Textural evolutiona plate boundary problem

结构演化板块边界问题

• 批准号: 1928827
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2017
• 资助国家: 英国
• 起止时间: 2017 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-1928827
• 关键词: Textural; evolutiona; plate; boundary; problem

Plate tectonics is a first order feature of our planet that is integral to the geosciences. Yet despite its importance, our understanding remains 'kinematic' rather than 'dynamic'. In order to produce plates, strain needs to be localised into thin boundaries allowing the plate interior to remain approximately rigid. Modelling approaches have demonstrated that the power-law rheologies associated with deformation mechanisms are insufficient to localise strain. Consequently, more exotic physics is required to understand plate tectonics.More recently, non-linear feedbacks associated with fabric elements such as grain-size, crystallographic preferred orientation (CPO) and melt preferred orientation (MPO) have been proposed as key mechanisms of generating strain localisation. The grain-size feedback operates in a heterogeneous microstructure, where large grains generate small grains as they strain which are then able to readily deform through diffusion creep. The positive feedback between increasing strain and increasing strain rate generates an instability which can cause strain to localise. The CPO and MPO feedback occurs due to viscous anisotropy associated with preferred orientation: rocks which are favourably orientated deform much more readily than rocks in other orientations. Since both CPO and MPO form and strengthen with increasing strain, there is the potential for a feedback which localises deformation.However, each of these fabric elements have problems when applied to the plate tectonic problem. The small grain sizes associated with the grain size feedback are not thought to be long lived enough to explain the reactivation of old plate boundary scars. The potential for CPO based strain localisation may be limited due to the progressive rotation of crystallographic axis from their optimal orientation with increasing strain. In addition, CPO is generally thought to form in the dislocation creep regime, so if the grain size is too small due to the grain size feedback, CPO may be unable to form. Recent experiments have also demonstrated an interaction between CPO and MPO termed the 'a-c' switch. Clearly an integrated approach which considers the dynamical interaction of grain-size, CPO and MPO together is needed to assess the potential role of these fabric elements and their respective feedbacks in the formation and operation of plate boundaries.To assess the role of each fabric element, models must be developed which describe the evolution of each element as a function of time and other variables such as strain rate. Models exist for each of these fabric elements but are problematic when applied to the problem of plate boundaries. For example, grain size evolution models typically parameterise grain size as a single scalar variable and so cannot model the microstructural heterogeneity needed to develop the grain size feedback. Although a model for MPO development exists, it too parameterises the strength of MPO as a single scalar variable, and in the parameterisation of the model important physics is missing such as a treatment of the reduction in strength of MPO as strain rate decreases. Despite these challenges, new approaches to modelling these fabric elements have opened new avenues for addressing the problem. A recent model of CPO evolution has tackled the issue of olivine not fulfilling the von Mises criterion by seeking a best fit solution instead of the more technical approaches used in the past. This approach has both improved the accuracy and significantly decreased the complexity of CPO modelling. A yet unpublished model of grain size approaches the problem through constraints on the rate of dissipation of entropy.

板块构造是地球的一级特征，是地球科学的组成部分。然而，尽管它的重要性，我们的理解仍然是'运动'，而不是'动态'。为了生产板材，应变需要局限于薄的边界，允许板材内部保持近似刚性。建模方法已经证明，与变形机制相关的幂律流变性不足以使应变局部化。最近，与组构元素相关的非线性反馈，如晶粒尺寸、晶体择优取向（CPO）和熔体择优取向（MPO），被认为是产生应变局部化的关键机制。晶粒尺寸反馈在异质微结构中起作用，其中大晶粒在它们应变时产生小晶粒，然后小晶粒能够通过扩散蠕变容易地变形。增加的应变和增加的应变速率之间的正反馈产生不稳定性，其可导致应变局部化。CPO和MPO反馈的发生是由于与优选取向相关的粘性各向异性：有利取向的岩石比其他取向的岩石更容易变形。由于CPO和MPO都是随着应变的增加而形成和加强的，因此存在着使变形局部化的反馈的可能性。然而，当应用于板块构造问题时，这些组构元素中的每一个都有问题。与粒度反馈相关的小粒度被认为不足以解释旧板块边界疤痕的重新激活。基于CPO的应变局部化的潜力可能是有限的，这是由于随着应变的增加，晶轴从其最佳取向逐渐旋转。此外，CPO通常被认为是在位错蠕变状态下形成的，因此如果由于晶粒尺寸反馈而导致晶粒尺寸太小，则CPO可能无法形成。最近的实验也证明了CPO和MPO之间的相互作用，称为“a-c”开关。显然，需要一个综合的方法，认为动态的相互作用的粒度，CPO和MPO一起评估这些组构元素的潜在作用和各自的反馈在形成和操作的板块boundarys.To评估的作用，每个组构元素，必须开发的模型，描述每个元素的演变作为一个函数的时间和其他变量，如应变率。这些组构元素的模型存在，但当应用于板块边界的问题时是有问题的。例如，晶粒尺寸演化模型通常将晶粒尺寸参数化为单个标量变量，因此不能对开发晶粒尺寸反馈所需的微观结构异质性进行建模。尽管存在MPO发展的模型，但它也将MPO的强度参数化为单个标量变量，并且在模型的参数化中缺少重要的物理学，例如随着应变率降低MPO强度降低的处理。尽管存在这些挑战，但对这些织物元素进行建模的新方法为解决这一问题开辟了新的途径。最近的一个CPO演化模型解决了橄榄石不满足冯米塞斯标准的问题，通过寻求最佳的解决方案，而不是过去使用的更技术的方法。这种方法既提高了准确性，又显着降低了CPO建模的复杂性。一个尚未发表的晶粒尺寸模型通过对熵耗散率的约束来解决这个问题。