New Insights into the Fine-scale Behavior of Subducted Slab Around the Transition Zone and in the Deep Mantle

北美大陆的形成和演化的特点是在过去的30亿年中，较小的大陆块的聚集和分散。因此，了解该大陆的起源为长期地幔动力学提供了重要线索，同样，了解地幔动力学的全球框架对于正确解释碎片地质记录并将其同化为大陆演化的统一模型至关重要。该项目的关键在于这样一个概念，即如果在全球框架内同时进行开发，则在区域角度上的实质性进展将最有效地实现，并且受到USAray提供的独特而令人兴奋的机会的激励，以在解决地幔对流结构及其化学演化等一阶问题方面取得相当大的进展。地幔对流的类型在过去的几十年里一直存在激烈的争论，偏移成像有可能取得突破，通过提供一种新的地震证据补充层析成像。俯冲板片不仅比周围地幔温度低，而且由两个化学性质不同的层组成，即薄的富石榴石洋壳和富橄榄石地幔岩石圈。俯冲板片的热特征在地幔深部变得更加难以探测，因为随着压力的增加，地震速度对温度的敏感性降低，但其化学特征仍然很强，这可以通过分析地震阵列数据来探测。此外，地幔分异负责俯冲板块的化学分层点迁移成像在理解化学地球动力学，即俯冲洋壳的命运的地震映射中发挥的另一个重要作用。为了加深我们对地幔对流类型及其化学演化的理解，PI和他的同事将新一代迁移方法应用于USAray数据，以成像中美洲地幔中细尺度散射体的三维分布。中美洲是少数几个根据一些层析成像模型俯冲板片似乎穿透到下地幔的地区之一，就震源和接收器的分布而言，它所处的位置很好，即，南美洲的深聚焦地震和北美的大孔径地震阵列（USAray）。这项研究还与本科教育相结合，通过设计一个新的实验室会议的核心课程在地球物理学和招收学生阵列地震数据处理。计划分发高级迁移方法的源代码以及用户手册和教程。

The formation and evolution of the North American continent is characterized by the assembly and dispersal of smaller continental masses over the last three billion years. Understanding the origin of this continent thus provides important clues to long-term mantle dynamics, and by the same token, understanding the global framework of mantle dynamics is vital to properly interpret fragmentary geological records and assimilate them into a unifying model for continental evolution. This project hinges on the notion that substantial progress in a regional perspective would be achieved most efficiently if concurrent development is also made in a global framework, and is motivated by a unique and exciting opportunity provided by the USArray to make considerable progress toward resolving first-order issues such as the structure of mantle convection and its chemical evolution. The style of mantle convection has been debated intensely in the last few decades, and migration imaging has a potential to make a breakthrough, by providing a new kind of seismic evidence complementary to tomographic imaging. Subducted slab is not only cooler than the ambient mantle but also composed of two chemically distinct layers, namely thin garnet-rich oceanic crust and olivine-rich mantle lithosphere. The thermal signature of subducted slab becomes more difficult to detect in the deep mantle because seismic velocity becomes less sensitive to temperature with increasing pressures, but its chemical signature remains robust, which may be detected by analyzing seismic array data. Also, mantle differentiation responsible for the chemical layering of subducted slabs points to another important role played by migration imaging in understanding chemical geodynamics, namely the seismic mapping of the fate of subducted oceanic crust. In order to advance our understanding on the style of mantle convection and its chemical evolution, the PI and his colleague apply new-generation migration methods to the USArray data to image the three-dimensional distribution of fine-scale scatterers throughout the mantle beneath Central America. Central America is one of the few regions where subducted slab seems to penetrate into the lower mantle according to a number of tomographic models, and it is well situated in terms of the distribution of seismic sources and receivers, i.e., deep-focused earthquakes in South America and a large-aperture seismic array in North America (USArray). This research is also coupled with undergraduate education, by designing a new laboratory session for a core course in geophysics and recruiting students to array seismology data processing. The source code of the advanced migration methods is planned to be distributed, together with a user manual and a tutorial.