Stephan V. Sobolev

In contrast to the traditional mechanism of passive continental rifting (driven by far‐field tectonic forces), the active rifting‐to‐break‐up processes (caused by rising mantle plumes) are still poorly understood. However, most episodes of fragmentation of the last supercontinent Pangea were relatively shortly preceded (within ∼10 Myr) by the emplacement of Large Igneous Provinces, indicating that a link between lithospheric ruptures and mantle plumes is very close and frequent. In this study, we present a systematic numerical modeling of purely active continental rifting and break‐up, that is, without far‐field extension, examining the following parameters: (a) the thermo‐rheological structure of the lithosphere, (b) the buoyancy of the thermo‐chemical mantle plume anomaly, and (c) the duration of the incoming plume flux. Thermo‐mechanical experiments show that a classic active rifting scenario, involving the complete break‐up of the lithosphere within less than 10 Myr following the arrival of a mantle plume, is achievable, though only under specific conditions. These include: (a) a continuously fed plume with an elevated thermal and/or compositional density deficit ( ≤ −30 kg m ⁻³ ) and (b) a relatively warm overlying continental plate, characterized by an above‐average Moho temperature ( = 750°C). Although both prerequisites do not seem entirely unrealistic, their simultaneous occurrence in the Phanerozoic Earth is unlikely. We therefore conclude that for a successful transition to lithospheric break‐up, plume‐activated continental rifting events in the Mesozoic–Cenozoic time must be accompanied by external tectonic stresses.

Many aspects of solid Earth's evolution such as crustal growth, tectonic regime and the onset of plate tectonics remain debated. Here, the influence of composite rheology and water on Earth’s dynamics is investigated using 2D global compressible convection models. When compared to models with just diffusion creep, models incorporating a composite rheology (diffusion creep and dislocation creep proxy) for the upper mantle show higher production of continental crust, more efficient planetary cooling and higher recycling of denser oceanic crust. These models also show higher mobilities and a tectonic regime fluctuating between mobile-lid and plutonic-squishy-lid during the Hadean and early Archean, accompanied by rapid crustal growth and intensive crustal recycling, consistent with the new geochemical data. As the water budget of mantle and surface remains poorly constrained, different in-gassed values of water are tested, which show a strong correlation with continental crust production. Upon incorporating water-dependent density, these models showcase a significant drop in surface mobility and are less efficient at planetary cooling due to excess of water. Once the surface rocks are partially saturated to better fit the petrological constraints and the effect of water on viscosity of rocks is considered, subduction becomes active and a fluctuating tectonic regime is observed again. Finally, the reduction in effective friction of the oceanic lithosphere during the mid-Archean, resembling the lubrication effect of continental sediments, stabilises the mobile-lid regime and signifies a transition to plate tectonics.

The rates of continental crust growth and recycling on early Earth remain unclear due to the lack of information resulting from the extensive alteration of ancient rocks. Melt inclusions trapped and shielded from alteration in Archean high-Mg olivine crystals offer a solution to this problem. We report an unprecedented unradiogenic Sr mantle source component (⁸⁷Sr/⁸⁶Sr = 0.69932 ± 0.00024, 95% confidence interval) of melts included in olivine from 3.27 Ga komatiitic lava flows in the Barberton Greenstone Belt, South Africa. This component indicates a model age of 4.31 ± 0.19 Ga and significant chemical fractionation (Nb/U = 36.9 ± 1.5, Ce/Pb=16.7 ± 1.1), suggesting up to 80% ± 16% of the present-day continental crust’s mass was extracted by the late Hadean from the whole mantle. Geodynamic models support this finding, explaining geochemical data by producing 40% to 70% of the present-day continental crust mass during the Hadean in a variable tectonic regime with tens of millions of years-long periods of massive impulsive subduction induced by mantle plumes.

Earth’s size and composition make it comparable to Venus but these planets exhibit contrasting surface expressions due to their different tectonic regimes and surface recycling processes. Earth’s efficient recycling of its surface and the operation of plate tectonics are facilitated by the formation of extensive global subduction networks. During the Wilson Cycle, these networks drive periods of supercontinental breakup and active plate tectonics. We hypothesize that the formation of global subduction networks on Earth is promoted by the presence of water-rich continental sediments that reduce lithospheric friction at convergent margins. This is critical for inducing large-scale motion and reorganization of lithospheric plates, a key defining feature of modern plate tectonics. To explore this hypothesis, we developed a series of 3D global geodynamic models using the ASPECT code. These models reproduce 2 scenarios: (i) self-consistent plume-induced regional subduction and its reorganization into global subduction networks, (ii) prescribed inherited plate boundaries at 1Ga, demonstrating that sustained subduction activity is possible thanks to local frictional strength reduction. On Earth, such frictional reductions may fluctuate over time, driven by climatic events like Snowball Earth, which increase sediment flux and lubricate convergent plate boundaries. We compare these results with the results of models for Venus, where there is no liquid water at the surface, which implies higher frictional strength. We infer that without localized reduction of friction, regional subduction-like deformation on Venus’s dry surface is short-lived, failing to establish global subduction networks. However, in long-term, Venus can still experience episodic resurfacing as its lithosphere becomes unstable and collapses into the asthenosphere. Comparison of Earth’s and Venus’s tectonic styles highlights the role of surface water and water-rich sediments in sustaining large-scale and long-term subduction and in the development of a global network of subduction zones and plate boundaries, which is a characteristic of modern plate tectonics.

Ocean on the Earth is a key feature, which is likely responsible for the onset and further operation of plate tectonics as well as for the origin of life. Geochemical data suggests that ocean existed on the Earth already since at least the middle Hadean time. Recent studies also infer that after the solidification of the magma ocean, mean concentration of water in the Earth’s mantle could have been up to few 1000 ppm and that extraction of part of it formed the surface ocean. However, a clear understanding of this process is still lacking.Here, we report results of modelling of Earth’s evolution during its first 1.5 Gyr with a focus on water cycle and generation of the continental crust. We use geodynamic code StagYY in 2D spherical annulus geometry that generates both basaltic and felsic melts, includes cooling of the core and uses an advanced treatment of water. We also included the effect of water on density of crustal and mantle materials based on experimental data and thermodynamic calculations.Our models start just after solidification of magma ocean with assumed initial mantle potential temperature of 1900K and core temperature of 5000K. We run models with different initial mean water content in the mantle reaching up to 1500 ppm. In all the models, most of the water is initially concentrated in the mantle transition zone (MTZ), because of its higher water storage capacity. Due to the lower density of the water-containing materials, this leads to Rayleigh–Taylor instabilities and hot and “wet” mantle plumes rapidly rise to the surface. As a result, a large amount of mantle water is outgassed forming the surface ocean in just a few million years. Simultaneously, a significant amount of continental crust is produced. Masses of the produced ocean and continental crust depend on the initial concentration of water in the mantle. For instance, for the initial mean water concentration of 1000 ppm, ocean mass of about 1.5 times recent ocean masses (OM) and continental crust of about 0.7 times present-day continental crust mass (CCM) is produced during 7 Myr. Water outgassing from the mantle dominates during the first 100 Myr till ocean mass reaches about 2 OM. Afterwards, the outgassing by plumes and in-gassing by subduction are mostly balanced with a tendency of the surface ocean mass to decrease with time during the 1.5 Gyr.Interestingly, in all models, MTZ behaves as a buffer for water cycle and despite it’s high water storage capacity, it’s mean water content mostly remains below 400 ppm, rising to up to 1500 ppm only for the short time periods when a number of cold slabs are resting in MTZ. We will show results from a set of models and compare the model-predicted trace elements ratios with the recent geochemical data.

Even though surface water is essential for Earth's habitability, the estimates of total amount of water (at the surface and in the deep interior) throughout Earth's evolution vary from 5-15 ocean masses (OM) based on magma ocean solidification models [Hamano et al., 2013] to 1.2-3.3 OM based on petrological studies [Hirschmann, 2006]. Previous numerical models of coupled surface-mantle system have estimated a lower bound of 9-12 OM [Nakagawa et al., 2018]. Experiments have shown that water lowers the melting temperature, density and viscosity of rocks and it is also required for the generation of felsic magmas. In this work, we use global convection models [Tackley, 2008] spanning the age of the Earth to elucidate the effect of water on mantle dynamics in terms of planetary cooling, surface mobility and production of continental crust.Our models self-consistently generate oceanic and continental (Archean TTGs) crust while considering both plutonic and volcanic magmatism and incorporate a composite rheology for the upper mantle. Pressure-, temperature-, and composition-dependent water solubility maps calculated with Perple_X [Connolly, 2009] control the ingassing and outgassing of water between the mantle and the surface [Jain et al., 2022]. Irrespective of the initial water content used, our models exhibit mobile-lid regime (high surface mobility with subduction) throughout the 4.5 Gyr with episodes of short-lived plutonic-squishy-lid regime (low surface mobility with delamination or dripping) in the Hadean. These models are also consistent with the cooling history of the Earth inferred from petrological observations [Herzberg et al., 2010]. A strong positive correlation is observed between continental crust production and the total amount of water available, with the former's cumulative mass increasing by roughly three times when water in the planetary system is raised from 1 OM to 10 OM.Models that consider a reduction in the density of crustal and mantle materials in the presence of water exhibit mobile-lid regime for the initial 200 Myr. Afterwards, the mobility stays low as the hydrated oceanic crust is less dense and does not subduct. It thickens over time and eventually collapses as global resurfacing events. Mantle stays comparatively warm and a much lower amount of continental crust is produced. This motivated us to make the following improvements to achieve more realistic models. First, mantle minerals only in the top 5 km of the computational domain (as opposed to 10 km considered previously) are ingassed with water. Second, instead of fully saturating the rocks based on their solubility maps, they are partially saturated to control the input of surface water into the lithosphere. Third, different partition coefficients for water are considered: 0.01 for pyrolite to basalt melting and 0.25 for basalt to TTG melting. These changes help in increasing the surface mobility, cooling down the planet and producing more continental crust. These trends are further amplified in models that additionally consider a viscosity reduction of mantle materials in the presence of water.

The Earth is the only planet in our Solar System with active plate tectonics. Answering questions such as why plate tectonics started on Earth and which tectonic regime came before are fundamentally important for understanding the evolution of the early Earth. Currently, the most popular answers are (i) before plate tectonics on Earth, there was stagnant lid or plutonic-squishy-lid tectonic regime with no or minor contribution of subduction; (ii) plate tectonics took over when the initially high mantle temperature on Earth dropped by 100-200K due to secular cooling. In this work, we challenge both these statements based on published and new data and models.Plenty of observations suggest that plate tectonics started on Earth during mid-Archean. At the same time, the numerical models and petrological data on the thermal evolution of the Earth show that mid-Archean was likely the time of the highest temperature of the Earth’s mantle and that significant secular cooling took place later in the Proterozoic. Moreover, previously published and our new thermochemical models also suggest that among all proposed tectonic regimes, only mobile-lid regime (i.e. plate tectonics) can lead to significant cooling of the Earth. Therefore, we conclude that plate tectonics in mid- or early Archean was unlikely to be initiated due to the significant secular cooling of the Earth’s mantle. The existence of no-subduction regimes, such as stagnant-lid or plutonic-squishy-lid, prior to plate tectonics are challenged by the new geochemical data which suggest extensive subduction and continental crust production already in the Hadean and early Archean.Here we present global geodynamic models of Earth’s evolution computed using StagYY and ASPECT codes in 2D spherical annulus and 3D geometries respectively. StagYY models suggest that during the Hadean and the early Archean, the tectonic regime was oscillating between plume-induced subduction and plutonic-squishy-lid. The mantle temperature remains high during this time, but significant amount of continental crust is produced in these models, which is in agreement with the new geochemical data (melt inclusions from Weltevreden komatiites). After the emergence of continents in the mid- to late Archean, we decrease the effective friction of the oceanic lithosphere in the models to mimic the lubricating effect of continental sediments in subduction channels. This leads to a transition of the tectonic regime from oscillatory to continuous mobile-lid and to an efficient secular cooling of Earth, which is consistent with petrological observations. Being 2D, all plume-induced subduction zones in StagYY models are global. With the preliminary 3D ASPECT models, we show how a number of plume-induced regional subduction zones in early Earth evolve into a global network of plate boundaries and result in plate tectonics.

Releasing and restraining bends are complementary features of continental strike-slip faults. The Dead Sea Basin of the strike-slip Dead Sea Fault is a classical example of a releasing bend with an asymmetric, deep basin structure. However, the intrinsic relationship to its northern counterpart, the restraining bend that created the Lebanese mountains, remains unclear.Here, we present 3D coupled geodynamic and landscape evolution models that include both the releasing and the restraining bend in a single framework. These simulations demonstrate that the structural basin asymmetry is a consequence of strain localization processes, while sediments control the basin depth. Local extension emerges due to strength heterogeneities and a misalignment of faults and the overall stress field in an area where regional tectonics are dominated by strike-slip motion. Furthermore, we reveal a crustal thinning and thickening pattern that intensifies with surface process efficiency. Along-strike deformation is linked through coupled crustal flow driven by gravitational potential energy which is opposed by deposition at the releasing bend and enhanced by erosion around the restraining bend. Due to the generic nature of our models, our results provide templates for the evolution of fault bends worldwide.

Magmas from mantle plumes are potentially the best monitors of Earth's compositional and thermal evolution over time. However, their erupted products are commonly modified by syn- and post-magmatic processes and thus do not fully retain original information about their mantle sources. Such data can be recovered from melt inclusions in olivine phenocrysts in the most primitive magmas from mantle plumes. Such inclusions, shielded by host olivine, retain original isotopic and critical trace element signatures of deep mantle sources even for Archean and Hadean Eons.We will present the results of a study of chemical and Rb-Sr isotope composition (EPMA, LA-ICP-MS and RAMAN) of melt inclusions and chemical (EPMA, LA-ICP-MS) compositions of host olivines for komatiites and plume-related picrites with eruption age from 3.3 Ga to 1 Ka.Recent advances in in-situ split stream LA-ICP-MS measurements of 87Sr/86Sr ratios and trace element contents of olivine-hosted melt inclusions revealed significant mantle source heterogeneities of magmas from individual plumes. The results are confirmed by geodynamic modelling (Jain et al., this meeting).We show that the melt inclusions of most studied mantle plumes display heterogeneous populations in age-corrected 87Sr/86Sr ratios and include groups with model ages more than 1 Ga older than the emplacement age. The oldest inclusion groups found in Archean komatiites correspond to Hadean (4.3±0.2Ga, Vezinet et al., in review) and Eo-Paleoarchean (3.6±0.2 Ga) model ages. These and most inclusions from studied komatiites and picrites display Nb/U, Nb/Th and Ce/Pb significantly higher than in BSE.Evolution over time of canonical proxies of continental crust generation (Nb/U, Th/U and Ce/Pb, Hofmann et al., 1986) in mantle plumes, combined with geodynamic modelling, suggests:Most of the continental crust was generated in several Hadean and Archean pulses by plume-induced subduction and melting of the hydrated mafic/ultramafic crust or mantle. Hadean continental crust was subducted or/and reworked. Restites left after extraction of continental crust were continuously subducted to the core-mantle boundary from the mid-Hadean and later recycled in Archean mantle plumes. Active formation of both continental and oceanic crust in Hadean was governed by plume-induced subduction, which ceased after cold subducted material hindered the propagation of large plumes at the core-mantle boundary. After heating the recycled lithosphere at the core-mantle boundary, the process repeats, producing oscillating subduction and crustal formation in Hadean-Archean.

The evolution of modern plate tectonics is described by the Wilson cycle, which portrays the dynamics of the supercontinental cycle through the interaction of the oceanic plate with the continental plate over periods of hundreds of millions of years. This cycle is characterized by a phase of supercontinent assembly and enhanced orogenic collision, followed by a phase of supercontinent fragmentation and dispersal, as shown by the geological record. The dynamics of the Wilson cycle is intrinsically linked to mantle convection and subduction dynamics. While the assembly phase appears to follow a degree-2 mantle convection style, the mechanism responsible for supercontinent fragmentation is still debated. We hypothesize that the dispersal phase is mostly governed by trench roll-back from subductions and mantle plumes. To test this hypothesis, we have built a series of 2D and 3D geodynamic models of the Earth on a global scale using the ASPECT code. We have tested different scenarios in which we prescribe the distribution of the supercontinent Rodinia at 1Ga or Pangea at 250 Ma and let the models evolve self-consistently. In some model variants, the strength of the supercontinent and that of the surrounding oceanic area is changed. We will present our preliminary results and discuss the dynamics of continental dispersal and its link to subduction and mantle dynamics. In particular, 3D models will demonstrate how regional plume-induced retreating subduction zones evolve into a global network of subduction zones and tectonics plate boundaries which ultimately leads to the break-up of the supercontinent.