Currently three supercontinent cycles have been identified and existed supercontinents named from youngest to oldest: Pangea, Rodinia and Nuna/Columbia. Recently Wang et al. (2020) suggested that supercontinent amalgamation were each preceded by ~200 Myr by the assembly of long-lasting megacontinent aking to Gondwana.

The Congo-São Francisco (C/SF) craton is a main building block in Gondwana due to its central location, but its participation to Rodinia is controversial. Salminen et al. (2018) presented 1.11 Ga paleomagnetic and geochronological data from a prominent Epembe-Huila swarm of gabbronoritic dykes in the southern part of the Congo craton in Namibia and in Angola. This paleomagnetic pole yields a relatively low paleolatitude for the C/SF craton at ca. 1.11 Ga and permits a direct connection between Congo and Kalahari cratons. This connection supports an earlier qualitative comparison (Ernst et al., 2013), that the mafic Epembe-Huila swarm was an integral component of the Umkondo Large Igneous Province (LIP). The 1.11 Ga Umkondo LIP is widespread across Kalahari craton, and coeval mafic magmatism has been identified in several of the world’s other late Mesoproterozoic cratons: Laurentia, India, Amazonia, and Antarctica (Grunehogna). Were these coeval provinces spatially linked at the time of emplacement during the amalgamation of Rodinia? Robust paleomagnetic and geochronological data from Laurentia and Kalahari have demonstrated substantial separation between those two blocks at 1.11 Ga (Swanson-Hysell et al., 2015). However, based on similar tholeiitic magmatism Choudhary et al. (2019) proposed that Kalahari and C/SF together with Amazonia and northern India constituted “Umkondia” at 1.11 Ga. It has been proposed that Umkondia occupied an intermediary “megacontinental” role in the Nuna-Rodinia transition analogous to Gondwana in Rodinia-Pangea evolution (Wang et al., 2020). Contradicting Gondwana the proposed Umkondia was not long-lasting, since it has been proposed that Kalahari and Congo separated after 1.10 Ga to form a vast ocean (ca. 6000 km) during the formation of Rodinia and widespread juvenile intra-oceanic magmatism along the present-day central Brazil indicates a large ca. 0.94 Ga ocean between C/SF and Amazonia (Cordani et al., 2003).

Choudhary et al. 2019. Precambrian Research 332, 105382.

Cordani et al. 2003. Gondwana Research 6, 275-283.

Ernst et al. 2003. Lithos 174 1-14.

Salminen et al. 2018. Geology 46, 1011-1014.

Swanson-Hysell et al. 2015. Geophysical Journal International 203, 2237-2247.

Wang et al. 2020. Geology 49, https://doi.org/10.1130/G47988.1

How to cite: Salminen, J.: Congo-São Francisco at 1.11 Ga and the megacontinent Umkondia , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10714, https://doi.org/10.5194/egusphere-egu21-10714, 2021.

The West African Craton (WAC) is one of the major cratons in the Rodinia jigsaw puzzle (~1000–750 Ma). In the Rodinian models, the position of West Africa is mainly constrained by the assumption that it had been a partner of Amazonia since the Paleoproterozoic. Unfortunately, no paleomagnetic data are available for these cratons when the Rodina supercontinent is considered tectonically stable (~1000-750 Ma). Thus, every new reliable paleomagnetic pole for the West African Craton during the Neoproterozoic times is of paramount importance to constrain its position and testing the Rodinia models. In this study we present a combined paleomagnetic and geochronological investigation for the Manso dyke swarm in the Leo-Man Shield, southern West Africa (Ghana). The ~860 Ma emplacement age for the NNW-trending Manso dykes is thus well-constrained by two new U-Pb apatite ages of 857.2 ± 8.5 Ma and 855 ± 16 Ma, in agreement with baddeleyite data. Remanence of these coarse-to-fine grained dolerite dykes is carried by stable single to pseudo-single domain (SD-PSD) magnetite. A positive baked-contact test, associated to a positive reversal test (Class-C), support the primary remanence obtained for these dykes (13 sites). Moreover, our new paleomagnetic dataset satisfy all the seven R-criteria (R=7). The ~860 Ma Manso pole can thus be considered as the first key Tonian paleomagnetic pole for West Africa. We propose that the West Africa-Baltica-Amazonia-Congo-São Francisco were associated in a long-lived WABAMGO juxtaposition (~1100–800 Ma).

Keywords: West Africa, Neoproterozoic, Tonian, Rodinia, paleomagnetism.

How to cite: Antonio, P. Y. J., Baratoux, L., Trindade, R. I. F., Rousse, S., Ayite, A., Lana, C., Macouin, M., Adu, E. W. K., Sanchez, C., Silva, M., Firmin, A.-S., Martinez Dopico, C. I., Proietti, A., Amponsah, P. O., and Sakyi, P. A.: Paleomagnetism of the ~860 Ma Manso dyke swarm, West Africa: implications for the assembly of Rodinia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13336, https://doi.org/10.5194/egusphere-egu21-13336, 2021.

The paleomagnetic record during the middle Neoproterozoic (~825-780 Ma) displays rapid apparent polar wander variations leading to large discrepancies in paleogeographic reconstructions. Some authors propose that these data may represent true polar wander events, which correspond to independent motion of the mantle and lithosphere with respect to Earth’s rotation axis. An alternative explanation might be a perturbation of the geomagnetic field, such as a deviation from a predominantly dipole field or a hyper-reversing field. To test these hypotheses, we sampled 1200 oriented cores over a stratigraphic height of 100 metres in sedimentary rocks of the 820-810 Ma Laoshanya Formation in South China. We will present preliminary paleomagnetic and rock magnetic analyses together with results of petrologic and geochemical experiments to better understand the origin of the paleomagnetic signal.

How to cite: Tonti-Filippini, J., Robert, B., Muller, É., Wack, M., Zhao, X., and Gilder, S.: The Neoproterozoic geomagnetic field: new insights from a high-resolution paleomagnetic study in South China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4671, https://doi.org/10.5194/egusphere-egu21-4671, 2021.

Paleomagnetic data obtained from Neoproterozoic glacial and glacier-associated sedimentary rocks indicate that they were formed at near equatorial latitudes. Based on these data, the Snowball Earth hypothesis was proposed [Kirschvink, 1992]. According to this hypothesis, during the Neoproterozoic glaciations, the entire planet (including the oceans) was completely covered with ice. Although evidence is emerging that does not support this hypothesis, there is still no conclusive evidence that it is not true [Sansjofre et al., 2011].

It is worth noting that the Snowball earth hypothesis is based on paleomagnetic data. At the same time, the available paleomagnetic data for the Neoproterozoic-Early Cambrian [Meert, Van der Voo, 2001; Shatsillo et al, 2005; Abrajevitch, Van der Voo, 2010; Pavlov et al., 2018] difficult to interpret in terms of the Geocentric Axial Dipole hypothesis. This imposes serious restrictions on the possibility of correctly constructing paleomagnetic reconstructions.

For the development and testing of a model of the geomagnetic field of the Neoproterozoic, it is necessary to obtain a lot of high-quality paleomagnetic data. Data from well-dated magmatic bodies are especially valuable.

Within the framework of this work, we obtained paleomagnetic data from three carbonatite dikes (7 to 30 cm thickness) exposed in the Udzha river bank on the Udzha uplift in the northeastern part of the Siberian platform. These dikes are associated with the large alkaline Tomtor massif located 15 km to the west. Ar/Ar dating of phlogopite megacrysts gives an intrusion age of the dikes of 706.1±8.8 Ma. Coordinates of the virtual geomagnetic pole, calculated from the direction of the high-temperature component of magnetization: Φ=-20.7°; Λ=88.6°; Α95=3.4°.

Our report will present preliminary interpretation of these data, as well as their comparison with paleomagnetic data on close-aged objects in Siberia.

The research was supported by the Russian Science Foundation grant (19-77-10048).

References:

How to cite: Pasenko, A., Alexey, I., Sergey, M., and Alexey, T.: Geochronological and paleomagnetic studies of small carbonatite intrusions of the Udzha uplift (Tomtor massif, northeast of the Siberian platform), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12614, https://doi.org/10.5194/egusphere-egu21-12614, 2021.

The Ediacaran (635-541 Ma) is the last geological period of the Precambrian during which major changes occurred in the superficial layers of the Earth (biosphere, cryosphere, oceans, atmosphere). The paleomagnetic data from the main continents of this epoch display very fast polar wander excursions, which seemed to occur simultaneously on several continents. Two main competing hypotheses have been proposed in the literature to explain these data: (1) very fast True Polar Wander episodes (TPW), which represent the global movement of the mantle and the crust with respect to the Earth's spin axis, or (2) perturbations of the Earth’s magnetic field. On geological timescales, the TPW is speed-limited to some degrees per million years while magnetic field changes could be much faster (degrees per kyrs). The velocity of the polar wander excursions of the Ediacaran is therefore a critical parameter to distinguish these two families of solutions. The volcanic rocks of the Ouarzazate group (575-545 Ma) in the Anti-Atlas belt recorded a large polar wander excursion from ~571 to ~565 Ma, which is also observed in Laurentia and Baltica at about the same time. Because the age uncertainties are too high, the existing SHRIMP U-Pb ages obtained on zircons are not precise enough to distinguish these two hypotheses. In this study, we bring new high-precision CA ID-TIMS ages on zircons from seven tuff layers that recorded the rapid paleomagnetic variations. Our results show, in most of the samples, a large spread in age, indicating either the presence of inherited zircons or strong Pb loss in some of the zircons. Four of the samples display a good consistency in the zircon ages, and could represent the age of the tuff emplacement. In this presentation, we will discuss the two hypotheses based on these new geochronological constraints.

How to cite: Robert, B., Corfu, F., and Blein, O.: New age constraints on the Ouarzazate Group (Morocco): implications on the hypothesis of True Polar Wander during the Ediacaran, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9564, https://doi.org/10.5194/egusphere-egu21-9564, 2021.

The Neoproterozoic is marked by unusual perturbations of the climate system (global glaciations), biogeochemical cycles (e.g. oxygenation, Carbon), and life diversity that will lead to a world as we know it today. These upheavals can be considered from the point of view of paleogeographic reconstructions to decipher the forcing mechanisms and consequences. The paleopositions of the continents and their geology impact the continental weathering, a fundamental element in the feedbacks driving the climate. Besides, the position of the supercontinent Rodinia and the nature of its margins influence degassing, itself a major factor in biogeochemical cycles. Neoproterozoic paleogeographic reconstructions are based on some reliable paleomagnetic data and geological evidence of kinship between the cratons. Uncertainties in Neoproterozoic paleogeographies hinder our understanding of the relationship between deep Earth and superficials layers. Notably, the positions of the cratons that will constitute the Arabian-Nubian shield are poorly constrained. The paleomagnetic results we obtained in Oman on well-dated mafic dykes, indicate a mid-latitude position at the dawn of the Sturtian glaciation. These results show the potential of these small cratons, which represent a zone of arcs and arc collisions, in our understanding of the geodynamics of the Rodinia supercontinent. We then propose a refined configuration at ca. 720 Ma, highlighting the extent of snowball Earth deposits.

How to cite: Macouin, M., Rousse, S., and Antonio, P.: Solving uncertainties in Neoproterozoic paleogeographic reconstructions: the key to understanding the links between the Earth's outer and inner envelopes?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14668, https://doi.org/10.5194/egusphere-egu21-14668, 2021.

Full-plate reconstructions describe the history of both past continental motions and how plate boundaries have evolved to accommodate these motions. The fluxes of material into and out of the mantle at plate boundaries is thought to deeply influence the evolution of deep Earth structure, surface environments and biological systems through deep time. Traditionally, plate tectonic reconstructions have relied on geophysical data from the oceans, which provides details of how Pangea broke apart (since ca. 200 Myr) while paleomagnetism is the primary quantitative constraint prior to Pangea formation. However, these data do not directly constrain the extent of subduction zones or other plate boundaries, so reconstructing the past plate configurations of past supercontinents must rely on alternative methods. One source of data that can resolve this problem is to use observations from detrital zircons. Previous studies have proposed classification schemes to determine tectonic settings where samples were deposited, based on the different characteristic shapes of detrital zircon age spectra found in convergent, collisional and extensional settings.

Here, we investigate the applicability of this method to test and refine global full-plate tectonic reconstructions in deep time, using a published database of zircon ages. We first use reconstructions for relatively recent times (<100 Ma), where reconstructions are reasonable well constrained, to evaluate the effectiveness of the classification method. For older times, where uncertainties in the reconstructions are far larger, we can use the results to discriminate between competing models. We analysed the proximity between reconstructed plate boundaries and zircon sample sites assigned to different tectonic classifications, and found that the classification method does well (~64－79% success depending on distance threshold used) in distinguishing convergent settings. The ability of the classification to define extensional settings such as rift basins is less clear, though samples in this class do lie preferentially further from convergent settings. Based on these insights, we apply the method to evaluate full-plate reconstructions for the Neoproterozoic as well as other competing models for the configuration of Rodinia.

How to cite: Jian, D., Williams, S., Yu, S., and Zhao, G.: Implications of the detrital zircon record for global plate tectonic reconstructions in deep time, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10078, https://doi.org/10.5194/egusphere-egu21-10078, 2021.

Paleo-digital elevation models (paleoDEM) based on plate tectonic and paleogeographic reconstructions use age grids of ocean floor to determine ocean bathymetry. In recent years, such age grids have also been developed for now-subducted oceans from the far geological past, as far back as the Neoproterozoic, using geology and paleomagnetism-based estimates of ocean opening. In such reconstructions, mid ocean ridges are drawn based on estimated Euler poles and rotations, and conceptual knowledge on the geometry consisting of spreading ridges and transform faults.

Current procedures to draw mid ocean ridges in plate tectonic reconstructions are laborious, as new ridges are drawn every time the Euler pole location changes. Fortunately this is also a task that can be automated. We have written an algorithm using pyGPlates that takes as input a smooth curve at the approximate position of the reconstructed mid ocean ridge at the moment of its formation, and then calculates spreading and transform segments according to their typical geometries in modern oceans, assuming symmetric spreading. The algorithm allows gradual readjustment of ridge orientations upon Euler pole changes comparable to documented cases in the modern oceans (e.g., in the Weddell Sea). The algorithm also contains modules that can convert the calculated mid ocean ridges with other plate boundaries to boundary topologies – which can be used as input for the recently published TracerTectonics algorithm, produce isochrons which can be converted to age grids, check for subduction of isochrons and subsequently create bathymetry grids. We illustrate the use of the MORGEN algorithm with recently published reconstructions of subducted, as well as future oceans.

How to cite: van der Linden, T. and van Hinsbergen, D.: MORGEN – The Mid Ocean Ridge GENerating algorithm, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9494, https://doi.org/10.5194/egusphere-egu21-9494, 2021.