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

对过渡带周围和地幔深部俯冲板块精细尺度行为的新见解

• 批准号: 0842753
• 负责人: Jun Korenaga
• 金额: $ 26.68万
• 依托单位: Yale University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-01-01 至 2011-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0842753&HistoricalAwards=false
• 关键词: New; Insights; into; Fine; scale

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.

北美大陆的形成和演化的特点是在过去的30亿年里较小的大陆块的聚集和分散。因此，了解这块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它们同化为大陆演化的统一模型至关重要。该项目基于这样一种理念，即如果在全球框架内同时进行开发，那么在区域角度上取得实质性进展将是最有效的，并且受到USArray提供的独特而令人兴奋的机会的推动，可以在解决地幔对流结构及其化学演化等一级问题方面取得重大进展。在过去的几十年里，地幔对流的类型一直争论不休，而运移成像有可能通过提供一种新的地震证据来补充层析成像，从而取得突破。俯冲板块不仅比周围地幔温度低，而且由两层化学性质截然不同的岩石圈组成，即富含石榴石的薄洋壳和富含橄榄石的地幔岩石圈。随着压力的增加，地震速度对温度的敏感性降低，俯冲板块在深部地幔中的热特征更难探测，但其化学特征仍然保持稳定，可以通过分析地震阵列数据来探测。此外，对俯冲板块的化学分层负责的地幔分异指出了迁移成像在理解化学地球动力学方面的另一个重要作用，即对俯冲海洋地壳命运的地震测绘。为了加深我们对地幔对流及其化学演化的认识，PI和他的同事将新一代迁移方法应用于USArray数据，对中美洲整个地幔的精细尺度散射体的三维分布进行了成像。从许多层析模型来看，中美洲是少数俯冲板块似乎能渗透到下地幔的地区之一，并且在震源和震源接收器的分布上，即南美洲的深震和北美的大孔径地震阵（USArray）都处于良好的位置。本研究也结合本科教育，为地球物理学的核心课程设计一个新的实验课程，并招募学生进行阵列地震资料处理。高级迁移方法的源代码计划与用户手册和教程一起发布。