A coupled model for phase mixing, grain damage and shear localization in the lithosphere: comparison to lab experiments
A coupled model for phase mixing, grain damage and shear localization in the lithosphere: comparison to lab experiments
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岩石圈中相混合、颗粒损伤和剪切局部化的耦合模型：与实验室实验的比较
DOI：
10.1093/gji/ggac428
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发表时间：
2022
影响因子：
2.8
通讯作者：
Skemer, Philip
中科院分区：
文献类型：
--
作者：
Bercovici, David;Mulyukova, Elvira;Girard, Jennifer;Skemer, Philip
地球板块构造的发生植根于岩石圈韧性弱化和剪切局部化的物理学。岩石圈剪切带中糜棱岩的普遍性是一个关键的证据，本地化与矿物粒度的减少。大多数岩石圈糜棱岩是多矿物和矿物相之间的相互作用，如橄榄石和辉石，特别是通过齐纳钉扎，阻碍正常的晶粒生长，同时可能提高晶粒损伤，这两个促进晶粒尺寸减小和弱化，在实验室实验和现场观察中很明显。然而，钉扎的功效依赖于矿物相在颗粒尺度上的混合和分散，其中良好的混合状态导致更大的糜棱岩化。为了在连续尺度上模拟不同相之间的颗粒混合，我们以前开发了一种理论，将颗粒尺度过程视为相之间的扩散，但由作用于相之间边界的压缩应力驱动。在这里，我们提出了一个新的模型剪切岩石，结合我们的理论扩散颗粒混合，2-D非牛顿流和两相颗粒损伤。该模型的几何形状是专门设计的比较扭转剪切变形实验。变形要么是由恒定速度或恒定应力边界条件。当层变形时，不同矿物单元之间的混合区经历增强的粒度减小和弱化，特别是在高应变下。对于恒速边界实验，应力以约4的应变向初始测压平台下降;这也是单相实验的典型情况，对于单相实验，该初始平台是最终的稳态应力。然而，多相实验可以在10-20的应变下经历第二次大的应力下降，并且这与增强的相混合和所得的晶粒尺寸减小和弱化有关。多相介质与晶粒混合和损伤的模型计算捕获的实验行为时，相之间的界面损伤是适度缓慢或效率较低的晶界损伤。其他因素，如第二相的分布和体积分数，以及颗粒混合扩散率也影响第二应力下降的时间。对于常应力边界条件，应变速率在弱化和局部化过程中增加。对于单相介质，理论上应变速率增加一次达到测压稳定状态。但对于多相模型，应变速率经历了第二次突然增加，其时机再次控制的界面损伤和晶粒混合。通过混合和变形的非均匀性的演变，以及粒度分布也比较好的实验观察。总的来说，理论与变形实验的比较为指导未来的实验提供了一个框架，将微结构物理学扩展到地球动力学应用，并证明了颗粒混合和损伤对板块构造边界形成的重要性。
The occurrence of plate tectonics on Earth is rooted in the physics of lithospheric ductile weakening and shear-localization. The pervasiveness of mylonites at lithospheric shear zones is a key piece of evidence that localization correlates with reduction in mineral grain size. Most lithospheric mylonites are polymineralic and the interaction between mineral phases, such as olivine and pyroxene, especially through Zener pinning, impedes normal grain growth while possibly enhancing grain damage, both of which facilitate grain size reduction and weakening, as evident in lab experiments and field observations. The efficacy of pinning, however, relies on the mineral phases being mixed and dispersed at the grain scale, where well-mixed states lead to greater mylonitization. To model grain mixing between different phases at the continuum scale, we previously developed a theory treating grain-scale processes as diffusion between phases, but driven by imposed compressive stresses acting on the boundary between phases. Here we present a new model for shearing rock that combines our theory for diffusive grain mixing, 2-D non-Newtonian flow and two-phase grain damage. The model geometry is designed specifically for comparison to torsional shear-deformation experiments. Deformation is either forced by constant velocity or constant stress boundary conditions. As the layer is deformed, mixing zones between different mineralogical units undergo enhanced grain size reduction and weakening, especially at high strains. For constant velocity boundary experiments, stress drops towards an initial piezometric plateau by a strain of around 4; this is also typical of monophase experiments for which this initial plateau is the final steady state stress. However, polyphase experiments can undergo a second large stress drop at strains of 10–20, and which is associated with enhanced phase mixing and resultant grain size reduction and weakening. Model calculations for polyphase media with grain mixing and damage capture the experimental behaviour when damage to the interface between phases is moderately slower or less efficient than damage to the grain boundaries. Other factors such as distribution and bulk fraction of the secondary phase, as well as grain-mixing diffusivity also influence the timing of the second stress drop. For constant stress boundary conditions, the strain rate increases during weakening and localization. For a monophase medium, there is theoretically one increase in strain rate to a piezometric steady state. But for the polyphase model, the strain rate undergoes a second abrupt increase, the timing for which is again controlled by interface damage and grain mixing. The evolution of heterogeneity through mixing and deformation, and that of grain size distributions also compare well to experimental observations. In total, the comparison of theory to deformation experiments provides a framework for guiding future experiments, scaling microstructural physics to geodynamic applications and demonstrates the importance of grain mixing and damage for the formation of plate tectonic boundaries.