Displays

Numerous works also suggested that mantle plumes or mantle upwellings associated with LLSVPs in a degree-2 mantle state play a major role in driving the break-up of a supercontinent. However, subduction and mantle downwelling may play an increasing role in the leadup to the assembly of the next supercontinent. Anderson (1994) noticed that continents tend to gather at mantle downwelling zones, which was later developed into the hypothesis of orthoversion assembly of supercontinents by Mitchell (2012). Zhong et al. (2007) conceptualised the assembly of supercontinents through the merger or absorption of mantle downwellings, leading to the assembly of supercontinents over a superdownwelling in a degree-1 mantle. Here we present a revised global paleogeographic reconstruction featuring an extroversion assembly of Pangea (i.e. through the closure of the Mirovoi superocean) over a pre-existing yet dynamic mantle downwelling zone (Li et al., 2019). In particular, we show that the Paleozoic world was dominated by two major subduction (dowelling) cells, one associated with the newly assembled Gondwana, and the other associated with the assembly of Laurasia. The two cells gradually merged together by the Carboniferous time, forming the supercontinent Pangea over a mantle superdownwelling (Zhang et al., 2010). It was during the merger of the two dowelling cells that continental and arc terranes was successively transported from Gondwana margin to future Laurasia.

References:

Anderson, D.L., 1994. Superplume or supercontinents? Geology 22, 39-42.

Huang, C., Zhang, N., Li, Z.-X., Ding, M., Dang, Z., Pourteau, A., Zhong, S., 2019. Modeling the Inception of Supercontinent Breakup: Stress State and the Importance of Orogens. Geochemistry, Geophysics, Geosystems 20, 4830-4848.

Li, Z.X., Mitchell, R.N., Spencer, C.J., Ernst, R., Pisarevsky, S., Kirscher, U., Murphy, J.B., 2019. Decoding Earth’s rhythms: Modulation of supercontinent cycles by longer superocean episodes. Precambrian Research 323, 1-5.

Mitchell, R.N., Kilian, T.M., Evans, D.A.D., 2012. Supercontinent cycles and the calculation of absolute palaeolongitude in deep time. Nature 482, 208-211.

Zhang, N., Zhong, S., Leng, W., Li, Z.-X., 2010. A model for the evolution of the Earth's mantle structure since the Early Paleozoic. Journal of Geophysical Research: Solid Earth 115, B06401.

Zhong, S., Zhang, N., Li, Z.-X., Roberts, J.H., 2007. Supercontinent cycles, true polar wander, and very long-wavelength mantle convection. Earth and Planetary Science Letters 261, 551-564.

How to cite: Li, Z.-X., Collins, W., Wu, L., and Pisarevsky, S.: From Rodinia to Pangea: an extroversion process driven first by plume push followed by downwelling pull, absorption and merging , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12348, https://doi.org/10.5194/egusphere-egu2020-12348, 2020.

There is a view that a supercontinent, called Pannotia, existed for a short time at the end of the Neoproterozoic. This hypothetical continent requires collision between Neoproterozoic India, Australia-Mawson and the African and South American continents to occur before formation of Iapetus as Laurentia rifted off Amazonia.

Data from the last decade demonstrate the complexity of consumption of the Mozambique Ocean that separated Neoproterozoic India from the African Neoproterozoic continents (Congo-Tanzania-Bangweulu, the Sahara Metacraton and Kalahari). In particular, the presence of pre-Neoproterozoic terranes that lie within the East African Orogen of Arabia, east Africa, Madagascar and South India demonstrate the multi-phase collision of the this ocean closure?. Here we examine the Cryogenian to Cambrian tectonic geography of the closure of the Mozambique Ocean from a full-plate perspective. We focus on the northern East African Orogen, where Gondwana-formation shortening and crustal thickening has been considerably less than seen in East Africa/Madagascar/South India. We focus on the Neoproterozoic India–Azania–Sahara Metacraton collision represented by the northernmost part of Madagascar (the Bemarivo Domain), and throughout Arabia. We conclude that final ocean closure and formation of central Gondwana occurred in the latest Ediacaran and into the Cambrian, along a suture that passes under the Rub' al Khali region of Arabia and through the northeast of Madagascar. It separates the extended Neoproterozoic India margin (now in Oman, The Seychelles and the northern Bemarivo Domain), from the growing kernel of Gondwana (the east-most parts preserved in Saudi Arabia, Yemen and Central Madagascar).

Considering the early Ediacaran formation of Iapetus, there is growing evidence that Pannotia never existed as connected continental crust, yet the ‘Pannotian geodynamic cell’ with lithosphere divided into continental and oceanic hemispheres had formed. The closure of the Mozambique Ocean represented the termination of >500 million years of subduction at this locale. The termination of this subduction with the formation of Gondwana, and the initiation of the Terra Australis Orogen led to the present geodynamic configuration.

How to cite: Collins, A., Blades, M., Foden, J., Armistead, S., Razakamanana, T., Alessio, B., and Merdith, A.: Pannotia didn’t exist, but the “Pannotian geodynamic cell” formed as the Mozambique Ocean closed and Gondwana amalgamated—the view from Arabia and the East African Orogen, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3173, https://doi.org/10.5194/egusphere-egu2020-3173, 2020.

The late Neoproterozoic is a time interval of dramatic changes affecting the biosphere, the cryosphere and the lithosphere, including the final disaggregation of the supercontinent Rodinia and the formation of Gondwana. The Iapetus Ocean opened during the breakup of Rodinia, i.e. resulting from the separation of the three major continental blocks: Laurentia, Baltica and Amazonia. Protracted continental extension to rifting from 750 to 530 Ma is observed along the involved continental margins and may indicate several ocean openings in addition to the Iapetus Ocean. Breakup timing is still much debated in the literature, as it remains unclear how to best interpret the fragmentary geological record along the rifted margins, and because only few reliable paleomagnetic data are available for this period of time. Three distinct times for the breakup are proposed for Laurentia and Amazonia: at (1) 750-700 Ma, (2) 615-570 Ma and (3) 550-530 Ma. Various terranes are also involved in the opening of the Iapetus Ocean and may have drifted along with or independently of Amazonia.

In this study, we reviewed the geological observations of each of the involved margins and the available paleomagnetic data from 750 to 520 Ma to test these scenarios. Paleomagnetic data from Laurentia and Amazonia-West Africa constrain the breakup age to occur before 575 Ma, discarding the possibility of a late Ediacaran/Early Cambrian opening. Geological observations, better preserved in Laurentia and Baltica, indicate two main phases of (attempted) continental rifting, from 750 to 680 Ma and from 615 to 550 Ma. The second phase is usually interpreted as leading to the breakup of Laurentia, Amazonia and Baltica, as in scenarios (2) and (3). Nevertheless, it cannot easily explain (i) the absence of the Central Iapetus Magmatic Province in West Amazonia, (ii) the dynamics of accreted terranes now observed in South America and (iii) the distinct late Neoproterozoic detrital zircon age population in Phanerozoic sediments along West Amazonia (which are moreover absent in East Laurentia). These observations are better explained by a model wherein Laurentia and Amazonia broke apart during the first rifting phase around 750-680 Ma. In this scenario, the second phase of rifting (615-550 Ma) leads, in the west, to drifting of small terranes southward and toward Amazonia, and in the east, to the final breakup between Laurentia and Baltica.

How to cite: Robert, B., Domeier, M., and Jakob, J.: A diachronous opening of the Iapetus Ocean in the Neoproterozoic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6927, https://doi.org/10.5194/egusphere-egu2020-6927, 2020.

The Istanbul Zone (NW Turkey) is regarded as the eastward elongation of Avalonia in Central Europe. Its Paleozoic stratigraphy is characterized by continuous sedimentation from Early Ordovician to Late Carboniferous. However, the Intra-Pontide Suture between the Istanbul and Sakarya zones is regarded as a Neotethyan Suture representing an oceanic domain of Permo-Triassic to Cretaceous age. Here, we present U-Pb ages and Lu-Hf isotopic compositions of the detrital zircons from the Upper Silurian-Lower Devonian, Upper Carboniferous, Permian and Upper Triassic sandstones of the Istanbul Zone. Detrital zircon ages from the Upper Silurian-Lower Devonian sandstone are dominated by Mesoproterozoic zircons (1950-900 Ma), with subordinate peaks at the latest Neoproterozoic to Silurian and Mid-Archean (2850-2750 Ma) confirming its Avalonian affinity. Detrital zircons from Carboniferous to Triassic sandstones yielded a major peak at Carboniferous-Early Permian (360-270 Ma) and a minor peak at Late Neoproterozoic-Cambrian (700-480 Ma) while Mesoproterozoic zircons become insignificant. The εHf (t) values of the detrital zircon grains from Upper Silurian-Lower Devonian, Upper Carboniferous, and Upper Triassic sandstones exhibit a wide range from -21.3 to +11.7, and over 62% of zircon grains have negative values, suggesting mixing derivation of both mantle and crustal melts. Apart from the Permo-Triassic magmatism, the Istanbul Zone is devoid of Carboniferous igneous and metamorphic events. Therefore, abundant Carboniferous zircons and disappearance of the Mesoproterozoic zircons in the Carboniferous to Upper Triassic clastic rocks of the  Istanbul Zone require juxtaposition with a continental domain similar to the Sakarya and Rhodope‐Strandja zones, which are characterized by widespread Carboniferous magmatism. We suggest that the Intra-Pontide Suture probably represents trace of the Rheic Suture in Turkey, along which Avalonia and Armorica collided during Early Carboniferous.

Key words: Intra-Pontide Suture, Istanbul Zone, Rheic Suture, detrital zircon, U-Pb ages, provenance, Hf isotopes

How to cite: Akdoğan, R., Hu, X., Okay, A. I., and Topuz, G.: Provenance of the Paleozoic-Mesozoic siliciclastic rocks of the Istanbul Zone: Constraints on the location of the Rheic suture in Turkey, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5706, https://doi.org/10.5194/egusphere-egu2020-5706, 2020.

The Brno Massif forms a part of larger tectonostratigraphic unit named the Brunovistulian Terrane (BVT) that is one of crustal block of Europe with the Neoproterozic basement.  However, the Neoproterozoic orogenic belt was developed in wide area i.e. along the Gondwana margin and near the present day eastern and southern edge of the East European Craton. For more than 20 years, the problem of primary setting of the BVT inside the Neoproterozic orogenic  belt have been discussed. Also the path of their drift and  time of their final accretion have been a matter of debate. To solve these problems the paleomagnetic and isotope studies of vertical intrusions cutting the BVT basement near Brno in Moravia have been undertaken. Preliminary isotope dating of granitic and basaltic intrusions points to the early Silurian age of them. Results of demagnetization of paleomagnetic samples from three localities revealed the presence of stable components with a steep inclination, at that time characteristic for the northern margin of Gondawana but not for the Baltica paleocontinent that during the Silurian was situated between the equator and 30^oS. The Emsian  “old red” type deposits may indicate that final amalgamation of the BVT took place some-time between the Silurian and the Devonian. This time of joining of the BVT  to Baltica and quite high (50 – 60^oS) paleolatitudes obtained from the early Silurian rocks of the Brno Massif  point to a rapid drift of the BVT across the Rheic Ocean during the Silurian.

How to cite: Nawrocki, J., Leichmann, J., and Pańczyk, M.: Origin and the Silurian odyssey of the Brunovistulian terrane: paleomagnetic evidence from the Brno Massif (central Europe)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5299, https://doi.org/10.5194/egusphere-egu2020-5299, 2020.

Proterozoic microcontinents are widespread in the western part of the Central Asian Orogenic Belt, but their origin remains poorly constrained. The U-Pb dating of detrital zircons in Proterozoic rocks of the southern Kazakhstan and Kyrgyz North Tianshan elucidate depositional ages and evolution of the Precambrian basins and characterize possible links of Precambrian microcontinents in these regions with Gondwana and other cratons.

Distributions of U-Pb detrital zircon ages in 13 samples from ca 5 km thick flysch-like succession of the Talas and Malyi Karatau ranges (Ishim-Middle-Tianshan microcontinent) show significant similarity. They are characterized by a widespread occurrence of Neoproterozoic grains with peaks at ca 820-800 and 910–860 Ma, almost complete absence of Mesoproterozoic grains and distinct peaks at ca 2040–1990 and 2500–2465 Ma for Paleoproterozoic grains. Archean grains occur in small amount. Close similarity is supported by K-S test indicating that samples have the same or similar provenance, also implying rapid accumulation and similar depositional ages. Main peaks resemble those in the Tarim Craton, suggesting Tarim as likely provenance and pointing to the Gondwana affinity of the Ishim-Middle-Tianshan microcontinent.

In contrast, detrital zircon populations in 3 samples from the Neoproterozoic quartzites of the North Tianshan microcontinent are dominated by Mesoproterozoic grains ranging in age from ca 1500 to 1000 Ma and contain few Paleoproterozoic grains ca 1800-1650 Ma. Distributions of U-Pb zircon ages in all 3 samples are very similar and resemble those in the early Neoproterozoic quartzites from the Kokchetav area of northern Kazakhstan, recently reported by Kovach et al. (2017). Age peaks in these samples are very different from the ages of magmatic pulses in Gondwana and point that the North Tianshan microcontinent did not have connection with Gondwana.

The Ishim-Middle-Tianshan microcontinent was rifted out from the Gondwana in late Neoproterozoic and travelled to the north. Origin and travel paths of the North Tianshan microcontinent remain poorly constrained. Widespread occurrence of Mesoproterozoic zircons implies possible links with Baltica, North America or east Siberia, but more detailed study is required to define exact provenance. These two microcontinents welded together in the middle to late Ordovician during amalgamation of the Kazakhstan paleocontinent and were jointly incorporated in Eurasia during the late Paleozoic collisions of the Kazakhstan continent with Siberia, Baltica and Tarim.

The study was supported by RFBR grant 20-05-00252

How to cite: Khudoley, A. K., Alexeiev, D. V., and DuFrane, S. A.: Proterozoic microcontinents in the western Central Asian Orogenic Belt (Kyrgyzstan and Kazakhstan): relationship with Gondwana, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10716, https://doi.org/10.5194/egusphere-egu2020-10716, 2020.

Five major oceans (Iapetus, Rheic, Proto-Tethys, Paleo-Tethys and Paleo-Asian) formed during or after assembly of the Gondwana continent. However, the relationship between them is poorly understood, largely due to the complex and disputed evolution of NE Gondwana in the early Paleozoic. Here we present a summary of early Paleozoic tectono-thermal events in the NE Gondwana and discuss their tectonic settings. Early Paleozoic magmatic rocks are widely distributed in the Himalaya, Lhasa, Southern Qiangtang, Baoshan, Sibumasu and Tengchong terranes, and their ages were loosely constrained to be ca. 530-430 Ma. However, after a critical review of these dating results, we propose the magmatic rocks were mostly formed between ca. 500-460 Ma. Although bimodal, they are dominated by granitoid rocks distributed over an area of >2500 km × 900 km. Thus, they constitute a typical silicic large igneous province. Almost all granitoid rocks were derived from partial melting of sedimentary rocks, but a few show A-type characteristics. Coeval amphibolite-facies metamorphic rocks yield ages of 490-465 Ma. A sedimentary hiatus marked by either a disconformity or angular unconformity coeval with the major magmatic flare-up period is evident in all terranes. Thus, present evidence doesn’t favor either the conventional Andean-type subduction model, in which these magmatic rocks reflect subduction of Proto-Tethys oceanic lithosphere beneath the northern Gondwanan margin, or a post-collision setting, in which extension is associated with the collapse of the Pan-African orogeny in NE Gondwana. The tectonic setting for this magmatic province is tentatively related to a plume in a far-field subduction zone.

How to cite: Dan, W., Murphy, J. B., Tang, G.-J., Zhang, X.-Z., and Wang, Q.: An early Paleozoic silicic large igneous province in NE Gondwana: a preliminary synthesis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8193, https://doi.org/10.5194/egusphere-egu2020-8193, 2020.

Controversy about the status of Pannotia (Laurentia + Baltica + Gondwana) as an Ediacaran supercontinent centers on palaeomagnetic data (which is permissive not conclusive) and geochronology (which implies breakup commenced before full assembly). But evidence of past supercontinent assembly is not limited to these two criteria and can be found in many other phenomena that accompany the process. Irrespective of whether Pannotia qualifies as a supercontinent, a key unanswered question is whether the legacy of its amalgamation influenced global mantle convection patterns because such patterns are generally ignored in models claiming the transition from Rodinia to Pangaea represents a single supercontinent cycle. We contend that the proxy signals of assembly and breakup in the Ediacaran are unmistakable and indicate profound changes in mantle circulation. These changes correlate with a wealth of geologic data for Pan-African collisional orogenesis, reflecting the amalgamation of the Gondwana, and for tectonothermal activity along the Gondwanan portion of Pannotia’s periphery.

Collisional orogenesis necessitates subduction of oceanic lithosphere between the converging continental blocks. By analogy with the amalgamation of Pangea, the subducted oceanic lithosphere should have congregated to form a “slab graveyard” along the core-mantle boundary that would have generated a superplume beneath the Gondwanan component of Pannotia, the effects of which can be seen along its margins. We suggest that such dramatic changes in mantle convection patterns can indeed be recognized, they provide insights into the processes responsible for the opening of the Iapetus and Rheic oceans, and a potential explanation for some of the enigmatic tectonothermal events that characterize the Late Neoproterozoic-Early Paleozoic tectonic evolution of the margin of Gondwana.

How to cite: Murphy, J. B., Nance, R. D., and Heron, P. J.: The assembly of Pannotia: a thermal legacy for Pangaea?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5258, https://doi.org/10.5194/egusphere-egu2020-5258, 2020.

Synorogenic basins could be linked to a wide variety of sedimentary environments, from continental to deep-marine, in distinct geodynamic settings. The sedimentary evolution of synorogenic basins is mainly controlled by the existence of relief rejuvenation and denudation within and in the surroundings areas. Accumulation of sediment in such basins could react to changes in tectonic settings. Successive extensional or contractional events that are common during the formation of an orogenic belt can induce variations on basin depth, basin depocenter migration and/or repetition of sedimentation-erosion cycles.

Detrital zircon age fingerprinting of sedimentary basins has proven to be a very sensitive tool for analyzing large and local scale changes in source-terranes, contributing to refine regional paleogeographic models. Recognition of potential source areas could be done by using statistically robust techniques. Kolmogorov-Smirnoff test (K-S) and Multidimensional Scaling (MDS) has been successfully applied to define the fingerprints of sedimentary rocks using detrital zircon age populations and compare with those from potential terrane sources. Comparative statistical analysis of detrital zircon age populations from particular sources and basin strata may be useful to prove sedimentary provenance and distance from source areas, to identify intra-basin sediment recycling and to track multi-source mixing along drainage systems.

During the Late Devonian-Carboniferous amalgamation of Pangea extensive marine sedimentation occurred in the Variscan orogen on both Laurussia and Gondwana collision margins. Remains of such synorogenic basins are currently located in different sectors of the European Variscan belt, including Iberia.

Recent provenance studies conducted in SW Iberia Variscan basins have distinguished the contribution of three distinct terrane sources “Gondwana-”, “Laurussia-” and “Variscan magmatic arc-” types, in some cases admitting sediment recycling and mixing of sources. Statistical analysis of detrital zircon age population from NW Iberia Variscan basin allowed us to distinguish two major sources a “Middle Ordovician-Silurian magmatic episode”-type and a “Gondwana”-type. These two types appear to correspond to source areas belonging to the nearby autochthonous and allochthonous units. Gondwanan-type source includes six sub-types whose contributions varied throughout synorogenic basins evolution, indicating that where sedimentary recycling seems to have been relevant.

Provenance studies on Variscan basins proved to be essential to test if whether or not NW Iberia and SW Iberia synorogenic basins have developed in geographical proximity of Paleozoic Laurussian- or Gondwanan-terrane sources. The differences found between the sources of NW and SW Variscan basins suggest that they would be geographically separated and influenced by independent drainage systems. This finding has provided a better understanding of the framing of Iberia synorogenic basins in paleographic models of Pangea amalgamation.

Acknowledgements: This study was supported by SYNTHESIS3 project DE-TAF-5798, by “Estímulo ao Emprego Científico – Norma Transitória” by CGL2016-78560-P (MICINN) and by FCT- project UID/GEO/50019/2019 - Instituto Dom Luiz.

How to cite: Dias da Silva, Í., Pereira, M. F., González Clavijo, E., Martínez Catalán, J. R., Gómez Barreiro, J., Silva, J. B., Linnemann, U., Hofmann, M., Gärtner, A., and Zieger, J.: Detrital zircon age fingerprinting of NW and SW Iberia Variscan basins: Constraints for the pre-Pangea terrane assemblage analysis and paleogeography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-29, https://doi.org/10.5194/egusphere-egu2020-29, 2020.

The Variscan belt is the result of the Pangea accretion, a prominent feature of the European continental lithosphere (von Raumer et al., 2003) . The debate on the number of oceans and the geodynamic evolution of the Variscan belt is still open (Faure et al., 2009; Franke et al., 2017). Two scenarios have been proposed:

1. Monocyclic scenario: assumes a single long-lasting south-dipping subduction of a large oceanic domain. Armorica remained more or less closed to Gondwana during its northward drift, in agreement with lack of biostratigraphic and paleomagnetic data that suggests a narrow oceanic domain (lesser than 1000 km; Matte, 2001; Lardeaux, 2014);

2. Polycyclic scenario: this geodynamic reconstruction envisages two main oceanic basins opened by the successive northward drifting of two Armorican microcontinent and closed by two opposite subductions (Lardeaux, 2014; Franke et al., 2017). The northern oceanic basin is identified as the Saxothuringian ocean, while the southern basin is identified as the Medio-European ocean (Lardeaux, 2014).

Models of single and double subduction have been developed to verify which scenario better fits with Variscan P-T evolutions from the Alps and the French Central Massif (FCM). From the comparison between model predictions and natural Variscan P-T-t estimates results that data from the Alps with high P/T ratios better fit with the double subduction model, supporting that a polycyclic scenario is more suitable for the Variscan belt evolution. Differently, data from the FCM with high P/T ratios that fit with both models have poorly constrained geological ages and, therefore, are not suitable to actually discriminate between mono- and polycyclic scenarios (Regorda et al., 2020). Moreover, the predictions of the models open to the possibility that rocks of the Upper Gneiss Unit of the FCM could derive from tectonic erosion of the upper plate and not only from the ocean-continent transition of the lower plate.

References

Faure M., Lardeaux J.-M. and Ledru P.; 2009: A review of the pre-Permian geology of the Variscan French Massif Central. Comptes Rendus Geoscience, 341, 202-213.

Franke W., Cocks L.R.M. and Torsvik T.H.; 2017: The Palaeozoic Variscan oceans revisited. Gondwana Research, 48, 257-284.

Lardeaux J.-M.; 2014: Deciphering orogeny: a metamorphic perspective. Examples from European Alpine and Variscan belts. Part II: Variscan metamorphism in the French Massif Central – A review. Bull. Soc. géol. France, 185(5), 281-310.

Matte P.; 2001: The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of theArmorica microplate: A review. Terra Nova, 13(2), 122-128.

Regorda A., Lardeaux J-.M., Roda M., Marotta A.M. and Spalla M.I.; 2020: How many subductions in the Variscan orogeny? Insights from numerical models. Geoscience Frontiers, 10.1016/j.gsf.2019.10.005.

von Raumer J. F., Stampfli G.M. and Bussy, F.; 2003: Gondwana-derived microcontinents – the constituents of the Variscan and Alpine collisional orogens. Tectnophysics, 365, 7-22.

How to cite: Regorda, A., Lardeaux, J.-M., Roda, M., Marotta, A. M., and Spalla, M. I.: How many subductions in the Variscan orogeny? Insights from numerical models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9816, https://doi.org/10.5194/egusphere-egu2020-9816, 2020.

In today’s western Mediterranean region, Variscan and Alpine thrusts and shear zones combine to hamper a correct identification and palinspastic reconstruction of Cambro-Ordovician sequences. However, gap-related stratigraphic, climatically sensitive facies associations, sedimentary, volcanosedimentary, biogeographic, biodiversity and detrital zircon data mainly made available during the last two decades allow envisaging a new palaeogeographic scenario by linking proximal-to-distal transects across the western and eastern branches of the Ibero-Armorican Arc. Variscan parautochthonous and autochthonous domains are represented palaeogeographically by, from SW to NE: (i) the Central Iberian, West Asturian-Leonese and Cantabrian zones of the Iberian Massif and their laterally correlative Central Armorican Domain, fringed marginally by the Ossa-Morena and North Armorican thinned outer margin of Gondwana, reminiscent of the rift axis during the Cambrian; and (ii) the southeastern Pyrenees, Occitan and SW Sardinia domains, fringed marginally by the slope-to-basinal South Armorican, Thiviers-Payzac, Albigeois and northeastern Pyrenees domains. These proximal-to distal transects of West Gondwana record a diachronous SW-to-NE migration of evaporites, phosphorites and maximum peak of trilobite diversity, related to the counter-clockwise migration of the Gondwana supercontinent, supported by a gradual modification of detrital zircon provenance. Both branches of the Ibero-Armorican Arc also display a diachronous migration of Cambro-Ordovician rift-to-drift conditions associated with distinct igneous manifestations (volcanosedimentary and plutonic). This migration is related to the development of the Furongian (Toledanian) to Mid-Late Ordovician (Sardic) geodynamic events, in response to gap-related thermal doming, subaerial denudation and magmatic activity evolving from calc-alkaline to tholeiitic affinity.

How to cite: Quesada, C., Álvaro, J. J., and Casas, J. M.: Reconstructing the pre–Variscan puzzle of Cambro–Ordovician basement rocks in the western Mediterranean region of Gondwana, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9215, https://doi.org/10.5194/egusphere-egu2020-9215, 2020.

A regional South Portuguese Zone (SPZ) mapping and stratigraphic program in SW Iberia is presented. It is being developed by LNEG and IGME and financed by the GEO_FPI Project (www.geo-fpi.eu).

The SPZ is the southwesternmost geotectonic unit of the Variscan Orogeny in Iberia. The following domains are considered: Pulo do Lobo (early Frasnian -late Famennian); Iberian Pyrite Belt (IPB, late Famennian-late Visean), Baixo Alentejo Flysch Group (late Visean-late Moscovian) and Southwest Portugal (late Strunian-mid Bashkirian). The mapping program also includes the Mesozoic sequences of the Lusitanian, Santiago do Cacém, and Algarve basins and the Cenozoic Lower Tagus, Alvalade and Guadalquivir/Algarve basins. Proper research was conducted in the IPB, considered one of the most important metallogenetic VHMS deposit provinces worldwide with significant Cu, Zn, Pb, Ag, Au, Sn, In, Se and Ge resources. Currently, mining is being undertaken both in Portugal (Aljustrel, Neves-Corvo) and Spain (Las Cruces, Aguas Teñidas, La Magdalena, Sotiel, Riotinto). Field surveys were done using common stratigraphic and GIS database methodologies, developed in cooperation involving the Portuguese and Spanish Geological Surveys. A joint fieldwork was carried out in the border region (Guadiana and Chança river sections), allowing a better integration and correlation of geological data. Palynological studies performed at LNEG allowed dating of 113 Palaeozoic sediment samples in outcrop and drill hole sections. The same approach was used for U/Pb zircon geochronology using 31 samples of plutonic and volcanic rocks. Rock dating results obtained are important to constrain the geological structures of the IPB Volcano-Sedimentary Complex (VSC) that host the massive sulphide and stockwork mineralization. Key ore horizons, important to identify, are dated late Famennian (late Strunian) age in felsic volcanic and in sedimentary sequences and Tournaisian age felsic volcanic sequences. For upper VSC, zircon ages ca. 340–330 Ma were reported for the first time, suggesting new geodynamic interpretations. The main project outputs are the first 1/200.000 scale cross border and the 1/400.000 scale SPZ Geological Maps. The latter covers SW Iberia from Lisbon to Seville along 330 km. This scale was also considered in the following thematic maps developed by LNEG, IGME and JA: mineral occurrences, mining, and geological heritage. Another project activity was the development of a drill hole database and equipment acquisition for the Aljustrel (LNEG) and Peñarroya (IGME) drill core sheds. LNEG and Aljustrel Municipality also promoted mining and geological studies in the Algares (Aljustrel) mine sector on gossan, underground gallery mapping and mineral characterization. GEO_FPI Project has improved the geological knowledge of the cross border region and promoted IPB as a key mining region in Europe. Therefore, since 2010, exploration campaigns led to the discovery of the Semblana, Monte Branco, La Magdalena, Sesmarias, Lagoa Salgada Central and Elvira deposits. Regional surveys carried out to promote a common approach to SW Iberia and improve new business initiatives focused on mineral resources and territory management. These activities could predict a larger mapping program to be developed in central and northern sectors of the Portuguese-Spanish border. Acknowledgement: EU/Interreg-VA/Poctep/0052_GEO_FPI_5_E Project/ funded by European Regional Development Fund/ERDF.

How to cite: Matos, J. X. and the João Xavier Matos: The South Portuguese Zone 1/400 000 LNEG-IGME-Junta de Andalusia common mapping program. A contribution for the Iberian Pyrite Belt VHMS exploration in Portugal and Spain, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19381, https://doi.org/10.5194/egusphere-egu2020-19381, 2020.

The amalgamation of Pangea formed the contorted Variscan-Alleghanian orogen suturing Gondwana and Laurussia during Carboniferous. From all swirls of this orogen, a double curve stands out in Iberia, the coupled Cantabrian Orocline and Central Iberian Curve. The Cantabrian Orocline formed subsequent to Variscan orogeny (ca. 315-295 Ma). The mechanisms of formation for this orocline are disputed being the most prominent:

1) An Avalonian (Laurussia) indenter at SW Iberia, that would form the Cantabrian Orocline in a sinistral transpressive orogenic phase.

2) A change in the stress field that buckled the orogen. This change in stress would be potentially far-field and linked to subduction of the Paleo-tethys and/or diachronous collision in the Variscan belt.

In contrast, the geometry and kinematics of the Central Iberian curve are largely unknown. Whereas some authors defend both curvatures are genetically linked, others support they are distinct and formed at different times. Such uncertainty adds an extra layer of complexity into our understanding of the final stages of Pangea amalgamation.

We have performed a paleomagnetic analysis of several tectonostratigraphic units in SW Iberiat to solve the late Carboniferous Variscan kinematics. Our results show differential counterclockwise rotations, ranging from 20˚  and up to 70˚ at late Carboniferous. These results are coincident with the kinematics expected in the southern limb of the Cantabrian Orocline and discard a concomitant formation of both Cantabrian and Central Iberian curvature. The Avalonian portion of Laurussia rotated with the Cantabrian Orocline at both limbs: the northern one (Ireland, Pastor-Galán et al., 2015) and the southern one (South Portugese Zone, this study). The coherent rotation of Avalonia confirms the Greater Cantabrian Orocline hypothesis and discards the Avalonian indenter as a mechanism of formation for the Cantabrian Orocline. The Greater Cantabrian Orocline extended beyond the Rheic Ocean suture affecting both Laurussia and Gondwana margins and probably formed by a late Carboniferous change in the stress field, due to a still unidentified cause.

How to cite: Leite Mendes, B. D., Pastor-Galan, D., Dekkers, M. J., and Krijgsman, W.: Avalonia, get bent! Paleomagnetism from SW Iberia confirms the Greater Cantabrian Orocline, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13074, https://doi.org/10.5194/egusphere-egu2020-13074, 2020.

Oceanic accretion complexes along active continental margins contain a mixture of oceanic and potential continental tectonic elements, among which oceanic island volcanoes are the most prominent one. Here, we report an example from the pre-Alpine Austroalpine amphibolite-grade metamorphic basement of Eastern Alps, which contains several undated ophiolitic sutures and accompanying amphibolite-rich micaschist units. All of them have been considered to have formed not later than during Variscan plate collision. Major portions of this basement are then overprinted by Permian rift processes including Permian low-pressure metamorphism. The location of a Paleotethyan suture has not been considered to extend into the Alps.

Here we report preliminary results of an extensive survey with U-Pb zircon ages, Hf isotopes on zircon and whole rock geochemistry from the Plankogel and overlying Amphibolite-Micaschist complexes in Eastern Alps, which are directly overlying the Eclogite-Gneiss unit with Cretaceous high-pressure metamorphism. The Plankogel complex is composed of coarse-grained garnet-micaschist as a matrix and plagioclase-rich biotite schist, within which hectometer-sized lenses of marble, spessartine-quartzite, amphibolite and ultramafic rocks occur. According to the new data, the amphibolites have either (1) a N-MOR-basalt geochemical signature or (2) show ocean island basalt characteristics. Metasedimentary rocks like the garnet-biotite-micaschist show a large population of Early-Middle Triassic age, partly euhedral zircons implying an age of the sedimentary precursor rocks not older than Middle Triassic, and a significant Middle Triassic volcanic component. The manganese quartzites are explained as siliceous deep-sea sediments and show a large Permian to Early Triassic volcanic components (244±6 – 282±8 Ma) with a ~340 Ma peak and minor > 630 Ma peak ages of detrital zircons. Two N-MORB amphibolites exhibit late Permian/Early Triassic protolith ages (227±10 Ma-254±6.3 Ma). Positive εHf(t) values from zircons of Permian and Triassic age reveal uniform crustal model ages between 0.92 and 1.20 Ga.

Thick biotite-amphibolites from the overlying Amphibolite-Micaschist exhibit the geochemical characteristics of ocean island alkali basalts and have U-Pb zircon ages of 415±11 Ma and 413 ± 13 Ma. Again, εHf(t) values of zircons indicate a uniform crustal model ages clustering at ca. 1.2 Ga. The youngest detrital zircons of accompanying metasediments is at 450 Ma revealing that the age of host rocks is Silurian or younger. Consequently, this succession is interpreted as part of the accretionary wedge with ocean island volcanoe relics at margin of the Paleotethyan ocean.

Our dating results are entirely unexpected and require a re-evaluation of the tectonic history of the Austroalpine units. Based on these results, we conclude that the Plankogel complex represents a Triassic ophiolite-bearing mélange with oceanic trench sediments and components from a deep-sea environment as well continental components. The detritus is rich in Permian to Middle Triassic volcanic components. The volcanic components indicate the subduction of the Paleotethyan Ocean, and oceanic lithospheric elements were incorporated into the trench sediments.

Together, the new data reveal the accretion of an ocean island into the Plankogel subduction complex. Furthermore, this accretionary system was active up to Triassic times and can be considered to relate to the Paleotethyan suture in Eastern Alps.

How to cite: Liu, B., Liu, Y., Neubauer, F., Chang, R., Yuan, S., Genser, J., Bernroider, M. B., Guan, Q., Jin, W., and Huang, Q.: A Paleotethyan oceanic accretion complex in the Eastern Alps: the Plankogel and overlying Amphibolite-Micaschist complexes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19610, https://doi.org/10.5194/egusphere-egu2020-19610, 2020.

Investigations on the late Neoproterozoic to early Paleozoic sedimentary strata of western South China and northern Indochina reveal a provenance affinity between the two, which was mainly derived from the local western part of South China. The newly discovered provenance featured differently from that of the typical Indian-Australian Gondwana siliciclastic source. Basin types and sedimentation histories of the two sedimentary basins in western South China and northern Indochina are also comparable. Furthermore, previous studies discovered the geochronological, petrological and geochemical similarities of the early Paleozoic magmatic rocks between these two regions, suggesting a connection between the two during the subduction of the proto-Tethys ocean towards the northern Gondwana and the accretion of Asian continents onto the Gondwana mainland. Utilizing all such geological information, we speculate in this study that South China and Indochina were probably in the neighbourhood on the northern Gondwana margin when the Gondwana semi-supercontinent was assembled. Specifically, Indochina was likely located to the southwest of South China during the late Neoproterozoic to early Paleozoic. Apart from sedimentation, neither Indochina nor the western part of South China got much deformational and metamorphic impaction from the collision between South China and northern Gondwana during that time.

How to cite: Yao, W., Wang, J., Spencer, C., Martin, E., and Li, Z.-X.: The South China and Indochina neighborhood in the assembled Gondwana, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12874, https://doi.org/10.5194/egusphere-egu2020-12874, 2020.

The Proto-Tethys Ocean is principally defined as an ancient ocean distributed to the northern margin of the Gondwana landmasses, which initiated during the breakup of the Rodinia supercontinent and closed in the Early Paleozoic during the final assembly of Gondwana. Major continental blocks of China, including Tarim, Qaidam, South China and North China, were distributed in this ocean. Locally in the Altyn-Tagh UHP belt in the southeastern margin of the Tarim Craton, the ocean is referred to as the Altyn Ocean. The Kulamulake ophiolitic mélange occur within the South Altyn Terrane and was extensively sheared and deformed, with its southern and northern margins of the ophiolitic mélange delineated by a top to the northwest thrusting fault zone and a ductile shearing zone, respectively. The mélange thrust on to the latest Mesoproterozoic-Neoproterozoic Altyn group and Neoproterozoic-Paleozoic Bashikuergan group on its northern and southern margins, respectively. Stratigraphically from bottom to top it is composed of sheared serpentinite on the basal thrust, layered dunite-harzburgite, pyroxene peridotite, layered olivine pyroxenite and fine-grained meta-gabbro, along with exotic blocks of marble. Pillow basalt and plagio-granite have also been reported from within the mélange, which might be upper components of the ophiolite stratigraphy. All the exposed lithostratigraphic sequences occur as structural blocks. Therefore, overall lithologies and structures resembles those of ophiolitic mélanges. Meta-gabbro components of the mélange yield concordia ages of 518 ± 2 Ma, along with juvenile zircon Hf and whole rock isotopic signatures. The analyzed mafic-ultramafic samples display chemical characters that are comparable to E-MORB, but with some island-arc signatures, resembling those of SSZ type ophiolite. In addition, correlations of major and trace element compositions of all analyzed samples are indicative of fractional crystallization from a depleted mantle source. The overall lithological assemblages, isotopes and chemical compositions are consistent with a disrupted ophiolitic mélange during initial oceanic subduction environment. Therefore, we concluded the Kulamulake ophiolite recorded the initiation of oceanic subduction within the Paleo-Tethys Ocean in northern Gondwana margin. This research was supported by NSFC Projects (41730213 and 41190075) and Hong Kong RGC GRF (17307918 and 17301915).

How to cite: Yao, J., Zhao, G., Han, Y., Liu, Q., Dong, Z., Li, J., Wang, P., and Yu, S.: Constraining the initiation of oceanic subduction of the Proto-Tethys Ocean beneath the Tarim block in northern Gondwana margin , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3821, https://doi.org/10.5194/egusphere-egu2020-3821, 2020.

As the largest and highest plateau on Earth, the Tibetan Plateau is distinguished from most other ranges and liner continental orogenic belts (e.g., the Alps) by its broad and flat topography. According to influential numerical and theoretical models, the (former) existence of ductile and molten mid-to-lower crust was an essential contributor to the topographic smoothing process. However, the question of whether the Tibetan Plateau has undergone widespread crustal melting remains highly controversial and hard to prove due to the scarcity of direct evidence from the deep crust. Here we first report on a series of hydrous crustal xenoliths entrained in 28 Ma host lavas from central and northern Tibet. Our new results document the former existence of hydrous crust at 28 Ma as a potentially highly fertile magma source. Quantitative modeling reveals a thermal gradient reaching about 680 ℃ to 790 ℃ at a depth of 14 to 40 kilometers, which is significantly lower than that of recent (since 2.3 Ma) evidence for hot Tibetan crust. Petrological data suggest that the initial crustal melting beneath Tibet began at 28 Ma at depths of 23–40 km (and even deeper) with 0.5–9.6 vol. % melts, which would lead to a significant reduction of seismic speeds similar to the low-velocity zones observed in the present Tibetan mid-to-lower crust. As the geothermal gradient continued to rise from 28 to 2.3 Ma, wholesale crustal melting (> 20–30 vol. %) of the mid-to-lower crust beneath Tibet was inevitable and created the modern flat Tibetan Plateau.

How to cite: Zhang, X.-Z., Wang, Q., and Dan, W.: Widespread crustal melting since 28 Ma creates the modern flat Tibet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12351, https://doi.org/10.5194/egusphere-egu2020-12351, 2020.

A lamprophyre dyke has been found in Ramba area within the Tethyan Himalaya. It intruded into the Late Triassic low-grade metasedimentary rocks (Langjiexue Group) and show typical porphyritic textures, with phlogopite as the dominant phenocrysts. In this study, we performed phlogopite 40Ar/39Ar dating and whole-rock major and trace element as well as Sr and Nd isotope geochemical analyses on the lamprophyre. The ⁴⁰Ar/³⁹Ar plateau ages (13.1 ± 0.2 Ma and 13.5 ± 0.2 Ma) of the phlogopites from two samples are both in excellent agreement with the inverse isochron ages of 13.1 ±0.3 Ma and 13.6 ± 0.3 Ma, recording the times at which the lamprophyre dyke has cooled below ~300 °C. The lamprophyre has low contents of SiO₂ (51.43–55.15 wt%) and Al₂O₃ (11.10–11.85 wt%), high Fe₂O_3T (8.57–9.27 wt%) and MgO (9.14–9.49 wt %) contents with Mg^# of 66–69, higher content of K₂O (3.26–5.57 wt%) relative to Na₂O (0.50–1.39 wt%) with K₂O/Na₂O of 2.3–11.1. Furthermore, the lamprophyre has high abundances of large ion lithophile elements (e.g., Rb, Ba, Sr), shows depletions in high field strength elements (e.g., Nb, Ta, Ti), and displays enrichment in light rare-earth elements over heavy rare earth elements with (La/Yb)_N of 42.3~47.0. Besides, the lamprophyre is characterized by high initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7196~0.7204 and negative ε_Nd(t) values of -10.7~-10.8. Geochemical data suggest that the Ramba lamprophyre was likely generated by partial melting of a metasomatized, phlogopite-bearing harzburgite lithospheric mantle source, followed by crystal fractionation and varying degree of crustal assimilation. The studied lamprophyre provides a window into the composition of the subcontinental lithospheric mantle (SCLM) in the northern margin of the Indian plate. We suggest that the northern Indian plate might be involved in the Andean-type orogeny from the subduction of the Proto-Tethys Ocean during Cambrian to Early Ordovician.

How to cite: Liu, Z.-C., Wang, J.-G., and Liu, X.-C.: Geochemical and Sr-Nd isotopic characteristics of the lamprophyre in the Tethyan Himalaya, South Tibet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12552, https://doi.org/10.5194/egusphere-egu2020-12552, 2020.

The Adelaide Rift Complex is a large sedimentary superbasin in South Australia that formed resultant of Rodinia’s breakup and subsequent evolution of the Australian passive margin of the Pacific basin. It holds a globally significant and exceptionally well-preserved Neoproterozoic–early Cambrian succession. Much work has been done over the last century describing the lithostratigraphy and sedimentology of this vast basin. The rift complex contains evidence for major changes in Earth’s systems, yet, the rocks are poorly dated, and the sediment provenance, and link with tectonic evolution, is remarkably poorly known.

This work provides a centralised database of the currently available, and previously unpublished, detrital zircon geochronology for the Neoproterozoic of the Adelaide Rift Complex, highlighting where the available data is from, and the stratigraphic and spatial gaps in our knowledge. By subjecting the U–Pb detrital zircon data to data analytical techniques, we provide a first look overview of the change in provenance, and subsequently (generalised) palaeo-tectonogeography that this suggests during the Neoproterozoic. These data show a change from dominantly local sources in the middle Tonian, to dominantly far-field sources as the rift-basin develops over time. The Cryogenian icesheets punctuate this with an ephemeral return to more local sources from nearby rift shoulders. This effect is particularly apparent during the Sturtian Glaciation than in the younger Marinoan Glaciation. In the Ediacaran, we see an increasingly stronger influence of younger (<700 Ma) detrital zircons from an enigmatic source that we interpret to be from southern (i.e. Antarctic) sources. We also note that we see a slight shift in the late Mesoproterozoic age peaks, from ca. 1170 Ma to ca. 1090 Ma, with a corresponding decrease in older ca. 1600 Ma detritus.

This work forms the basis of continuing work to improve our understanding of the geochronology, provenance and palaeo-tectonogeography of the Adelaide Rift Complex.

How to cite: Lloyd, J., Blades, M., Counts, J., Collins, A., Amos, K., Hall, J., Hore, S., Wade, B., Job, A., Shahin, S., and Drabsch, M.: The State-of-play of geochronology and provenance in the Neoproterozoic Adelaide Rift Complex, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3184, https://doi.org/10.5194/egusphere-egu2020-3184, 2020.

There is still little known about the occurrence, formation and spatial distribution of long-lived cratonic basins that form during hundreds of millions of years of subsidence. Their histories often span multiple phases of super-continent break-up, dispersal and amalgamation. Each of these phases resulted in the modification of sedimentation rates and drainage within the basins but the broader basin persisted. These changing conditions are recorded in the detrital zircon record, providing a tool for understanding the basin evolution and consequently its palaeogeography.

The informally termed greater McArthur Basin is a regionally extensive Proterozoic basin that overlies the North Australian Craton. It is a vast sedimentary system that stretches across the northern part of the Northern Territory from north-eastern Western Australia to north-western Queensland. It includes Palaeo- to Mesoproterozoic successions of the McArthur and Birrindudu basins, the Tomkinson Province and likely the Lawn Hill Platform and South Nicholson Basin (to the south-east); all interpreted to be contemporaneous systems. However, the full extent of the greater McArthur Basin sedimentary system is still being unravelled. The basin records nearly one billion years of Earth history, from ca. 1.82 Ga to ca. 0.85 Ma. This sedimentary system temporally overlaps with episodes of Palaeo- to Mesoproterozoic tectonism and igneous activity that affected underlying and adjacent terranes, including the Aileron, Warumpi and Musgrave provinces to the present-day south, Pine Creek Orogen and Arnhem Province to the north, Halls Creek Orogen and Tanami Region to the west, and Mount Isa and Murphy provinces to the east.  

LA-ICP-MS detrital zircon U–Pb geochronology and Lu–Hf isotope data provide new constraints on the lower sedimentary successions of the McArthur Basin (Tawallah and Katherine River Groups) and demonstrate they are coetaneous with the Tomkinson Province (Tomkinson Creek Group). U–Pb detrital zircon data show major ²⁰⁷Pb /²⁰⁶Pb peaks at ca. 1860 Ma and ca. 2500–2400 Ma in both the McArthur Basin and Tomkinson Province sediments. Combined with Lu–Hf isotope data, the detrital zircon age data from the McArthur Basin show similarities to the Aileron Province (to the south) and magmatic rocks of the Gawler Craton, suggesting that these terranes might be possible source areas. Comparatively, the oldest succession within the Tomkinson Province (Hayward Creek Formation), shows similar spectra to units within the Lawn Hill Platform succession (McNamara Group, Surprise Creek Sandstone and Carrara Range Group) possibly suggesting a correlation between the two areas.

Here we explore the links between the North Australia Craton and surrounding continents to further elucidate the evolution of this enigmatic basin throughout the Proterozoic. New palaeogeographic reconstructions link the ‘greater’ McArthur basin to the Yanliao Basin and coeval rocks in the North China Craton. The ‘greater’ McArthur basin may also have extended into southern Australia, Laurentia and Siberia as a vast intra-continental gulf (the McArthur-Yanliao Gulf) within the core of the supercontinent Nuna/Colombia.

How to cite: Blades, M., Collins, A., Yang, B., Cruz, C., Cassidy, E., Payne, J., Farkas, J., Glorie, S., and Munson, T.: Using detrital geochronology to unravel the Proterozoic greater McArthur Basin of Northern Australia , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12412, https://doi.org/10.5194/egusphere-egu2020-12412, 2020.