Grain size reduction by plug flow in the wet oceanic upper mantle explains the asthenosphere's low seismic Q zone

The prominent seismic low-velocity zone (LVZ) in the oceanic low-viscosity asthenosphere is approximately coincident with the low seismic Q zone (LQZ; a zone of high seismic attenuation). Small olivine grain sizes could link these seismic and rheological properties because they reduce viscosity, seismic velocity and seismic Q. Because rock deformation reduces grain sizes, the asthenosphere's seismic properties can place constraints on asthenospheric flow. To determine dominant flow patterns, we develop a self-consistent analytical 1-D channel flow model that accounts for upper mantle rheology and its dependence on flow-modified grain-sizes, water content, and melt fraction, for flow driven by both surface plate motions (Couette flow) and/or horizontal pressure gradients (Poiseuille flow). We find that Couette flow dominates if the upper mantle is dry, and plug flow (a Poiseuille flow for power law rheology) dominates if it is weakened by wet conditions. A plug flow configuration spanning the upper 670 km of the mantle best explains the LQZ in the asthenosphere, and can be attributed to significant grainsize reduction due to extensive shearing across the asthenosphere. This flow configuration also explains high seismic Q values below the asthenosphere associated with minimal shear deformation and large grain sizes above the mantle transition zone. We suggest that the asthenospheric LQZ and LVZ seismic anomalies can be largely explained by grain-size variations associated with plug flow in the wet upper mantle. & COPY; 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/).

Automated summary

This study investigates the relationship between the prominent seismic low-velocity zone (LVZ) and the low seismic Q zone (LQZ) in the oceanic low-viscosity asthenosphere, and suggests that small olivine grain sizes may be the cause of these seismic and rheological properties. A self-consistent analytical 1-D channel flow model is developed to determine the dominant flow patterns in the upper mantle, taking into account its rheology and its dependence on flow-modified grain-sizes, water content, and melt fraction. The study finds that a plug flow configuration best explains the LQZ in the asthenosphere, and this can be attributed to significant grain-size reduction due to extensive shearing across the asthenosphere.