Insights in the exhumation history of the NWZagros frombedrock and detrital apatite fission-track analysis: evidence for a long-lived orogeny

We present the ¢rst ¢ssion-track (FT) thermochronology results for theNWZagrosBelt (SWIran) in order to identify denudation episodes that occurred during the protracted Zagros orogeny. Samples were collected from the two main detrital successions of the NWZagros foreland basin: the Palaeocene^early Eocene Amiran^Kashkan succession and theMiocene Agha Jari and Bakhtyari Formations. In situ bedrock samples were furthermore collected in the Sanandaj-Sirjan Zone. Only apatite ¢ssion-track (AFT) data have been successfully obtained, including 26 ages and11track-length distributions. Five families of AFTages have been documented from analyses of in situ bedrock and detrital samples: pre-middle Jurassic at 171 and 225Ma, early^late Cretaceous at 91Ma, Maastrichtian at 66Ma,middle^late Eocene at 38Ma and Oligocene^early Miocene at 22Ma.The most widespread middle^late Eocene cooling phase, around 38Ma, is documented by a predominant grain-age population in Agha Jari sediments and by cooling ages of a granitic boulder sample. AFTages document at least three cooling/denudation periods linked to major geodynamic events related to the Zagros orogeny, during the lateCretaceous oceanic obduction event, during the middle and late Eocene and during the earlyMiocene. Both late Cretaceous and early Miocene orogenic processes produced bending of the Arabian plate and concomitant foreland deposition. Between the two major £exural foreland episodes, the middle^late Eocene phase mostly produced a long-lasting slow- or nondepositional episode in the inner part of the foreland basin, whereas deposition and tectonics migrated to theNE along the Sanandaj-Sirjan domain and its Gaveh Rud fore-arc basin. As evidenced in this study, the Zagros orogeny was long-lived and multiepisodic, implying that the timing of accretion of the di¡erent tectonic domains that form the Zagros Mountains requires cautious interpretation

Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?

Zircon and apatite fission track (ZFT and AFT) and (U-Th)/He, 40Ar/39Ar hornblende, and U-Pb zircon ages from the granites of Tinos Island in the Aegean Sea, Greece, suggest, together with published ZFT data, that there are three extensional detachments on Tinos. The Tinos granites crosscut the Tinos detachment. Cooling of the granites was controlled by the Livadi detachment, which occurs structurally above the Tinos detachment. Our U-Pb zircon age is 14.6 ± 0.2 Ma and two 40Ar/39Ar hornblende ages are 14.4 ± 0.4 and 13.7 ± 0.4 Ma. ZFT and AFT ages go from 14.4 ± 1.2 to 12.2 ± 1.0 Ma and 12.8 ± 2.4 to 11.9 ± 2.0 Ma. (U-Th)/He ages are from 10.4 ± 0.2 to 9.9 ± 0.2 Ma (zircon) and 11.9 ± 0.5 to 10.0 ± 0.3 Ma (apatite). All ages decrease northeastward in the direction of hanging wall transport on the Livadi detachment and age-distance relationships yield a slip rate of 2.6 (+3.3 / −1.0) km Ma−1. This rate is smaller than a published slip rate of 6.5 km Ma−1 for the Vari detachment, which is another detachment structurally above the Tinos detachment. Because of the different rates and because published ZFT ages from the footwall of the Vari detachment are ∼10 Ma, we propose that the Vari detachment has to be distinguished from the older Livadi detachment. We discuss various models of how the extensional detachments may have evolved and prefer a scenario in which the Vari detachment cut down into the footwall of the Livadi detachment successively exhuming deeper structural units. The thermochronologic ages demonstrate the importance of quantitative data for constraining localization processes during extensional deformation

Tectono-thermal history of an exhumed thrust-sheet-top basin : an example from the south Pyrenean thrust belt

This paper presents a new balanced structural cross-section of the Jaca thrust-sheet-top basin of the southern Pyrenees combined with paleo-thermometry and apatite fission track (AFT) thermochronology data. The cross-section, based on field data and interpretation of industrial seismic reflection profiles, allows refinement of previous interpretations of the south-directed thrust system, involving the identification of new thrust faults, and of the kinematic relationships between basement and cover thrusts from the middle Eocene to the early Miocene. AFT analysis shows a southward decrease in the level of fission track resetting, from totally reset Paleozoic rocks and lower Eocene turbidites (indicative of heating to Tmax>~120°C), to partially reset middle Eocene turbidites and no/very weak resetting in the upper Eocene-lower Oligocene molasse (Tmax<~60°C). AFT results indicate a late Oligocene-early Miocene cooling event throughout the Axial Zone and Jaca Basin. Paleo-maximum temperatures determined by vitrinite reflectance measurements and Raman spectroscopy of carbonaceous material reach up to ~240°C at the base of the turbidite succession. Inverse modelling of AFT and vitrinite reflectance data with the QTQt software for key samples show compatibility between vitrinite-derived Tmax and the AFT reset level for most of the samples. However, they also suggest that the highest temperatures determined in the lowermost turbidites correspond to a thermal anomaly rather than burial heating, possibly due to fluid circulation during thrust activity. From these results, we propose a new sequential restoration of the south Pyrenean thrust system propagation and related basin evolution

Upper Cretaceous feldspars in the Cenozoic Limagne Basin: a key argument in reconstructing the palaeocover of the Massif Central (France)

peer reviewe

Post-Paleozoic evolution of the northern Ardenne Massif constrained by apatite fission-track thermochronology and geological data

International audienceThe exhumation history of basement areas is poorly constrained because of large gaps in thesedimentary record. Indirect methods including low temperature thermochronology may be used to estimateexhumation but these require an inverse modeling procedure to interpret the data. Solutions from suchmodeling are not always satisfactory as they may be too broad or may conflict with independent geologicaldata. This study shows that the input of geological constraints is necessary to obtain a valuable and refinedexhumation history and to identify the presence of a former sedimentary cover presently completely eroded.Apatite fission-track (AFT) data have been acquired on the northern part of the Ardenne Massif close to theVariscan front and in the southern Brabant, in particular for the Visean ash-beds. Apatite fission-track agesfor surface samples range between 140 ± 13 and 261 ± 33Ma and confined tracks lengths are rangingbetween 12.6 ± 0.2 and 13.8 ± 0.2 mm. Thermal inversion has been realized assuming that (1) samples wereclose to the surface (20–40 °C) during Triassic times, this is supported by remnants of detrital UpperPermian–Triassic sediments preserved in the south of the Ardenne and in the east (border of the Roer Grabenand Malmédy Graben), and (2) terrestrial conditions prevailed during the Early Cretaceous for the ArdenneMassif, as indicated by radiometric ages on paleoweathering products. Inversion of the AFT datacharacterizes higher temperatures than surface temperatures during most of the Jurassic. Temperature rangeis wide but is compatible with the deposition on the northern Ardenne of a significant sedimentary cover,which has been later eroded during the Late Jurassic and/or the Early Cretaceous. Despite the presence ofsmall outliers of Late Cretaceous (Hautes Fagnes area), no evidence is recorded by the fission-track data forthe deposition of a significant chalk cover as highlighted in different parts of western Europe. These resultsquestion the existence of the London-Brabant Massif as a permanent positive structure during the Mesozoic.L’évaluation des épaisseurs érodées sur les socles n’est pas immédiate car l’absence fréquente de couverture sédimentaire rend muette leur quantification sur une grande période de temps. Des méthodes indirectes comme la thermochronologie basse température permettent d’appréhender l’érosion à condition d’inverser correctement les données par modélisation. Les résultats de l’inversion ne sont pas toujours en accord avec les données géologiques ou sont trop imprécis pour être pertinents. Cette étude montre que la prise en compte de contraintes géologiques est nécessaire pour obtenir une histoire cohérente, définir l’ampleur de l’érosion et identifier la présence d’une couverture sédimentaire aujourd’hui érodée. Des données traces de fission dans les cristaux d’apatite ont été réalisées dans le nord du massif de l’Ardenne à proximité du front varisque et au sud du Massif du Brabant, en particulier sur des échantillons de cinérites viséennes. Les âges traces de fission des échantillons prélevés à la surface varient entre 140 ± 13 et 261 ± 33 Ma et la longueur des traces confinées horizontales est comprise entre 12,6 ± 0,2 et 13,8 ± 0,2 μm. L’inversion thermique de ces données a été réalisée en prenant comme hypothèses : (1) la proximité des échantillons de la surface (20–40 °C) au cours du Trias, hypothèse étayée par des témoins de couverture du Permien supérieur et du Trias au sud et à l’est du massif de l’Ardenne (grabens de la Roer et de Malmédy) ; (2) l’existence de conditions continentales au cours du Crétacé inférieur, en accord avec des âges de cette période pour des profils d’altération datés localisés dans le massif de l’Ardenne. Le résultat de cette inversion suggère des températures assez élevées au cours du Jurassique. Ces températures sont interprétées comme le résultat du dépôt d’une couverture sédimentaire qui a ensuite été érodée au Jurassique supérieur et/ou au Crétacé inférieur. Malgré la présence de quelques témoins du Crétacé supérieur (notamment dans les Hautes Fagnes), les données ne permettent pas de détecter le dépôt d’une série épaisse datée du Crétacé supérieur comme c’est le cas dans plusieurs régions d’Europe de l’Ouest. Ces résultats remettent en question l’existence de la structure positive du massif de Londres-Brabant au cours du Jurassique en identifiant des mouvements verticaux significatifs

U-series and radiocarbon cross dating of speleothems from Nerja Cave (Spain): Evidence of open system behavior. Implication for the Spanish rock art chronology

Two stalagmites from Nerja cave (Andalusia, Spain) were studied. The cave is well known because of its long human occupation from the Upper Palaeolithic to the Chalcolithic and its abundant parietal prehistoric Art. The aims of this study were twofold: i) to compare uranium/thorium (Th/U) and Carbon-14 (C) ages obtained all along the growth axis of the stalagmites in order to understand the consequences of diagenetic processes on the validity of radiometric ages; ii) as one of the stalagmites contains black layers, attributed to combustion soot, to establish when these intense hearths were used and by which culture. Th/U and C ages were coupled with mineralogical studies using FTIR (Fourier-transform infrared spectroscopy) and thin section observations. The first stalagmite (GN16-9b) displays Th/U ages in stratigraphic order, and compatible with C ages corrected for a few percent of dead carbon. Homogeneous composition of aragonitic crystals characterized by their needle-like texture is observed throughout this speleothem. For the second stalagmite (GN16-7), in contrast, Th/U ages display large significant inversions and discordant results on the upper part and at the base of the stalagmite, suggesting a possible open system behavior for this chronometer. Interestingly, C ages are in stratigraphic order all along the stalagmite and are compatible with Th/U ages only in its central part. Mineralogical studies display evidence of aragonite to calcite transformation at the top and a complex mineralogical assemblage with interlayered silicates (possibly clays) and calcitic mineralogy for the base of GN16-7. In these parts, discordant Th/U ages were measured. In the middle part of the stalagmite, however, where the fibrous aragonite is well preserved, the C and Th/U ages agree. Our data suggest that in the case of aragonite to calcite transformation as shown here, Th/U ages are biased, but C ages seem to remain accurate, as already observed in aragonitic marine bio minerals. C ages obtained are used for the chronology of the soot layer, determined here between 7900 and 5500 years Cal BP, coherent with previous analysis of charcoals in the same sector of the cave. This study highlights the importance of working with at least two chronometers when stratigraphic age verification is not possible, as is the case of some parietal CaCO thin layers used for rock art dating. Recent Th/U ages published for carbonate deposits on Spanish parietal Art are discussed in light of this demonstration.This research was funded by ANR (grant number ANR-18-CE27- 0004, ApART project) and supported by the Paris Ile-de-France Region – DIM “matérieux Anciens et Patrimoniaux” for FTIR analysis. The authors thank LMC14 staff (Laboratoire de Mesure du Carbone-14), ARTEMIS national facility, for the results obtained with the Accelerator Mass Spectroscopy method, and the PANOPLY analytical platform. This research is part of the “Proyecto General de Investigación aplicada a la conservación de Cueva de Nerja” authorised by the Junta de Andalucía and financed by the Fundación de Servicios Cueva de Nerja. The authors also wish to thank the “Instituto de Investigación Cueva de Nerja” for supporting this research. M.A.Medina-Alcaide has a Postdoctoral Fyssen Grant; the results presented in this paper are included in the PID2019-107262GB-I00 and PDC2021-121501-I00 grants funded by MCIN/AEI/10.13039/501100011033

Helium trapping in apatite damage: insights from (U-Th-Sm)/He dating of different granitoid lithologies

Apatite (U-Th-Sm)/He (AHe) thermochronometry is widely used to constrain thermal histories and rates of tectonic, exhumation, and erosion processes. However, data interpretation is often challenging, especially when the thermal history includes extended residence time in the He partial retention zone (HePRZ), with highly dispersed dates revealing the complexity of diffusion processes in natural systems. This study investigates chemical and physical factors that may have impacted He diffusion in apatite over long timescales in a context of protracted residence in the HePRZ. Nine samples from the Ploumanac'h pluton and North Tregor (Armorican Massif, France) were collected in granitoids, differing in petrography and chemisty. This area was chosen because these samples underwent a similar thermal history since ~300 Ma. We report new (U-Th Sm)/He dates, along with apatite fission-track (AFT) data, as well as lithological and chemical characterization. The results show dispersed (U-Th-Sm)/He dates, ranging from 87 ± 7 to 291 ± 23 Ma, whereas central AFT dates vary from 142 ± 6 to 199 ± 9 Ma. Current predictive models for He diffusion and fission-track annealing in apatite could not reproduce the two datasets together. However, this apparent discrepancy gives insight into the parameters influencing He diffusion at geological timescales. The data confirm that radiation damage enhances He trapping, as the AHe dates are positively correlated to effective uranium (eU) concentration. The He age dispersion for constant eU content cannot be explained just by variations in grain size or chemical composition. To explore the potential influence of recoil damage trapping behavior and annealing kinetics on AHe dates, we tested a new diffusion model from Gerin et al. (2017). Given the expected model of the thermal history provided by AFT inversion, we investigated the influence of the trapping energy on AHe dates. The AHe date variations can be explained only if the trapping energy evolves from one crystal to another, increasing with the amount of damage. For a given trapping energy, minor variations in the recoil-damage annealing rate can consistently explain most of the remaining dispersion of the AHe dates

A new multimethod approach for dating cave calcite: application to the cave of Trou du Renard (Soyons, France)

A multimethod approach aimed at characterizing carbonate parietal deposits and at proposing a chronology for these carbonate crusts is described. Dating was performed by radiometric methods (C-14 for recent samples and U series) on samples that had been characterized beforehand using optical and cathodoluminescence microscopy and Fourier transform infrared microspectroscopy. For the U series, high precision on U–Th ages was achieved using liquid phase multicollector inductively coupled plasma mass spectrometry (ICP-MS) applied to large samples, while laser ablation single collector inductively coupled plasma sector field mass spectrometry (ICP-SFMS) provided information on the reliability of the sampling with a high spatial resolution. This methodology, based on the combination of these two techniques reinforced by the information obtained by the calcite characterization methods, was applied to carbonate deposits from the cave of Trou du Renard (Soyons, France). The ages obtained with the two U–Th dating techniques are comparable and illustrate that different laminae were deposited at different rates in the samples. In the future, this procedure, based on the mineralogical and geochemical characterization of the samples and their dating by radiometric methods, will be applied to the layers of parietal carbonates deposited on Palaeolithic decorated walls. When the crystallization is slow, the U/Th dating method by imaging technique is of interest, as well as that by multicollector ICP-MS in liquid phase. The development of robust dating methods on very small quantities of material will make it possible to define the chronological framework of cave rock art.</p