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FRINATEK-Fri prosj.st. mat.,naturv.,tek

PLATONICS - Shaping PLAnetary tecTONICS by solid-state convection incorporating damage and inheritance

Alternative title: PLATONIKK - Utforming av PLAnetarisk tekTONIKK ved fast-fase konveksjon med inkorporering av skade og arv

Awarded: NOK 7.7 mill.

Many planetary surfaces are textured due to surface tectonics, of which Earth features a peculiar form: plate tectonics. Earth's surface is fragmented into a set of moving plates, born at mid-ocean ridges, and destroyed at subduction zones. No other planetary body currently shows a similar mechanism that allows for efficient recycling of crustal material into the deep interior and so controls the bodies' long-term evolution. Why Earth is so unique remains an enigma whose resolution is crucial for understanding planetary evolution across the solar system. Other bodies feature different forms of surface tectonics, like the hot rocky planet Venus or Jupiter's icy moon Europa. The observed diversity tells us about the interplay of different mechanisms that shape planetary surfaces. PLATONICS is based on the hypothesis that surface tectonics is dominantly driven by processes inside the planetary body. These include solid-state convection (i.e., large-scale material transport) thanks to which heat can leave the interior efficiently. Convection induces stress and deform the crust depending on the level of stress and on the strength of the crust. The strength is controlled by the large-scale tectonic environment, but also by processes at the (small) grain scale of crustal material - whether ice or rock - and may change with time. Grains shrink upon deformation causing weakening, but once deformation has ceased, grains grow and material strength recovers. Previously damaged ice or rock thus remembers its history. Determining the scales and history of deformation, how it is inherited and how small- and large-scale processes interact to shape the evolution of planetary tectonic is the key purpose of PLATONICS. In this project, we developed a numerical framework linking the different scales and compare its predictions to the Earth, Venus, and Europa. The goal is to advance our insight into planetary tectonics and why Earth is unique in featuring plate tectonics. To achieve this, the methodological framework has been continuously improved and applied to more specific questions during the project so far. Upon implementing the possibility of multiple coexisting deformation mechanisms, either dominated by diffusion or dislocation of grain boundaries, we investigated which mechanism dominates in which regions of a rocky mantle, such as the Earth mantle. Depending on the mechanism, different regimes of surface tectonics are observed with different rates of surface motion and with different styles of subduction. Next, we made the rheology of mantle rocks sensitive to grain size and found pronounced effects on the upper mantle and on the viscosity structure of deep mantle plumes. This additional sensitivity also allows for the development of weak scars in the crust, which can possibly serve as zones of tectonic activation. Considering grain-size-sensitive rheology does not substantially widen the feasibility window of a plate- like tectonic regime, however, it tends to reduce episodicity making the plate-like regime more continuous. We also investigated this memory effect in a simpler parameterization based on strain accumulation and found this to make the development of plate-like tectonic behaviour, as seen on Earth, more feasible. For a given strength of intact oceanic lithosphere it is more likely to observe plate-like behaviour including a memory effect on previous deformation. For strong continental lithosphere, however, the effects are secondary, making the employed rheology insufficient to explain continental rifting. We transferred the grain-size sensitive rheology to Venus and Europa. For Venus, we investigated how its increased surface temperature alters damage accumulation and dampens inheritance effects. Results suggest that the plastic strength of the lithosphere is still the controlling agent of the tectonic regime. Venus-like surface temperatures still alter the style of tectonic resurfacing, but this is not primarily driven by grain-size evolution. For Europa, we implemented additional deformation mechanisms relevant for ice. Grain-size reduction affects the strength of Europa’s surface layers, but not sufficiently to overcome the high strength due to the low surface temperature. Without additional weakening, the surface of Europa remains largely undeformable, inconsistent with observed crater densities. Tidal heating of the icy shell favours thinning of the immobile surface layer under specific conditions but would in turn cause substantial melting of the ice shell. Synthesis of the achieved results suggests that grain-size evolution and tectonic inheritance do affect planetary interior structure and evolution, but not as strongly as temperature and plastic failure of surface rocks do. In particular, the generation of some initial weakness that can be inherited is crucial. Grain-size evolution was not found to be an efficient mechanism to do so, at least in present-like planetary mantles.

This project pushed forward the treatment of rock rheology in the long-term evolution of planetary rocky mantles and ice shells. The project demonstrated that approximations commonly used in the community of mantle dynamics do not capture well the dynamic behaviour of subducting slabs and the coupling between surface tectonics and global mantle flow. The evolution of the materials grain size through space and time is an important agent to determine how and at which rate materials deform and therefore couple the interior and the surface. The project led to a numerical framework which has been used, and can be used further in future, to exploit interior-surface evolution for a manifold of planetary objects with solid surfaces. In the light of upcoming space missions, especially those to Venus and to Jupiter’s Galilean satellites in the upcoming decade, this framework can prove useful for future research directing towards our understanding of planetary surface tectonics. The framework can straight-forwardly be upgraded to simulate other planetary objects than investigated here (Earth, Venus, Europa), refined parameter values from field and laboratory measurements can easily be tested, and additional physical processes such as surface processes like erosion and sedimentation can be added to make the framework more realistic compared to real planets. This project had a specific focus on the role of tectonic inheritance in the sense that the previous accumulated deformation history matters for the subsequent evolution of surface tectonics. Indeed, it was found for example that the accumulation of strain preserved in near-surface rocks can make Earth-like tectonics more feasible in terms of lithospheric strength. Also, grain size evolution in subduction zones is capable of creating weak zones in the lithosphere that may subsequently get reactivated and it tends to make surface tectonics less episodic. On the other hand, this study indicated that those features may only refine a planet’s tectonic regime, but do not determine it on first order. Since the detailed role of grain size evolution and how it leads to tectonic inheritance was found to be rather sensitive to some poorly constrained parameters, the results obtained invite for future collaborative studies between mantle dynamic modelling and experimental rock deformation, and, regarding icy satellite evolution, the community of terrestrial glaciology.

Many planetary surfaces in our solar system are strongly textured and feature e.g. mountains, valleys, and rifts. Surface tectonics describes the origin of many of such features. Earth's peculiar form of tectonics, 'plate tectonics', is unique in our solar system, but tectonic activity is observed on a variety of bodies, from Earth's neighbour Venus to icy satellites orbiting the outer planets, like Jupiter's moon Europa. Planetary observations are often sparse and surface tectonics is thus of major importance for our understanding of these bodies as it provides insight into the dynamic processes acting on them. Prominent among these processes is solid-state convection, which likely occurs (occurred) in terrestrial mantles and icy planetary shells across the solar system. Convection-induced stress may lead to deformation at the surface and generates features such as Earth's plates. This is governed by rheology, which links deformation to the induced stress. However, rheology is not only a snapshot depending on the instantaneous stress, but depends on the preceding history, e.g. on Earth, zones of pre-existing weakness persist over long geological time due to slow healing from previous damage and focus subsequent deformation: a memory that impacts the further tectonic evolution. To consistently grasp the complex feedback between convection inducing stress, surface deformation, and its inheritance a 3D dynamic framework is necessary, but is yet missing. The PLATONICS project will implement such a framework based on convection models incorporating the grain size evolution of rocky (or icy) material and its impact on rheological damage and inheritance. It will thereby boost our understanding of how Earth's plate boundary network has evolved and will reveal the variety of tectonic patterns on tectonically active bodies. This is of key interest in the light of upcoming space missions exploring these worlds some of which may even harbour life below their tectonic shell.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek