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

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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.