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.

地幔是固体，但在高压和高温下流动非常缓慢，就像一种粘稠的液体。这种流动影响着重要的地质过程，如地震和火山爆发，但它发生在地球表面以下很远的地方，无法直接观察到。在俯冲带，即大洋板块下降到大陆之下的地方，地幔流动模式被认为是非常复杂的。虽然这些模式可以用计算机模型来估计，但我们需要真实的数据来检验它们的准确性。间接获得地幔流数据的一种方法是利用地震产生的地震波。穿过地幔的地震波受到地幔矿物随流动排列而形成的细层的影响。这些地震波在地球表面的记录可以被解释为识别地幔区域，那里的矿物排列整齐，显示出三维的流动模式（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在选定的俯冲带开发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地震波穿过这个模型地幔到达地球表面时的样子。通过将这些预测与地震仪的数据进行比较，他们可以测试这个模型，并充分描述地幔流动的几何形状。该项目将为地震学和科学计算机编程方面的一名博士后学者和一名本科生提供培训。该软件和科学成果将用于明尼苏达大学的一门地球物理学本科课程，并将免费公开分享。用于推断矿物取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）。地震产生的横波穿过上地幔，并被极化成两个正交分量，以不同的速度传播。测量快速分量的偏振方向和两个分量之间的延迟时间提供了地幔中矿物的CPO信息，从而提供了地幔流动模式。然而，根据CPO和地幔流动方向对SWS参数（即快速方向和延迟时间）的解释并不简单，并且存在很大的不确定性，特别是在倾斜板块收敛的俯冲带，导致复杂的地幔楔流模式。先前资助的项目为具有一定范围俯冲倾角的一般俯冲系统生成了一套三维稳态耦合运动学动力学模型，以研究其对地幔楔流模式的影响。该模型预测的地幔楔流模式随局部俯冲倾角的变化，使基于局部俯冲倾角的地幔楔流模式沿缘变化预测成为可能，适用于全球俯冲带。利用计算得到的稳态地幔速度场，模拟了地幔楔体中橄榄石和辉石CPOs的演化。将计算得到的CPO分布输入到SWS正演模拟中。结果表明，地幔楔体中不同类型的CPO的复杂演化及其对SWS参数的影响。本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)量化由于边缘走向和板状结构的沿边缘变化而导致的CPO分布和SWS参数的变化。(2)在日本东北部进行了足够详细的SWS分析，将模型预测的SWS参数与观测到的SWS参数进行比较。(3)研究了板块回滚和推进过程中地幔楔内的张拉应变和张拉应变对CPO演化和SWS影响的相对重要性。研究结果为认识俯冲带几何形态和板块运动对地幔楔变形和CPO演化的影响以及SWS参数对CPO分布的依赖性提供了基础。将结果与先前资助项目的结果综合起来，将用于解决全球俯冲系统内部和之间SWS参数的观察变化。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。