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

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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 teams 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.