Constraining multi-scale interactions between slabs and mantle flow within Western Pacific subduction zones

University Of Miami, Coral Gables FL

Investigators

Abstract

Subduction zones, where one tectonic plate descends beneath another, are responsible for forming some of the Earth's most dramatic mountain ranges - and for some of its deadliest earthquakes and volcanoes. In the underlying mantle, where the descending plate is referred to as a “slab”, it is surrounded by hotter material which behaves as a fluid over geological time. The pattern and speed of mantle flow around the slab governs directions and magnitudes of forces acting on it and on the overlying plates. These forces play an important role in driving plate deformation, including mountain building and earthquakes, but they cannot be observed directly. They must be estimated, using sophisticated mathematical models. In this project, Holt and his team will develop such models and run several sets of calculations to investigate mantle flow and its effect on slabs and plates in the Western Pacific region. This region was chosen because there is abundant data that can be used to test the model calculations, and because the geometry of slabs and plates is complex (and hence interesting). Integrating a model with this complexity into a global-scale mantle flow model presents an immense technical challenge which has already been partially met by Holt. His group's new models will first address how much mantle flow may differ from estimates given by older, simplified models. After they have been adjusted to be consistent with geological and geophysical data from the Western Pacific, the models will shed new light on mantle flow and forces on plates in this complex region. The PI and team will develop and freely share software tools, opening the door for other groups to model similarly complex, earthquake-prone regions (e.g., the Caribbean and Southeast Asia). This project will support a graduate student and an early-career professor. The models and outputs will be used for teaching model visualization and data analysis to Geodynamics students at the University of Miami. The overarching goal of this research project is developing a mechanical framework for interactions between the mantle and lithospheric plates, with a focus on the geometrically complex Western Pacific. This framework will be developed by using a suite of global subduction models to predict the mantle flow regime and associated forces in the vicinity of the main Western Pacific slabs (e.g., Kuril-Japan, Ryukyu-Nankai, and Izu-Bonin-Mariana). After the team compiles a database of Western Pacific observables sensitive to mantle dynamics, global numerical models incorporating highly resolved Western Pacific slabs and weak plate boundaries will be run to quantify the mantle flow and viscosity fields consistent with these observations. The team will also use a suite of targeted numerical tests to examine how the flow-field and associated forces are impacted by both global-scale flow and mantle flow localization (due to a power-law viscous rheology). Three inter-linked questions related to lithosphere-mantle flow interactions will be addressed. At a regional scale: Does mantle out-flow from beneath the Philippine Sea Plate (PSP) occur and produce forces that dictate Western Pacific subduction observables? Does this out-flow, which occurs through narrow gaps between slabs, require flow localization due to power-law creep? At a global scale: Do forces imposed by large-scale mantle flow strongly impact the Western Pacific slabs (e.g., do they produce the very shallow dip of the Japan slab)? In addressing these questions, this study will provide transferrable insight on the relative contributions of complex slab geometries and global-scale mantle flow to subduction zone mechanics. 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.