ORCID Profile
0000-0003-2252-2451
Current Organisation
University of Oslo - Centre for Earth Evolution and Dynamics (CEED)
Does something not look right? The information on this page has been harvested from data sources that may not be up to date. We continue to work with information providers to improve coverage and quality. To report an issue, use the Feedback Form.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-6948
Abstract: It is well-established that the mantle exerts a strong control on the geodynamic processes and their expressions observed at the surface. One of the key components in mantle-surface interaction is rheology, yet, the details of this rheological coupling remain not well understood. This is particularly true for large to global scales, which are difficult to assess in the field or in the lab. For instance, rheological inheritance is thought to influence the onset location of plate boundaries and their subsequent evolution. But the originating physical processes and timescales over which such rheological inheritance applies are still debated. Moreover, differences in the rheological coupling between mantle and surface are expected to change first-order surface observables, such as topography.Here, we use numerical geodynamic models of whole-mantle convection to test the sensitivity of surface tectonics to different rheological assumptions, for instance in terms of deformation mechanisms at play and rheological inheritance. We show that the self-consistent generation of plate tectonics from mantle convection is altered by the use of a composite rheology (with co-existing diffusion and dislocation creep), by the consideration of mantle grain-size evolution, as well as by simple parameterisations of rheological memory such as strain-weakening. Using a set of quantitative diagnostics, we also demonstrate how such rheological complexities affect surface topography.
Publisher: Authorea, Inc.
Date: 03-2023
DOI: 10.22541/ESSOAR.167768126.61592439/V1
Abstract: Our understanding of Earth’s paleogeography relies heavily on paleomagnetic apparent polar wander paths (APWPs), which represent the time-dependent position of Earth’s spin axis relative to a given block of lithosphere. However, conventional approaches to APWP construction have significant limitations. First, the paleomagnetic record contains substantial noise that is not integrated into APWPs. Second, parametric assumptions are adopted to represent spatial and temporal uncertainties even where the underlying data do not conform to the assumed distributions. The consequences of these limitations remain largely unknown. Here, we overcome these challenges with a bottom-up Monte Carlo uncertainty propagation scheme that operates on site-level paleomagnetic data. To demonstrate our methodology, we present an extensive compilation of site-level Cenozoic paleomagnetic data from North America, which we use to generate a high-resolution APWP. Our results demonstrate that even in the presence of substantial noise, polar wandering can be assessed with unprecedented temporal and spatial resolution.
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-4083
Abstract: & & Global scale temporal assignments for planetary surfaces are made based on craters statistics. Age assignments require a cratering chronology model that links normalized crater frequencies and s le ages. A challenge is that rocks on planetary surface may not belong to the site from which they are collected. We use detailed spectral mapping, cratering statistics and impact basin ejecta column estimates to suggest stratigraphic relationships for lunar s ling sites, and most specifically the Apollo 14 landing site. We confirm previously recognised resurfacing in the crater size-frequency distribution in agreement with a single blanketing layer. Based on our and previously published crater statistics, and mineral relation derived from spectral data, we suggest a cratering chronology model that effectively increase the surface age of the Moon. In this presentation, we will document and outline our reasons and data. This change in cratering chronology model implies significant changes for the early solar system evolution time-scale assessment, small body flux and geological dates and rates on all planetary surface when crater statistics is used to determine ages.& &
Publisher: Copernicus GmbH
Date: 23-09-2022
DOI: 10.5194/EPSC2022-621
Abstract: & & & strong& Introduction& & & & & & & & & & & & & & /strong& The surfaces of Venus and Earth display striking differences in geology, tectonism, and volcanic activity, which is particularly intriguing given the similar radius and bulk composition of the two planets. The most evident difference is the lack of well-developed plate tectonics on Venus & em& [e.g., 1]& /em& . However, Venus& #8217 tectonic mode also differs from that of the classical stagnant-lid bodies & #8211 Mars, Mercury and the Moon & em& [2]& /em& . A profound description and definition of Venus& #8217 tectonic regime remains to be made & em& [3]& /em& , but it has to explain Venus& #8217 relatively young and uniformly-aged surface under the condition of rather limited large-scale horizontal surface motion.& & & & One promising hypothesis to explain the lack of mobile lid tectonics on Venus is that rheological healing is enhanced on Venus& #8217 surface, because the high surface temperature causes faster growth of grains. Faster healing causes more rapid recovery from previous deformation that caused grain shrinking and weakening of crustal rocks & em& [e.g., 4]& /em& . In principle, this should the planet to form a global network of coherent plates. Although this possibility has been demonstrated with theoretical scalings and thin-sheet modelling & em& [4]& /em& , no systematic study exists to investigate this process in fully dynamic models of mantle convection that self-consistently generate a range of tectonic modes relevant for Venus and Earth& em& [e.g., 5,6]& /em& . Using such models coupled to grain size evolution (GSE), we test the hypothesis that enhanced grain growth & #8211 due to higher, Venusian surface temperature & #8211 is capable of shutting down mobile lid tectonics.& & & & & & & & & & strong& Methods & /strong& & & & & & & & To test our hypothesis, we compute 2D models of mantle flow coupled to GSE in spherical annulus geometry using StagYY & em& [7]& /em& in the extended Boussinesq approximation. We employ a rheology that accounts for diffusion and dislocation creep in a composite Arrhenius formulation, depending on stress and grain size. In addition, the models feature a yield stress, the maximum stress rocks can sustain before deforming plastically. Upon reaching it, the model rheology is dominated by pseudoplastic yielding, leading to locally reduced viscosity as described in previous works & em& [see e.g., 5,6].& /em& Grain size & em& (D)& /em& is tracked using Lagrangian tracers following the equation & em& [8]& /em& :& & & & & img src=& quot & quot alt=& quot & quot width=& quot & quot height=& quot & quot /& & & & & where& em& & k& /em& & em& & & /em& and & em& c& /em& are (semi-)empirical factors for grain growth and reduction, respectively. & em& E& /em& & is the activation energy controlling the thermal sensitivity of the grain growth, & em& & /em& & is the grain growth exponent, & em& f& /em& & is the temperature-dependent fraction of the dissipation & em& & #968 & /em& that is used to reduce grain size. In our systematic investigation, we vary the GSE parameters as well as the surface temperature of the planetary body to predict interior dynamics and surface tectonic modes as a function of the lithospheric yield stress. We then evaluate how applicable the models are to Earth and Venus.& & & & & & & & & & strong& Results& /strong& & & & & & & & & & & An ex le model assuming Earth-like surface temperature & #8211 representative of the Earth-like mobile lid regime & #8211 is shown in Figure 1. This model almost continuously features at least one site of major subduction, which cools the mantle to comparably moderate internal temperature. Low grain size is obtained mainly in subduction zones, while high grain size is characteristic of (hot) plumes. As a result, viscosity variations are smoothened with respect to models without grain size. An exception to that correlation is the low viscosity area below plates, where high temperatures correlate with low grain sizes, resulting in a weaker asthenosphere.& & & & & img src=& quot & quot alt=& quot & quot width=& quot & quot height=& quot & quot /& & & & & & strong& & em& Figure 1& /em& & /strong& & em& : Snapshot of the (left) viscosity and (right) grain size field for a simulation with low surface temperature, representative of the mobile-lid regime. Values are dimensionless and are with respect to reference values of 6.21 Pa s for viscosity and 2.89x10& sup& & /sup& m for grain size.& /em& & & & & Upon increasing the yield stress, the simulations promote a more episodic and, eventually, a stagnant-lid behavior with a continuously immobile lithosphere, as it has been described before. However, the GSE parameters affect the transition from the mobile to stagnant via the episodic regime. In particular, episodic subduction occurs preferentially when grain growth is boosted with respect to grain reduction (increased ). However, neither enhanced grain reduction (increased ) nor enhanced grain growth seem to strongly change the critical yield stress for entering the stagnant lid regime& & & & Cases with substantially higher surface temperature (as relevant for Venus) are to be performed for this abstract. According to the GSE equation, higher & should boost grain growth and therefore enhance episodicity. However, effects of increasing & are expected to differ from that of increasing & as above, because the former may have less impact on the deeper mantle than the latter, with possible implications for the mantle-lithosphere coupling. These differences will be presented.& & & & & & & & & & strong& Link to future Venus missions& /strong& & & & & & & & & & & & & & Our study aims to shed light on the characteristics of Venus& #8217 tectonic regime and its thermomechanical origin, thereby pointing to potential differences in lithospheric strength and structure as well as in plate-mantle coupling on Venus and Earth. The array of missions during the upcoming decay of Venus will deliver new observables linked to lithospheric structure, which will help to constrain such generic numerical models. In particular, VERITAS aims to map & #8211 structurally and compositionally & #8211 the surface of Venus, which will help to infer the stress state of the lithosphere and the interior of the planet & em& [9,10]& /em& . Regardless, many other lithospheric and upper mantle characteristics will remain difficult to pinpoint with the next missions. Therefore, modelling studies such as ours provide important complementary insight, in particular with respect to Venus& #8217 evolution to its current state.& & & & & & & & & & strong& References& /strong& & & & & & em& [1] Phillips & Hansen, 1994, Annu. Rev. EPS 22, 597-654 [2] Tosi & Padovan, 2020, AGU Geophysical Monograph, 263, 455-489 [3] Byrne, 2021, PNAS 118 [4] Bercovici & Ricard, 2014, Nature, 508, 513-516 [5] Armann & Tackley, 2012, JGR Planets, 117(2) [6] Rolf et al., 2018, Icarus, 313, 107-123, [7] Tackley, 2008, PEPI, 171, 7-18 [9] Freeman et al., 2016, IEEE [10] Cascioli et al., 2021, The Planetary Science Journal, 2(6)& /em& & &
Publisher: Authorea, Inc.
Date: 04-05-2023
DOI: 10.22541/ESSOAR.168319739.97166236/V1
Abstract: Earthâ\\euro™s upper mantle rheology controls lithosphere-asthenosphere coupling and thus surface tectonics. Rock deformation experiments and seismic anisotropy measurements indicate that composite rheology (co-existing diffusion and dislocation creep) occurs in the Earth’s uppermost mantle, potentially affecting convection and surface tectonics. Here, we investigate how the spatio-temporal distribution of dislocation creep in an otherwise diffusion-creep-controlled mantle impacts the planform of convection and the planetary tectonic regime as a function of the lithospheric yield strength in numerical models of mantle convection self-generating plate-like tectonics. The low upper-mantle viscosities caused by zones of substantial dislocation creep produce contrasting effects on surface dynamics. For strong lithosphere (yield strength $ $35 MPa), the large lithosphere-asthenosphere viscosity contrasts promote stagnant-lid convection. In contrast, the increase of upper mantle convective vigor enhances plate mobility for lithospheric strength $ $35 MPa. For the here-used model assumptions, composite rheology does not facilitate the onset of plate-like behavior at large lithospheric strength.
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-15790
Abstract: & & Earth& #8217 s lithospheric behavior is tied to the properties and dynamics of mantle flow. In particular, upper mantle rheology controls the coupling between the lithosphere and the asthenosphere, and therefore partly dictates Earth& #8217 s tectonic behavior. It is thus important to gain insight into how Earth& #8217 s upper mantle deforms in order to understand the evolution of plate tectonics. The presence of seismic anisotropy in the uppermost mantle suggests the existence of mineral lattice-preferred orientation (LPO) caused by the asthenospheric flow. Together with laboratory experiments of mantle rock deformation, this indicates that Earth& #8217 s uppermost mantle can deform in a non-Newtonian way, through dislocation creep. Although such a deformation mechanism can significantly impact both mantle flow and the surface tectonic behavior, most numerical studies of whole-mantle convection use a viscoplastic rheology involving diffusion creep as the only deformation mechanism in the mantle.& & & & Here, we investigate the effects of using a composite rheology (with both diffusion and dislocation creep) on the surface tectonic behavior in 2D-cartesian whole-mantle convection models that self-consistently generate plate-like tectonics. We vary the proportion of dislocation creep in the mantle by imposing different temperature- and depth-dependent transitional stresses between diffusion and dislocation creep. Using different yield stresses, we investigate how the amount of dislocation creep affects the planform of convection and promotes surface plate-like or stagnant-lid behavior. In particular, we show that for a given yield stress promoting plate-like behavior in diffusion-creep-only models, a progressive increase in the amount of dislocation creep affects the shape and dynamics of slabs, eventually leading to stagnant-lid convection. We discuss the spatio-temporal distribution of dislocation creep in the mantle in light of the observed geometry of slabs and the spatial distribution of seismic anisotropy in Earth& #8217 s upper-mantle.& &
Publisher: Springer Science and Business Media LLC
Date: 28-11-2022
DOI: 10.1007/S11214-022-00937-9
Abstract: The dynamics and evolution of Venus’ mantle are of first-order relevance for the origin and modification of the tectonic and volcanic structures we observe on Venus today. Solid-state convection in the mantle induces stresses into the lithosphere and crust that drive deformation leading to tectonic signatures. Thermal coupling of the mantle with the atmosphere and the core leads to a distinct structure with substantial lateral heterogeneity, thermally and compositionally. These processes ultimately shape Venus’ tectonic regime and provide the framework to interpret surface observations made on Venus, such as gravity and topography. Tectonic and convective processes are continuously changing through geological time, largely driven by the long-term thermal and compositional evolution of Venus’ mantle. To date, no consensus has been reached on the geodynamic regime Venus’ mantle is presently in, mostly because observational data remains fragmentary. In contrast to Earth, Venus’ mantle does not support the existence of continuous plate tectonics on its surface. However, the planet’s surface signature substantially deviates from those of tectonically largely inactive bodies, such as Mars, Mercury, or the Moon. This work reviews the current state of knowledge of Venus’ mantle dynamics and evolution through time, focussing on a dynamic system perspective. Available observations to constrain the deep interior are evaluated and their insufficiency to pin down Venus’ evolutionary path is emphasised. Future missions will likely revive the discussion of these open issues and boost our current understanding by filling current data gaps some promising avenues are discussed in this chapter.
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-9133
Abstract: & & The core-mantle boundary (CMB) is the most prominent compositional boundary inside the Earth. Its topography provides insight on lower mantle flow and the thermochemical structure above the CMB. Yet, CMB topography remains challenging to observe and estimates from seismology vary substantially. Numerical models of mantle convection provide complementary means to estimate CMB topography. Classically, topography is determined from the normal stresses acting on the CMB. However, this is known to face severe complications when applied to the surface boundary of the mantle, leading to non-Earth-like topographic scales and a different style of subduction. A (quasi-)free surface yields more Earth-like predictions, but for the CMB this comparison has never been made.& & & & Here, we compare CMB topography predicted from mantle convection modelling using different treatments of the CMB. Specifically, we test the role of a & #8216 sticky core& #8217 , a quasi-fluid approximation the core. We compare results predicted by different codes (with either sticky core or true free base) and compare to a simple analytical case. Also, we simulate the evolution of subduction and deep thermochemical provinces to compare the topography of the (quasi-)free CMB and the free-slip approach. Initial results indicate that the sticky core approach can reproduce CMB topography reasonably well, but has rather high computational cost (grid resolution, number of particles). In analogy to the sticky air at the surface, the viscosity contrast of the sticky core layer determines the quality of predicted topography, with larger contrasts (& #8805 & sup& & /sup& ) leading to acceptable levels of artificial CMB topography. In dynamic flow cases with vigorous mantle convection, entrainment by plumes further complicates application of the sticky core, but can be tackled with an unmixing procedure. A true free base tends to better accuracy than the sticky core approach and avoids the problem with entrainment, but it also comes with additional computational costs as various forces at the CMB have to be taken into account.& &
Publisher: Copernicus GmbH
Date: 03-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-540
Abstract: & & Earth's tectonic evolution and its link to global mantle dynamics are controlled by the pre-existing structure of the lithosphere which guides how strain localizes and causes the necessary weakness to (re-)activate plate boundaries. Recent models of global-scale mantle convection have self-consistently reproduced Earth-like tectonic regimes, consistent with several aspects of today& #8217 s observed tectonics. In many cases these models ignore the memory on pre-existing deformation though. Here, a mantle convection model is advanced to include the associated rheological inheritance via a parameterization of strain-induced plastic (brittle) weakening. Based on more than 180 simulations in a wide 2D cartesian box, the control of strain-induced weakening on the resulting tectonic regime is demonstrated. Strain-induced brittle weakening impacts the stability fields of the different tectonic regimes observed, but to first order it does not generate new tectonic regimes or change the dynamics of a given regime (e.g., its characteristic surface mobility). A time-dependent plate-like regime similar to Earth's becomes more feasible with decreasing critical strain at (and above) which maximum weakening is observed. It is less feasible with increasing temperature-dependence of the healing rate, but remains a possibility at small critical strain. While the critical yield stress that still allows for plate-like behavior is apparently larger with strain-induced weakening considered, the effective shift (incorporating the yield stress reduction due to strain weakening) is relatively small and only about 10% under the tested conditions. Strain accumulation in stable continental lithosphere is generally small because of the necessity of high rheological strength. This holds true even for continental collision events, although at least some strain is accumulated and preserved following such events in the immediate proximity of the colliding continental margins.& & & & & & / &
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-6653
Abstract: & & The last few years have been marked by a number of motivating novel ideas and methodological advancements in paleomagnetic analysis (e.g. trans-hierarchical uncertainty propagation), observational and theoretical geodynamics, and paleogeographical modeling (e.g. optimisation and Bayesian approaches). Many of these developments offer new insights on, and/or approaches to estimating, the past motions of tectonic plates& #8212 but so far these developments have largely unfolded in isolation of one another. In November 2021 an international group of 15 young scientists with highly complementary backgrounds (spanning the aforementioned fields) gathered to explore and discuss these exciting new developments and to brainstorm strategies that may enable their integration. We anticipate that the integration of these erse new ideas and methods will open new frontiers in plate tectonic research, and notably lead to much better-constrained paleogeographic models. In this presentation, we will share some of the insights and strategies that emerged from the workshop, including the advantages of conducting paleomagnetic analysis at the site-level, the application of emerging paleomagnetic Euler pole analysis frameworks, and the use of insights extracted from Earth-like geodynamic models (which self-generate plate tectonic behavior) to further constrain the results of these paleomagnetic methods. We also present some preliminary results of early experiments putting these strategies into practice on a paleomagnetic dataset from North America.& & & &
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-9112
Abstract: The origin of the observed differences between Earth and Venus remains a mystery. On Earth, surface deformation is focused at narrow plate margins resulting in plate tectonics (or a mobile-lid regime). On Venus, a global network of connected plate margins is absent, but the surface is young and has preserved evidence of at least regional crustal mobility. Therefore, the planet must be in a yet-to-be-defined regime distinct from plate tectonics, for ex le an episodic-lid regime. The array of Venus missions planned for the next decade provides us with an unprecedented chance to refine our knowledge of this tectonic regime, but to use the upcoming data, we need hypotheses to test and a physical framework in which to contextualize the data. To explain the discrepancy on the tectonic regime, a popular hypothesis is that Venus& #8217 higher surface temperatures foster a stiffer lithosphere due enhanced grain growth. Thermally assisted grain growth is supposed to increase the lithospheric viscosity, since diffusion creep depends on grain size, and therefore subduction becomes less efficient. In a previous work [Manj& #243 n-Cabeza C& #243 rdoba, A., Rolf, T., and Arnould, M: Feasibility of the mobile-lid regime controlled by grain size evolution. EGU General Assembly 2022], we showed that high grain reduction can decrease the interval of yield stresses for which the episodic regime applies, but the results on grain growth were not too conclusive. Here, we present a new set of convection models in spherical annulus geometry using different surface temperatures to specifically address the differences between Earth and Venus. Our results suggest that the effect of the climate thermal runaway depends on the strength of the lithosphere. For yield stresses that yield Earth-like behaviors at lower surface temperatures, an increase in surface temperature does not result in the episodic regime, but rather a sluggish-dripping regime with relatively low plateness. We conclude that either Venus is not in an episodic-regime, or a different explanation must be put forward for the tectonic regime of Venus (e.g., lack of liquid water at the surface).
Publisher: Springer Science and Business Media LLC
Date: 10-2023
Publisher: American Astronomical Society
Date: 08-2023
DOI: 10.3847/PSJ/ACDC16
Abstract: Times and rates of planetary surface-modifying processes are crucial for the sequence and correlation of events on planetary bodies. For most planetary surfaces, superposition principles and crater densities are commonly used methods to collect relative age information. Lunar-based cratering-chronology models, which pair crater densities and s le ages from several lunar landing and s ling sites, calibrate the relative age information in absolute time. Here, we propose calibration pairs based on new crater statistics and spectrally supported s le-age assignments for the lunar cratering-chronology model. The resulting model reflects modern high-precision, radiometric ages, compositional and spectral information, and an up-to-date crater-production function. This revision supports a crater-forming projectile flux with monotonic decay, similar to previous standard models, but of distinctively lower flux. This originates from lower crater densities identified in spectrally and morphologically defined reference units, and from assigning more precise s le ages accounting for spectral resemblance between reference unit and s le. The observed maximal values for crater densities and ages provide the oldest and most densely cratered calibration pair. Because of the nature of highland s les, age constraints for the Luna 20 and Apollo 16 s ling sites remain challenging, which restricts the confidence in times for in idual basin-formation events older than Orientale Basin. The new cratering-chronology model, when transferred to other planetary bodies, would cause aging of the surfaces, because of the lower overall flux.
Publisher: American Geophysical Union (AGU)
Date: 08-2017
DOI: 10.1002/2017JE005283
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-11133
Abstract: & & Earth& #8217 s surface dynamics and topography are tied to the properties and dynamics of mantle flow. In particular, upper mantle rheology controls the coupling between the lithosphere and the asthenosphere, and therefore partly dictates Earth& #8217 s surface tectonic behaviour and topographic response to mantle convection (dynamic topography). The presence of seismic anisotropy in the uppermost mantle suggests the existence of mineral lattice-preferred orientation (LPO) caused by the asthenospheric flow. Together with laboratory experiments of mantle rock deformation, this indicates that Earth& #8217 s uppermost mantle can deform in a non-Newtonian way, through dislocation creep. Although several studies suggest the potentially significant effect of upper-mantle non-Newtonian rheology on mantle convection (e.g. Schulz et al., 2020) and topography (e.g. Asaadi et al., 2011, Bodur and Rey, 2019), it is usually not considered in whole-mantle models of mantle convection self-generating plate tectonics.& & & & & & & & & Here, we investigate the effects of using a composite rheology (with both diffusion and dislocation creep) on surface tectonics and dynamic topography in 2D-spherical annulus models of mantle convection with plate-like tectonics and continental drift. We systematically vary the amount of dislocation creep by changing the activation volume for dislocation creep and the reference transition stress between diffusion and dislocation creep. We show that for low yield stresses promoting plate-like behavior in diffusion-creep-only models, modeling a composite rheology in the mantle favors more surface mobility while for large yield stresses which still generate plate-like motions in diffusion-creep-only models, a progressive increase in the amount of dislocation creep leads to stagnant-lid convection. We then compare the litudes and spatio-temporal distribution of dynamic topography in models with and without dislocation creep, in light of observed Earth present-day residual topography characteristics.& & & & & & & & & References:& & & & Schulz, F., Tosi, N., Plesa, A. C., & Breuer, D. (2020). Stagnant-lid convection with diffusion and dislocation creep rheology: Influence of a non-evolving grain size. & em& Geophysical Journal International& /em& , & em& & /em& (1), 18-36.& & & & Asaadi, N., Ribe, N. M., & Sobouti, F. (2011). Inferring nonlinear mantle rheology from the shape of the Hawaiian swell. & em& Nature& /em& , & em& & /em& (7348), 501-504.& & & & Bodur, & #214 . F., & Rey, P. F. (2019). The impact of rheological uncertainty on dynamic topography predictions. & em& Solid Earth& /em& , & em& & /em& (6), 2167-2178.& &
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-7226
Abstract: Earth& #8217 s upper mantle rheology controls lithosphere-asthenosphere coupling and thus its surface tectonics. Although rock deformation experiments and seismic anisotropy measurements indicate that dislocation creep can occur in the Earth's uppermost mantle, the role of composite rheology (including both diffusion and dislocation creep) on global-scale mantle dynamics and surface tectonics remains largely unexplored.Here, we investigate the influence of composite rheology on the planform of convection and on the planetary tectonic regime as a function of the lithospheric yield strength in numerical models of mantle convection with plate-like tectonics. We show that the consideration of composite rheology in the upper mantle leads to the self-generation of a discontinuous asthenosphere evolving fast, with a low-viscosity and a maximal thickness that depend on the rheological parameters for diffusion and dislocation creep. In mobile-lid models, the spatio-temporal evolution of the asthenosphere is mainly controlled by the location of slabs and plumes that generate regions of mantle deforming dominantly through dislocation creep. Moreover, the low upper-mantle viscosities caused by composite rheology produce substantial and contrasting effects on surface dynamics. For a strong lithosphere (high yield stress), the large lithosphere-asthenosphere viscosity contrasts promote stagnant-lid convection, while the increase of upper-mantle convective vigor enhances plate mobility for low lithospheric strength (small yield stress). We further show that composite rheology does not facilitate the onset of plate-like behavior at large lithospheric strength due to decoupling between the asthenosphere and the lithosphere.
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-3889
Abstract: & & One of the most discussed issues of whole-mantle geodynamic models is the need of an 'ad hoc' yield stress which is lower than any strength measurement of natural s les in the brittle or plastic regimes. It is commonly believed that grain size evolution, in particular grains size reduction due to dynamic recrystallization, may decrease the strength of the lithosphere and therefore aid the onset and persistence of the mobile-lid regime. In this work, we carry out an investigation of 2D whole-mantle annulus models with varying yield stress. We compare cases with different grain growth and grain reduction parameters to cases with constant grain size to make inferences on the feasibility of a plate-like convective regime as a function of the yield strength of the lithosphere.& & & & Our results show that viscosity profiles of models with dynamic grain-size evolution are inherently different to those with constant grain size, and that those profiles vary little when changing grain-size evolution parameters. In this context, the lower mantle shows greater variations in viscosity than the upper mantle: with viscosity contrasts between upper and lower mantle and plume widths comparable to those of the Earth, in particular in models with enhanced grain growth. Furthermore, our models show that, while enhancing grain size reduction reduces episodicity and increases mobility up to some point, increasing grain growth favors mobile-lid convection even more. This is at odds with previous conceptions of the grain-size-evolution-induced mobile-lid regime, where grain groth should promote healing of the lithosphere and therefore inhibit subduction. We hypothesize that increased stiffness of the bottom of the lithosphere, together with a more viscous lower mantle, are the main reasons for the grain-grouth-favored mobile-lid regime.& &
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-10000
Abstract: Our understanding of paleogeography through Earth history relies heavily on apparent polar wander paths (APWPs), which represent the time-dependent position of Earth& #8217 s spin axis relative to a given tectonic plate. However, there are a number of limitations associated with conventional approaches to APWP construction. First, the paleomagnetic record contains significant uncertainty in in idual pole positions that is not propagated into APWPs. This traditional approach makes it difficult to incorporate age and positional uncertainty into synthesized paths and assigns equal weight to paleomagnetic poles with vastly different numbers of underlying sites. Second, the effective propagation of site-level uncertainties into the APWP requires a transformation that renders traditional parametric assumptions (i.e., Fisher statistics) on the pole level ineffective. Here, we overcome these limitations with a bottom-up Monte Carlo uncertainty propagation scheme that operates on site-level paleomagnetic data. To demonstrate our methodology, we present a large compilation of site-level Cenozoic paleomagnetic data from North America, which we use to generate a high-resolution APWP. We show that even in the presence of significant noise, polar wandering can be assessed with unprecedented temporal and spatial resolution.
Publisher: American Geophysical Union (AGU)
Date: 06-2018
DOI: 10.1029/2017JE005463
Publisher: Springer Science and Business Media LLC
Date: 31-10-2023
Publisher: American Geophysical Union (AGU)
Date: 12-08-2023
DOI: 10.1029/2023GL104146
Abstract: Earth's upper mantle rheology controls lithosphere‐asthenosphere coupling and thus surface tectonics. Rock deformation experiments and seismic anisotropy measurements indicate that composite rheology (co‐existing diffusion and dislocation creep) occurs in the Earth's uppermost mantle, potentially affecting convection and surface tectonics. Here, we investigate how the spatio‐temporal distribution of dislocation creep in an otherwise diffusion‐creep‐controlled mantle impacts the planform of convection and the planetary tectonic regime as a function of the lithospheric yield strength in numerical models of mantle convection self‐generating plate‐like tectonics. The low upper‐mantle viscosities caused by zones of substantial dislocation creep produce contrasting effects on surface dynamics. For strong lithosphere (yield strength 35 MPa), the large lithosphere‐asthenosphere viscosity contrasts promote stagnant‐lid convection. In contrast, the increase of upper mantle convective vigor enhances plate mobility for lithospheric strength MPa. For the here‐used model assumptions, composite rheology does not facilitate the onset of plate‐like behavior at large lithospheric strength.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-6132
Abstract: Europa& #8217 s outermost layer is a shell of water ice with a probable thickness of a few to a few dozens of km. It is most likely underlain by a liquid water ocean in direct contact with mantle rock, which makes Europa a prime target for understanding habitability. Europa& #8217 s surface is heavily deformed and the mean surface age is low ( ~100 Myr), implying active resurfacing, perhaps even through subduction-like processes. While this requires future confirmation, convection in Europa& #8217 s icy shell is a viable mechanism to drive such processes. However, the pattern of convection and its link to resurfacing is poorly understood. Here, we use 2D numerical simulations to shed light on these aspects and implement a composite rheology featuring the different slip mechanisms potentially relevant for ice: diffusion creep, basal slip (BSL), grain-boundary sliding (GBS), and dislocation creep. We couple this to grain-size evolution (GSE) and test in basally and mixed basally-tidally heated cases in a 20 km-thick shell the parameters governing the deformation mechanism and GSE.Without imposing a yield stress to modulate pseudo-plastic deformation, we typically observe an immobile layer at the top of the ice shell. This layer tends to deform via GBS/BSL and features very small grain-sizes ( & #181 m), while grains are on the order of cm in the warmer deeper parts, due to stronger grain growth. The thickness of the immobile layer decreases with enhancing the rate of tidal heating and with the sensitivity of grain growth to temperature variations. The immobile layer is thinnest (10-20% of the total thickness), if grain growth in the interior is only moderately enhanced compared to the cold shallow parts, while a large contrast in grain growth increases the layer thickness until eventually convection in the ice shell ceases completely. The omnipresence of an immobile layer (no matter how thick) appears at odds with Europa& #8217 s strongly deformed surface and its low age, unless other processes can explain this aspect. Preliminarily, mobilization of the surface layer is possible in our models by imposing a small finite yield stress. Using a very low coefficient of friction, surface velocities can reach rates of up to tens of centimeters per year, under which the surface would be recycled efficiently. & & &
Publisher: American Geophysical Union (AGU)
Date: 06-06-2023
DOI: 10.1029/2023GL103436
Abstract: Our understanding of Earth's paleogeography relies heavily on paleomagnetic apparent polar wander paths (APWPs), which represent the time‐dependent position of Earth's spin axis relative to a given block of lithosphere. However, conventional approaches to APWP construction have significant limitations. First, the paleomagnetic record contains substantial noise that is not integrated into APWPs. Second, parametric assumptions are adopted to represent spatial and temporal uncertainties even where the underlying data do not conform to the assumed distributions. The consequences of these limitations remain largely unknown. Here, we address these challenges with a bottom‐up Monte Carlo uncertainty propagation scheme that operates on site‐level paleomagnetic data. To demonstrate our methodology, we present an extensive compilation of site‐level Cenozoic paleomagnetic data from North America, which we use to generate a high‐resolution APWP. Our results demonstrate that even in the presence of substantial noise, polar wandering can be assessed with unprecedented temporal and spatial resolution.
Location: Norway
No related grants have been discovered for Tobias Rolf.