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1. Simplified equations for lower crustal flow driven by lateral pressure gradients

   https://doi.org/10.1093/gji/ggab125  

   Perry-Houts, Jonathan; Humphreys, Eugene (May 2021, Geophysical Journal International)

   null (Ed.)

   SUMMARY Evidence from seismology, geology and geodynamic studies suggests that regional-scale lower crustal flow occurs in many tectonic settings. Pressure gradients caused by mantle processes and crustal density heterogeneity can provide driving force for lower crustal flow. Numerically modelling such flow can be computationally expensive. However, by exploiting symmetry in the physical system, it is possible to represent the vertical component of flow in terms of its lateral components, thereby reducing the problem’s spatial dimension by one. Here, we present a mathematical formulation for flow in a viscous channel below an elastic upper plate, which is optimized for solution by common numerical methods. Our formulation drastically reduces the computational load required to simulate lower crustal flow over large areas and long timescales. We apply this model to two example problems, with and without an elastic upper plate, identifying combinations of parameters that are capable of generating measurable geologic uplift. 

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2. An entropy method for geodynamic modelling of phase transitions: capturing sharp and broad transitions in a multiphase assemblage

   https://doi.org/10.1093/gji/ggac293  

   Dannberg, Juliane; Gassmöller, Rene; Li, Ranpeng; Lithgow-Bertelloni, Carolina; Stixrude, Lars (July 2022, Geophysical Journal International)

   SUMMARY Phase transitions play an important role for the style of mantle convection. While observations and theory agree that a substantial fraction of subducted slabs and rising plumes can move through the whole mantle at present day conditions, this behaviour may have been different throughout Earth’s history. Higher temperatures, such as in the early Earth, cause different phase transitions to be dominant, and also reduce mantle viscosity, favouring a more layered style of convection induced by phase transitions. A period of layered mantle convection in Earth’s past would have significant implications for the secular evolution of the mantle temperature and the mixing of mantle heterogeneities. The transition from layered to whole mantle convection could lead to a period of mantle avalanches associated with a dramatic increase in magmatic activity. Consequently, it is important to accurately model the influence of phase transitions on mantle convection. However, existing numerical methods generally preclude modelling phase transitions that are only present in a particular range of pressures, temperatures or compositions, and they impose an artificial lower limit on the thickness of phase transitions. To overcome these limitations, we have developed a new numerical method that solves the energy equation for entropy instead of temperature. This technique allows for robust coupling between thermodynamic and geodynamic models and makes it possible to model realistically sharp phase transitions with a wide range of properties and dynamic effects on mantle processes. We demonstrate the utility of our method by applying it in regional and global convection models, investigating the effect of individual phase transitions in the Earth’s mantle with regard to their potential for layering flow. We find that the thickness of the phase transition has a bigger influence on the style of convection than previously thought: with all other parameters being the same, a thin phase transition can induce fully layered convection where a broad phase transition would lead to whole-mantle convection. Our application of the method to convection in the early Earth illustrates that endothermic phase transitions may have induced layering for higher mantle temperatures in the Earth’s past. 

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3. Direct observations of transient weakening during phase transformations in quartz and olivine

   https://doi.org/10.1038/s41561-025-01703-6  

   Cross, Andrew J; Goddard, Rellie M; Kumamoto, Kathryn M; Goldsby, David L; Hansen, Lars N; Chen, Haiyan; Hein, Diede; Thom, Christopher A; Nehring, M Adaire; Breithaupt, Thomas; et al (June 2025, Nature Geoscience)

   Phase transformations are widely invoked as a source of rheological weakening during subduction, continental collision, mantle convection and various other geodynamic phenomena. However, despite more than half a century of research, the likelihood and magnitude of such weakening in nature remain poorly constrained. Here we use experiments performed on a synchrotron beamline to reveal transient weakening of up to three orders of magnitude during the polymorphic quartz to coesite (SiO2) and olivine to ringwoodite (Fe2SiO4) phase transitions. Weakening becomes increasingly prominent as the transformation outpaces deformation. We suggest that this behaviour is broadly applicable among silicate minerals undergoing first-order phase transitions and examine the likelihood of weakening due to the olivine-spinel, (Mg,Fe)2SiO4, transformation during subduction. Modelling suggests that cold, wet slabs are most susceptible to transformational weakening, consistent with geophysical observations of slab stagnation in the mantle transition zone beneath the western Pacific. Our study highlights the importance of incorporating transformational weakening into geodynamic simulations and provides a quantitative basis for doing so. 

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4. Mantle dynamics on large spatial and temporal scales

   https://doi.org/10.6038/cjg2021P0530  

   Zhong, Shijie (October 2021, Chinese journal of geophysics)

   This article provides a review on the studies of large temporal and spatial scale dynamics of the Earth’s mantle. The review focuses on relevant observations and their geodynamic interpretations and implications. These observations include present-day Earth’s plate tectonics, long- and intermediate-wavelength geoid and gravity anomalies, and mantle seismic structures, as well as important tectonism and magmatism that have happened in the last one billion years, associated with the formation and breakup of supercontinents Pangea and Rodinia. Much of the discussion is centered on how these observations have motivated geodynamic studies and modeling that seek to understand and interpret the observations. This review covers four topics. The first is on the primary characteristics of mantle seismic structure and their dynamic origin. The present-day Earth’s mantle is predominated by long-wavelength structures (i.e., degree-2 in the lower mantle and LLSVPs near the core-mantle boundary) and linear structures in subduction zones, both of which can be interpreted as a result of mantle convection modulated by surface plate motion history in the last 100 million years. The second is on the long- and intermediate-wavelength geoid and gravity anomalies and their dynamic interpretation. The geoid anomalies are explained by mantle flow that is driven by buoyancy associated with the mantle structure. Such studies indicate that the upper mantle is at least one magnitude weaker than the lower mantle and strongly suggest the existence of a weak asthenosphere. Third, the cyclic process of formation and breakup of supercontinents Pangea and Rodinia is surface manifestation of time-dependent mantle convection. During supercontinent formation and its early stage, mantle structure is predominately degree-1 with cold downwellings in one hemisphere and hot upwellings in the other hemisphere. However, the degree-1 structure starts to transition to degree-2 mantle structure with two major antipodal upwelling systems (e.g., the present-day Earth) in the late stage of a supercontinent, leading to supercontinent breakup. Abundant observational and dynamic evidence support the 1-2-1 model for supercontinent cycle and mantle structure evolution. The fourth is on the origin of plate tectonics and long-term thermal evolution of the Earth which is a fundamentally important but also controversial topic in the studies of earth science. 

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5. The morphology, evolution and seismic visibility of partial melt at the core-mantle boundary: Implications for ULVZs

   https://doi.org/10.1093/gji/ggab242  

   Dannberg, Juliane; Myhill, Robert; Gassmöller, René; Cottaar, Sanne (July 2021, Geophysical Journal International)

   null (Ed.)

   Seismic observations indicate that the lowermost mantle above the core-mantle boundary is strongly heterogeneous. Body waves reveal a variety of ultra-low velocity zones (ULVZs), which extend not more than 100 km above the core-mantle boundary and have shear velocity reductions of up to 30 per cent. While the nature and origin of these ULVZs remain uncertain, some have suggested they are evidence of partial melting at the base of mantle plumes. Here we use coupled geodynamic/thermodynamic modelling to explore the hypothesis that present-day deep mantle melting creates ULVZs and introduces compositional heterogeneity in the mantle. Our models explore the generation and migration of melt in a deforming and compacting host rock at the base of a plume in the lowermost mantle. We test whether the balance of gravitational and viscous forces can generate partially molten zones that are consistent with the seismic observations. We find that for a wide range of plausible melt densities, permeabilities and viscosities, lower mantle melt is too dense to be stirred into convective flow and instead sinks down to form a completely molten layer, which is inconsistent with observations of ULVZs. Only if melt is less dense or at most ca. 1 per cent more dense than the solid, or if melt pockets are trapped within the solid, can melt remain suspended in the partial melt zone. In these cases, seismic velocities would be reduced in a cone at the base of the plume. Generally, we find partial melt alone does not explain the observed ULVZ morphologies and solid-state compositional variation is required to explain the anomalies. Our findings provide a framework for testing whether seismically observed ULVZ shapes are consistent with a partial melt origin, which is an important step towards constraining the nature of the heterogeneities in the lowermost mantle and their influence on the thermal, compositional, and dynamical evolution of the Earth. 

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