Dating faulted alluvial fans with cosmogenic 10Be in the Gurvan Bogd mountain range (Gobi-Altay, Mongolia): climatic and tectonic implications

The Gurvan Bogd mountain range is a fault system characterized by strong earthquakes (M ∼ 8) separated by long periods of quiescence. Further to the previous works in the area, our study provides new data concerning the tectonic and climatic processes in the Gobi-Altay. To quantify the slip rates along the faults, we dated offset alluvial fans analysing the in situ produced ¹⁰Be along profiles at depth. The slip rates along the Bogd strike–slip fault and its associated thrust faults over the Upper Pleistocene–Holocene period are 0.95 ± 0.29 mm yr⁻¹ and comprised between 0.12 ± 0.02 and 0.13 ± 0.02 mm yr⁻¹, respectively. The surfaces ages account for a cyclic formation of the fans over the past ∼360 ka, in correlation with the terminations of the marine isotope stages 2, 6, 8 and 10.     

Late Pleistocene glacial history of Jameson Land, central East Greenland, derived from cosmogenic 10Be and 26Al exposure dating

Previous work has presented contrasting views of the last glaciation on Jameson Land, central East Greenland, and still there is debate about whether the area was: (i) ice-free, (ii) covered with a local non-erosive ice cap(s), or (iii) overridden by the Greenland Ice Sheet during the Last Glacial Maximum (LGM). Here, we use cosmogenic exposure ages from erratics to reconcile these contrasting views. A total of 43 erratics resting on weathered sandstone and on sediment-covered surfaces were sampled from four areas on interior Jameson Land; they give ¹⁰Be ages between 10.9 and 269.1 kyr. Eight erratics on weathered sandstone and till-covered surfaces cluster around ∼70 kyr, whereas ¹⁰Be ages from erratics on glaciofluvial landforms are substantially younger and range between 10.9 and 47.2 kyr. Deflation is thought to be an important process on the sediment-covered surfaces and the youngest exposure ages are suggested to result from exhumation. The older (>70 kyr) samples have discordant ²⁶Al and ¹⁰Be data and are interpreted to have been deposited by the Greenland Ice Sheet several glacial cycles ago. The younger exposure ages (≤70 kyr) are interpreted to represent deposition by the ice sheet during the Late Saalian and by an advance from the local Liverpool Land ice cap in the Early Weichselian. The exposure ages younger than Saalian are explained by periods of shielding by non-erosive ice during the Weichselian glaciation. Our work supports previous studies in that the Saalian Ice Sheet advance was the last to deposit thick sediment sequences and western erratics on interior Jameson Land. However, instead of Jameson Land being ice-free throughout the Weichselian, we document that local ice with limited erosion potential covered and shielded large areas for substantial periods of the last glacial cycle.     

Chronology of deglaciation based on 10Be dates of glacial erosional features in the Grimsel Pass region, central Swiss Alps

Surface exposure dating, using in situ produced cosmogenic ¹⁰Be, is applied to determine the time since deglaciation of bedrock surfaces in the Grimsel Pass region. Nine ¹⁰Be dates from bedrock surfaces corrected for cover by snow are minimum ages for deglaciation of the pass. Four ¹⁰Be dates from surfaces below 2500 meters above sea level (m a.s.l.) on Nägelisgrätli, east of Grimsel Pass, yield ages that range from about 14 000 to 11 300 years. Three ¹⁰Be dates from locations above 2600 m a.s.l. on Nägelisgrätli are between about 11 700 and 10 400 years. Two ¹⁰Be dates from locations at 2560 m a.s.l. below Juchlistock are about 12 100 and 11 000 years. The geographical distribution of ¹⁰Be dates on Nägelisgrätli either may show the timing of progressive deglaciation of Grimsel Pass or may reflect differences in subglacial erosion of bedrock in the pass region. All dates are discussed in the context of deglaciation of the late Würmian Alpine ice cap and deglaciation from Last Glacial Maximum (LGM) ice extents in other regions.     

10Be chronology of the last deglaciation of County Donegal, northwestern Ireland

A well-preserved moraine on the northern coast of County Donegal, Ireland, has played a critical role in our understanding of the glacial history of this sector of the Irish Ice Sheet (IIS). Because of a lack of numerical dating of the moraine, however, previous interpretations of its age and significance to the glacial history of this region have varied widely. Here we report eight in situ cosmogenic ¹⁰Be ages on boulders sampled from the moraine. Two of these ages are outliers, with the remaining six ranging from 18.8±1.0 ¹⁰Be kyr to 20.9±1.3 ¹⁰Be kyr, with an uncertainty-weighted mean age of 19.4±0.3 ¹⁰Be kyr (19.4±1.2 kyr accounting for production rate uncertainty). Our results confirm one previous ¹⁰Be age obtained from the moraine, with the combined data ( n =7) constraining the age of initial deglaciation of the IIS from its LGM position on the continental shelf to be 19.3±0.3 ¹⁰Be kyr (19.3±1.2 kyr accounting for production rate uncertainty). These ages are in excellent agreement with calibrated ¹⁴C ages that constrain retreat of the IIS margin from the continental shelf elsewhere in northwestern and western Ireland and the Irish Sea Basin associated with the start of the Cooley Point Interstadial (≥20–≤18.2 cal. kyr BP), suggesting widespread deglaciation of the IIS ∼19.5–20 kyr ago.     

Long-term (105) or short-term (103) δ13C excursion near the Palaeocene-Eocene transition: evidence from the Tethys

Expanded sedimentary records from the Tethys reveal unique faunal and isotopic changes across the Palaeocene-Eocene (P-E) transition. Unlike in the open oceans, the Tethys exhibits a gradual decrease of 1.5% in δ¹³C values prior to the rapid δ¹³C excursion. Associated with the 6¹³C excursion is a decrease in calcite burial, increase in detrital content and appearance of a unique opportunistic planktic foraminifera1 assemblage (e.g. compressed acarininids). The existence of a prelude decrease in δ¹³C values in the Tethys suggests that the P-E δ¹³C excursion may have occurred in two steps and over a few hundred thousand years, rather than as one step over a few thousand years as previously suggested. This slower excursion rate is readily explained by changing organic carbon weathering or burial rates and avoids the need of invoking ad hoc scenarios.     

Cosmogenic 10Be exposure age dating across Early to Late Weichselian ice-marginal zones in northwestern Russia

Rock samples from the Kanin Peninsula and the Timan Ridge were analysed for in situ cosmogenic ¹⁰Be for exposure age dating purposes. Crystalline rocks were sampled at four sites on the Kanin Peninsula, either from bedrock outcrops or from glacial erratics, giving overall similar ¹⁰Be ages. Outcropping sandstone and crystalline erratics were available from three sites at the Timan Ridge. The highly weathered sandstone gives substantially younger ¹⁰Be ages than the adjacent erratics. The exposure ages from the Kanin Peninsula suggest that the last deglaciation of this area took place between 55 and 37 ¹⁰Be kyr ago, in agreement with a preceding Kara Sea glaciation (55-45 kyr BP). The northwest coast of the peninsula was probably just outside the maximum limit of the last Scandinavian glaciation (20-17 kyr BP). Glacial erratic exposure ages from the Timan Ridge suggest that the 55-45 kyr BP Kara Sea glaciation reached the northern part of the ridge. The exposure dates do not show conclusive evidence regarding the existence of a Timan Ridge ice cap.     

Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps

The Schistes Lustrés (SL) suture zone occupies a key position in the Alpine chain between the high‐pressure (HP) Brianconnais domain and the ultrahigh‐pressure (UHP) Dora Maira massif, and reached subduction depths ranging from c. 40–65 km (Cottian Alps). In order to constrain the timing of HP metamorphism and subsequent exhumation, several phengite generations were differentiated, on the basis of habit, texture, paragenesis and chemistry, as belonging to the first or second exhumation episode, respectively, D2 or D3, or to earlier stages of the tectono‐metamorphic evolution. Ten carefully selected samples showing D2, D3 (D2 + D3), or earlier (mostly peak temperature) phengite population(s) were subjected to laser probe ⁴⁰Ar/³⁹Ar analysis. The data support the results of the petrostructural study with two distinct age groups (crystallization ages) for D2 and D3 phengite, at 51–45 and 38–35 Ma, respectively. The data also reveal a coherent age cluster, at 62–55 Ma, for peak temperature phengite associated with chloritoid which were preserved in low strain domains. The age of the D3 event in the SL complex appears very similar to ages recently obtained for greenschist facies deformation on the border of most internal crystalline massifs. Exhumation rates of the order of 1–2 mm yr^?1 are obtained for the SL complex, which are compatible with velocities documented for accretionary wedge settings. Similarly, cooling velocities are only moderate (c.5 °C Myr^?1), which is at variance with recent estimates in the nearby UHP massifs.     

40Ar/ 39Ar dating of Penninic Front tectonic displacement (W Alps) during the Lower Oligocene (31–34 Ma)

Direct absolute dating of the Penninic Frontal Thrust tectonic motion is achieved using the ⁴⁰Ar/³⁹Ar technique in the Pelvoux Crystalline Massif (Western Alps). The dated phengites were formed syn-kinematically in shear zones. They underline the brittle-ductile stretching lineation, pressure-shadow fibres and slickensides consistent with underthrusting of the European continental slab below the propagating Penninic Thrust. Chlorite–phengite thermobarometry yields 10–15 km and T ∼280 °C, while ⁴⁰Ar/³⁹Ar phengite ages mainly range between 34 and 30 Ma, with one younger age at 27 Ma. This Early Oligocene age range matches a major tectonic rearrangement of the Alpine chain. Preservation of prograde ⁴⁰Ar/³⁹Ar ages is ascribed to passive exhumation of the Pelvoux shear zone network, sandwiched between more external thrusts and the Penninic Front reactivated as an E-dipping detachment fault. Partial resetting in the Low Temperature part of argon spectra below 24 Ma is ascribed to brittle deformation and alteration of phengites.     

Superimposed tectonic and hydrothermal events during the late-orogenic extension in the Western French Massif Central: a structural and 40Ar/39Ar study

In the Western French Massif Central, the Argentat fault is a major structure through which As–Au fluids percolated in the Late Carboniferous along brittle fractures. New petrostructural investigations show that an early ductile normal-dextral faulting, coeval to leucogranite emplacement took place during the Late Visean syncollisional extension of the belt and was accompanied by a hydrothermal event marked by the growth of muscovites whose ⁴⁰Ar/³⁹Ar ages cluster around 335 Ma. This early fluid channelling is associated with brittle deformation only in the hangingwall of the Argentat fault, whereas ductile deformation is restricted to the footwall. These results provide new evidence for the upper crust implication during the syncollisional extension in the French Massif Central. This study stresses the interest of a detailed multimethod analysis to characterize hydrothermal processes, especially in basement areas where the tectonic, plutonic and metamorphic evolution is polyphased.     

Flood and earthquake disturbance of 210Pb geochronology (Lake Anterne, NW Alps)

Dating recent lake sediment records yielding disturbed ²¹⁰Pb profiles has been a problem of wide interest in palaeoclimatic and palaeoseismic studies over the last few centuries. When applied to an alpine lake sedimentary record, a high-resolution sedimentological study reveals that the ²¹⁰Pb profile is disturbed by the occurrence of single-event deposits triggered by two different mechanisms: flood events deposits and gravity reworking. Removing disturbed layers from the ²¹⁰Pb profile yields a logarithmic depth–activity relationship. Using a simple ²¹⁰Pb decay model (CFCS) provides an assessment of mean accumulation rate of `continuous sedimentation', as opposed to `event-linked sedimentation'. The correlation of the thickest four gravity-reworked deposits with historically known earthquakes permits both validation and refinement of the age–depth relationship. This refinement highlights variations in accumulation rate consistent with post-Little Ice Age climatic variations.     

Geochronological Constraints Using 40Ar/39Ar Dating on the Mineralization of the Hishikari Epithermal Gold Deposit, Japan

Abstract. Ages for thirty adularia samples collected from various veins were in the Hishikari gold deposit determined by ⁴⁰Ar/³⁹Ar dating to constrain the timing of adularia‐quartz vein formation and to determine the temporal change in temperature of hydrothermal fluid. Plateau ages were obtained from all adularia samples, and significant excess ⁴⁰Ar is not recognized from inverse isochrones. The duration of mineralization within individual veins was determined by adularia ages from the early and late stages of mineralization within the same vein. The durations of mineralization in the Daisen‐1, Daisen‐3, Hosen‐2 and Keisen‐3 veins in the Honko‐Sanjin zone were 7,000, 140,000, 160,000 and 170,000 years, respectively. The durations of mineralization in the Seisen‐2 and Yusen‐1–2 veins in the Yamada zones were 360,000 and 320,000 years, respectively. Mineralization lasted for a relatively longer period in individual veins at the Yamada zone. Mineralization ages from the Honko‐Sanjin zone range from 1.04 to 0.75 Ma, and most mineralization ages are concentrated in a short period from 1.01 to 0.88 Ma. In contrast, mineralization ages for the Yamada zone range from 1.21 to 0.64 Ma. These results indicate that fracturing and subsequent vein formation lasted for a longer period in the Yamada zone (about 570,000 years) compared with those events in the Honko‐Sanjin zone (about 290,000 years). The homogenization temperatures of liquid‐rich fluid inclusions in columnar adularia used for age determination were determined to be 223°C on average, and most of these temperatures range from 180 to 258d?C. No significant temporal change in homogenization temperature is recognized in this study. However, adularia in the Keisen veins indicated higher homogenization temperatures compared with elsewhere in the deposit, suggesting that the principal ascent of mineralizing hydrothermal fluid was via the Keisen veins.     

Motion on the Kaparelli fault (Greece) prior to the 1981 earthquake sequence determined from 36Cl cosmogenic dating

ABSTRACT In February and March 1981, three successive destructive earthquakes occurred at the eastern end of the Gulf of Corinth. The third shock (March, 4, Ms ≈ 6.4) ruptured the Kaparelli fault. About 40 cm of a limestone fault scarp was exhumed by the earthquake. Each major prehistoric earthquake has added new surface to this cumulative scarp exposing fresh material to cosmic-ray bombardment. Using ³⁶Cl cosmic ray exposure dating we have obtained the continuous exposure history for this 4–5-m-high limestone surface at two sites about 50 m apart. The results suggest that the Kaparelli fault has ruptured three times prior to 1981 at 20 ± 3 ka, 14.5 ± 0.5 ka and 10.5 ± 0.5 ka with slip amplitudes between 0.6 m and 2.1 m. The Kaparelli fault appears to have been inactive for 10 thousand years prior to the 1981 event.     

Timing of rockfalls in the Mont Blanc massif (Western Alps): evidence from surface exposure dating with cosmogenic <Superscript>10</Superscript>Be

Rockfalls and rock avalanches are a recurrent process in high mountain areas like the Mont Blanc massif. These processes are surveyed due to the hazard they present for infrastructure and alpinists. While rockfalls and rock avalanches have been documented for the last 150 years, we know very little about their frequency since the Last Glacial Maximum (LGM). In order to improve our understanding, it is imperative to date them on a longer timescale. A pilot campaign using Terrestrial Cosmogenic Nuclide (TCN) dating of five samples was carried out in 2006 at the Aiguille du Midi (3842 m a.s.l.). In 2011, a larger scale study (20 samples) was carried out in five other test sites in the Mont Blanc massif. This paper presents the exposure ages of the 2011 TCN study as well as the updated exposure ages of the 2006 study using newer TCN dating parameters. Most of these exposure ages lie within the Holocene but three ages are Pleistocene (59.87?±?6.10 ka for the oldest). A comparison of these ages with air temperature and glacier cover proxies explored the possible relationship between the most active rockfall periods and the warmest periods of the Holocene: two clusters of exposure ages have been detected, corresponding to the Middle Holocene (8.2–4.2 ka) and the Roman Warm Period (c. 2 ka) climate periods. Some recent rockfalls have also been dated (<?0.56 ka).     

Low δ18O eclogites from the Kokchetav massif, northern Kazakhstan

Oxygen isotopic compositions of silicates in eclogites and whiteschists from the Kokchetav massif were analyzed by whole‐grain CO₂‐laser fluorination methods. Systematic analyses yield extremely low δ¹⁸O for eclogites, as low as ?3.9‰ for garnet; these values are comparable with those reported for the Dabie‐Sulu UHP eclogites. Oxygen isotopic compositions are heterogeneous in samples of eclogite, even on an outcrop scale. Schists have rather uniform oxygen isotope values compared to eclogites, and low δ¹⁸O is not observed. Isotope thermometry indicates that both eclogites and schists achieved high‐temperature isotopic equilibration at 500–800 °C. This implies that retrograde metamorphic recrystallization barely modified the peak‐metamorphic oxygen isotopic signatures. A possible geological environment to account for the low‐δ¹⁸O basaltic protolith is a continental rift, most likely subjected to the conditions of a cold climate. After the basalt interacted with low δ¹⁸O meteoric water, it was tectonically inserted into the surrounding sedimentary units prior to, or during subduction and UHP metamorphism.     

Exhumation of the Central Alps: evidence from 40Ar/39Ar laserprobe dating of detrital white micas from the Swiss Molasse Basin

⁴⁰Ar/³⁹Ar single-grain laserprobe dating of detrital white micas from early Oligocene to middle Miocene (31–14 Ma) sedimentary rocks of the central Swiss Molasse basin reveals three distinct clusters of cooling ages for the hinterland. Two Palaeozoic age clusters reflect cooling after the Variscan orogeny with only limited reheating during the Alpine orogeny. The third Tertiary age cluster reflecting late Alpine cooling is restricted to sediments younger than 20 Myr old. Micas with cooling ages < 30 Myr are interpreted to originate from the footwall of the Simplon detachment fault, thus representing formerly exposed upper levels of the present-day Lepontine metamorphic dome. Erosion of these levels is reflected by an increase of low-grade metamorphic lithic grains in the sandstones. This interpretation puts constraints on the timing of exhumation as well as on the evolution of the drainage pattern of the Central Alps.     

Biostratigraphical and palaeoenvironmental implications of isotopic studies (18O, 13C) of middle Miocene (Badenian) foraminifers in the Central Paratethys

The cause of the middle Miocene Badenian salinity crisis in the Central Paratethys is addressed by examining the palaeotemperature evolution of Badenian waters before and after the deposition of evaporites. Selected foraminifer taxa ( Globigerinoides spp., Globigerina bulloides , and Uvigerina ) characterizing, respectively, the near-surface, intermediate, and bottom layers of the water column, were studied in two boreholes of SW Poland. The δ¹⁸O and δ¹³C values for these taxa show distinct differences which can be explained by the temperature difference between surface and bottom waters during deposition. These values also show temporal changes corresponding to the water temperature evolution in the Badenian basin. Different and quickly changing environmental conditions have been inferred from changes in foraminifer assemblages. They explain why biostratigraphic subdivisions based on well-recognized assemblages are the most accurate approach for determining the biostratigraphy of middle Miocene deposits in the Central Paratethys. The results of isotopic studies indicate that evaporites occur in a part of the Badenian section that was characterized by the lowest temperatures in the studied sections.     

A 40Ar/39Ar laser probe study of micas from the Sesia Zone, Italian Alps: implications for metamorphic and deformation histories

Two samples from the Eclogite Micaschist Complex (EMC) and the Seconda Zona Diorito–Kinzigitica (IIDK) of the Sesia Zone have been studied using a high-spatial resolution laser probe ⁴⁰Ar/³⁹Ar technique with the aim of investigating the complexities of argon behaviour in metamorphic rocks and comparing their thermal histories. Data from a single large phengite grain from the EMC show a range of ages from mid-Jurassic to Upper Cretaceous. These ‘apparent age’ variations are spatially related to both location within the grain and to intragrain microstructure. Modelling of the data shows that the profile formed by the diffusion of an excess argon component into the grain, parallel to the mica cleavage. Profile asymmetry is explained by temporal variations in microstructural development enabling excess argon to enter the grain at different times in different places. The temperatures of the initiation of deformation and the possible time-scales for the deformation can be calculated as a function of cooling rate. All estimates suggest deformation at greenschist facies, in accord with the observed retrograde mineral assemblage. Absolute temperature estimates for deformation vary by less than 22 °C for different cooling rates of 10 and 30 °C Ma^?1 but vary by 80 °C with different estimates of diffusion parameters. The duration of deformation was for at least 2 Ma at 10 °C Ma^?1 or 0.7 Ma at 30 °C Ma^?1. Biotites from the IIDK sample record a Permian to Upper Cretaceous age range that correlates with grain size, the smallest grain sizes yielding the youngest ages. This relationship is best explained by a partial resetting of biotites during an Alpine thermal event initiated not more than 70 Ma ago. Modelling of these data suggest that the sample never exceeded 300 °C during the Alpine. The profoundly different thermal histories of the two units—the EMC recrystallized at 550 °C whilst the IIDK remained below 300 °C—suggests that they may not have been juxtaposed until much later than the eclogite facies metamorphism.     

Dating cleavage formation in slates and phyllites with the 40Ar/39Ar laser microprobe: an example from the western New England Appalachians, USA

Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high-spatial-resolution ⁴⁰Ar/³⁹Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S₂, S₃, and S₄) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S₃ and S₄ are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative ⁴⁰Ar/³⁹Ar geochronology. Mean ⁴⁰Ar/³⁹Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S₃ and 345.5 ± 1.7 Myr for S₄. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S₃ and S₄, these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region.