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，但是在高压和温度下流动非常缓慢，就像厚的液体一样。这种流程会影响重要的地质过程，例如地震和火山喷发，但它发生在地球表面以至于直接观察到地球表面。在俯冲带中，海洋板下降到大陆以下，地幔流模式被认为非常复杂。尽管可以使用计算机模型来估算这些模式，但我们需要实际数据来检查它们的准确性。间接获得地幔流数据的一种方法是使用地震产生的地震波。穿过地幔的地震波会受到地幔矿物在响应流动时排队时形成的细小分层的影响。可以解释在地球表面的此类波的记录，以识别矿物排列的地幔区域，显示3D中流动模式的外观（至少在许多记录的地震中已经经过的部分地幔中）。在这项研究中，Wada博士和她的小组将在选定的俯冲带中开发地幔流的计算机模型，并预测地幔矿物应如何排队。接下来，他们将使用不同的软件来预测当它们到达地球表面时穿过该模型地幔的地震波应该是什么样的。通过将这些预测与地震仪的数据进行比较，它们可以测试模型并充分描述地幔流的几何形状。该项目将为博士后科学和科学计算机编程的本科生提供培训。该软件和科学结果将在明尼苏达州的本科生地球物理学课程中使用，并将免费共享。用于推断矿物方向和地幔流方向的间接观察之一是剪切波分裂（SWS）观测值。地幔中的矿物质，尤其是橄榄石，相对于地幔流动方向而变得一致，形成了晶体优选方向（CPO）。地震产生的剪切波穿过上地幔，并变成两个正交组件，它们以不同的速度传播。快速组分的极化方向和两个组件之间的延迟时间的测量提供了有关地幔中矿物质CPO的信息，从而提供了地幔流图。然而，在CPO和地幔流方向方面，SWS参数（即快速方向和延迟时间）的解释并不是微不足道的，并且仍然存在较大的不确定性，尤其是在具有影响复杂地幔楔子流动模式的斜板收敛的俯冲区中。先前资助的项目生成了一套3D稳态耦合运动动力模型，用于具有一系列俯冲倾斜范围的通用俯冲系统，以检查其对地幔楔流的影响。地幔楔流模式的模型预测变化具有局部俯冲倾斜度，可以预测基于局部俯冲倾斜度的地幔楔流模式的沿线楔形流量变化，以便全球俯冲带。使用计算出的稳态地幔速度场，模拟了地幔楔中橄榄石和辉石CPO的演化。 CPO的计算分布被馈入SWS的正向建模。结果强调了针对不同CPO类型的地幔楔中CPO的复杂演化及其对SWS参数的影响。拟议的研究旨在通过以下三个活动来填补先前工作中的空白：（1）量化CPO分布和SWS参数的变化，这是由于保证金打击和平板几何形状的差异变化而引起的。 （2）对日本NE的SWS参数测试模型预测的SWS参数，并进行了足够的详细SWS分析。 （3）调查在平板回滚过程中，在地幔楔中变换性和透射菌株的影响的相对重要性，并在CPO进化和SWS上进行前进。结果提供了对俯冲区几何形状和平板运动对地幔楔形变形和CPO演化的影响的基本理解，以及SWS参数对CPO分布的依赖性。结果与先前资助的项目的合成将用于解决全球俯冲系统内部和俯冲系统中观察到的变化。该奖项反映了NSF的法定任务，并被认为是通过基金会的智力优点和更广泛的影响标准来评估通过评估来获得的支持。