Dynamic Component of the Asthenosphere: Lateral Viscosity Variations Due To Dislocation Creep at the Base of Oceanic Plates

The asthenosphere is commonly defined as an upper mantle zone with low velocities and high attenuation of seismic waves, and high electrical conductivity. These observations are usually explained by the presence of partial melt, or by a sharp contrast in the water content of the upper mantle. Low viscosity asthenosphere is an essential ingredient of functioning plate tectonics. We argue that a substantial component of asthenospheric weakening is dynamic, caused by dislocation creep at the base of tectonic plates. Numerical simulations of subduction show that dynamic weakening scales with the surface velocity both below the subducting and the overriding plate, and that the viscosity decrease reaches up to two orders of magnitude. The resulting scaling law is employed in an apriori estimate of the lateral viscosity variations (LVV) below Earth's oceans. The obtained LVV help in explaining some of the long-standing as well as recent problems in mantle viscosity inversions. This work has been supported by the Czech Science Foundation through project No. 23-06345S

 

Figure 1. Dynamic weakening below Earth’s oceanic plates. Vectors show the absolute plate velocities derived by Müller et al. (2019), obtained with a freely available software package GPlates. Sublithospheric dynamic weakening, w, showed in color, is computed from these velocities using our empirical law, w = 1.5 [1 − exp(−0.3 v_P )]. The quantity w represents a first-order estimate of the LVV in the asthenosphere. Grey areas depict Earth’s continents, black and white lines show the major trenches and ridges, respectively (Coffin, 1998).

References:

Patočka, V., Čížková, H., & Pokorný, J. (2024). Dynamic component of the asthenosphere: Lateral viscosity variations due to dislocation creep at the base of oceanic plates. Geophysical Research Letters, 51, e2024GL109116. https://doi.org/10.1029/2024GL109116

Müller, R. D., Zahirovic, S., Williams, S. E., Cannon, J., Seton, M., Bower, D. J., Gurnis, M. (2019). A global plate model including lithospheric deformation
along major rifts and orogens since the triassic. Tectonics, 38 (6), 1884-1907, doi: 10.1029/2018TC005462

Coffin, M. (1998). Present-day plate boundary digital data compilation. University of Texas Institute for geophysics technical report, 174 , 5.