TS10.1

Several isotopic systems can potentially be used to provide absolute chronology of carbonate minerals; these include Rb-Sr, Sm-Nd, U-Pb and U-Th. The production of a robust date requires incorporation of the parent isotope during formation, and ideally low abundance of the daughter isotope. Variable parent-daughter (P/D) abundance during formation additionally can increase the robustness of the resulting isochron. The ability to use high spatial resolution sampling via laser ablation (LA-) ICP-MS, makes it the most attractive technique, as varying P/D ratios can be sampled within single age domains, whether these be crystals, growth bands, or other textural domains. Of the systems available in carbonate, U-Pb is the only one that is commonly applied with LA-ICP-MS methods, although the others are all possible with modern instrumentation. Of note, collision-cell technology means that Rb-Sr is regaining popularity as an in situ dating method. Carbonate geochronology can be achieved at a range of timescales, with U-Th ranging from 100s yrs to ca. 500 ka, and U-Pb ranging from 100s ka to 100s Ma. The potential for isotopic disequilibrium effecting measured U-Pb ages, means that young (< 10 Ma) U-Pb dates are susceptible to inaccuracy. Published LA-ICP-MS U-Pb dates suggest that this method can be pushed well into the Precambrian.

The application of U-Th and U-Pb geochronology to provide direct timing constraints on deformation gained ground around 10 and 5 years ago, respectively. Because LA-ICP-MS instrumentation is relatively common, and because ancient carbonates provide undated material of significant interest, U-Pb in particular has become a rapidly growing technique. The biggest advance in LA-ICP-MS U-Pb dating has been the characterisation of matrix-matched calcite reference materials (RMs). The observation of minor matrix-related effects between carbonate matrices however, means that the availability of well characterised RMs for minerals such as dolomite and siderite, are a limiting factor in the accuracy of these non-calcite dates. In terms of deformation, most existing data corresponds to calcite.

Calcite precipitates from fluid at a range of temperatures in the upper crust, with fluid-flow typically being enhanced by brittle deformation, i.e. faulting and fracturing. To link calcite dates to the timing of specific deformational events, such as fault slip or fracture-opening, various ‘syn-tectonic’ or ‘syn-kinematic’ vein types have ben utilised. These include slickenfibres, breccia cements, and various types of vein arrays. Each of these structures has variable ability to faithfully record the timing of fault slip, and the ability to link calcite mineralisation to the timing of fault slip remains one of the most assumptive parts of this method. Detailed petrographic and compositional characterisation and documentation are required, for which a range of methods are available, such as cathodoluminescence and trace element mapping. Along with a summary of the advances in carbonate geochronology, various examples of vein structures and of methods for characterisation will be discussed, including examples where there is evidence for overprinting by later fluid-flow.

How to cite: Roberts, N. and Lee, J.: Progress and pitfalls in dating deformation with carbonate geochronology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12068, https://doi.org/10.5194/egusphere-egu21-12068, 2021.

Recently, U-Pb dating of fracture-filling carbonates has revealed as a powerful tool to constrain the absolute timing of deformation in fold and thrust belts. However, geochronological studies of these minerals have to be combined with petrological observations and geochemical analyses to decipher if measured dates document fluid flow synchronously to deformation or post-kinematic events.

The Pyrenean compressional belt formed from Late Cretaceous to Oligocene due to the stacking of three thrust sheets and a deformed foreland basin. From top-and-older to bottom-and-younger, these consist of the Bóixols-Upper Pedraforca, Lower Pedraforca and Cadí thrust sheets and the Ebro foreland basin. Here, we quantify the duration of thrust sheet emplacement and shortening rates in the SE Pyrenees using U-Pb dating of 43 calcites filling fractures and interparticle porosity.

Four fracture sets related to compressional tectonics and one set related to extension are identified. The compressive sets include: 1) N-S, NNW-SSE and NNE-SSW trending veins; 2) E-W trending folding-related veins; 3) E-W trending reverse faults; and 4) NW-SE and NE-SW trending strike-slip faults. Fractures related to extension are NNW-SSE and NW-SE trending normal faults.

Elongated blocky, blocky and bladed calcite textures of the dated cements are observed. Elongated textures are observed in reverse, strike-slip and normal faults and occasionally in N-S, NNW-SSE and NNE-SSW and E-W veins. In these fractures, calcite crystals are arranged parallel, oblique, or perpendicular to fracture walls and provide evidence for syn-kinematic growth. Blocky and bladed textures have been identified in N-S, NNW-SSE and NNE-SSW veins, E-W folding-related veins, reverse and strike-slip faults and in calcite precipitated between sedimentary breccia clasts. Although these textures indicate precipitation after vein opening or at lower rates than vein opening, their presence in crack-seal veins and in stepped slickensides also indicates syn-kinematic growth. Moreover, clumped isotope temperatures measured in several blocky and bladed calcites precipitated in veins and faults indicate that most of them precipitated from fluids in thermal disequilibrium with host rocks, revealing rapid fluid flow and precipitation just after fracturing. Contrarily, low temperatures measured in blocky and bladed calcite precipitated in the interparticle porosity of sedimentary breccias indicate late fluid migration.

U-Pb dating applied to fracture-filling calcites in the SE Pyrenean fold and thrust belt yielded 46 ages from 70.6 ± 0.9 Ma to 2.8 ± 1.8 Ma (Cruset et al., 2020). The results reveal minimum durations for the emplacement of each thrust sheet (18.7 Myr for the Bóixols-Upper Pedraforca, 11.6 Myr for the Lower Pedraforca and 14.3 Myr for the Cadí), and that piggy-back thrusting was accompanied by post-emplacement deformation of upper thrust units above the lower ones during tectonic transport. These estimated durations, combined with the minimum shortening established for the Bóixols-Upper Pedraforca, Lower Pedraforca and Cadí thrust sheets by other methods, allows calculating shortening rates of 0.6 mm/yr, 3.1 mm/yr and 1.1 mm/yr, respectively. Finally, the results also reveal the development of local normal faults at late Oligocene times during the final stages of compression and exhumation.

References:

Cruset et al. (2020). Geological Society of London. 177, 1186-1196.

How to cite: Cruset, D., Vergés, J., and Travé, A.: U-Pb dates measured in fracture-filling calcites from the SE Pyrenees: syn- or post-kinematic mineral growth?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8814, https://doi.org/10.5194/egusphere-egu21-8814, 2021.

Zircon is ubiquitously used to nail down the geological events for both terrestrial and extraterrestrial materials. The U-Pb system and other trace elements in zircon plausibly remain stable and robust in normal metamorphic processes on Earth, while under the extremely shock condition, trace element behaviors in zircon could be unstable and differential due to the generated extraordinary deformations and thermal annealing. Since the systematic deformations in zircon recovered from the Chicxulub impact structure, such as planar fractures (PFs), reidite and granular zircon, the phenomenon of partially or completely age resetting are discovered in zircons from impact melt, breccia, ejecta and meteorites. In effect, element migration during the shock or post-shock setting is the most critical question, which may yield age resetting in nature. The enrichment of elements in shock-deformed zircon regions (PFs and reidite) are revealed, such as Y, Al, Ca, U, Th and Pb. Due to the limitation of resolution and lack of typical shock deformations, the straightforward correlations among deformations, element migration and chronology in zircon by traditional means have not been illustrated clearly so far. Here we systematically analyzed the correlations between shock deformations (from low to high degree: PFs, reidite and granular zircon) and element distribution in zircon by high-resolution Nano-SIMS mapping data. This can be used to interpret the chronology of shock products both from terrestrial and extraterrestrial bodies.

How to cite: Jiawei, Z., Long, X., Qi, H., and Zhiyong, X.: Chronology and Element Distribution of Shock-deformed Regions in Zircon from the Chicxulub Impact Structure, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3856, https://doi.org/10.5194/egusphere-egu21-3856, 2021.

The northern Adamello is crosscut by ductile shear zones and pseudotachylyte-bearing faults, both compatible with the same stress field, with ductile shear zones crosscut by brittle faults. These relations are coherent with the re-equilibration of the pluton-related thermal anomaly to temperatures typical of the base of the seismogenic continental crust (T = 250 – 300°). Our new ⁴⁰Ar-³⁹Ar ages help to constrain the absolute age and duration of each deformation phase.

Samples included wall-rock biotite, bulk ultramylonites and pseudotchylytes. Before stepwise heating ⁴⁰Ar-³⁹Ar measurements, samples were characterized by microstructural, geochemical and petrological analyses.

The wall-rock biotite is 33.4±0.1 Ma old, independently of grainsize. Mylonites feature complex age spectra between 28-31 Ma, including biotite and altered feldspar. Four pseudotachylyte matrices are clustered around 30-31.5 Ma, and two samples have 25-26 Ma ages.

Ductile shearing active 2 Ma after wall-rock emplacement indicates either low strain rates, or a long-lasting thermal anomaly, which might be due to high emplacement depth, and/or the progressive assemblage of adjacent plutons through small magma pulses. Seismogenic faulting overlaps with mylonitization around 31 Ma; younger pseudotachylyte ages may be due to late-stage reactivation.

How to cite: Mittempergher, S., Zanchetta, S., Caldiroli, F., Bistacchi, A., Zanchi, A., and Villa, I. M.: The timescale of the aseismic to seismic deformation in a cooling pluton: 40Ar-39Ar ages of the solid-state deformation in the Adamello (Southern Italian Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-791, https://doi.org/10.5194/egusphere-egu21-791, 2021.

The Periadriatic Fault System (PFS) is one of the most important tectonic element in the Alps, separating the Europe-verging collisional wedge from the S-verging Southern Alps. The PFS developed in a dextral transpressional regime during the Cenozoic, following the Adria-Europe collision. The area between the Passeier and the Eisack rivers (Meran, NE Italy) is a key area for the understanding of the interactions among the PFS, the Giudicarie Fault and the fault network here active in the middle to late Cenozoic. Here the elsewhere E-W trending PFS rotates to a NE-SW trend, impliying significant changes in the fault kinematics and evolution.

The NE-SW strand of the PFS, known as the Meran-Mauls fault, is connected to the North Giudicarie Fault to the west and to the Pustertal segment of the PFS to the east. A general evolution from the ductile to brittle deformation regime has been recognized on the base of field-based structural analysis and microstructural analysis of fault rocks. Pseudotachylytes occur all along the fault zone, testifying to the seismic activity of the Meran-Mauls fault. ⁴⁰Ar-³⁹Ar dating of pseudotachylytes provided ages in the 32-22 Ma time interval, indicating that the PFS experienced a prolonged seismic activity during middle Cenozoic times. Several pseudotachylytes veins show a re-activation as cm-thick ductile shear zones, indicating that the plastic-brittle transition was not sharp in time.

Combining the structural analysis of the PFS with other adjacent faults connected in space and time (Passeier fault, Faltleis fault, Val Nova fault and other minor faults) we reconstructed a marked reverse dip-slip kinematics of the Meran-Mauls Fault during a progressive transition across the plastic-brittle regime, followed in time by a dextral transpression. Paleostress reconstructions performed on these faults populations indicate a progressive switch of the main direction of compression from NW-SE to N-S. This switch likely occurred when the Meran-Mauls segment of the PFS definitively passed to a brittle deformation regime.

How to cite: Zanchetta, S., Montemagni, C., Mascandola, C., and Zanchi, A.: Time constraints and fault kinematic evolution of the Periadriatic Fault System along the Meran-Mauls segment (N Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2162, https://doi.org/10.5194/egusphere-egu21-2162, 2021.

Sredna Gora Zone in Bulgaria is confined between the Balkan fold-thrust belt to the north and the Rhodope metamorphic complex to the south. The pre-Mesozoic basement of the central parts of the zone consists of Variscan high-grade metamorphic rocks intruded by Late Carboniferous granitoid plutons. They are transgressively overlaid by Triassic epicontinental, arc-related Upper Cretaceous volcaniclastic and Paleocene continental deposits. The Paleogene-Neogene sediments of the Thrace basin cover unconformably the older rock sequences. The zone experienced several compressional and extensional events during the Alpine time followed by post-orogenic extension in the Cenozoic.

We performed apatite fission-track analysis on rocks from the central, topographically highest parts of the Sredna Gora Zone in order to constrain the cooling history of the Variscan basement. With this aim four granitic samples were collected at different altitude (between 565 and 1604 m) from the tectonically uninterrupted section along the slope of Sredna Gora Mountains. The samples were processed and analyzed in the newly established Low-Temperature Thermochronology Laboratory in Bulgaria using LA-ICP-MS technique.

The samples yield apatite FT ages between 41.6 ± 2.6 (the highermost sample) and 39.4 ± 3.1 (the lowermost sample). The obtained confined mean tracks lengths are between 12.81 and 14.06 µm with standard deviation between 0.99 and 2.11 µm. The Dpar values vary from 1.75 µm to 1.46 µm (with standard deviation of approx. 0.20 µm).

The obtained positive age-altitude correlation suggests indeed that the studied part of the basement has cooled as one single block. The apparent exhumation rate is estimated to 0.46 mm/year. However, the positive Dpar-age correlation implies that the age dispersion could be influenced by apatite kinetic variability and hence relatively different closure temperature for the analysed samples may be suggested. Therefore, we consider the estimated apparent exhumation rate as only the minimum possible rate. The thermal models of the analysed samples (using HeFTy software) also show moderate cooling rates in the period between 45 and 35 Ma. This cooling could be related to the period of post-orogenic denudation and extension during the Eocene, associated with formation of the Thrace basin to the south-southeast. This extensional event, known from the whole Balkan Peninsula, is well documented in the neighbouring Balkan fold-thrust belt and the Rhodope metamorphic complex from where much faster exhumation rates were reported.

Acknowledgements. The study is supported by the grant 04/9 funded by the National Science Fund, Ministry of Education and Science, Bulgaria.

How to cite: Balkanska, E., Georgiev, S., Kounov, A., Peytcheva, I., Tagami, T., and Sueoka, S.: Timing and rate of exhumation of Central Sredna Gora Zone basement, Bulgaria, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3862, https://doi.org/10.5194/egusphere-egu21-3862, 2021.

The Serbo-Macedonian Massif represents a belt of medium to lower amphibolite facies metamorphics situated along the European continental margin between the Pannonian Basin in the north and the Aegean Sea in the south. Structurally, it comprises the innermost segments of the Dacia mega-unit of the European affinity and is juxtaposed against the Adria-derived units of the Dinarides across the Adria-Europe zone of collision. The peak metamorphic event in the Serbo-Macedonian Massif is Variscan in age, while its magmatism had a complex pre-Alpine evolution, with the youngest stage being related to the crustal extension during the Triassic opening of the northern branch of Neotethys Ocean (or the Vardar Ocean). The subsequent Late Jurassic–Paleogene closure of the Vardar Ocean led to the E-ward subduction of the Neotethys oceanic lithosphere beneath the upper European plate (i.e., the Sava subduction system). The retreating and steepening of subducting lithosphere during the Late Cretaceous triggered syn-subductional extension in the upper plate of the Sava subduction system. The Late Cretaceous extension exhumed and structurally juxtaposed the high-grade Serbo-Macedonian metamorphics against the low-grade metamorphics of the Carpathians Supragetic Unit. The contact is marked by the E-dipping shear zone that can be traced along the eastern margin of Serbo-Macedonian Massif, from the Vršac Mts in the north, across the Jastrebac Mts and further towards the south in the Central Serbo-Macedonian sub-unit of south-eastern Serbia. The Late Cretaceous extension exhumed the Serbo-Macedonian metamorphic core, concurrently creating subsidence in a forearc basin along the frontal part of the European continental margin.

Due to its unique position in the interference zone of the two retreating Carpathian and Dinaridic slabs, the Northern Serbo-Macedonian sub-unit between the Vršac Mts in the north and the Jastrebac Mts in the south was strongly influenced by processes associated with the Oligocene–Miocene Pannonian extension. Hence, large segments of the Northern Serbo-Macedonian sub-unit including its contact with the Supragetic Unit were buried beneath the Neogene sediments of the Morava Valley Corridor, as the southern prolongation of the Pannonian Basin. In order to segregate and quantify the effects of the Oligocene–Miocene extension we have conducted a coupled kinematic, petrological and thermochronological study in the segments of Northern Serbo-Macedonian sub-unit adjacent to the Dinarides and Carpathians. The recent tectonic uplift of the Vršac Mts occurred in the Middle to Late Miocene along the WSW-dipping normal faults that control deposition in the adjacent Zagajica depression. The ENE-WSW oriented extension, which was triggered by the retreat of Carpathian slab, exhumed the core of the mountains and exposed the Late Cretaceous Serbo-Macedonian\Supragetic extensional contact. South from the Vršac Mts such exhumation was hampered by the presence of rigid Moesian indenter. Tectonic exhumation of the Jastrebac Mts, together with a cluster of Serbo-Macedonian gneiss domes that emerge from the surrounding Neogene sediments in the western-central part of the Morava Valley Corridor, was induced by corrugated detachment faults during the Oligocene–Miocene E-W oriented Dinaridic extension.

How to cite: Kostić, B., Stojadinović, U., Krstekanić, N., Ružić, M., and Luković, A.: Alpine tectonic evolution of the Northern Serbo-Macedonian sub-unit: inferences from kinematic and petrological investigations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-202, https://doi.org/10.5194/egusphere-egu21-202, 2021.