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- Title
Constraints on Mantle Viscosity From Slab Dynamics.
- Authors
Liu, Hao; Gurnis, Michael; Leng, Wei
- Abstract
The radial viscosity of the mantle is generally thought to increase by ∼10–100 times from the upper to lower mantle with a putative, abrupt increase at 660 km depth. Recently, a low viscosity channel (LVC) between 660 and 1,000 km has been suggested. We conduct a series of time‐dependent flow models with viscosity either increasing or decreasing at 660 km depth while tracking slab structure, state‐of‐stress, and geoid. We find that a LVC will lower the amplitude of long wavelength (>5,000 km) geoid highs over slabs, with amplitudes <10 m in height, while increasing the slab dip angle and downdip tension in the upper 300 km of slabs. A viscosity increase at 660 km gives rise to strong downdip compression throughout a slab and this pattern will largely go away with the introduction of the LVC. In addition, the endothermic phase change at 660 km depth can substantially affect the stress distribution within slabs but has a minor influence on the geoid. Models that fit the observed long wavelength geoid and observed focal mechanism in the western Pacific favor models without the presence of the LVC between 660 km and 1,000 km depths. Plain Language Summary: The viscosity of the mantle plays a key role in the thermochemical evolution of the Earth. It is generally believed that the mantle has a viscosity jump of 10–100 times at 660 km depth. Recently, some scholars suggest that there is a low viscosity channel (LVC) between 660 and 1,000 km depths. Here, we developed a series of geodynamic models with viscosity either increasing or decreasing at 660 km depth to track slab structure, state‐of‐stress, and geoid. We find that the existence of the LVC will reduce the amplitude of long wavelength geoid (>5,000 km) over slabs to less than 10 m, and increase the slab dip angle and downdip tension in the upper 300 km of slabs. An increase in viscosity at 660 km will lead to strong downdip compression throughout a slab, however, this pattern will disappear with the introduction of the LVC. In addition, the postspinel phase transition has a large influence on the stress state within slabs, but has a minor influence on the geoid. Compared with the observed long wavelength geoid and focal mechanism in the western Pacific, our models do not support the existence of the LVC between 660 and 1,000 km depths. Key Points: A low viscosity channel (LVC) between 660 and 1,000 km lowers the geoid over slabsThe LVC increases downdip tension in the upper 300 km within slabsOur results favor viscous models without the presence of the LVC
- Subjects
EARTH'S mantle; VISCOSITY; STRESS concentration; WAVELENGTHS; PHASE transitions
- Publication
Journal of Geophysical Research. Solid Earth, 2021, Vol 126, Issue 8, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2021JB022329