Orogenic Stage Research Articles

Paleoproterozoic sediment-derived magmas reveal a late orogenic stage in the southern São Francisco Craton, Brazil: Evidence from petrogenesis of the 2.0 Ga Cupim Pluton leucogranites

Sediment-derived granites (S-type) can provide detailed information about continental crust reworking processes and tectonic regimes through time. Their compositions reflect the nature of sedimentary rocks that melted to form them. Inherited accessory minerals in these granites can provide information regarding the age and nature of the crustal rocks that contributed to the formation of their sedimentary source rock. In addition, S-type granites typically form during collisional stages in modern orogens, and their Precambrian records can provide clues about ancient tectonics. In this contribution, we present a detailed petrogenesis of the Cupim Pluton, located in the Paleoproterozoic Mineiro Belt (Brazil). The Cupim pluton is composed of two-mica and garnet-bearing leucogranites that are strongly peraluminous (ASI > 1.1), high-K (K2O/Na2O > 1), low MgO + FeO + TiO2 (<2 wt%). Its composition is similar to that of melts experimentally produced by muscovite/biotite dehydration melting reactions. Rb/Sr and Rb/Ba ratios suggest that these leucogranites are formed by the melting of heterogeneous sedimentary source(s). Zircon crystals from the Cupim Pluton unveil a protracted crustal melting period (2.01–1.98 Ga) hitherto unknown in the Mineiro Belt, which itself discloses a late collisional orogenic stage in southern São Francisco Craton. Igneous zircon crystals exhibit εHf(t) between −16.4 and − 0.6, and δ30Si between −1.16 and − 0.26 ‰, which is typical for zircon from sediment-derived granites. Hafnium isotope variations reflect mixing between at least two crustal sources, which is in line with inherited zircon cores that evidence major Archean and minor Paleoproterozoic sediment contributions. The emplacement of the Cupim Pluton leucogranites ends a subduction-collision cycle, following a long period of TTG-sanukitoid magma production, and promoted the stabilization of the southern São Francisco Craton.

Tectonothermal model and magmatism evolution of the postcollisional (pre-plume) development stage of the Kara orogen (Northern Taimyr, Central Arctic)

Tectonothermal model and magmatism evolution of the postcollisional (pre-plume) development stage of the Kara orogen (Northern Taimyr, Central Arctic)

From Triassic metamorphism to Early Cretaceous anatexis in the Dabie orogen, central China: Constraints from in-situ U-Pb age and Hf and O isotopes of zircon from migmatites

The Dabie orogen underwent a ‘complete’ orogenic cycle from Triassic collisional orogeny to Early Cretaceous tectonic collapse. Zircons of the migmatite in the North Dabie terrane recorded the processes from Triassic granulite-facies metamorphism to Early Cretaceous anatexis. Three types of zircon are recognized in the migmatite: the Neoproterozoic inherited magmatic zircon, the Triassic metamorphic zircon, and the Late Jurassic–Early Cretaceous anatectic zircon. The Neoproterozoic inherited magmatic zircons suggest the protolith of the migmatite has a tectonic affinity with the Yangtze Craton (YC). The Triassic metamorphic zircon domains have variable εHf(t) values from −4.1 to 3.6 and low δ18O compositions ranging from −4.36‰ to −0.30‰. The anatectic zircon domains have clearly negative εHf(t) values of −19.8 to 0.8 (mostly less than −10.0) and relatively high δ18O values from −4.24‰ to 3.49‰. Based on the chronological and isotopic data of migmatite and the results of previous studies, the Triassic granulite-facies metamorphism was closely related to the asthenosphere upwelling after the slab break-off during the orogenic stage, and the Early Cretaceous anatexis respond to widely heat event during the collapse stage of orogen.

Titanite geochemistry traces extreme differentiation of granitic magma in the collisional orogen

The genesis of granitic magma is important for the growth and evolution of continental crust. However, whether and how granitic magma can undergo efficient crystal fractionation from partial melting at the source to magma emplacement in the shallow crust level are still controversial. To address this problem, a combined study of whole-rock and titanite geochemistry was carried out for syn-exhumation granites from the Sulu ultrahigh-pressure metamorphic belt in eastern China. These high-Si granites were produced by partial melting of the deeply subducted continental crust. Significant geochemical differentiation has been revealed in these granites. Titanite in the granites can be divided into anatectic and magmatic origins, according to the Fe/Al ratios and trace element compositions. Both types of titanite yield similar UPb ages ranging from 209 ± 23 Ma to 233 ± 26 Ma, corresponding to the late collisional stage of the Sulu orogen. These titanite domains generally have consistent εNd(t) values, suggesting that they formed in a nearly closed system without significant magma mixing of different sources. The highly variable Nb/Ta ratios of titanite show good correlations with FeO, Al2O3, Ta, Sr, LREE and (Lu/Gd)N. This interesting feature is ascribed to the extensive crystal fractionation of titanite, plagioclase and allanite/epidote during magma evolution. The results suggest that the highly silicic granitic melts can experience extensive crystal separation from the liquid during granitic magma differentiation. This study shows that accessory minerals such as titanite play a key role in constraining the geochemical differentiation of granitic melts. Furthermore, titanite geochemistry has the potential to decode the complex processes from partial melting at the source to magma differentiation at shallow crust level in collisional orogens.

The Alpine Geological History of the Hellenides from the Triassic to the Present—Compression vs. Extension, a Dynamic Pair for Orogen Structural Configuration: A Synthesis

In this paper, the Hellenic orogenic belt’s main geological structure and architecture of deformation are presented in an attempt to achive a better interpretation of its geotectonic evolution during Alpine orogeny. This study was based not only on recent research that I and my collaborators conducted on the deformational history of the Hellenides but also on more modern views published by other colleagues concerning the Alpine geotectonic reconstruction of the Hellenides. The structural evolution started during the Permo–Triassic time with the continental breaking of the supercontinent Pangea and the birth of the Neotethyan ocean realm. Bimodal magmatism and A-type granitoid intrusions accompanied the initial stages of continental rifting, followed by Triassic–Jurassic multiphase shallow- and deep-water sediment deposition on both formed continental margins. These margins were the Apulian margin, containing Pelagonia in the western part of the Neotethyan Ocean, and the European margin, containing continental parts of the Serbo-Macedonian and Rhodope massifs in the eastern part of the Neotethyan ocean. Deformation and metamorphism are recorded in six main deformational stages from the Early–Middle Jurassic to the present day, beginning with Early–Middle Jurassic Neotethyan intra-oceanic subduction and ensimatic island arc magmatism, as well as the formation of a suprasubduction oceanic lithosphere. Compression, nappe stacking, calc-alkaline magmatism, and high-pressure metamorphic events related to subduction processes alternated successively over time with extension, orogenic collapse, medium- to high-temperature metamorphism, adakitic and calc-alkaline magmatism, and partial migmatization related to the uplift and exhumation of deep crustal levels as tectonic windows or metamorphic core complexes. A S- to SW-ward migration of dynamic peer compression vs. extension is recognized during the Tertiary Alpine orogenic stages in the Hellenides. It is suggested that all ophiolite belts in the Hellenides originated from a single source, and this was the Neotethyan Meliata/Maliac-Axios/Vardar ocean basin, parts of which obducted during the Mid–Late Jurassic on both continental margins, Apulian (containing Pelagonia) and European (containing units of the Serbo-Macedonian/Rhodope nappe stack), W-SW-ward and E-NE-ward, respectively. In this case, the ophiolite nappes should be considered far-traveled nappes on the continental parts of the Hellenides associated with the deposition of Middle–Late Jurassic ophiolitic mélanges in basins at the front of the adjacent ophiolite thrust sheets. The upper limit of the ophiolite emplacement are the Mid–Upper Jurassic time(Callovian–Oxfordian), as shown by the deposition of the Kimmeridgian–Tithonian Upper Jurassic sedimentary carbonate series on the top of the obducted ophiolite nappes. The lowermost Rhodope Pangaion unit is regarded as a continuation of the marginal part of the Apulian Plate (External Hellenides) which was underthrust during the Paleocene–Eocene time below the unified Sidironero–Kerdylia unit and the Pelagonian nappe, following the Paleocene–Eocene subduction and closure of a small ocean basin in the west of Pelagonia (the Pindos–Cyclades ocean basin). It preceded the Late Cretaceous subduction of the Axios/Vardar ocean remnants below the European continental margin and the final closure of the Axios/Vardar ocean during the Paleocene–Eocene time, which was associated with the overthrusting of the European origins Vertiskos–Kimi nappe on the Sidironero–Kerdylia nappe and, subsequently, the final collision of the European margin and the Pelagonian fragment. Subsequently, during a synorogenic Oligocene–Miocene extension associated with compression and new subduction processes at the more external orogenic parts, the Olympos–Ossa widow and the Cyclades, together with the lower-most Rhodope Pangaion unit, were exhumed as metamorphic core complexes.

U–Pb age constraints on the Carboniferous–Permian transition in continental basins of eastern equatorial Pangaea (France): implications for the depositional history and correlations across the late Variscan Belt

Intramontane late Carboniferous–Permian basins of western Europe developed during the latest orogenic stages of the Variscan Mountain Belt in eastern Pangaea, at equatorial palaeolatitudes. Their stratigraphic framework is mainly based on continental subdivisions (e.g. Stephanian and Autunian continental stages), which can be contentious owing to biostratigraphic biases, resulting in long-distance diachronous subdivisions. To provide precise inter-basinal and global correlations to the internationally recognized chronostratigraphic marine stages, this study reports new U–Pb geochronology from the Aumance and Decize–La Machine basins, located in the northern French Massif Central. Zircon grains extracted from three volcanic ash-fall layers give weighted mean 206 Pb/ 238 U ages of 299.11 ± 0.35, 298.73 ± 0.36 and 298.59 ± 0.35 Ma (2 σ total propagated uncertainty) by the chemical abrasion isotope dilution thermal ionization mass spectrometry method, coinciding with the Carboniferous–Permian transition (Gzhelian and Asselian stages). These ages imply that the northern Massif Central basins developed synchronously in relatively short periods of time (&lt;10 myr), reflecting substantial sedimentation rates. Finally, the new chronology of infilling of these basins confirms that they were connected during the late Carboniferous and early Permian periods, improving the knowledge on the late orogenic Variscan geodynamic setting in this area. Supplementary material: Operating conditions and complete analytical results are available at https://doi.org/10.6084/m9.figshare.c.6805228

Exploring paleoproterozoic metamorphism in the western Río de la Plata craton from drill-core evidence

Rhyacian basement is not exposed in the western part of the Río de la Plata craton, but deep borehole samples (depth of 1046–3340 m) provide information on its composition and metamorphic evolution. An amphibole schist sample from the Camilo Aldao borehole underwent two metamorphic stages: M1 and M2. The P-T conditions of M1 are not constrained whereas the M2 main metamorphic stage is characterized by conditions of 6.1–7.4 kbar and 575–605 °C. In contrast, an olivine gabbronorite sample from the Santiago Temple borehole records crystallization conditions of 1.7 ± 0.6 kbar and 1180 ± 10 °C based on clinopyroxene composition, and late magmatic to subsolidus low-pressure granulite facies metamorphism at 2.3 ± 1.2 kbar and 749 ± 85 °C based on multi-equilibrium thermobarometry. The Rhyacian evolution of the Río de la Plata craton is characterized by at least three thermo-tectonic orogenic stages: T1 (ca. 2200–2100 Ma), T2 (ca. 2100–2040 Ma), and T3 (ca. 2040–2010 Ma) that correspond respectively to: 1) early oceanic subduction and juvenile continental arc magmatism; 2) subsequent continental collision (clockwise intermediate P/T paths of metamorphism), and 3) final low-pressure metamorphism coeval with post-orogenic magmatism. The Camilo Aldao amphibole schist is representative of the T2 orogenic stage while the Santiago Temple gabbronorite is representative of T3 magmatism and metamorphism.

POST-TECTOGENIC EVENTS AND EXHUMATION PROCESSES IN SVEKOKARELIDES OF THE LADOGA REGION

Elements of the structure of the Svecokarelian pericratonic zone of the Karelian Massif (SE Fennoscandia), formed after the main events of the Svekofennian tectogenesis, have been identified and characterized. Dated age intervals of the orogenic stage and post-orogenic extensional collapse manifestation, determined the depth levels of the corresponding structures and visualized the scheme of differentiated exhumation of the deep complexes of the post-Proterozoic Savo-Ladoga movable zone. Approximate estimations of exhumation rates of deep material at certain stages of the Precambrian evolution of this zone are given.

Structural configuration and tectonics of the Flemish Cap, offshore Newfoundland, from newly acquired seismic reflection data

Although the Flemish Cap played a pivotal role in the opening of the North Atlantic, the tectonic history of this continental ribbon has been poorly constrained owing to insufficient seismic coverage. In this study, we present 13 newly acquired seismic reflection profiles over the Flemish Cap, on which seismic reflectors show highly variable seismic facies both on and beneath the top acoustic basement, with exceptional imaging of layered crustal structure. The upper crust is primarily characterized by transparent, chaotic amplitude reflectivity. The lower crust, particularly on the flanks of the cap, exhibits relatively bright and coherent reflection packages interpreted as Appalachian orogenic fabrics based on onshore–offshore correlations from pre-rift plate reconstructions. Extensional systems within the continental crust of the Flemish Cap record a transitional stage between Paleozoic orogenic collapse and pre-Jurassic rifting. The crustal architecture associated with Mesozoic rifting of the Flemish Cap is also mapped and the interpreted distinct rift domains display along-strike variations. Overall, the complex tectonic history of the Flemish Cap involved dominantly ductile deformation during the Paleozoic orogenic stage, multiple deformation styles (primarily ductile and brittle–ductile) during the orogen-to-rift transitional stage and brittle deformation during the major Jurassic–Cretaceous rifting stage.

Late Triassic magmatic rocks in the southern East Kunlun Orogenic Belt, northern Tibetan Plateau: Petrogenesis and tectonic implications

ABSTRACT The East Kunlun Orogenic Belt (E-KOB) documents successive subduction and accretion processes of the Paleo-Tethyan Ocean. However, the tectonic regime of the E-KOB at the late orogenic stage remains ambiguous. This study presents new results of whole-rock geochemistry and zircon U-Pb-Hf isotopes of the Xidatan pluton in the southern E-KOB. Detailed investigation reveals that the pluton is composed of quartz monzodiorite, host monzogranite, and igneous microgranular enclaves (IMEs). Zircon U-Pb dating results suggest that they are coeval magmatic rocks emplaced at ca. 206–203 Ma. Geochemical studies reveal that the quartz monzodiorite and IMEs share a common magma source, evidenced by consistent compositions of major elements, congruently enriched LREEs and LILEs, and consistently moderately depleted HFSEs and HREEs. Enriched to slightly juvenile zircon Hf isotopes (εHf(t) = −4.24 to +0.49) indicate that the primitive magmas of the quartz monzodiorite and IMEs involve the addition of mantle-derived magmas. The quartz monzodiorite and IMEs have identical ages, and consistent isotope features with the host monzogranite, together with the observation of ingestion of xenocrysts from host felsic rocks into the IMEs indicating that they were produced through the mixing of the mantle-derived mafic magmas and host felsic magmas. In contrast, The host monzogranite has moderately high SiO2 and Na2O+K2O, but low Fe2O3 T and MgO contents. It shows crust-like trace element patterns such as strong enrichments in LREEs and LILEs, but remarkable depletions in HREEs. Along with enriched zircon Hf isotopes (εHf(t) = −3.51 to −0.25), the host monzogranite was most likely to have originated from partial melting of lower crustal mafic rocks induced by underplated mantle-derived magmas. Considering abundant Late Triassic igneous rocks across the E-KOB, these new data provide further evidence that the Late Triassic magmatism in the E-KOB may be related to the delamination of thickened orogenic root under an extensional setting.

Geological bases and types of gold in carbonate deposits of the Birimian stage in West Africa

Relevance. The Paleoproterozoic region of the West African craton (WAC) includes numerous world-class gold deposits. These deposits are located in belts of volcanic and volcanic-sedimentary Birimian greenstone rocks. Gold mineralization of the orogenic type is spatially, chronologically, and genetically associated with the compression stage in the northwest and southeast. Several deposits are located in more or less unaltered and deformed zones west of Côte d’Ivoire (Guinea) and the Keniba–Kedugou Senegal-Mali salient. To date, the role of this type of rocks from the standpoint of the model of “local structural traps” for hydrothermal and (or) metamorphogenic gold-bearing fluids remains underestimated. The purpose of the research. The purpose and objectives of the research are to study and reveal the relationship of precious metal mineralization with regional and provincial geotectonic events, igneous and volcanic rock complexes of different ages, types of metamorphic and metasomatic transformation of ore-bearing rocks and types of ores within the West African Сraton. Particular attention is paid to the origin and location of gold mineralization in the carbonate formations of the lower (early) Birimian dating up to 2.12 billion years. Conclusions. The author demonstrates the differences between the Proterozoic gold mineralization of the preorogenic and, in fact, orogenic stages of the geological development of West Africa (Burkina Faso, Ghana) from modern regional-geological, geohistorical and metallogenic positions. The results of our own research have shown and proved the endogenous nature of the precious metal mineralization of the region. This is especially important for gold ores in sedimentary carbonate strata, which for a long time were considered syngenetic (placer).

Geochemistry, Geochronology, and Prospecting Potential of the Dahongliutan Pluton, Western Kunlun Orogen

Triassic granitoids are abundant on the northwestern margin of the Tibetan Plateau. The Dahongliutan pluton, located in the eastern Western Kunlun orogen, formed in the Late Triassic.Previous field studies have identified potential mixing of crustal and mantle magmas. In this study, we used zircon U–Pb ages and major and trace elemental analyses to investigate the tectonic evolution of the pluton, and to determine whether any exchange of mantle-derived material occurred between the pluton and the source area. We found that the pluton has relatively high SiO2 contents, and the aluminum saturation index is consistent with peraluminous high-K calc-alkaline granite. The pluton is enriched in light rare earth elements; both light and heavy rare earth elements are highly fractionated. The magma that formed the pluton was predominantly derived from the crust; however, a small amount of upper mantle material was involved in the early stages of magma formation. The pluton underwent composite emplacement as a result of tectonic extension and magmatic emplacement, which may have occurred in the late Triassic post-collisional orogenic stage. Late Triassic magmatism provided heat and ore-forming material for Pb–Zn, Cu, Fe, and rare metal mineralization, which is of considerable importance for geological prospecting.

Structural record of polyorogenic pre-Alpine and Alpine deformations within a major thrust nappe close to a suture zone (Internal-External Zones Boundary of the central Betic Cordillera, S Spain)

ABSTRACT The Malaguide Complex occupies the highest position in a thick-skinned Alpine thrust stack. It includes polydeformed, sedimentary and low-grade metamorphic rocks of Palaeozoic basements and their Meso-Cenozoic covers. NE of Granada the complex results severely sheared and stretched along the Betic suture zone in the metamorphic hinterland (orogenic backstop). The suture developed after the Burdigalian, due to oblique collision of the Alboran Domain (including the Malaguide Complex) with the South-Eastern Iberian Paleomargin. The orogenic evolution in the backstop region started with late Palaeogene to Early Miocene N-directed pre-collisional shortening and early thrust stacking, associated with low-grade metamorphism in the lowest Malaguide units increasing downwards into the underlying Alpujarride Complex. An accretionary prism developed at the expense of deep-marine (Flysch) sediments and the sedimentary covers of the Iberian and Alboran conjugate margins (External Zones and Frontal units, respectively). Later orogenic stages were characterized by SSE-directed back-folding and -thrusting in the Middle Miocene affecting both sides of the Betic suture related to dextral wrenching causing plurikilometric displacements. In this way, the suture evolved towards a several kilometres wide megashear zone (Cogollos-Diezma Corridor), prior to a final phase of SW-directed post-orogenic extension in the Late Miocene. Despite this protracted Alpine evolution, the Malaguide basements also preserve a record of orogenic activity during Late Carboniferous stages of the Variscan Orogeny in the form of a set of NE-SW trending folds with SE vergence and associated slaty cleavage, which are not found in the post-Carboniferous cover. The temperature conditions of this event (>300°C) in the less metamorphic Malaguide units are determined by colour alteration index (CAI) data from conodonts that have also allowed biostratigraphic dating of the Devonian-Carboniferous successions.

METATERRIGENIOUS ROCKS OF THE OLKHON TERRANE OF THE CENTRAL ASIAN OROGENIC BELT: U-Pb ZIRCON AGE, GEOCHEMICAL CHARACTERISTICS, AND FORMATION MODELS OF SEDIMENTARY PROTOLITHS

The paper presents a petrographic, geochemical, and Sm-Nd isotopic data on the gneisses from different tectonic zones of the Olkhon terrane of the Central Asian Orogenic Belt, as well as the composition of garnets and the age of zircons in these metaterrigenous rocks. The garnet-biotite gneisses of the Anga-Sakhurta zone, as well as the garnet-bearing and garnet-free gneisses (granulites) of the Chernorud zone may result from metamorphism of immature terrigenous rocks of polymictic or greywacke compositions similar in geochemical characteristics to rocks of continental arcs. At the same time, the gneisses of these zones show both similarities and some differences in geochemical and isotopic characteristics, including variations in ɛNd(T) values from –0.2 to –9.0, which may indicate different proportions of one or another source in their protoliths. The age of most detrital zircons in the gneisses of the Chernorud and Anga-Sakhurta zones corresponds to 530–1000 Ma, and the youngest detrital zircons have an age of 522–537 Ma. The geochemical and geochronological data on the gneisses of the Chernorud and Anga-Sakhyurta zones suggests that the protoliths of these gneisses could be formed from the same sources of predominantly Neoproterozoic age. We assumed that gneiss protoliths could initially be sediments of the continental slope of Neoproterozoic composite terrane assembled to the Siberian Platform at 600‒610 Ma. The transport of clastic material being gneiss protoliths from this composite terrane, took place towards the Paleo-Asian Ocean. The youngest zircons with an age of about 530 Ma could be generated from igneous complexes intruding the Neoproterozoic composite superterrane. The garnet-biotite gneisses of the Krestovskaya zone are similar in chemical composition to immature graywacke sandstones, The ɛNd(T) value in these gneisses is –3.7. Detrital zircons in the gneisses of the Krestovskaya zone form age peaks at 780–820 and 498–515 Ma. Based on geochemical and geochronological data we assume that these gneisses could be formed in an intracontinental basin formed at the orogenic stage during accretionary-collisional events at the amalgamation the Orso block and the Birkhin island arc into the Krestovskaya zone.All terrigenous rocks being gneiss protoliths were metamorphosed at 460–510 Ma under granulite or amphibolite facies associated with accretionary and collisional events, which led to the formation of the Early Paleozoic Olkhon composite terrane.

Mélange, Flysch and Cliffs in the Pieniny Klippen Belt (Poland): An Overview

The Pieniny Klippen Belt (PKB) is located in the suture zone between the Central and Outer (Flysch) Carpathians. Its structure is an effect of prolonged processes of the Cretaceous–Miocene folding, thrusting and uplifting. In this zone, tectonic components of different ages and features, including strike-slip-bounded tectonic blocks, thrust units, as well as toe-thrusts and olistostromes, result in the present-day mélange characteristics of the PKB, where individual tectonic units are difficult to distinguish. In the PKB, both tectonic and sedimentary events triggered the mélange creation. The name “Klippen Belt” is derived from cliffs (German Klippen). These cliffs form harder, more erosion-resistant elements of the mélange, residing within less competent clastic deposits, sandstones, shales and marls that form flysch complexes. The cliffs often represent olistoliths, which glided down from elevated areas to the deeper basinal zones. Two olistostrome belts were distinguished. The older one resulted from subduction of the southern part of the Alpine Tethys, and the younger originated in response to the northward shift of the accretionary wedge. The other cliffs were placed within the surrounding clastic by tectonic deformational processes. The flower structure of the PKB was formed during the collision and strike-slip movement of the lithospheric plates. This structure is limited on both sides by deep-rooted faults. Several evolutionary stages could be distinguished in these areas. The rift-related stage is expressed by the opening of the Alpine Tethys that contains two major basins—Magura and Pieniny (Złatne) basins, separated by Czorsztyn Ridge. The reorganization of the Alpine Tethys basins and the development of the accretionary prism happened during the synorogenic stage. This process was initiated by the movement of the Central Carpathians. Thick flysch sequences with olistostromes were deposited in these basins. The Czorsztyn Ridge was destroyed during the late orogenic stages.

Polyphase Permo-Carboniferous magmatism adjacent to the Intra-Sudetic Fault: constraints from U–Pb SHRIMP zircon study of felsic subvolcanic intrusions in the Intra-Sudetic Basin, SW Poland

The SHRIMP U–Pb dating of zircons from felsic subvolcanic rocks in the Carboniferous formations of the Intra-Sudetic Basin in SW Poland verifies previous views on the timing of Late Palaeozoic magmatic activity in this area and constrains the links between regional tectonics and intra-basinal volcanism in the mid-European Variscides. Two main stages of magmatism are identified: the Late Carboniferous late orogenic stage, and the Early Permian post-orogenic stage. The Carboniferous late orogenic subvolcanic to volcanic activity was contemporaneous with the formation of the Karkonosze granite pluton to the west and comprised episodic emplacement of predominantly felsic laccoliths and sills within the basin fill during the Westphalian–Stephanian in two sub-stages at ca. 313–310 Ma and 306–305 Ma. Local folding of the Carboniferous succession peaked at ca. 303–300 Ma adjacent to the Intra-Sudetic Fault, in relation to late-stage sinistral movements along this regional wrench fault. The emplacement of minor subvolcanic intrusions at ca. 293 Ma completed the activity in the northern Intra-Sudetic Basin; this Permian post-orogenic volcanism, however, developed fully in areas further south. The predominance of subvolcanic intrusions over extrusions in Carboniferous can be linked to the trapping of rising magmas within the several kilometres thick sedimentary basin fill. Zircon xenocrysts in the Carboniferous subvolcanic rocks indicate that older crustal lithologies, possibly metamagmatic and metasedimentary rocks of dominantly Neoproterozoic–Palaeozoic ages, were involved in petrogenesis as magma sources and/or contaminants. The crustal influence on magma formation was stronger during the earlier magmatic events.

Leucogranite Records Multiple Collisional Orogenies

AbstractThe geological evolution of orogens is the sum of events related to both their cycle of generation as well as features inherited from their basement units. The Himalayan Orogen provides a well‐constrained Cenozoic history of active plate convergence and orogenic processes overprinting an early Paleozoic orogen. A total of 1,627 Himalayan leucogranite zircons were depth‐profiled showing high resolution (0.6 μm) internal age structures. Zircon rim ages record a detailed and complex history of melting and deformation, postdating the North Indo‐Australie Orogen (ca. 490–460 Ma) documented by the zircon core age data. An interesting finding is the inconsistency of age groups between detrital zircons in the Himalaya and inherited cores of leucogranite zircons, re‐evaluating the argument that leucogranites are purely derived from melting of metasedimentary rocks. Variations in the thickness of zircon rims corresponds with changing Cenozoic orogenic stages with rims thicker than 4 μm indicating the extensional regime.

Tectonothermal Model for the Late Paleozoic Syncollisional Formation Stage of the Kara Orogen (Northern Taimyr, Central Arctic)

AbstractWe present a tectonothermal model for the late Paleozoic syncollisional formation stage of the Kara orogen in northern Taimyr in the Central Arctic. The model is based on new and published structural, petrological, geochemical, and geochronological data, as well as thermophysical properties obtained for the Kara orogen. The latter hosts a significant volume of granites formed as a result of the collision between the Kara microcontinent and the Siberian craton. Based on geological, geochemical, and U–Th–Pb isotope data, the granites were differentiated into syncollisional and postcollisional intrusions that were emplaced in the intervals 315–282 and 264–248 Ma, respectively. The presented tectonothermal model covers only the syncollisional formation stage of the Kara orogen, during which anatectic granites formed. The 2D models help to reconstruct the main tectonothermal processes of the syncollisional stage of formation of this structure, taking into account the local peculiarities of the thermal state of the Earth’s crust in the region. The model shows the mechanisms of increase in the lower crust temperature necessary for the formation of syncollisional anatectic granites. The estimates obtained from the model constrain the time interval between collision/tectonic stacking and the granite formation. The modeling also showed the general regularities typical for orogens on syncollisional stages.

Long-term assessment of creep and water effects on tunnel lining loads in weak rocks using displacement-based direct back analysis: an example from northwest of Iran

The time-dependent stability of tunnels is an important and challenging topic, mainly when the tunnel is excavated in incompetent and weak rocks. The creep property of rock is one of the crucial mechanical properties of weak rock and the main factor affecting the long-term stability of rock masses. Also, water as an important environmental factor influences both the short-term and long-term behavior of rocks and is one of the causes of geotechnical engineering disasters, such as tunnel collapse, slope sliding, surface subsidence, etc. In this research, the effects of rock’s creep behavior and underground water on the long-term stability of the Shibli tunnels were analyzed. Geological maps and reports of Shibli tunnels show a highly jointed condition in the surrounding rocks which have been crushed by two orogenic stages. The Burger-creep visco-plastic model was used to simulate the tunnel host rock creep behavior. The model's parameters were adopted based on the displacement-based direct back analysis technique using a univariate optimization algorithm. In addition, the influence of underground water is assessed under the condition of the varying water table. Support capability diagrams were used to evaluate the loading created on the tunnel’s permanent lining due to the creep behavior of rock mass and underground water. This study suggests that the weak rock's creep behavior and underground water significantly affect the time-dependent stability of tunnels. Results show that the induced stresses due to the rock's creep behavior and underground water are more considerable in the tunnel spring-line. Also, the increasing 20 m in the water table approximately decreases ten years of tunnel lining stability time at the fault zone.Article highlightsRocks creep behavior and underground water significantly affect the time-dependent stability of tunnels in weak rocks.Displacement-based direct back analysis using a univariate optimization algorithm was used to determine the CVISC model’s properties.Increasing 20 m in the water table approximately decreases ten years of tunnel lining stability time at the fault zone.

Characterizing terranes and a Neoproterozoic suture zone in Central Hoggar (Tuareg Shield, Algeria) with airborne geophysics and Landsat 8 OLI data

Integration of multisource data into a geographic information system has been a powerful tool to characterize contrasting terranes and their boundaries, such as suture zones with oceanic rocks juxtaposed to continental blocks. We present an integration study of geological, Landsat 8 OLI, and airborne geophysical data for the Wadi Takalous region (Central Hoggar, Algeria), covering the SAA (Sérouenout, Aghefsa, Aguendis) domain, rich in juvenile oceanic rocks, flanked by two continental domains, LATEA (Laouni, Aouilène, Tefedest, Egéré-Aleksod and Azrou N'Fad) to the West, and TAI (Tazat and Assodé-Issalane) to the East. The Sérouenout terrane, comprising tectonic slices of metamafic-ultramafic rocks (including eclogites) enveloped by metapelites and covered by Eopaleozoic sandstones, stands out for its low magnetic intensities and low radiometric responses. To the East, granitic rocks with abnormally high radiometric responses and high magnetic intensities highlight the TAI domain, a basement block rich in Pan-African collisional and post-collisional intrusions. To the West of Sérouenout terrane, a heterogeneous magnetic and radiometric pattern characterizes the eastern LATEA that has been interpreted as a Neoproterozoic passive margin. Our integrated data also suggests that the Afara terrane is more likely correlated with LATEA (instead of TAI). Deep magnetic lineaments outline major shear zones marking terrane boundaries, such as the crustal-scale (hundreds-km long and dozens-km thick) 7°30′E thrust zone limiting the SAA and TAI domains. Structures and kinematic indicators related to collisional deformation point to an overall NE-SW to E-W crustal shortening which induced thrusting and tight folding, followed by strike-slip shear zones related to transpressional deformation in late orogenic stage. In this scenario, the SAA domain may represent an accretionary complex associated to a Pan-African suture zone and thrust onto the LATEA passive margin.