Comprehensive set of seismic and potential field data from the whole European Variscan belt is used to interpret the structure and evolution of the European Variscides as defined by Martínez Catalán et al. (2021). The gravity data show the presence of high amplitude, short-wavelength gravity anomalies correlated with the outcrops of eclogites, ultramafic rocks and ophiolites delineating the main body of the Mid-Variscan Allochthon (MVA) and the Devonian Mid-Variscan suture (MVS). The medium amplitude and elongated long-wavelength gravity highs, aligned parallel to the Variscan structural grain, correspond to the low-grade Proterozoic rocks of the MVA and Devonian arc – back-arc system. On the other hand, the short wavelength negative gravity anomalies developed in the central part of the belt coincide with Carboniferous (330–310 Ma) per- to meta-aluminous magmatic bodies. The magnetic data show two belts correlated with Carboniferous Rhenohercynian and Devonian Mid-Variscan magmatic arc granitoids. The Rhenohercynian and Mid-Variscan subduction systems are also well-imaged by moderately dipping primary reflectors in reflection seismic lines. Younger moderately dipping reflectors in the upper-middle crust coincide with outcrops of Carboniferous detachments, limiting granite plutons and core complexes along-strike the core of the Variscan orogeny. Deep crustal reflectors are considered as an expression of lower crustal flow resulting from extensional re-equilibration of the previously thickened Variscan crust. A P-wave velocity logs synthesis shows a high-velocity cratonic crust surrounding a thin Variscan orogenic crust defined by low-velocity lower and middle crusts. The latter crustal type coincides with regional outcrops of 330–310 Ma per- to meta- aluminous granitoids and associated gravity lows along-strike the belt. All these data are used to define the primary polarity of Devonian subduction systems defining the European Variscan belt (Schulmann et al., 2022) and discuss the Carboniferous extension forming specific structure of the Variscan crust. This geodynamic evolution is integrated into a paleomagnetically constrained model of the movements of continental plates and intervening oceans (Edel et al., 2018; Martínez Catalán et al., 2021).REFERENCES:Catalan, J.R.M., Schulmann, K. and Ghienne, J.F., 2021. The Mid-Variscan Allochthon: Keys from correlation, partial retrodeformation and plate-tectonic reconstruction to unlock the geometry of a non-cylindrical belt. Earth-Science Reviews, 220, 1–65.Edel, J.B., Schulmann, K., Lexa, O. and Lardeaux, J.M., 2018. Late Palaeozoic palaeomagnetic and tectonic constraints for amalgamation of Pangea supercontinent in the European Variscan belt. Earth-science reviews, 177, 589-612.Schulmann, K., Edel, J.B., Catalán, J.R.M., Mazur, S., Guy, A., Lardeaux, J.M., Ayarza, P. and Palomeras, I., 2022. Tectonic evolution and global crustal architecture of the European Variscan belt constrained by geophysical data. Earth-Science Reviews, 234,  p.104195.
The Orlica-Śnieżnik dome comprises large orthogneiss bodies interbedded with amphiblite-grade metasediments and minor metavolcanics. New U-Pb and Pb-Pb SHRIMP zircon ages for two major gneiss units of the dome, the Śnieżnik and Gierałtów gneiss, yielded similar ages of ca. 500 Ma. This is interpreted to reflect the magmatic crystallization age from the same or similar igneous precursors, in agreement with the geochemical characteristics of these rocks. Some zircon cores in both gneisses, interpreted to be inherited xenocrysts, have ages of ca. 530–540 Ma, and, additionally, of ca. 565 Ma and 2.6 Ga in the Śnieżnik gneiss. Igneous grains in both gneiss types have high-U rims, which are dark under cathodoluminescence. They are much better developed in the Gierałtów gneiss and they yield a well-defined weighted mean U-Pb age of 342 ± 6 Ma. These high-U rims are interpreted to have grown close to the peak of HT metamorphism which is responsible for the migmatitic texture of the Gierałtów gneiss. The Visean HT–LP metamorphism in the Orlica-Śnieżnik dome is interpreted as a result of rapid uplift and decompression due to overthrusting of high grade rocks over the Moravo-Silesian nappe pile. Our data support geodynamic models that ascribe a predominant influence in the tectonic evolution of the West Sudetes to the Variscan orogenic events. This is suggested by the inheritance of zircon xenocrysts from the Cadomian basement and by the Late Cambrian–Early Ordovician magmatic event, both typical of the Armorican terrane assemblage, as well as by the Early Carboniferous age of the metamorphism. Le dôme d'Orlica-Śnieżnik est constitué de grands corps d'orthogneiss intercalés avec des sédiments métamorphisés dans le faciès amphibolite et des roches métavolcaniques en quantité mineure. De nouvelles analyses U-Pb et Pb-Pb, à l'aide de la sonde ionique « SHRIMP », mettent en évidence un âge commun d'environ 500 Ma pour les deux unités principales (Śnieżnik et Gierałtów). Cet âge est interprété comme celui de la cristallisation magmatique des protolithes ignés de ces roches. Quelques cœurs de 530–540 Ma ont été décelés dans les deux types de gneiss, ainsi que dans les gneiss de Śnieżnik seulement, des cœurs âgés d'environ 565 Ma et 2,6 Ga. Ces âges anciens sont interprétés en termes de xénocristaux hérités. Dans les deux types de gneiss, les images de cathodoluminescence des zircons magmatiques montrent des surcroissances riches en U. Ces surcroissances sont plus développées dans les gneiss de Gierałtów, où elles permettent de mesurer un âge de 342 ± 6 Ma (Viséen), interprété comme celui du climax du métamorphisme de HT et de la migmatisation des gneiss de Gierałtów. Cet épisode métamorphique de HT–BP du dôme d'Orlica-Śnieżnik est interprété comme le résultat de la remontée rapide et de la décompression causées par le charriage des roches de haut degré métamorphique sur l'empilement d'unités des nappes Moravo-Silesiennes. Nos données confirment les modèles géodynamiques qui attribuent une importance prédominante aux événements orogéniques Varisques dans l`évolution tectonique des Sudètes Occidentales. La présence de xenocristaux de zircon d'âge cadomien et l'âge Cambrien supérieur–Ordovien inférieur de l'épisode magmatique sont deux caractéristiques typiques du « terrane » Armoricain, de même que le métamorphisme du Carbonifère inférieur.
Abstract Detailed X‐ray compositional mapping and microtomography have revealed the complex zoning and growth history of garnet in a kyanite‐bearing eclogite. The garnet occurs as clusters of coalesced grains with cores revealing slightly higher Ca and lower Mg than the rims forming the coalescence zones between the grains. Core regions of the garnet host inclusions of omphacite with the highest jadeite, and phengite with the highest Si, similar to values in the cores of omphacite and phengite located in the matrix. Therefore, the core compositions of garnet, omphacite, and phengite have been chosen for the peak pressure estimate. Coupled conventional thermobarometry, average P–T , and phase equilibrium modelling in the NCKFMM n ASHT system yields P–T conditions of 26–30 kbar at 800–930°C. Although coesite is not preserved, these P–T conditions partially overlap the coesite stability field, suggesting near ultra‐high–pressure ( UHP ) conditions during the formation of this eclogite. Therefore, the peak pressure assemblage is suggested to have been garnet–omphacite–kyanite–phengite–coesite/quartz–rutile. Additional lines of evidence for the possible UHP origin of the Międzygórze eclogite are the presence of rod‐shaped inclusions of quartz parallel to the c‐axis in omphacite as well as relatively high values of Ca‐Tschermak and Ca‐Eskola components. Late zoisite, rare diopside–plagioclase symplectites rimming omphacite, and minor phlogopite–plagioclase symplectites replacing phengite formed during retrogression together with later amphibole. These retrograde assemblages lack minerals typical of granulite facies, which suggests simultaneous decompression and cooling during exhumation before the crustal‐scale folding that was responsible for final exhumation of the eclogite.
Significant E‐W extension and/or compression must have been generated by displacements along the Red River Fault (RRF) since its curvature does not match a small circle centered at the Euler pole for the Indochina–south China plate pair. The amount of extension perpendicular to the RRF offshore Vietnam depends on the magnitude of left‐lateral displacement along the RRF. In general, the larger the left‐lateral displacement along the fault, the smaller the amount of E‐W extension. All purely strike‐slip models of the opening of the South China Sea that assume large displacements (>250 km) along the RRF encounter major problems because they imply little extension, or even considerable shortening, offshore east Vietnam. This is inconsistent with the presence of large elongated basins offshore Vietnam. Using a plate tectonic model, we compare continental extension values implied by different magnitudes of displacement along the RRF with crustal stretching estimates derived from 2‐D profiles modeled from gravity data. We utilize 2‐D gravity forward models to restore the extended continental margin crust to its original position prior to extension. We find that substantial amounts of extension for offshore Vietnam can only be modeled assuming moderate displacements along the RRF compatible with the presence of a southward subducting proto–South China Sea. The total amount of ENE‐WSW extension offshore northern Vietnam constrained by our 2‐D gravity profiles and gravity inversion increases southward from 36 to 89 km along the Yinggehai Basin. These values of ENE‐WSW extension are consistent with 250 km of left‐lateral displacement along the RRF.
Atmospheric responses to earthquakes or volcanic eruptions have become an interesting topic and can potentially contribute to future forecasting of these events. Extensive anomalies of the total electron content (TEC) are most often linked with geomagnetic storms or Earth-dependent phenomena, like earthquakes, volcanic eruptions, or nuclear explosions. This study extends rarely discussed, but very frequent, interactions between tectonic plate boundaries and the ionosphere. Our investigations focus on the very frequent occurrence of TEC enhancements not exclusively linked with individual seismic phenomena but located over tectonic plate boundaries. The objective of this study is to provide a review of the global spatiotemporal distribution of TEC anomalies, facilitating the discussion of their potential relations with tectonic activity. We apply a Kriging-based UPC-IonSAT quarter-of-an-hour time resolution rapid global ionospheric map (UQRG) from the Polytechnic University of Catalonia (UPC) IonSAT group for the detection of relative vertical TEC (VTEC) changes. Our study describes global relative and normalized VTEC variations, which have spatial and temporal behaviours strongly indicating their relationship both with geomagnetic changes and the tectonic plate system. The variations in geomagnetic fields, including the storms, disturb the ionosphere and amplify TEC variations persisting for several hours over tectonic plate boundaries, mostly over the diverging ones. The seismic origin of the selected parts of these TEC enhancements and depletions and their link with tectonic plate edges are suspected from their duration, shape, and location. The changes in TEC originating from both sources can be observed separately or together, and therefore, there is an open question about the directions of the energy transfers. However, the importance of geomagnetic field lines seems to be probable, due to the frequent common occurrence of both types of TEC anomalies. This research also proves that permanent observation of global lithosphere–atmosphere–ionosphere coupling (LAIC) is also important in time periods without strong earthquake or volcanic events. The occurrence of TEC variations over diverging tectonic plate boundaries, sometimes combined with travelling anomalies of geomagnetic origin, can add to the studies on earthquake precursors and forecasting.
