Effects of 3-D Mantle Wedge Flow and Crystal Preferred Orientation on Shear-Wave Splitting in Subduction Zones

3-D 地幔楔流和晶体择优取向对俯冲带剪切波分裂的影响

• 批准号: 2321144
• 负责人: Ikuko Wada
• 金额: $ 21.03万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-09-01 至 2025-08-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2321144&HistoricalAwards=false
• 关键词: Effects; Mantle; Wedge; Flow; Crystal

The Earth’s mantle is solid, but it flows very slowly under high pressure and temperature, like a thick fluid. This flow affects important geological processes such as earthquakes and volcanic eruptions, but it occurs too far below the Earth’s surface to be observed directly. In subduction zones, where oceanic plates descend below continents, mantle flow patterns are thought to be very complex. Though these patterns can be estimated using computer models, we need real data to check their accuracy. One way to indirectly obtain mantle flow data is to use seismic waves that are generated by earthquakes. Seismic waves that travel through the mantle are affected by fine layering that forms when mantle minerals line up in response to flow. Recordings of such waves at the Earth’s surface can be interpreted to identify mantle regions where minerals are lined up, showing what the flow patterns look like in 3D (at least, in parts of the mantle that many of the recorded seismic waves have passed through). In this study, Dr. Wada and her group will develop computer models of mantle flow in selected subduction zones, and predict how mantle minerals should line up. Next, they will use different software to predict what seismic waves travelling through this model mantle should look like when they reach the Earth’s surface. By comparing these predictions with data from seismometers, they can test the model and fully describe the geometry of mantle flow. The project will provide training for a postdoctoral scholar and an undergraduate student in seismology and scientific computer programming. The software and scientific results will be used in an undergraduate geophysics course at Minnesota and will be freely and publicly shared. One of the indirect observations that are used to infer mineral orientations and mantle flow directions are shear-wave splitting (SWS) observations. Minerals in the mantle, in particular olivine, become aligned relatively to the mantle flow direction, developing crystal preferred orientations (CPO). Shear waves that are generated by earthquakes travel through the upper mantle and become polarized into two orthogonal components that travel at different speeds. Measurements of the polarization direction of the fast component and the delay time between the two components provide information about the CPO of minerals in the mantle and thus the mantle flow pattern. However, the interpretation of the SWS parameters (i.e., the fast direction and the delay time) in terms of CPO and mantle flow directions is not trivial, and large uncertainties remain, particularly in subduction zones with oblique plate convergence that induces complex mantle wedge flow patterns. The previously funded project generated a suite of 3-D steady-state coupled kinematic-dynamic models for generic subduction systems with a range of subduction obliquity to examine its effect on the mantle wedge flow pattern. The model-predicted variation in mantle wedge flow pattern with the local subduction obliquity allowed prediction of along-margin variations in the mantle wedge flow pattern based on the local subduction obliquity, for subduction zones worldwide. Using calculated steady-state mantle velocity fields, the evolution of olivine and pyroxene CPOs in the mantle wedge was simulated. The calculated distributions of CPO were fed into the forward modeling of SWS. The results highlighted the complex evolution of CPOs in the mantle wedge for different CPO types and their impact on the SWS parameters. The proposed study is aimed to fill the gaps in the previous work through the following three activities: (1) Quantifying the variations in the CPO distribution and SWS parameters due to along-margin variations in the margin strike and the slab geometry. (2) Testing the model-predicted SWS parameters against observed SWS parameters in NE Japan where sufficient detailed SWS analyses have been performed. (3) Investigating the relative importance of the impact of transtensional and transpressional strains in the mantle wedge during slab rollback and advance on the CPO evolution and SWS. The results provide foundational understanding of the impacts of subduction zone geometry and slab motion on deformation and CPO evolution in the mantle wedge and the dependence of SWS parameters on the CPO distribution. The synthesis of the results with those from the previously funded project will be used to address the observed variations in SWS parameters within and among subduction systems globally.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

地球的地幔是固体的，但它在高压和高温下流动非常缓慢，就像一种厚厚的流体。这种流动会影响地震和火山喷发等重要的地质过程，但它发生在地球表面以下太远，无法直接观察到。在俯冲带，海洋板块下降到大陆以下，地幔流动模式被认为是非常复杂的。虽然这些模式可以使用计算机模型来估计，但我们需要真实的数据来检查它们的准确性。间接获取地幔流动数据的一种方法是使用地震产生的地震波。在地幔中传播的地震波受到地幔矿物对流动的响应而形成的精细分层的影响。在地球表面记录的这种波可以被解释为识别矿物排列的地幔区域，以3D方式显示流动模式(至少在许多记录到的地震波经过的地幔部分)。在这项研究中，和田博士和她的团队将开发选定俯冲带中地幔流动的计算机模型，并预测地幔矿物应该如何排列。接下来，他们将使用不同的软件来预测穿过这个模型地幔的地震波到达地球表面时应该是什么样子。通过将这些预测与地震仪的数据进行比较，他们可以检验该模型，并完全描述地幔流动的几何形状。该项目将为一名博士后学者和一名本科生提供地震学和科学计算机编程方面的培训。该软件和科学成果将用于明尼苏达州的一门本科生地球物理学课程，并将免费和公开分享。用于推断矿物方向和地幔流动方向的间接观测之一是剪切波分裂(SWS)观测。地幔中的矿物，特别是橄榄石，相对于地幔流动方向排列，形成晶体优先取向(CPO)。地震产生的横波通过上地幔传播，并被极化成两个以不同速度传播的垂直分量。对快成分的极化方向和两个成分之间的延迟时间的测量提供了关于地幔矿物的CPO的信息，从而提供了地幔流动模式的信息。然而，用CPO和地幔流动方向解释SWS参数(即快方向和延迟时间)并不是微不足道的，仍然存在很大的不确定性，特别是在板块倾斜会聚的俯冲带，导致复杂的地幔楔形流动模式。之前资助的项目为具有一定俯冲倾角范围的通用俯冲系统生成了一套3-D稳态运动学-动力学耦合模型，以检查其对地幔楔形流动模式的影响。模型预测的地幔楔形流动模式随局部俯冲倾角的变化允许根据全球俯冲带的局部俯冲倾角预测地幔楔形流动模式的沿边缘变化。利用计算的稳态地幔速度场，模拟了橄榄石和辉石在地幔楔体中的演化过程。将计算得到的CPO分布输入到SWS正演模拟中。结果表明，不同CPO类型下地幔楔体中CPO的复杂演化及其对SWS参数的影响。这项研究旨在通过以下三个方面的工作来填补以往工作的空白：(1)量化边缘走向和板坯几何形状沿边缘变化引起的CPO分布和SWS参数的变化。(2)将模式预报的SWS参数与日本东北部观测到的SWS参数进行对比检验，其中已进行了足够详细的SWS分析。(3)研究了在板坯回滚和推进过程中，地幔楔体内的扭张应变和扭压应变对CPO演化和SWS的相对重要性。这些结果为了解俯冲带几何形状和板块运动对地幔楔形变和CPO演化的影响，以及SWS参数对CPO分布的依赖关系提供了基础。这些结果与之前资助的项目结果的综合将被用来解决在全球俯冲系统内部和之间观察到的SWS参数的变化。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。