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In the absence of global plate tectonics, mantle convection and plume–lithosphere interaction are the main drivers of surface deformation on Venus. Among documented tectonic structures, circular volcano-tectonic features known as coronae may be the clearest surface manifestations of mantle plumes and hold clues to the global Venusian tectonic regime. Yet, the exact processes underlying coronae formation and the reasons for their diverse morphologies remain controversial. Here we use three-dimensional thermomechanical numerical simulations of impingement of a thermal mantle plume on the Venusian lithosphere to assess the origin and diversity of large Venusian coronae. The ability of the mantle plume to penetrate into the Venusian lithosphere results in four main outcomes: lithospheric dripping, short-lived subduction, embedded plume and plume underplating. During the first three scenarios, plume penetration and spreading induce crustal thickness variations that eventually lead to a final topographic isostasy-driven topographic inversion from circular trenches surrounding elevated interiors to raised rims surrounding inner depressions, as observed on many Venusian coronae. Different corona structures may represent not only different styles of plume–lithosphere interactions but also different stages in evolution. A morphological analysis of large existing coronae leads to the conclusion that at least 37 large coronae (including the largest Artemis corona) are active, providing evidence for widespread ongoing plume activity on Venus. Thermomechanical modelling shows that the formation and diverse morphologies of coronae on Venus can be explained by interactions between the lithosphere and impinging mantle plumes. Some corona structures are consistent with ongoing plume activity.

International audience; Our planet is in crisis! The latest report of the United Nations Intergovernmental Panel on Climate Change (IPCC AR6) confirms that human influence is causing widespread, rapid, and intensifying changes in our weather and climate that are affecting every region on Earth in multiple ways. With every additional ton of carbon we emit, the frequency and intensity of storms, floods, droughts, and fires become greater and the effects on the environment and on human health and civilization become more severe.As geoscientists and journal editors, most of us have been accustomed to being on the leading edge of human knowledge and understanding of climate change, where we deal in objectivity, uncertainty, and debate, but now we find ourselves at the core of this climate crisis. It is no longer scientific discoveries at stake, but also humanity itself. This is an uncomfortable place for many of us. We are trained to be dispassionate observers and cautious thinkers, yet the alarming rate of recent climate change impels us to turn our attention directly toward mitigating this impending crisis.We are making a plea for collective action: we must make the switch to a green economy, put a just and effective price on carbon now, and consider a portfolio of other equitable public investments in climate solutions. These actions will ensure that the true costs and risks of burning fossil fuels are accounted for and global carbon emissions are rapidly reduced. Rich countries must lead the way in making drastic cuts to carbon emissions and in helping low- and middle-income countries to develop sustainably. We are running out of time.For decades, American Geophysical Union (AGU) journals have been at the forefront of documenting human-caused climate change and warning of a worsening climate crisis. Over 2,000 publications from AGU journals are cited in the new IPCC AR6 report. But we too can do more than just document and scientifically explain the ongoing crisis—our profession must help lead the way to solutions.Finding solutions and adapting to change have become not only necessary, but essential in ensuring safe, sustainable, and healthy human communities in the future. The geosciences have an essential role to play in these efforts by pivoting toward more cross-sector, solution-based science. To help lead this vision, the AGU is adding a new publication forum for community science in partnership with associations outside the geosciences. This forum will enhance interactions among AGU's existing, more disciplinary journals and give local communities a voice in leading solutions to global challenges.We are scientists, but we also have families and loved ones alongside our fellow citizens on this planet. The time to bridge the divide between scientist and citizen, head and heart, is now. The lead-up to the 2021 UN Climate Change Conference, COP26, being held in Glasgow in November, is our “last best chance” to urge world leaders to come together and commit to keeping climate change and its devastating impacts in check.

SUMMARY Inversions of planetary gravity are aimed at constraining the mass distribution within a planet or moon. In many cases, constraints on the interior structure of the planet, such as the depth of density anomalies, must be assumed a priori, to reduce the non-uniqueness inherent in gravity inversions. Here, we propose an alternative approach that embraces the non-uniqueness of gravity inversions and provides a more complete view of related uncertainties. We developed a Transdimensional Hierarchical Bayesian (THB) inversion algorithm that provides an ensemble of mass distribution models compatible with the gravitational field of the body. Using this ensemble of models instead of only one, it is possible to quantify the range of interior parameters that produce a good fit to the gravity acceleration data. To represent the interior structure of the planet or moon, we parametrize mass excess or deficits with point masses. We test this method with synthetic data and, in each test, the algorithm is able to find models that fit the gravity data of the body very well. Three of the target or test models used contain only point mass anomalies. When all the point mass anomalies in the target model produce gravity anomalies of similar magnitudes and the signals from each anomaly are well separated, the algorithm recovers the correct location, number and magnitude of the point mass anomalies. When the gravity acceleration data of a model is produced mostly by a subset of the point mass anomalies in the target model, the algorithm only recovers the dominant anomalies. The fourth target model is composed of spherical caps representing lunar mass concentration (mascons) under major impact basins. The algorithm finds the correct location of the centre of the mascons but fails to find their correct outline or shape. Although the inversion results appear less sharp than the ones obtained by classical inversion methods, our THB algorithm provides an objective way to analyse the interior of planetary bodies that includes epistemic uncertainty.

International audience; Peer reviewing is the selfless act of evaluating and providing feedback on scientific papers submitted in journals like JGR: Planets. Every review is written with the objective of clarifying the article and often contains suggestions for additional analyses or topics of discussion. Reviewers often spend hours examining the rigor of the methods, the validity of the results, and the significance of the conclusions in the submitted manuscripts. The extra effort of our pool of reviewers in a year when we all needed to adjust our daily routines in response to a pandemic has not gone unnoticed. In 2020, JGR: Planets benefited from 1,168 reviews provided by 705 unique reviewers. We are particularly grateful to the scientists who remained involved in the review process. We also understand that many of us had to reduce our reviewing commitment this year. Hopefully, the coming year will see a return to normalcy in your lives. To our reviewers, past, present, and future, the entire editorial board of JGR: Planets wishes to express here our most heartfelt gratitude. Thank you!

The interaction between the seismogenic portion of faults and their ductile roots is central to understanding the mechanics of seismic cycles. It is well established that faults are highly localized within the cold and brittle upper crust, but less is known about fault and shear zone structure in the warmer, more ductile, lower crust and in the upper mantle. Increasing temperature with depth causes two transitions in behavior: a frictional transition from seismic to aseismic fault behavior and a transition from brittle to ductile off-fault deformation (BDT). To explore the effects of these two transitions on seismic cycle characteristics (e.g., recurrence interval, nucleation depth, and down-dip limit of coseismic rupture), we simulate seismic cycles on a 2D strike-slip fault. All phases of the earthquake cycle are simulated, allowing the model to spontaneously generate earthquakes and to capture aseismic fault slip and off-fault viscous flow in the interseismic period. The fault is represented with rate-and-state friction. In the off-fault material, distributed viscous flow occurs through dislocation creep. We also consider two possible weakening mechanisms that may be active in lower crustal shear zones: shear heating and grain size reduction, which changes the ductile rheology from dislocation to diffusion creep. This model makes it possible to self-consistently simulate the variations of stress, strain rate, and grain size in the vicinity of a strike-slip fault.We find that the viscous shear zone beneath the fault (defined as the region of elevated viscous strain rate) is roughly elliptically shaped, extending up to 10 km below the fault and with a width of 1 to 3 km. When weakening mechanisms are neglected, the BDT occurs below the depth of the transition from seismic to aseismic fault slip. In these cases, seismic cycle characteristics are similar to those of a traditional elastic cycle simulation that neglects viscoelastic deformation. However, the inclusion of shear heating, which produces a thermal anomaly relative to the background geotherm, shallows the BDT enough to limit the down-dip propagation of coseismic slip in some cases. In these cases, earthquakes penetrate 1-2 km into the shear zone, consistent with observations of zones in which both viscous flow and coseismic slip occur. Also, in these simulations, very little aseismic fault slip occurs. Instead, tectonic plate motion is accommodated primarily through coseismic slip and bulk viscous flow. Preliminary simulations that include the effects of grain size reduction within the shear zone show similar effects. Both weakening mechanisms narrow the shear zone by up to 20%, suggesting that the fault also plays a large role in controlling shear zone localization.

Venus Corona and Tessera Explorer (VeCaTEx) would use an aerobot to descend repeatedly beneath the dense clouds for imaging targeted area of the surface in the near infrared to address six of the prime investigations prioritized by VEXAG. The technologies needed could be matured during the next decade.

In the absence of global plate tectonics, mantle convection and plume-lithosphere interaction are the main drivers of surface deformation on Venus. Whether Venus is geologically active today remains in question: the apparent young surface age and random distribution of impact craters on the planet, initially ascribed as resulting from a global resurfacing event 500-700 Myr ago1,2,3, can also be explained by equilibrium processes, suggestive of ongoing regional resurfacing4,5. Moreover, recent studies have identified active hotspots6,7,8,9 and young individual lava flows10 on the planet. Among documented tectonic structures, circular volcano-tectonic features known as coronae are perhaps the clearest surface manifestations of mantle plumes and may hold clues to the global Venusian tectonic regime. Coronae are characteristic quasi-circular volcano-tectonic features that are abundant on the Venusian surface and generally associated with volcanism, topographic relief, and concentric or radial faulted patterns9,11,12 (left panels of Fig. 1). They feature a wide range of sizes and morphology but typically display an annulus of closely spaced concentric fractures and/or ridges superimposed on a raised rim, with a central relief ranging from domes to depressions12. The exact processes underlying their development and the reasons for their diverse morphologies remain controversial. We conducted a systematic 3D numerical study of plume-lithosphere interaction that links the morphological diversity of large coronae to lithospheric structure and provide guidance for identifying which coronae are currently active. The morphology of at least thirty-seven coronae, dominantly located in a region covering Themis Regio, Lada Terra, and Alpha Regio, is consistent with present-day activity. We thereby provide evidence for widespread plume activity on the planet. The work presented here has just been published in Nature Geoscience13. Fig 1. Venusian coronae (left), their topographic signatures (middle) and comparison with numerical models (right)133D numerical experiments of coronae formationWe ran 3D high-resolution thermomechanical numerical simulations of impingement of a thermal mantle plume into the Venusian lithosphere to assess the origin and diversity of large Venusian coronae. We systematically varied plume size and temperature, the lithospheric strength and crustal thickness in the models. Our results reveal four regimes of plume-lithosphere interactions underlying corona development at the surface (Figure 2): (1) lithospheric dripping, (2) short-lived subduction, (3) embedded plume, and (4) plume underplating. The ratio of plume buoyancy over lithospheric strength majorly controls these dynamic regimes. This found dependency of plume-lithosphere dynamics on plume buoyancy and lithospheric configuration is key for future studies on plume-lithosphere interactions on Venus or (early) Earth. During the first three plume-lithosphere interaction scenarios (regimes (1)-(3)), plume penetration and spreading induce crustal thickness variations that eventually lead to a final topographic isostasy-driven topographic inversion that turns circular trenches surrounding elevated interiors into raised rims surrounding inner depressions.Reasons behind the morphological diversity of coronaeThe temporal evolution of the topographic profiles of the modelled coronae show that different corona morphologies represent no­­­t only different styles of plume-lithosphere interactions, but also different stages in evolution. Coronae with rims and/or trenches are only produced by mantle plumes that (partially) penetrate into the Venusian lithosphere. The common occurrence of coronae displaying rims on Venus suggests that most plumes that formed coronae were able to penetrate at least partially into the lithosphere. In addition, for coronae formed by a penetrative mantle plume, we are able to distinguish active from inactive structures: active coronae feature an outer trench and rise that imply ongoing suction above downwards-moving lithospheric material, as well as elevated interior supported by plume buoyancy­­­­. By contrast, inactive coronae show an inverted topographic profile with an outer rim and an inner depression linked to a thinned lithosphere.Observational evidence for coronae activity and implications for Venus' geodynamicsWe evaluated the possibility of present-day corona activity by systematically analysing the surface topography of large coronae on Venus. Each corona is labelled as currently “active” if it features a clear outer rise and trench, “inactive” if no outer rise but a rim and inner depression is evident, or“unclassified” if the presence of these features is ambiguous. We found expressions of current activity on thirty-seven coronae on Venus (including Artemis Corona, Fig. 1a), whereas thirty-five other coronae were marked “inactive” (including Thouris Corona, Fig. 1b). The remaining coronae are marked “inconclusive”, either because their topography was not well resolved by the dataset, presented ambiguous features, or was markedly different from that obtained in our models. Fig 2. Global distribution of coronae identified as inactive (white circles) or showing ongoing plume activity (red circles)13 Our study presents new evidence for recent tectonic and magmatic activity in the lithosphere Venus, complementing other indications of such activity6,8,10,15,16. Coronae activity spanning a variety of surface ages implies a gradual resurfacing behavior of Venus, akin to Earth-like volcanic and interior processes4,5. Moreover, the global arrangement of active coronae (Fig. 2) suggests a large-scale organization of tectono-magmatic activity on the planet, with a broad active region covering Themis Regio, Lada terra, and Alpha Regio, as well as Eistla Regio, in contrast to regions where large active coronae are absent (around Beta and Phoebe Regiones). Finally, our suggested regions of extensive recent corona activity may serve as interesting targets for detailed investigation by future spacecraft missions.References 1. Turcotte, D.L. (1993) J. Geophys. Res. 98, 17061-17068 2. Strom, R.G. et al. (1994) J. Geophys. Res. 99, 10899-10926 3. Romeo, L. (2013) Planetary and Space Science 87, 157-172 4. Herrick, R.R., & Rumpf, M. E. (2011) J. Geophys. Res. 116, E2 5. Bjonnes, E.E. et al. (2012) Icarus 217, 451-461 6. Smrekar, S.E. et al. (2010) Science 30, 605-608 7. Phillips, R.J. et al. (1991) Science 252, 651-658 8. Stofan, E.R. et al. (1995) J. Geophys. Res. 100, 23317-23327 9. Stofan, E.R. et al. (1992) J. Geophys. Res. 97, 13347-13378 10. Filiberto J. et al. (2020) Science Adv. 6, eaax7445 11. Stofan, E.R. et al. (1991) J. Geophys. Res. 96, 20933-20946 12. Roberts, K. and Head, J. W. (1993) Geophys. Res. Let. 20, 1111-1114 13. Gülcher, A.J.P. et al. (2020) Nat. Geosci., https://doi.org/10.1038/s41561-020-0606-1 14. Gerya, T.V. (2014) Earth Planet. Sci. Lett. 391, 183-192 15. Sandwell, D.T., & Schubert, G. (1992) Science 5071, 766-770 16. Zampa, L.S. et al. (2018) Planetary and Space Science 157, 48-62

International audience; Science is at its best when shared widely both within the scientific community and to the broader public. To maintain trust in science while simultaneously maximizing openness, it is important that every paper is carefully vetted. For this we rely on many volunteers, especially our peer reviewers. Their work is key for maintaining the integrity of scientific journals and to ensure the highest standards of scientific rigor. Reviewers verify that the assumptions, the methods, and the inferences made in our papers are sound. Many spend hours going over derivations, analyses, and evaluating the novelty and significance of the submitted manuscript. In 2019, JGR Planets benefited from 901 reviews provided by 584 unique reviewers. On behalf of the entire editorial board of JGR Planets and the entire planetary science community, we want to express here our most heartfelt gratitude to all of you. Thank you!

The Moon, near Earth asteroids, and the martian moons Phobos and Deimos are all Solar System Exploration Research Virtual Institute (SSERVI) target bodies as they present a wide variety of natural wonders and are potential hosts to resources that will one day enable human exploration of the Solar System. Our SSERVI project, Geophysical Exploration of the Dynamics and Evolution of the Solar System (GEODES) is exploring a suite of natural resources on these bodies through multidisciplinary geophysical investigations. Geophysical methods have been incredibly successful in identifying resources on Earth as they provide a means of characterizing and mapping the sub-surface using data gathered on and above the surface. We focus our geophysical investigations on four essential resources that will enable future human space exploration: I) Lava tubes and void spaces, capable of hosting people and infrastructure; II) Ice deposits, subsurface bodies that can be used for volatile extraction; III) Regolith, which covers the surface of all target bodies, potentially serving as a building material but also presenting a hazard to human and robotic operations and health; and IV) Magma-tectonic Systems, which mobilize, concentrate, and trap volatiles, unique rocks, and ore minerals.Our investigations use an "orbit to outcrop" approach by analyzing existing geophysical data, conducting geophysical exploration of field analog sites on Earth, and creating models that link these analog studies to SSERVI target bodies. Analog sites enable the development, ground-truthing, and integration of (a) exploration strategies, (b) multiple geophysical methods, and (c) modeling capabilities. The GEODES team is integrating field methods and results to create a scientific modeling framework that facilitates the joint inversion of data sets and will share these results with the community via cyber infrastructure, data management, and outreach. The unifying goal behind GEODES is to develop geophysical detection and exploration methods to characterize these natural resources and enable in situ resource utilization at SSERVI target bodies.

The shape and location of density anomalies inside the Moon provide insights into processes that produced them and their subsequent evolution. Gravity measurements provide the most complete data set to infer these anomalies on the Moon [1]. However, gravity inversions suffer from inherent non-uniqueness. To circumvent this issue, it is often assumed that the Bouguer gravity anomalies are produced by the relief of the crust-mantle or other internal interface [2]. This approach limits the recovery of 3D density anomalies or any anomaly at different depths. In this work, we develop an algorithm that provides a set of likely three-dimensional models consistent with the observed gravity data with no need to constrain the depth of anomalies a priori.The volume of a sphere is divided in 6480 tesseroids and n Voronoi regions. The algorithm first assigns a density value to each Voronoi region, which can encompass one or more tesseroids. At each iteration, it can add or delete a region, or change its location [2, 3]. The optimal density of each region is then obtained by linear inversion of the gravity field and the likelihood of the solution is calculated using Bayes’ theorem. After convergence, the algorithm then outputs an ensemble of models with good fit to the observed data and high posterior probability. The ensemble might contain essentially similar interior density distribution models or many different ones, providing a view of the non-uniqueness of the inversion results.We use the lunar radial gravity acceleration obtained by the GRAIL mission [4] up to spherical harmonic degree 400 as input data in the algorithm. The gravity acceleration data of the resulting models match the input gravity very well, only missing the gravity signature of smaller craters. A group of models show a deep positive density anomaly in the general area of the Clavius basin. The anomaly is centered at approximately 50°S and 10°E, at about 800 km depth. Density anomalies in this group of models remain relatively small and could be explained by mineralogical differences in the mantle. Major variations in crustal structure, such as the near side / far side dichotomy and the South Pole Aitken basin are also apparent, giving geological credence to these models. A different group of models points towards two high density regions with a much higher mass than the one described by the other group of models. It may be regarded as an unrealistic model. Our method embraces the non-uniqueness of gravity inversions and does not impose a single view of the interior although geological knowledge and geodynamic analyses are of course important to evaluate the realism of each solution.References: [1] Wieczorek, M. A. (2006), Treatise on Geophysics 153-193. doi: 10.1016/B978-0-444-53802-4.00169-X. [2] Izquierdo, K et al. (2019) Geophys. J. Int. 220, 1687-1699, doi: 10.1093/gji/ggz544, [3] Izquierdo, K. et al., (2019) LPSC 50, abstr. 2157. [4] Lemoine, F. G., et al. ( 2013), J. Geophys. Res. 118, 1676–1698 doi: 10.1002/jgre.20118.

Understanding the rheological structure of the lithosphere is important for inferring loading rate on faults and their potential downdip extensions. To this end, we compare viscosity estimates from geodynamic models and predictions of lithosphere rheology. At each point on a grid covering Southern California, we first produce deviatoric stress estimates averaged from the surface to 100 km depth obtained by modelling variations of crustal structure (gravity potential energy) and effective viscosity. This geodynamic model is evaluated on the basis of surface geodetic data. In a complementary approach, we generate a strength envelope at each grid point based on various community products provided by the Southern California Earthquake Center (SCEC) and associated researchers. For example, we use the thermal model of Shinevar et al. (2018) and crustal thickness variations from Shen and Ritzwoller 2016). At each depth, the lowest of the stress associated with dislocation or diffusion creep is retained. Eight alternative rheological models are developed, that consider either wet or dry rheologies, a uniform grain size (1mm) or a grain size tied to a piezometer, and a maximum allowed stress of 300 MPa or 30 MPa. We use the flow laws of feldspar from Rybacki et al. (2006) is used in the crust and that of Hirth and Kohlstedt (2004) for olivine in the mantle. At this point, lithological variations in the crust are neglected, although we find evidence in our results that they are probably important. The strength envelop is integrated with depth for various strain rates to produce an effective rheology of the lithosphere. We then determine the strain rate associated with the geodynamically-inferred stress and use it to produce a viscosity estimate from the rheological model The first result of this analysis is that the effective rheology of the lithosphere is highly non-linear (effective stress exponent between 10 and 30). Therefore only a limited range of stress is expected at any given location. In general, that stress is quite high so that the viscosity estimates from the geodynamic model can be explained only when using the weakest crust and mantle rheologies. Although certain viscosity variations are consistent between the models (strong Sierra Nevada block, weak Salton trough area), in other places they are not. In particular, the Great Basin block appears strong in the geodynamic model but weak in the rheological model due to high temperatures there. This study shows that there are likely important rheological variations due to mineralogy. It is expected that the Great Basin is underplated by gabbroic rocks that are not included here but would increase the effective viscosity of the rheological model. Mineralogical variations would also allow a great variety of accessible stress at each location. Finally, it is also likely that the stress does not reach failure at every depth, which is the basis for considering low saturation values for the strength envelop.

As the Earth's primary mode of planetary cooling, the oceanic plate is created at mid-ocean ridges, transported across the planet's surface, and destroyed at subduction zones. The evolution of its buoyancy and rheology during its lifespan maintains the coherence of the plate as a distinct geological entity and controls the localised deformation and vertical material exchange at plate boundaries, which enables the horizontal ocean-plate movements. These motions intimately link the oceanic plate to the overarching overturn of Earth's mantle: The plate forms out of rising mantle material at spreading ridges; it cools the Earth's interior as the cold thermal boundary layer to mantle convection; and its sinking portions drive not only the plate itself but also dominate global flow in the mantle. We scrutinise here the entire life cycle of the oceanic plate, starting with its birth at the mid-ocean ridge, including the thermal, rheological, and chemical conditions of initiation, followed by plate maturation as it ages and cools while crossing the seafloor, and finishing with the dynamics of plate destruction as it retires at the subduction zone to become a deeper part of Earth's convective system. We find that the full range of dynamic behaviour of the oceanic plate, including its forcing and overall framework within Earth's convecting system, is not fully captured by the existing concept of Plate Tectonics, which describes solely the horizontal surface kinematics of all plates. Therefore, we introduce a more specific and at the same time more integral concept named “Ocean-Plate Tectonics” that more specifically describes the dynamic life of the oceanic plate and accounts for the knowledge gained during the past 50 years. This “Ocean-Plate Tectonics” must have emerged on Earth at least 1 Billion years ago, and dominates Earth's dynamics today.

MeltMigrator is a MATLAB®-based melt migration software developed to process three-dimensional mantle temperature and velocity data from user-supplied numerical models of mid-ocean ridges, calculate melt production and melt migration trajectories in the mantle, estimate melt flux along plate boundaries, and predict crustal thickness distribution on the seafloor. MeltMigrator is also capable of calculating compositional evolution depending on the choice of petrologic melting model. Programmed in modules, MeltMigrator is highly customizable and can be expanded to a wide range of applications. We have applied it to complex mid-ocean ridge model settings, including transform faults, oblique segments, ridge migration, asymmetrical spreading, background mantle flow, and ridge-plume interaction. In this technical report, we include an example application to a segmented mid-ocean ridge. MeltMigrator is available as a supplement to this paper, and it is also available from GitHub and the University of Maryland Geodynamics Group website.

Subduction dynamics can be understood as the result of the balance between driving and resisting forces. Previous work has traditionally regarded gravitational slab pull and viscous mantle drag as the main driving and resistive forces for plate motion respectively. However, this paradigm fails to explain many of the observations in subduction zones. For example, subducting plate velocity varies significantly along-strike in many subduction zones and this variation is not correlated to the age of subducting lithosphere. Here we present three-dimensional and time-dependent numerical models of subduction. We show that along-strike variations of the overriding plate thermal structure can lead to along-strike variations in subducting plate velocity. In turn, velocity variations lead to significant migration of the Euler pole over time. Our results show that the subducting plate is slower beneath the colder portion of the overriding plate due to two related mechanisms. First, the mantle wedge beneath the colder portion of the overriding plate is more viscous, which increases mantle drag. Second, where the mantle wedge is more viscous, hydrodynamic suction increases, leading to a lower slab dip. Both factors contribute to decreasing subducting plate velocity in the region; therefore, if the overriding plate is not uniform, the resulting velocity varies significantly along-strike, which causes the Euler pole to migrate closer to the subducting plate. We present a new mechanism to explain observations of subducting plate velocity in the Cocos and Nazca plates. These results shed new light on the balance of forces that control subduction dynamics and prove that future studies should take into consideration the three-dimensional structure of the overriding plate., This work was supported by Spanish Ministry of Economy and Competitiveness projects CGL2012-37222 and CGL2014-58821-C2-1-R. M.I. Billen acknowledges support from NSF grant 1246864. L.G.J. Montesi acknowledges support from NSF grant OCE 10-60878.

Observations of dunite channels in ophiolites and uranium-series disequilibria in mid-ocean ridge basalt suggest that melt transport in the upper mantle beneath mid-ocean ridges is strongly channelized. We present experimental evidence that spatial variations in mineralogy can also focus melt on the grain-scale. This lithologic melt partitioning, which results from differences in the interfacial energies associated with olivine-melt and orthopyroxene-melt boundaries, may complement other melt focusing mechanisms in the upper mantle such as mechanical shear and pyroxene dissolution. We document here lithologic melt partitioning in olivine/orthopyroxene-basaltic melt samples containing nominal olivine to orthopyroxene ratio of 3 to 2 and melt fractions of 0.02 to 0.20. Experimental samples were imaged using synchrotron-based X-ray micro-computed tomography at a resolution of 700 nm per voxel. By analyzing the local melt fraction distributions associated with olivine and orthopyroxene (opx) grains in each sample, we found that the melt partitioning coefficient, i.e., the ratio of melt fraction around olivine to that around orthopyroxene grains, varies between 1.1 and 1.6. The permeability and electrical conductivity of our digital samples were estimated using numerical models and compared to those of samples containing only olivine and basaltic melt. Our results suggest that lithologic melt partitioning and preferential localization of melt around olivine grains might play a role in melt focusing, potentially enhancing average melt ascent velocities.

Estimates of melt content beneath fast-spreading mid-ocean ridges inferred from magnetotelluric tomography (MT) vary between 0.01 and 0.10. Much of this variation may stem from a lack of understanding of how the grain-scale melt geometry influences the bulk electrical conductivity of a partially molten rock, especially at low melt fraction. We compute bulk electrical conductivity of olivine–basalt aggregates over 0.02 to 0.20 melt fraction by simulating electric current in experimentally obtained partially molten geometries. Olivine–basalt aggregates were synthesized by hot-pressing San Carlos olivine and high-alumina basalt in a solid–medium piston-cylinder apparatus. Run conditions for experimental charges were 1.5 GPa and 1350 °C. Upon completion, charges were quenched and cored. Samples were imaged using synchrotron X-ray micro-computed tomography (μ-CT). The resulting high-resolution, 3-dimensional (3-D) image of the melt distribution constitutes a digital rock sample, on which numerical simulations were conducted to estimate material properties. To compute bulk electrical conductivity, we simulated a direct current measurement by solving the current continuity equation, assuming electrical conductivities for olivine and melt. An application of Ohm's Law yields the bulk electrical conductivity of the partially molten region. The bulk electrical conductivity values for nominally dry materials follow a power-law relationship σ bulk = C σ melt ϕ m with fit parameters m = 1.3 ± 0.3 and C = 0.66 ± 0.06 . Laminar fluid flow simulations were conducted on the same partially molten geometries to obtain permeability, and the respective pathways for electrical current and fluid flow over the same melt geometry were compared. Our results indicate that the pathways for flow fluid are different from those for electric current. Electrical tortuosity is lower than fluid flow tortuosity. The simulation results are compared to existing experimental data, and the potential influence of volatiles and melt films on electrical conductivity of partially molten rocks is discussed.

Crustal thickness differences between oceanic transform faults and associated mid-ocean ridges may be explained by melt migration and extraction processes. Slow-slipping transform faults exhibit more positive gravity anomalies than the adjacent spreading centers, indicating relative thin crust in the transform domain, whereas at intermediate-spreading and fast-spreading ridges transform faults are characterized by more negative gravity anomalies than the adjacent spreading centers, indicating thick crust in the transform domain. We present numerical models reproducing these observations and infer that melt can be extracted at fast-slipping transforms, but not at slow-slipping ones. Melt extraction is modeled as a three-step process. (1) Melt moves vertically through buoyancy-driven porous flow enhanced by subvertical dissolution channels. (2) Melt accumulates in and travels along a decompaction channel lining a low-permeability barrier at the base of the thermal boundary layer. (3) Melt is extracted to the surface when it enters a melt extraction zone. A melt extraction width of 2–4 km and a melt extraction depth of 15–20 km are needed to fit the tectonic damages associated with oceanic plate boundaries that reach into the upper mantle. Our conclusions are supported by the different degrees of magmatic activities exhibited at fast-slipping and slow-slipping transforms as reflected in geological features, geochemical signals, and seismic behaviors. We also constrain that the maximum lateral distance of crust-level dike propagation is about 50–70 km.

Global compilations of subslab shear wave splitting parameters show a mix of trench-parallel and trench-perpendicular fast directions that often directly contradict predictions from two-dimensional models of slab-entrained flow. Here we show that subslab anisotropy is consistent with three-dimensional geodynamic models that feature the interaction between subducting slabs and regional mantle flow. Each model represents a specific region for which high-quality source-side shear wave splitting data are available. We compare the distribution of finite strain in the models with shear wave splitting observations, showing that both trench-parallel and trench-perpendicular fast directions can be explained by deflection of regional mantle flow around or beneath subducted slabs. Subslab maximum elongation directions calculated from our models depend on a combination of geometry factors (such as slab dip angle and maximum depth), mechanical parameters (such as decoupling between the slab and the subjacent mantle), and the orientation and magnitude of the regional mantle flow.

Three-dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench-parallel subslab seismic anisotropy. Subslab flow becomes inherently three-dimensional when slab-driven flow interacts with a wide variety of ambient background mantle flow conditions. This interaction depends on slab geometries, mechanical coupling parameters, and lower mantle viscosities. Deflection of subslab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient, background mantle flow. Background mantle flow can become trench-perpendicular or trench-parallel subslab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or toward the edge of the slab. The first case is especially prominent in models with short slabs that do not penetrate into the lower mantle. The second case is especially prominent in models with long, steep slabs. The results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it. High levels of coupling create a boundary layer of trench-perpendicular entrained flow, pushing the deflection due to the obstructing slab away from the slab. We compare our subslab flow model predictions with a global set of seismic anisotropy fast directions in the subslab mantle, and find generally good agreement between the anisotropy observations (dominantly trench-parallel or trench-perpendicular) and the mantle flow directions predicted for decoupled systems.

Jupiter’s second Galilean satellite, Europa, is a Moon-sized body with an icy shell and global ocean approximately 100 km thick surrounding a rocky interior. Its surface displays extensive tectonic activity in a geologically recent past. Europa’s most ubiquitous surface features, double ridges, have a central trough flanked by two raised edifices. Double ridges can extend hundreds of kilometers and appear genetically related to cracks formed in the Europan ice shell. The origin of the raised flanks has been the center of much debate and many models have been proposed. There are also ridges without a central trough, single ridges. These ridges are far less common than their double ridge counterparts. However, there are locations where along-strike changes in ridge type appear to occur. We explore an elastic model in which the ridges form in response to crystallization of a liquid water intrusion. In our model, liquid water fills tension cracks that open in the Europan crust in response to tidal stress or perhaps overpressure of a subsurface ocean. The crack would be long and essentially continuous, similar to dikes on Earth, explaining the remarkable continuity and lack of segmentation of Europan ridges. The freezing of the water would cause a volume expansion, compressing and buckling the adjacent crust. We find that the geometry of the intruding water body controls the shape of the resulting ridges, with single ridges forming above sill-like intrusions and double ridges above dike-like intrusions. In order to match the ridge heights observed for double ridges we would need approximately 1.5 km2 of water intruded at a shallow depth in the ice shell, potentially over the course of multiple events. Deeper intrusions result in a broader, lower amplitude ridge than shallow intrusions.

Melt percolation in mantle rocks is currently poorly constrained, especially at low melt fractions. At mid-ocean ridges, for example, geochemical and geophysical observations produce divergent estimates of how much melt is present in the mantle and how quickly it moves. Accurate estimates of permeability and grain-scale melt distribution in mantle rock are necessary to reconcile these observations. We present three-dimensional (3-D), 700 nm-resolution images of olivine–basalt aggregates, containing nominal melt fractions ( ϕ n ) between 0.02 and 0.20. Samples were prepared from a powdered mixture of San Carlos olivine and high-alumina basalt and hot-pressed in a solid-medium piston–cylinder apparatus at 1350 °C and 1.5 GPa. Images were obtained using synchrotron X-ray microtomography (SXμT) from the Advance Photon Source at Argonne National Laboratory. Stokes flow simulations, conducted using the digital melt volume as the numerical domain, determine that the permeabilities of experimental charges range from 2 × 10 − 16 to 5 × 10 − 13 m 2 for ϕ n = 0.02 to 0.20, respectively. The simulation results are well represented by the power-law relation between permeability (k) and melt fraction (ϕ), k = ϕ n d 2 / C , where n = 2.6 ± 0.2 , and assuming a grain size of 35 μm in the experiments, C = 58 − 22 + 36 . These results place important new constraints on rates of melt migration and melt extraction within partially molten regions of the mantle.

Subduction-related transport of water into the mantle has significant dynamical and geochemical implications. Dehydration of hydrous phases within the slab can introduce water into the transition zone and lower mantle, potentially hydrating nominally anhydrous minerals (NAM) and impacting the viscosity and density structure of the mantle over a wide area. We present models of fluid transport and mantle hydration in the vicinity of a deeply subducting slab, focusing on the fate of water released by deep dehydration reaction in the subducted serpentinized mantle. A sharp decrease in water storage capacity across the lower boundary of the transition zone may produce “secondary dehydration” of hydrated NAM, leading to precipitation of a hydrous fluid and heterogeneous hydration of the transition zone. Rapid fluid migration relative to the solid flow field can lead to a broad region of diffuse hydration within the upper mantle wedge and the potential for localized melt regions at the top of the transition zone coincident with fluid pathways. Slower fluid migration instead implies that the fluid phase can be transported deep into the lower mantle. Water stabilized in NAM and as a free fluid can initiate upwelling within and above the transition zone. A less abrupt change in water storage capacity across the base of the transition zone leads to high NAM water contents in a channel adjacent to the slab where viscosity is reduced. However, seismic and electromagnetic observations of hydration in the transition zone are most compatible with a sudden drop of water storage capacity.

At the Galapagos triple junction, the Cocos and Nazca plates are broken by a succession of transient rifts north and south of the Cocos–Nazca (C–N) Rift. Modeling has suggested that each rift initiated at the East Pacific Rise (EPR), its location controlled by the distance of the C–N Rift tip from the EPR. Evidence on the Pacific plate confirms that each transient rift formed a true RRR triple junction with the EPR and clarifies the history of the region. At ∼1.5 Ma the triple junctions began jumping rapidly toward the C–N Rift suggesting that the C–N Rift tip moved closer to the EPR. Pacific abyssal hills became broad and shallow indicating enhanced magma supply to the region. At ∼1.4 Ma, the Galapagos microplate developed when extension became fixed on the southern transient rift to form the South scarp of the future Dietz rift basin. Lavas flooded the area and a Galapagos–Nazca magmatic spreading center initiated at the EPR. We suggest that a hotspot was approaching the southern triple junction from the west. The hotspot crossed to the Nazca plate ∼1.25 Ma. Dietz seamount formed within the young spreading center, dikes intruded Dietz rift basin, and eruptions built volcanic ridges. Since ∼0.8 Ma magmatic spreading has jumped northward twice, most recently to Dietz volcanic ridge. Amagmatic extension to the east has formed the large North scarp of Dietz rift basin. Northward jumping of the southern triple junction has maintained the microplate boundary close to the proposed hotspot.

Lithospheric deformation on Earth is localized under both brittle and ductile deformation conditions. As high-temperature ductile rheologies are fundamentally strain-rate hardening, the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature. In this contribution, I develop a localization potential that quantifies the weakening associated with these changes. The localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions. I provide analytical expressions for the localization potential associated with a temperature increase, grain size reduction, an increase in water fugacity, melt content, or the abundance of a weak mineral phase. I show that these processes cannot localize deformation from a mantle convection scale (103 km) to a ductile shear zone scale (1 km). To achieve this, is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers. This process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non-linear rheology. Micas, melt, and fine-grained aggregates – unless dry rheologies are used – have the necessary characteristics. As none of these phases is expected to be present in the dry lithosphere of Venus, this concept can explain why Venus, unlike the Earth, does not display a global network of plate boundaries. The diffuse plate boundary in the Central Indian Ocean may be as yet non-localized because serpentinization has not reached the ductile levels of the lithosphere.

The Rembrandt basin is crosscut by the largest fault scarp on Mercury, Enterprise Rupes, and a second scarp complex, Belgica Rupes, extends to the basin's rim. Topographic data derived from MESSENGER orbital stereo images show that these tectonic landforms bound a broad, relatively flat-floored valley with a mean width of ~400 km. Crosscutting relations suggest that the accumulation of structural relief likely postdates the formation and volcanic infilling of the Rembrandt basin. The valley floor, bound by fault scarps of opposite vergence, is significantly offset below the elevation of the back-scarp terrains. Along with an offset section of Rembrandt's rim, the elevation differences are evidence that the valley floor was lowered as a result of the formation of bounding fault scarps. The localization of the widely spaced thrust faults of Enterprise and Belgica Rupis and the offset of the valley floor may be the result of long-wavelength buckling of Mercury's lithosphere.

Seafloor bathymetry north of the Galapagos microplate in the Eastern Pacific Ocean contains evidence for a sequence of short-lived rifts cross-cutting abyssal hills adjacent to the East Pacific Rise. These secondary rifts are sub-parallel to the Incipient Rift that marks the current triple junction at 2°40′N and are nearly perpendicular to the direction of opening between the Cocos and Nazca plates. Secondary rifts likely initiated from the EPR by lithospheric cracking. Eventually, their activity stopped and they were carried away from the triple junction as part of the Cocos plate. We developed and analyzed numerical models of rift interaction to understand the evolution of rift configuration in this area. By varying the geometry and locations of rifts, we constrain the factors that have led to the location and orientation of secondary rifts at the northern Galapagos Triple Junction. Interaction between secondary rifts and the Cocos-Nazca Rift results in ~ 10° clockwise rotation of the secondary rift propagation direction, as observed. Furthermore, if a rift has become detached from the East Pacific Rise, a zone of reduced tension is present ahead of the rift tip, prohibiting its connection to the East Pacific Rise. Two zones of tensile stress enhancement develop along the East Pacific Rise next to the detached rift and control where new cracks are likely to form. Although the magnitude of tensile stress enhancement is controlled by the gap between the detached rift and the East Pacific Rise, whether the new rift forms north or south of the detached rift is controlled by position of the Cocos-Nazca Rift. Therefore, the sequence of ancient rifts found northeast of the current triple junction at 2°40′N represents the natural consequence of rift disconnection events and their position records the kinematic history of the gap between the East Pacific Rise and the Cocos-Nazca Rift.

The Mercury Laser Altimeter on the NASA MESSENGER mission has ranged to several ridges and lobate scarps during two equatorial flybys of the planet Mercury. The tectonic features sampled, like others documented by spacecraft imaging and Earth-based radar, are spatially isolated and have vertical relief in excess of 1 km. The profiles also indicate that the faulting associated with their formation penetrated to tens of kilometers depth into the lithosphere and accommodated substantial shortening. To gain insight into the mechanism(s) of strain accommodation across these structures, we perform analytical and numerical modeling of representative dynamic localization mechanisms. We find that ductile localization due to shear heating is not favored, given our current understanding of thermal gradients and shallow thermal structure of Mercury at the time of ridge and scarp formation, and is likely to be of secondary importance at best. Brittle localization, associated with loss of resistance during fault development or with velocity weakening during sliding on mature faults, is weakly localizing but permits slip to accumulate over geological time scales. The range of shallow thermal gradients that produce isolated faults rather than distributed fault sets under the assumption of modest fault weakening is consistent with previous models for Mercury’s early global thermal history. To be consistent with strain rates predicted from thermal history models and the amount of shortening required to account for the underlying large-offset faults, ridges and scarps on Mercury likely developed over geologically substantial time spans.

SUMMARY Volcanic gaps are segments of subduction zones that lack the volcanic activity usually found at these convergent margins. They are regions where the necessary conditions to produce melt may appear favourable, but where volcanoes are surprisingly absent from the surface. In this study, we present a new model that can explain the occurrence of such volcanic gaps. It is based on seismic imaging and geodynamic modelling of the Denali volcanic gap, a ∼400-km-wide region at the eastern end of the Alaska-Aleutian subduction zone. Here, the thick crust of the Pacific Plate and Yakutat terrane subduct at shallow angle beneath North America. A high-resolution seismic profile clearly images the subducting crust undergoing progressive dehydration between 50 and 120 km depth, and a negative subhorizontal velocity contrast at 60 km depth in the overlying mantle wedge. We interpret this 60 km discontinuity as marking the top of a layer of partial melt that pools at the base of the overriding plate. In steadystate subduction models, melt accumulates at the apex of a vaulted mantle wedge, the ‘pinch zone’, from where it may break through the overlying lithosphere to the surface. Beneath the Denali volcanic gap, the pinch zone is absent (or greatly reduced) because shallow subduction of the Yakutat terrane progressively cools the system, and causes the slab to advance and replace the hot core of the mantle wedge. This regime can be seen as the opposite of subduction rollback. It prevents the formation of a pinch zone, reduces the length of the melting column and causes melt to pool at the base of the overriding plate, thus inhibiting magma generation and extraction.

Geologic, experimental, and geodetic studies indicate that ductile shear zones do not deform in steady state. We present a theoretical model for an elastically loaded ductile shear zone deforming by combined dislocation and diffusion creep, in which grain size is allowed to evolve toward its equilibrium (recrystallized) value. Several grain size evolution laws are considered. Simulations of laboratory experiments show that grain size evolution can lead to changes in the dominant deformation mechanism that are accompanied by transient strain-weakening or strain-strengthening episodes reminiscent of laboratory observations. Ductile shear zones can also produce postseismic creep as observed in geodetic data following large earthquakes. Non-steady-state ductile creep is difficult to differentiate from frictional sliding with velocity-strengthening friction. Hence, ductile creep may play a role in the transition from velocity-weakening to velocity-strengthening friction with increasing temperature.

To understand how large submarine lava terraces form and why they are not commonly observed on land, we developed an isoviscous gravity flow model on an inclined surface to simulate the evolution and emplacement of lava flows under submarine conditions. By solving this preliminary model using a finite difference method, we are able to quantify how lava viscosity, pre-existing topographic slope, effusion rate, and lava volume affect meso-scale lava morphology. Our simulations show that, in general, high lava viscosity, gentle regional slope, and low effusion rate favor the formation of large terraces, but environmental conditions also play an important role. A gravity flow spreads more slowly underwater than subaerially. We also conclude that for low viscosity basaltic lava, the cooling of the lava body is one of the most critical factors that affect its shape. This study shows that the isoviscous model, though oversimplified, provides a quantitative tool to relate lava morphology to eruption characteristics. To gain a better understanding of the controls on submarine lava terrace formation, future models must take into account the temporal and spatial variation of lava viscosity, especially the effects of a brittle outer shell.

International audience; Much debate exists concerning the strength distribution of the continental lithosphere, how it controls lithosphere-scale strain localization and hence enables plate tectonics. No rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid-like behaviour of plate interiors. Here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution. Geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle, respectively. The positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remain

Triaxial extension experiments were conducted to investigate the influence of radial stress on porosity and permeability (for hydraulic flow along the axial direction) in three porous sandstones. The effective mean stresses were sufficiently high that the samples failed by cataclastic flow, with development of strain hardening and shear-enhanced compaction. Comparison of the new data with triaxial compression data from a previous study shows that the critical stress states for the onset of shear-enhanced compaction are comparable for the two different loading paths. The initial yield stress data for each sandstone map out an approximately elliptic envelope in the stress space. Stress-induced permeability anisotropy was inferred from synthesis of the triaxial compression and extension data. Before the onset of shear-enhanced compaction, permeability and porosity reduction are primarily controlled by the effective mean stress and stress-induced anisotropy is negligible. With the onset of shear-enhanced compaction and development of cataclastic flow, coupling of the deviatoric and hydrostatic stresses induces considerable permeability and porosity reduction. The permeability for flow along the direction of the maximum (compressive) principal stress is greater than that along the minimum principal stress. Microstructural observations on the shear-compacted samples show appreciable increase of grain crushing and pore collapse, which explain the overall decrease in permeability. The damage from grain crushing is highly anisotropic, with the stress-induced microcracks preferentially aligned with the maximum principal stress direction. Because more microcrack conduits are available to focus the flow in this direction, the permeability is relatively enhanced.

The Galapagos triple junction is not a simple ridge-ridge-ridge (RRR) triple junction. The Cocos-Nazca Rift (C-N Rift) tip does not meet the East Pacific Rise (EPR). Instead, two secondary rifts form the link: Incipient Rift at 2°40′N and Dietz Deep volcanic ridge, the southern boundary of the Galapagos microplate (GMP), at 1°10′N. Recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the GMP (∼1.5 Ma). South of C-N Rift a band of northeast-trending cracks cuts EPR-generated abyssal hills. It is a mirror image of a band of cracks previously identified north of C-N Rift on the same age crust. In both areas, the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the EPR spreading center and cut eastward into pre-existing topography. Each crack formed a short-lived triple junction until it was abandoned and a new crack and triple junction initiated nearby. Between 2.5 and 1.5 Ma, the pattern of cracking is remarkably symmetric about C-N Rift providing support for a crack interaction model in which crack initiation at the EPR axis is controlled by stresses associated with the tip of the westward-propagating C-N Rift. The model also shows that offsets of the EPR axis may explain times when cracking is not symmetric. South of C-N Rift, cracks are observed on seafloor as old as 10.5 Ma suggesting that this triple junction has not been a simple RRR triple junction during that time.