other previously published studies have focused at similar issues in other parts of Scandinavia. Both from the geomorphological record, as well as from the offshore sedimentary record, it seems evident that the development of the Northern Scandes differs in several respects from the development of its southern counterpart, the Southern Scandes range. The history of cooling and denudation in northern Scandinavia is presented first and is then compared to that of other North Atlantic margins, with emphasis on the eastern North Atlantic margins. Chapter 2 presents an overview of the Precambrian – Paleozoic evolution of the Fennoscandian shield. The main structures and tectonic units and also the geomorphological surfaces that are most relevant to the present study are introduced. The offshore basin evolution of the Norwegian Sea and the Barents Sea is briefly summarized. Chapter 3 offers an introduction to the theoretical backgrounds of Fission Track and (U-Th)/ He thermochronology. The different methods and techniques that have been applied in the lab are outlined. The potential applications of both Fission Track and (U-Th)/He thermochronology are discussed, as well as how one should interpret both types of data. Electron microprobe data for a selected set of apatite samples are included in this chapter. In Chapter 4 it is demonstrated that the pattern of Apatite Fission Track ages with respect to distance to the northern Norwegian Atlantic margin is very similar to that of models of passive margin evolution that represent a retreating scarp. On the margin itself the AFT ages are as young as 90 – 100 Ma and further inland, right across the Northern Scandes mountain range, the ages increase to more than 300 Ma on the gentle eastern flank of the range. Late Cretaceous – Paleogene (U-Th)/He ages in a vertical profile on Kebnekaise, the highest peak in the range, together with the observed age pattern suggest a rift shoulder origin for the range. Nevertheless, other uplift mechanisms have played an important role in the later development of the range. On the margin, the young AFT ages coincide spatially with a strong, negative gravity anomaly. Inverse modeling histories here indicate an important Neogene phase of denudation which is not recorded anywhere else in the mountain range by the AFT or (U-Th)/He systems. There is no clear pattern for the pattern of the AFT ages with distance to the southern Barents Sea margin. This is discussed in Chapter 5. Only a clear difference between relatively young ages (< 270 Ma) along the southwestern Barents Sea margin with respect to those further east (> 270 Ma) has been identified. For the region along the southwestern Barents Sea margin, inverse thermal histories provide a record of the Triassic and younger thermal history, whereas samples along the eastern part of the southern Barents Sea margins reveal the Paleozoic to Triassic thermal history only. Previously published apatite fission track and zircon fission track ages from the Kola Super Deep Borehole are used in conjunction with new apatite fission track data from surface samples. In Chapter 6, differential vertical movements between, and within, the Lofoten and Vesterålen domains are documented by the inverse thermal histories based on the new AFT data from both domains and extrapolation of offshore sedimentary units over the archipelago. Cooling and denudation on Vesterålen apparently was accompanied by Late Paleozoic – Early Mesozoic sedimentation and burial in the Lofoten domain. Cooling and denudation of the present-day Lofoten Ridge in the Jurassic and Cretaceous was contemporaneous with deposition of a sedimentary cover in the Vesterålen area, again indicating differential vertical movements between both domains. The boundary between both domains, which seems to be the Vesterålen transfer zone, lines up with an oceanic fracture zone. The youngest ages in the XII region, ~120 Ma, in the southern part of the Lofoten domain, coincide with a strong, positive gravity anomaly. Different mechanisms for the uplift of the Northern Scandes mountain range are discussed in Chapter 7. Asthenospheric diapirism has previously been proposed as an uplift mechanism for both the Southern and Northern Scandes mountains, but the new observations from the northern area do not satisfactorily fit the timing and pattern of cooling and denudation that is predicted by this mechanism. The initial uplift of the Northern Scandes range seems to be related to rift shoulder uplift, but a different mechanism must have been active as well in the Neogene. An important factor seems to be a large, low density granitic body within the upper crust, but the issue of what triggered the Neogene uplift in this region is not entirely clear yet. Although the observations are clear, tectonic modeling is required to get a better understanding of this phase. This is a first order study and it provides clear indications for the direction that future research into the low temperature evolution of the northern Scandinavian study area should take. This study provides the basis on which future research on a more detailed, smaller scale, can build further.

The Permian-Triassic transition in the Moatize-Minjova Basin, Karoo Supergroup, Mozambique: A Palynological Perspective

Article

Dec 2015
REV PALAEOBOT PALYNO

Termocronologia de baixa temperatura

Chapter

Jan 2011

Nesta obra organizada por Peter Christian Hackspacher, estão reunidos especialistas para analisar, de modo geral e sucinto, alguns importantes assuntos relacionados à Geologia estrutural, à Geomorfologia e à Estratigrafia: os conceitos de calor e de transporte na crosta terrestre; a aplicação de modelos numéricos termais na Geomorfologia; a evolução do relevo a partir da relação entre tectônica e sedimentação; os fatores exógenos de elaboração do relevo; a evolução do relevo a partir de escarpas e as antigas superfícies geomórficas; os métodos termocronológicos. Um livro original, que trata o tema de modo o mais próximo possível da realidade nacional e demonstra como a evolução do relevo implica diretamente uma diversidade de consequências quanto à forma e ao uso da terra. Uma importante contribuição para o meio acadêmico e científico sobre a dinâmica do relevo.

On etching, selection and measurement of confined fission tracks in apatite

Preprint

Full-text available

Jun 2023

Raymond Jonckheere

This work investigates the selection of horizontal confined tracks for fission-track modelling. It is carried out on prism sections of Durango apatite containing induced tracks with mean lengths of ~16, ~14, ~12, and ~10 μm. Suitable tracks are identified during systematic scans in transmitted light. The explicit selection criteria are that the tracks are horizontal and measurable. We measure the length, width, orientation, and cone angle of each selected track and in some cases other dimensions. The confined track selection is in the first place dependent on a threshold width and in the second place on the requirement that the tracks are etched to their ends. In most cases the first condition implies the second, which decreases in importance as the tracks are shortened following annealing. The widest confined tracks, which must also be the shallowest, come to intersect the surface and are excluded. In general, the selection is dominated by the width of the etched tracks. This, in turn, depends on their orientation relative to the c-axis and the apatite etch rates, and their effective etch times. Despite the different geometrical configuration of the unetched host tracks and confined tracks, neither the angular distribution nor the etch time distribution of the confined track sample depends on the degree of annealing. This illustrates the general principle that those entities are selected that have the right properties for being selected. In this case etching-related factors determining the track width are the most important, while the known geometrical biases are second order. The track etch rate exhibits no demonstrable variation along the track, but signifi-cant differences from track to track. Moreover, although the track etch rate of induced tracks is not correlated with the extent of partial annealing, it is on average twice as high as the value for fossil tracks. Our length measurements are in good agreement with the annealing models for this apatite and etch protocol. We submit that this is not fortuitous and that it is possible to select a representative confined track sample, and perform reproducible and meaningful confined track length measurements. Deliberate or inadvertent biasing, carelessness or inexperience will of course give different results, but these should be treated as statistical outliers, not as an indication that track lengths are fluid.

Mesozoic building of the Eastern Tianshan and East Junggar (NW China) revealed by low-temperature thermochronology

Article

Full-text available

Nov 2021
GONDWANA RES

The Eastern Tianshan and East Junggar orogenic belts are major constituents of the southwestern Central Asian Orogenic Belt. This study applies low-temperature thermochronology to constrain the thermo-tectonic history of these two domains. Apatite fission track (AFT) dating of Paleozoic basement samples from the Eastern Tianshan and East Junggar dominantly yield Cretaceous (∼126-70 Ma) AFT ages, except for two granitic samples from the East Junggar with older ages of ∼239 and ∼157 Ma, respectively. Thermal history modeling reveals that the Eastern Tianshan and southern part of the East Junggar experienced moderate to rapid basement cooling throughout the Cretaceous. We interpret it as a far-field effect of accretion and collision along the south Eurasia margin since the Early Cretaceous. Major faults were reactivated and thus may have played an important role in controlling localized fast uplift and cooling. We also dated seven Mesozoic sandstone samples collected from the eastern margin of the Junggar Basin. The detrital AFT age peaks, together with inverse thermal history modeling of the basement, reveal that the East Junggar underwent late Permian to Early Jurassic cooling episodes. These cooling events are thought to be related to post-orogenic transpression along major faults and distal effect of Qiangtang-Eurasia collision. Combined with already published evidence, our new data suggest that the Eastern Tianshan and East Junggar did not undergo significant exhumation during the Cenozoic.

Basics of Fission-track Geo- and Thermo-chronometry: Revisiting Its History to Explore Future Potentialフィッション・トラック年代測定の基礎—これまでの経緯と今後の発展の可能性—

Article

Full-text available

Mar 2021

Fission-track (FT) method observes linear dislocations in geological materials formed by nuclear fission of ²³⁸U. FT method provides information on thermal history of a sample below 300°C, together with geological dates. This report revisits the development history of FT method and suggests potential research target left unearthed.

Introduction: Recognizing Outstanding Ph.D. Research

Chapter

Jan 2018

Daniel Schauries

Thermal Annealing of Ion Tracks: Recognizing Outstanding Ph.D. Research

Chapter

Aug 2018

Daniel Schauries

At ambient temperatures, ion tracks in minerals are known to be stable and fairly constant in size. Even over geological timescales, in the range of hundreds of million years, only small reductions in length occur [1]. This changes dramatically when the tracks are exposed to elevated temperatures, leading to a recrystallisation of the damaged structure and a shrinkage in track size (i.e. length and radius). This process is extremely temperature-dependent, with a full recovery and disappearance of all tracks at 170–200

^{\circ }

C over geological timescales of 10

^{6}

years [2]. When annealed at 350–400

^{\circ }

C, however, a duration of less than 1 hr is sufficient to fully erase all tracks [3]. This is a result of the rate of recrystallization being typically associated with an exponential dependence of the diffusion rate of the displaced atoms incorporated in the ion tracks on temperature.

Crystallographic orientation dependence of mean etchable fission track length in apatite: An empirical model and experimental observations

Abstract

No full-text available

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