Temperature beneath continents as a function of continental cover and convective wavelength

International audienceGeodynamic modeling studies have demonstrated that mantle global warming can occur in response to continental aggregation, possibly leading to large-scale melting and associated continental breakup. Such feedback calls for a recipe describing how continents help to regulate the thermal evolution of the mantle. Here we use spherical mantle convection models with continents to quantify variations in subcontinental temperature as a function of continent size and distribution and convective wavelength. Through comparison to a simple analytical boundary layer model, we show that larger continents beget warming of the underlying mantle, with heating sometimes compounded by the formation of broader convection cells associated with the biggest continents. Our results hold well for purely internally heated and partially core heated models with Rayleigh numbers of 10(5) to 10(7) containing continents with sizes ranging from that of Antarctica to Pangea. Results from a time-dependent model with three mobile continents of various sizes suggests that the tendency for temperatures to rise with continent size persists on average over timescales of billions of years

Thermo-Mechanical Adjustment after Impacts during Planetary Growth

The thermal evolution of planets during their growth is strongly influenced by impact heating. The temperature increase after a collision is mostly located next to the shock. For Moon to Mars size planets where impact melting is limited, the long term thermo-mechanical readjustment is driven by spreading and cooling of the heated zone. To determine the time and length scales of the adjustment, we developed a numerical model in axisymmetric cylindrical geometry with variable viscosity. We show that if the impactor is larger than a critical size, the spherical heated zone isothermally flattens until its thickness reaches a value for which motionless thermal diffusion becomes more effective. The thickness at the end of advection depends only on the physical properties of the impacted body. The obtained timescales for the adjustment are comparable to the duration of planetary accretion and depend mostly on the physical properties of the impacted body

A sequential data assimilation approach for the joint reconstruction of mantle convection and surface tectonics

International audienceWith the progress of mantle convection modelling over the last decade, it now becomes possible to solve for the dynamics of the interior flow and the surface tectonics to first order. We show here that tectonic data (like surface kinematics and seafloor age distribution) and mantle convection models with plate-like behaviour can in principle be combined to reconstruct mantle convection. We present a sequential data assimilation method, based on suboptimal schemes derived from the Kalman filter, where surface velocities and seafloor age maps are not used as boundary conditions for the flow, but as data to assimilate. Two stages (a forecast followed by an analysis) are repeated sequentially to take into account data observed at different times. Whenever observations are available, an analysis infers the most probable state of the mantle at this time, considering a prior guess (supplied by the forecast) and the new observations at hand, using the classical best linear unbiased estimate. Between two observation times, the evolution of the mantle is governed by the forward model of mantle convection. This method is applied to synthetic 2-D spherical annulus mantle cases to evaluate its efficiency. We compare the reference evolutions to the estimations obtained by data assimilation. Two parameters control the behaviour of the scheme: the time between two analyses, and the amplitude of noise in the synthetic observations. Our technique proves to be efficient in retrieving temperature field evolutions provided the time between two analyses is 10 Myr. If the amplitude of the a priori error on the observations is large (30 per cent), our method provides a better estimate of surface tectonics than the observations, taking advantage of the information within the physics of convection

A deep subaqueous fan depositional model for the Palaeoarchaean (3.46 Ga) Marble Bar Cherts, Warrawoona Group, Western Australia

International audienceThe 3.46 Ga Marble Bar Chert Member of the East Pilbara Craton, Western Australia, is one of the earliest and best-preserved sedimentary successions on Earth. Here, we interpret the finely laminated thin-bedded cherts, mixed conglomeratic beds, chert breccia beds and chert folded beds of the Marble Bar Chert Member as the product of low-density turbidity currents, high-density turbidity currents, mass transport complexes and slumps, respectively. Integrated into a channel-levee depositional model, the Marble Bar Chert Member constitutes the oldest documented deep-sea fan on Earth, with thin-bedded cherts, breccia beds and slumps composing the outer levee facies tracts, and scours and conglomeratic beds representing the channel systems

Assessing the role of slab rheology in coupled plate-mantle convection models

International audienceAssessing the role of slab rheology in coupled plate-mantle convection models. Earth and Planetary Science Letters 430, 191-201. Abstract Reconstructing the 3D structure of the Earth's mantle has been a challenge for geodynamicists for about 40 years. Although numerical models and computational capabilities have substantially progressed, parameterizations used for modeling convection forced by plate motions are far from being Earth-like. Among the set of parameters, rheology is fundamental because it defines in a non-linear way the dynamics of slabs and plumes, and the organization of lithosphere deformation. In this study, we evaluate the role of the temperature dependence of viscosity (variations up to 6 orders of magnitude) and the importance of pseudo-plasticity on reconstructing slab evolution in 3D spherical models of convection driven by plate history models. Pseudo-plasticity, which produces plate-like behavior in convection models, allows a consistent coupling between imposed plate motions and global convection, which is not possible with temperature-dependent viscosity alone. Using test case models, we show that increasing temperature dependence of viscosity enhances vertical and lateral coherence of slabs, but leads to unrealistic slab morphologies for large viscosity contrasts. Introducing pseudo-plasticity partially solves this issue, producing thin laterally and vertically more continuous slabs, and flat subduction where trench retreat is fast. We evaluate the differences between convection reconstructions employing different viscosity laws to be very large, and similar to the differences between two models with the same rheology but using two different plate histories or initial conditions

On the predictability limit of convection models of the Earth's mantle

International audienceReconstructing convective flow in the Earth's mantle is a crucial issue for a diversity of disciplines, from seismology to sedimentology. The common and fundamental limitation of these reconstructions based on geodynamic modelling is the unknown initial conditions. Because of the chaotic nature of convection in the Earth's mantle, errors in initial conditions grow exponentially with time and limit forecasting and hindcasting abilities. In this work we estimate for the first time the limit of predictability of Earth's mantle convection. Following the twin experiment method, we compute the Lyapunov time (i.e. e-folding time) for state-of-the art 3D spherical convection models, varying rheology and Rayleigh number. Our most Earth-like and optimistic solution gives a Lyapunov time of 136±13 My. Rough estimates of the uncertainties in best guessed initial conditions are around 5%, leading to a limit of predictability for mantle convection of 95 My. Our results suggest that error growth could produce unrealistic convective structures over timescales shorter than that of Pangea dispersal

Compositional and thermal equilibration of particles, drops and diapirs in geophysical flows

International audienceCore formation, crystal/melt separation, mingling of immiscible magmas, and diapirism are fundamental geological processes that involve differential motions driven by gravity. Diffusion modifies the compo- sition or/and temperature of the considered phases while they travel. Solid particles, liquid drops and viscous diapirs equilibrate while sinking/rising through their surroundings with a time scale that depends on the physics of the flow and the material properties. In particular, the internal circulation within a liquid drop or a diapir favors the diffusive exchange at the interface. To evaluate time scales of chemical/thermal equilibration between a material falling/rising through a deformable medium, we propose analytical laws that can be used at multiple scales. They depend mostly on the non-dimensional Péclet and Reynolds numbers, and are consistent with numerical simulations. We show that equilibration between a particle, drop or diapir and its host needs to be considered in light of the flow structure complexity. It is of fundamental importance to identify the dynamic regime of the flow and take into account the role of the inner circulation within drops and diapirs, as well as inertia that reduces the thickness of boundary layers and enhances exchange through the interface. The scaling laws are applied to predict nickel equilibration between metals and silicates that occurs within 130 m of fall in about 4 minutes during the metal rain stage of the Earth's core formation. For a mafic blob (10 cm diameter) sinking into a felsic melt, trace element equilibration would occur over 4500 m and in about 3 years

Seismoacoustic coupling induced by the breakup of the 15 February 2013 Chelyabinsk meteor

International audienceOn 15 February 2013 around 03:20:00 UTC, the largest meteor reported since the 1908 Tunguska event was observed as a fireball traveling through the Earth's atmosphere, exploding in an air burst near the city of Chelyabinsk, Russia. The rarity of such an event provides a unique window on the physics of meteoroid collision. We report the fine seismic detection of Rayleigh waves produced by the coupling of ground motion with the incident shock wave at distances up to 4000 km from the event. Combining information from seismic beam-forming analysis, recon- structed trajectory from casual video records, and remote sensing, we identify the Rayleigh waves as being initiated by the shock wave produced by the main blast that occasioned damages and injuries in Chelyabinsk. From the Rayleigh wave observations, we report a magnitude Ms ~ 3.7 seismic source

Subduction controls the distribution and fragmentation of Earth’s tectonic plates

International audienceThe theory of plate tectonics describes how the surface of the Earth is split into an organized jigsaw of seven large plates 1 of similar sizes and a population of smaller plates, whose areas follow a fractal distribution 2,3. The reconstruction of global tectonics during the past 200 My 4 suggests that this layout is probably a long-term feature of our planet, but the forces governing it are unknown. Previous studies 3,5,6 , primarily based on statistical properties of plate distributions, were unable to resolve how the size of plates is determined by lithosphere properties and/or underlying mantle convection. Here, we demonstrate that the plate layout of the Earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere. Using 3D spherical models of mantle convection with plate-like behaviour that match the plate size-frequency distribution observed for Earth, we show that subduction geometry drives the tectonic fragmentation that generates plates. The spacing between slabs controls the layout of large plates, and the stresses caused by the bending of trenches, break plates into smaller fragments. Our results explain why the fast evolution in small back-arc plates 7,8 reflects the dramatic changes in plate motions during times of major reorganizations. Our study opens the way to use convection simulations with plate-like behaviour to unravel how global tectonics and mantle convection are dynamically connected