Collaborative Research: Investigating formation of stagnant slabs and implications for subduction dynamics

板块构造理论建立至今已有几十年了。科学家们仍然在争论构造板块在下降的过程中是如何随着时间的推移循环进入地球内部的。生成地球“地震断层图像”（类似于医学CT扫描）的进展提供了这些沉没板块（或“板”）的越来越详细的图像，其中许多被困在地球表面以下约300至600英里处。这些图像代表了对流地幔的现代快照，其随时间的演变可以通过俯冲过程的数值模拟来研究。这项合作提案结合了数据驱动建模和地震层析成像的最新发展。Liu，Tromp和他们的研究生将探索俯冲板块被困在中地幔中的不同方式，重点是两个相互竞争的假设：板块被粘在表面的板块的一端拉动，或者（2）它被周围地幔的水平流动沿着拖动。这项研究将提高我们对地球地幔如何流动的理解，以及它如何流动（以及地球板块如何移动）数千万年。用于板块建模和地幔成像的先进软件将得到显著改进，并通过公共网站与其他科学家免费共享。在500-1000 km地幔深度范围内广泛的快速地震异常被称为停滞板块，它们的起源对大陆构造和地幔动力学有影响。一个常用的机制是战壕撤退，但观察到的撤退量并不总是与模拟相匹配。另一个常见的假设是板块垂直下沉，即使这可能违反地质观测。这些问题的一个潜在的解决方案是考虑在中地幔深处的横向地幔流，其对板片停滞的影响仍然很少探索。另一个复杂的问题是断层图像的不一致性，目前的配置停滞板。PI团队将通过顺序数据同化（SDA）设计3D球形地球模型，并将结果与使用全波形反演（FWI）和源编码的改进层析成像图像进行比较，共同评估海沟退缩和横向地幔流对板块停滞的动态影响。SDA建模方法适用于测试板坯停滞的关键控制参数，同时考虑许多其他自然复杂性。改进后的FWI方法在中上地幔深度显著提高了地震分辨率，而中上地幔深度是停滞板的所在地。通过一组SDA模型，测试在区域和全球尺度模拟中实施的不同地理位置的沟槽运动历史的可用范围，该团队将量化沟槽退缩和横向地幔流对观察到的停滞板形成的各自贡献。最终，这项工作将产生新的洞察俯冲动力学和相关的构造记录。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

Many decades have passed since the establishment of the theory of plate tectonics. Scientists still debate how tectonic plates recycling into Earth’s interior evolve over time as they descend. Advances in generating 'seismic tomographic images' of the Earth (analogous to medical CT scans) provide increasingly detailed images of these foundered plates (or 'slabs'), many of which are trapped about 300 to 600 miles below the Earth's surface. These images represent a present-day snapshot of the convecting mantle, and its evolution over time may be investigated through numerical modeling of the subduction process. This collaborative proposal combines recent developments in data-driven modeling and seismic tomography. Liu, Tromp, and their graduate students will explore different ways that subducted slabs get trapped in the mid mantle, with a focus on two competing hypotheses: that the slab is being pulled by one end which is stuck to a plate at the surface, or (2) that it is dragged along with horizontal flow of the surrounding mantle. This research will improve our understanding of how the Earth's mantle flows, and how it has flowed (and how the Earth's plates have moved) over tens of millions of years. Sophisticated software for modeling the slabs and imaging the mantle will be significantly improved and freely shared with other scientists via public websites.Extensive fast seismic anomalies within the 500-1000 km mantle depth range are called stagnant slabs, and their origin has implications for continental tectonics and mantle dynamics. A commonly invoked mechanism is trench retreat, but the observed amount of retreat does not always match simulations. Another frequently made assumption is that slabs sink vertically, even though this can violate geological observations. A potential solution to these problems is to consider lateral mantle flow at mid-mantle depths, whose effect on slab stagnation remains poorly explored. Another complication is the inconsistency of tomographic images of the present-day configuration of stagnant slabs. The PI teams will collaboratively evaluate dynamic effects of trench retreat and lateral mantle flow on slab stagnation by designing 3D spherical Earth models through sequential data assimilation (SDA) and comparing the results with improved tomographic images using full-waveform inversion (FWI) with source encoding. The SDA modeling approach is appropriate for testing key controlling parameters for slab stagnation while simultaneously considering many other natural complexities. The improved FWI method increases seismic resolution significantly at upper-to-mid mantle depths, where stagnant slabs reside. Through a set of SDA models that test the available range of trench motion histories at different geographic locations implemented in both regional- and global-scale simulations, the team will quantify the respective contributions of trench retreat and lateral mantle flow on the formation of observed stagnant slabs. Ultimately, this exercise will generate new insight into subduction dynamics and the associated tectonic records.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.